Article pubs.acs.org/JPCA
Combined ab Initio Molecular Dynamics and Experimental Studies of Carbon Atom Addition to Benzene Michael L. McKee,*,‡ Hans Peter Reisenauer,§ and Peter R. Schreiner*,§ ‡
Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, United States Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, 35392 Giessen, Germany
§
S Supporting Information *
ABSTRACT: Car−Parrinello molecular dynamics was used to explore the reactions between triplet and singlet carbon atoms with benzene. The computations reveal that, in the singlet C atom reaction, products are very exothermic where nearly every collision yields a product that is determined by the initial encounter geometry. The singlet C atom reaction does not follow the minimum energy path because the bimolecular reaction is controlled by dynamics (i.e., initial orientation of encounter). On the other hand, in a 10 K solid Ar matrix, ground state C(3P) atoms do tend to follow RRKM kinetics. Thus, ab initio molecular dynamics (AIMD) results indicate that a significant fraction of C−H insertion occurs to form phenylcarbene whereas, in marked contrast to previous theoretical and experimental conclusions, the Ar matrix isolation studies indicate a large fraction of direct cycloheptatetraene formation, without the intermediacy of phenylcarbene. The AIMD calculations are more consistent with vaporized carbon atom experiments where labeling studies indicate the initial formation of phenylcarbene. This underlines that the availability of thermodynamic sinks can completely alter the observed reaction dynamics.
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INTRODUCTION The study of reactions with small or no barriers presents formidable challenges to experimental as well as computational methods, especially because such reactions are also subject to nonstatical dynamics kinetics.1−17 Carbon atom reactions are a particular example where most products are over 100 kcal mol−1 more stable than the reactants. In addition, a number of different products are possible and selectivity is expected to be low. Carbon atoms can be generated photolytically by vaporizing a graphite surface,18,19 electrically by striking a carbon arc,18,20 chemically by decomposing a suitable precursor,17,20−22 or in nucleogenic reactions.18,23,24 A mixture of electronic states (3P, 1D, and 1S) with relative energies of 0, 30, and 52 kcal mol−1, respectively, ensues. In arc generated carbon atom reactions, higher carbon clusters, i.e., C2, C3, etc. form.25 The initially formed products of C atom reactions are almost never observed. Rather, more stable products formed by rearrangement or by trapping are detected, and the primary reactions are inferred. Pioneering work by Wolf, Skell, Shevlin, and others, has led to a number of discoveries concerning the products of carbon atom reactions.26−39 There are four ways of studying atomic carbon reactions (1) carbon atom reactors,26−39 (2) crossed beams,40−50 (3) use of radioactive carbon atoms (11C or 14 C),23,24 and (4) matrix isolation of carbon atoms deposited in low-temperature matrices.25 In the last method, the initial kinetic energy and subsequent potential energy can dissipate into the matrix. In carbon-atom reactors, the temperature at © 2014 American Chemical Society
which the reaction takes place is very difficult to assess. However, a branching ratio is clearly observed. In the case of carbon atoms plus benzene derivatives, C−H insertion to form phenylcarbene has been suggested.18,36−39 This was first reported by Wolf and co-workers51 in the reaction of 11C with toluene where 43% of the benzocyclobutene arose from otolylcarbene formed by C−H insertion (eq 1).
More elaborate labeling studies were carried out on toluene and benzene (1) by Shevlin and co-workers using 13C rather than 11C.52 The reaction of 13C with C6D6 where HBF4 was added to the cold matrix, yielded only one H−C7D6+BF4− product (eq 2) labeled in the position expected from initial C− H insertion followed by the phenylcarbene ring-expansion to cycloheptatetrene.53−55 However, as benzene readily exchanges its protons for deuterons in the presence of strong acids,56,57 this conclusion is not entirely convincing. As we will show in the present work, there is a direct path to the final product cycloheptateraene (3) not involving phenylcarbene (2) as an intermediate. Received: January 31, 2014 Revised: March 22, 2014 Published: March 24, 2014 2801
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Due to the different goals of the present study and the fact that the present system is more complicated, the direct dynamics classical trajectory simulations applied in this study is simpler than that used for F− + CH3I. Specifically, the initial relative velocity of carbon and benzene is set to zero. At the starting coordinates of the trajectory, the interaction is attractive or repulsive depending on the position on the “reactivity surface” (see below).
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METHODS Computational Details. Gaussian0365 was used for the nondynamic electronic structure calculations. In the bimolecular reaction between a carbon atom and benzene, we assume that the initial trajectory of the atom will be random. As the atom approaches the benzene molecule, the electrons in the two fragments will begin to interact. The initial geometry for the trajectories were random positions on an isodensity surface at 0.0005 au at the B3LYP/6-31G(d) level. Values of 0.001 or 0.002 are sometimes used as estimates of molecular volume.66,67 Thus, the value of 0.0005 au encloses a volume somewhat larger than the van der Waals volume for individual atoms. However, carbon atoms placed on this contour level will interact with the substrate because the distance from an atom to the “reactivity surface” will be substantially smaller than the sum the van der Waals (vdW) radius of carbon (the vdW radius of C is 1.70 Å68) and the vdW radius of the substrate atom. The interaction energy will be either repulsive or attractive, depending on the donor−acceptor interactions at that position. The two carbon atom singlet states (1D and 1S) are known to be 29.1 and 61.8 kcal mol−1 above the C(3P) state.69 In the present AIMD computations the singlet wave function for a carbon atom is not properly symmetry adapted and the singlet state does not correspond to a pure 1D or 1S state. Therefore, it is not surprising that the singlet state (approximately the average of the 1D or 1S states) is 44.4 kcal mol−1 above the triplet state. The C(1D) state consists of five microstates with both closed-shell and open-shell character.70 At infinite distance all five microstates have the same energy. However, as the carbon atom approaches the molecule, the different configurations separate. As the carbon atom approaches benzene, the 5-fold degeneracy will split into different electronic states. In most cases the lowest energy state will have significant closed shell character because that state will have the strongest donor−acceptor interactions with the substrate. The gOpenMol program71 was used to extract a mesh of points on the surface of benzene (isodensity surface at 0.0005 au at the B3LYP/6-31G(d) level). The points were selected with positive x, y, and z values (8-fold reduction surface area). The mesh of points was further reduced to obtain a set of 53 equally spaced points for benzene. A series of input files was created with benzene plus a carbon atom at one of the mesh points. Preliminary computations were carried out at CASPT2/ CASSCF/6-31G(d) with Molcas4.172 where the lowest two states were calculated with an 8,6 active space. The computed surface represents a “reactivity surface” where the interaction energy is mapped onto the isodensity surface. The CASPT2 natural population indicates that at many points the wave function has substantial closed shell character on the singlet surface. Each point of the reactivity surface was used as a starting geometry for a Car−Parrinello molecular dynamics (CPMD) computation.73−75 We have used the generalized gradient approximation (GGA) using the Perdew−Burke−Ernzerhof
One way to distinguish between products from triplet carbon and singlet carbon is to run the reaction in the presence of molecular oxygen. The 3Σg− state of molecular oxygen reacts efficiently with C(3P) to produce CO plus O(3P). If a particular product remains constant in the presence of molecular oxygen, then it may be assumed that the reaction is between C(1D) and the substrate. −
C(3P) + O2 (3Σ g ) → CO + O(3P)
(3)
In a crossed beam experiment, reactant conditions can be more carefully controlled. The supersonic expansion ensures that reactions take place under single-collision conditions; i.e., all of the potential energy is converted into kinetic, vibrational, and rotational energy of the reaction complex. The observed product always results from a fragmentation accompanied with hydrogen atom loss. In the reaction of C(3P) + CnHm, the most common product is Cn+1Hm−1 + H.58−60 In contrast, in a matrix isolation experiment, a graphite surface can be vaporized with a short laser pulse. With a sufficient delay in introducing the substrate, the reactions of ground state carbon C(3P) can be selected; two of us used this method to demonstrate that C(3P) atoms do not react with water molecules.25 In the present work, we use ab initio molecular dynamics (AIMD) to demonstrate that the initial reaction between carbon atoms and benzene is controlled by dynamics. After the formation of the initial product, further unimolecular rearrangements appear to follow RRKM theory; i.e., the rate of rearrangement is slow compared to the rate of internal vibrational energy redistribution (IVR).61,62 The products are determined by the initial approach of the carbon atom to benzene. On the other hand, the experimental work in a lowtemperature matrix demonstrates that the reaction of C(3P) atoms with 1 does follow the minimum energy pathway, which does, however, not involve the intermediacy of 2. The purpose of the present AIMD calculations is not to determine rate constants, but rather to rationalize why phenylcarbene is the major product of the gas phase beam Catom reaction with benzene whereas the same reactants give mainly cycloheptatetraene in a low temperature matrix. In contrast to molecular beam studies, the experimental conditions in C-atom reactions are not well controlled. The interior walls of the reaction vessel are coated with substrate and cooled to liquid nitrogen temperature, a carbon arc is struck, and volatile products are extracted and analyzed. A reaction that has some similarity to the C+C6H6 reaction that has been studied by molecular beam is F− + H3CI where the major products are FCH3 + I−, but HF + H2CI− and CH3 + FI− also form.63 The reaction is barrierless and the major products form quite exothermically. The reaction was also studied by direct dynamics classical trajectory simulations,64 where collision energies and impact parameters were varied.63 2802
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Figure 1. Experimental setup A and B (schematic): horizontal cut through the lower part of the cryostats used in this study.
Figure 2. FTIR spectra of the matrix isolated (Ar, 10 K) reaction products of atomic carbon generated by laser ablation of graphite and (A) benzene (1) and (B) benzene-d6 (1-d6). Bands of unreacted 1 and 1-d6 have been removed by subtraction for clarity as far as possible.
(PBE)76,77 exchange−correlation functional. A plane-wave basis set with an energy cutoff of 70 Ry and the Martins−Trouiller PBE norm-conserving pseudopotentials78 have been used. The time step and the fictitious electronic mass were fixed at 0.12 fs and 600 au, respectively.
The dynamics runs were started with no initial excess kinetic energy and allowed to run for 2000 time steps where the atom movements were dramatic. After 2000 time steps, a thermostat was set to 500 K and the trajectory was allowed to run another 20 000 time steps. The thermostat removes most of the kinetic energy, and molecular species did not cross major barriers after 2803
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Figure 3. Spectrum A shows the 254 nm photolyses products of matrix isolated phenyldiazomethane (Ar, 10 K) with the IR bands of phenylcarbene (2) and cycloheptatetraene (3). Spectrum B: FTIR spectrum of the reactions products of atomic carbon with benzene: 3 forms predominantly.
1100 nm); ESR spectrometer Varian E4 (at X-band frequency, 0.1−6.5 mT).
the thermostat was turned on. This behavior resembles the usual carbon-arc matrix isolation experiments where excess energy effectively dissipates into the matrix material. In contrast, in experiments carried out in crossed-beam studies, the products do not dissipate excess kinetic energy to thirdbody collisions and eventually eject a “hot” hydrogen atom. Experimental Section. We used experimental setups A and B (Figure 1) for carbon atom generation.25 The excimer laser beam was focused on the surface of a rotating graphite disk, which was driven by a magnetically coupled motor. The main difference between device A and B is the distance between the graphite disk and the cold window (A, 4 cm; B, 8 cm). In setup A the collimated laser beam passes through a hole in the cold window before hitting the graphite target. The shorter distance allows a higher concentration of atomic carbon in the matrix. Matrix samples were prepared by codeposition of the products of the laser-ablated graphite plume with a reactant/Ar mixture on the surface of the cold window (KBr, 10−12 K). The gas mixture was stored in a 2 L glass bulb and entered the vacuum system through a stainless steel capillary. The gas deposition was performed either continuously via an adjustable needle valve during the laser ablation (1−3 Hz) or pulsed by a magnetic valve (pulse length in the order of several ms). In the latter case the gas pulse (0.5−2 Hz) triggers the laser pulse with a variable delay, which normally lies in the middle of the gas pulse. The substrate/argon concentrations were typically in the range 0.1−0.4%; depositon rates were in the range 1−2 mmol h−1. UV/vis and EPR (Figure S2, Supporting Information) spectra were taken directly after deposition and after subsequent annealing of the matrix for 10−30 min at 27−30 K. In doing so, matrix-isolated carbon atoms that have not yet reacted during the condensation period become mobile and react with the substrate molecules leading to a significant increase of the product bands. Technical Details (Figure 1). Cryostat: closed-cycle helium refrigerator system (APD cryogenics, HC-2/DE-202; Leybold, RW-2/RGD 210). Laser: excimer laser LPX 100 (Lambda Physik) with Kr/F (248 nm), maximum pulse energy approximately 100 mJ per pulse. Spectrometer: FTIR Bruker IFS 85, IFS 55 (4500−300 cm−1, resolution 0.7 cm−1); diodearray UV/vis spectrometer Hewlett-Packard HP8453 (190−
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RESULTS AND DISCUSSION As noted previously, the main reaction product of “hot” carbon atoms reacting with benzene (1) (Figure 2A) and hexadeuterobenzene (1-d6) (Figure 2B) in a low-temperature Ar matrix are cycloheptatetraene (3) and hexadeuterocycloheptatetraene (3-d6), respectively (eq 2). We have repeated these experiments with the key difference being the use of ground state triplet carbon atoms as briefly outlined above and in detail in ref 25. All compounds were identified by comparison of their IR bands taken from the literature79 or from an authentic sample, obtained from photolysis of matrix isolated phenyldiazomethane (Figure 3). This comparison also shows that the concentration of phenylcarbene (2), which is also a photoproduct of phenyldiazomethane, cannot be detected in our matrix sample using IR spectroscopy. EPR spectroscopy, which is much more sensitive and indicative for paramagnetic species, however, does show that traces of 2 also form in the reaction of ground state carbon atoms with benzene. However, the main product under matrix isolation conditions is 3. Highly exothermic reactions are expected to show nonArrhenius behavior.14 This is particularly true when there is no activation barrier and the internal modes are converted into vibrational modes of the product. The rate of IVR is slow compared to the reaction,61,62 and the products are determined by the initial orientations of the reactants. Thus, the reaction does not follow the mass-weighted intrinsic reaction pathway (MIRP) where all of the momentum is removed from the system as the system evolves in time; rather the trajectory evolves in time and the system can increase its vibrational and kinetic energy. However, when carbon atoms are deposited on a low-temperature matrix, the vibrational/kinetic energy can be rapidly dissipated into the matrix and the product distributions may be quite different; this is likely to be the case also for reactions in the solid state and at surfaces. When the “reactivity surface” between C(3P) and benzene was constructed at the 0.0005 au electron density contour, the computed interaction energies varied from +23.5 to −3.9 kcal mol−1 on the triplet surface and +18.8 to −8.3 kcal mol−1 on 2804
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CCSD(T)/CBS level. In this study, which also included 5 and 6, the largest deviation in relative energies from our G3B3 results was less than 1 kcal mol−1. From C(3P) plus benzene, the lowest energy pathway leads to a 1,2-addition complex (37) 16.0 kcal mol−1 more stable than the reactants (Figure 4) with a small barrier (2.6 kcal mol−1) to 3 3. A 1,3-addition complex (36) is 27.2 kcal mol−1 more favorable than the reactants and displays a 15.1 kcal mol−1 barrier to the 1,2-addition complex (37). The larger kinetic stability of the 1,3-complex (36) compared to 37 (ΔH‡ = 15.1 versus 2.6 kcal mol−1), probably accounts for the AIMD results (see below); five trajectories found the 1,3-addition complex (36) but no 1,2-addition complex (37) products were found (Figure 4). From an inspection of the activation barrier to 33 and 32 (Figure 4), the branching ratio from either triplet complex (1,2- or 1,3-) would strongly favor 33 over 32. Once 33 forms, rearrangement to 32 should be slow due to 3TS2, which is 47.1 kcal mol−1 higher than 33. Instead, intersystem crossing to 3 is expected to be fast as it is 26.7 kcal mol−1 more stable, and because the geometries of the two states are rather similar.55 On the C(1D) plus benzene potential energy hypersurface (Figure 5), two complexes were found, a 1,2-addition complex (7) 72.7 kcal mol−1 lower in enthalpy (30.0 + 42.7) than the reactants (C(1D) + C6H6) and a 1,4-addition complex (8), which is 86.4 kcal mol−1 lower in enthalpy (30.0 + 56.4). The first complex has a 1.9 kcal mol−1 barrier (42.7−40.8) for formation of 3 whereas the second complex has a 13.3 kcal mol−1 barrier (56.4−43.1) for formation of 3. In the AIMD results, no 1,2-addition complex (7) products were found whereas three trajectories led to the 1,4-addition complex 8 (Figure 5). In the construction of the PES at the G3B3 level, no direct low-energy pathway could be located from C(1D) plus benzene to 2. Instead, the initial low-energy product is 3, which has a 30.2 kcal mol−1 barrier to 2 (Figure 5). RRKM theory, which follows this PES, would indicate that only 33 or 3 should result from the addition of C(3P) or C(1D) carbon atoms to benzene.
the singlet surface (CASPT2(8,6)/6-31G(d)). As mentioned in the Methods, as the carbon atom position changes over the “reactivity surface”, the interactions with benzene are dominated by attractive donor−acceptor interactions or by Pauli repulsion. To compare the AIMD results (Table 1) with behavior expected for a reaction where excess internal energy is removed, Table 1. Number of Times a Product Was Formed out of 53 MD Runs of Triplet and Singlet Carbon Atoms Adding to Benzene
phenylcarbene (2) cycloheptatetraene (3) H2 loss benzocyclopropane (5) 7-norbornadienylidene (6) 1,2-complex (7) 1,4-complex (8) no reaction total
triplet (Figures 4 and 6A)
singlet (Figures 5 and 6B)
20 25 0 0 5 0
29 19 1 1 0
3 53
3 0 53
the PES of C7H6 was computed at the G3B3 level. Our results on both the triplet (Figure 4) and singlet (Figure 5) surfaces agree well with previous results.53−55,80,81 The singlet phenylcarbene rearrangement 2 → 3 is well-known48−50 and the current results are in good agreement with previous results. For example, the energies for 2/TS2/4/TS1/3 are (298 K) 17.4, 30.2, 14.1, 16.5, and 0.0 kcal mol−1 (Figure 5) versus 15.7, 30.2, 14.8, 17.5, and 0.0 kcal mol−1 at the CCSD(T)/cc-pVTZ// B3LYP/6-31G(d) level.55 In addition, bicyclo[4.1.0]hepta2,4,6-triene (4), benzocyclopropene (5), and 7-norbordienylidene (6) are relevant to early stages of the C addition to benzene. The computed 32/2 separation is 3.2 kcal mol−1 (triplet favored) and the 33/3 separation is −26.7 kcal mol−1 (singlet favored).82 In a recent study80,81 of the C7H6 potential energy surface, relative energies were computed at the
Figure 4. Computed G3B3 potential energy surface for triplet carbon atoms adding to benzene. The numbers in parentheses indicate the number of times that product was formed out of the 53 trajectories. 2805
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Figure 5. Computed G3B3 potential energy surface for singlet carbon atoms adding to benzene. The energy of C(1D) + C6H6 (shown in brackets) was taken from experiment (not computed). The numbers in parentheses indicate the number of times that product formed out of 53 trajectories (cf. Table 1).
6B). In two trajectories, the insertion of C into the C−C bond is accompanied by H-migration. In these two cases, 2 initially formed followed by ring-opening (2 → 4 → 3). In one example, 2 formed after 650 time steps. After 2300 time steps, 4 formed, which persisted for about 1150 time steps before collapsing again to 3. These results indicate that both 2 and 3 are important species expected in the addition of C(1D) to benzene. The AIMD results in this study suggest a direct route to phenylcarbene (2) in addition to a route to cycloheptatetraene (3) with a computed ratio of 60:40 2:3. In contrast, the conclusion from the PES would be that only 3, which is separated from 2 by a 30.2 kcal mol−1 barrier, must form first. The AIMD computations (formation of both 2 and 3) as opposed to the PES predictions (formation of only 3) are in better agreement with the experimental results of Shevlin and co-workers18 who found only C−H insertion to phenylcarbene (2) under carbon arc conditions. In the experimental system of Shevlin et al.,18 intersystem system crossing may occur because the reactive species are a mixture of C(1D) and C(3P) as well as carbon dimers and trimers. The crossing may take place before collision with benzene (C(1D) → C(3P)), after reaction (triplet → singlet phenylcarbene and triplet → singlet cycloheptatetraene), or during the reaction. However, in all scenarios, the predicted ratio of phenylcarbene:cycloheptatetraene is about 1:1. There appear to be three limiting behaviors: (a) carbon-arc generation of carbon atoms that insert into the C−H bond of benzene to give 2, (b) the C(3P) + benzene reaction where carbons atoms insert into the benzene CC bond to form 3, and (c) crossed beam reaction where a hydrogen atom is eliminated. Path (a) is controlled by chemical dynamics where the initial trajectory controls the product; path (b) is controlled by RRKM dynamics where the C(3P) atoms are thermalized in a low temperature matrix. In this matrix, the direct CC addition is clearly preferred over C−H insertion.
Indeed, this result is entirely consistent with our matrix isolation studies of C(3P) but not with the results from arcgenerated carbon atoms where the initial formation of 2 is indicated from labeling studies.18 In the matrix isolation study, only C(3P) atoms are present in the matrix (vide supra).25 This is a significant simplification over the carbon arc results where there is a mixture of C(3P), C(1D), and carbon clusters, C2, C3, etc. Of the 53 AIMD trajectories on the triplet surface (Figure 6A), 25 (47%)
Figure 6. “Reactivity surfaces” for triplet (A) and singlet (B) carbon plus benzene. The color indicates the product formed when a trajectory starts with a carbon atom at this position. A: C(3P) (A) with 3 2 = orange (25), 33 = brown (20), 36 = purple (5), no reaction = green (3). B: C(1D) with 2 = orange (29), 3 = brown (19), 8 = purple (3), and other = green (2). The numbers in parentheses indicate the number of times that product formed out of 53 trajectories.
produced 32 whereas 20 (38%) produced 33, 5 trajectories produced 36 (9%), and 3 trajectories were nonreactive. In contrast, all of the trajectories on the singlet surface were found to be reactive. Presumably, the greater stability of a triplet relative to a singlet carbon atom results in a reduced reactivity. Indeed, in the reaction of C(3P) with H2O, Schreiner and Reisenauer found that C(3P) was unreactive toward H2O.25 Twenty-nine trajectories on the singlet surface (out of 53) gave 2 (yellow) whereas 19 trajectories formed 3 (blue, Figure 2806
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Notes
The AIMD modeling studies suggest the formation of a mixture of 2 and 3 for both triplet and singlet carbon atom reactions with benzene whereas only C−H insertion is observed experimentally for C(1D).18 We do not have an explanation for the exclusive production the C−H insertion product with arc-generated carbon atoms. However, it is possible that the benzene molecules coated onto the walls of the vessel are orientated in the matrix in such a way as to favor C−H insertion. The parallel-displaced and T-shaped orientation of the benzene dimer are calculated to be nearly isoenergetic.83 However, if π-stacking is favored in the matrix, the exposure of the benzene π-system to incoming carbon atoms might be hindered which would favor in-plane approach.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.L.M. thanks Philip B. Shevlin, emeritus, for significant mentoring during his many years investigating carbon atom reactions. The Alabama Supercomputer Network is thanked for a generous allocation of computer time.
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CONCLUSIONS The reactions of triplet and singlet carbon atoms with benzene show significant dynamic behavior as almost all reaction channels proceed without activation. In the absence of fast energy dissipation (as in the condensed state, e.g., under matrix isolation conditions), the reaction path followed is controlled by the initial trajectories of reactants, and the reaction rates are faster than the internal vibrational energy redistribution. The potential energy surface would lead one to expect that the reactions start from an initial complex of the carbon atom with the unsaturated CC bond in benzene. However, the majority of trajectories are controlled by C-atom insertion into benzene C−H bonds forming 2 as an intermediate. Dynamic reactivity is likely to be common in carbon atom reactions where atoms are generated either thermally or photolytically and allowed to react with (and be stabilized by) a substrate. The initially formed products are quenched by the matrix and allowed to exist long enough to react with substrates. Previous experimental studies of carbon atoms reactions with benzene and other aromatic compounds are consistent with the formation of 2. The AIMD computations give a ratio of 55:35 for the C(1D) trajectories for the formation of 2 and 3, respectively, and a 47:38 ratio for the C(3P) trajectories. This is significantly different from a consideration of the potential energy hypersurfaces where only 3 is expected. However, the experimental results of Shevlin and co-workers18 indicate that only 2 initially forms. The present study suggests that the initial approach of the carbon atom is the decisive factor in determining the product. Thus, the reactivity surface can be divided into a surface fraction giving C−H insertion product and a surface area giving C−C addition.
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ASSOCIATED CONTENT
* Supporting Information S
Figures S1 and S2 with additional FTIR and EPR spectra. Table S1 gives total and relative energies of species in Figures 4 and 5. Table S2 gives the Cartesian coordinates of triplet and singlet species in Figures 4 and 5 optimized at the B3LYP/631G(2df,p) level. Full ref 65. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
(1) Bunker, D. L.; Hase, W. L. Non-RRKM [Rice-RamspergerKassel-Marcus] Unimolecular Kinetics. Molecules in General, and Acetonitrile in Particular. J. Chem. Phys. 1973, 59, 4621−4632. (2) Marcus, R. A.; Hase, W. L.; Swamy, K. N. RRKM and nonRRKM Behavior in Chemical Activation and Related Studies. J. Phys. Chem. 1984, 88, 6717−6720. (3) Borkovec, M.; Straub, J. E.; Berne, B. J. The Influence of Intramolecular Vibrational Relaxation on the Pressure Dependence of Unimolecular Rate Constants. J. Chem. Phys. 1986, 85, 146−149. (4) Carpenter, B. K. Dynamic Behavior of Organic Reactive Intermediates. Angew. Chem., Int. Ed. 1998, 37, 3340−3350. (5) Tse, J. E. Ab Initio Molecular Dynamics with Density Functional Theory. Annu. Rev. Phys. Chem. 2002, 53, 249−290. (6) Wang, Y.; Hase, W. L. Trajectory Studies of SN2 Nucleophilic Substitution. 9. Microscopic Reaction Pathways and Chemical Kinetics for Cl− + CH3Br. J. Chem. Phys. 2003, 118, 2688−2695. (7) Carpenter, B. K. Nonstatistical Dynamics In Thermal Reactions of Polyatomic Molecules. Annu. Rev. Phys. Chem. 2005, 56, 57−89. (8) Levine, R. D. Molecular Reaction Dynamics; Cambridge University Press: Cambridge, U.K., 2005. (9) Upadhyay, S. K. Chemical Kinetics and Reaction Dynamics; Springer: New York, 2006. (10) Litovitz, A. E.; Keresztes, I.; Carpenter, B. K. Evidence for Nonstatistical Dynamics in the Wolff Rearrangement of a Carbene. J. Am. Chem. Soc. 2008, 130, 12085−12094. (11) Lourderaj, U.; Park, L.; Hase, W. L. Classical Trajectory Simulations of Post-Transition State Dynamics. Int. Rev. Phys. Chem. 2008, 27, 361−403. (12) Lourderaj, U.; Hase, W. L. Theoretical and Computational Studies of Non-RRKM Unimolecular Dynamics. J. Phys. Chem. A 2009, 113, 2236−2253. (13) Glowacki, D. R.; Liang, C. H.; Marsden, S. P.; Harvey, J. N.; Pilling, M. J. Alkene Hydroboration: Hot Intermediates That React While They Are Cooling. J. Am. Chem. Soc. 2010, 132, 13621−13623. (14) Yamataka, H. Molecular Dynamics Simulations and Mechanism of Organic Reactions: Non-TST Behaviors. Adv. Phys. Org. Chem. 2010, 44, 173−222. (15) Berne, B. J.; Borkovec, M.; Straub, J. E. Classical and Modern Methods in Reaction Rate Theory. J. Phys. Chem. 1988, 92, 3711− 3725. (16) Manikandan, P.; Zhang, J.; Hase, W. L. Chemical Dynamics Simulations of X− + CH3Y− → XCH3 + Y− Gas-Phase SN2 Nucleophilic Substitution Reactions. Nonstatistical Dynamics and Nontraditional Reaction Mechanisms. J. Phys. Chem. A 2012, 116, 3061−3080. (17) Paranjothy, M.; Sun, R.; Zhuang, Y.; Hase, W. L. Direct Chemical Dynamics Simulations: Coupling of Classical and Quasiclassical Trajectories with Electronic Structure Theory. WIREs Comput. Mol. Sci. 2013, 3, 296−316. (18) Shevlin, P. B. Atomic Carbon. In Reactive Intermediate Chemistry; Moss, R. A., Platz, M. S., Jones, M., Eds.; Wiley: New York, 2004; pp 463−500. (19) Verdieck, J. F.; Mau, A. W. H. Some Laser-Induced Reactions in the Gas Phase. Chem. Commun. 1969, 226−226. (20) Skell, P. S.; Havel, J.; McGlinchey, M. J. Chemistry and the Carbon Arc. Acc. Chem. Res. 1973, 6, 97−105. (21) Moll, N. G.; Thompson, W. E. Reactions of Carbon Atoms with N2, H2, and D2 at 4.2 K. J. Chem. Phys. 1966, 44, 2684−2686.
AUTHOR INFORMATION
Corresponding Authors
*M. L. McKee: e-mail, [email protected]; telephone, +1-334-844-6953. *P. R. Schreiner: e-mail, [email protected]; telephone: +49641-9934300. 2807
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Acetylene and Its Isotopomers. J. Phys. Chem. A 2007, 111, 2980− 2992. (45) Leonori, F.; Petrucci, R.; Segoloni, E.; Bergeat, A.; Hickson, K. M.; Balucani, N.; Casavecchia, P. Unraveling the Dynamics of the C(3P,1D) + C2H2 Reactions by the Crossed Molecular Beam Scattering Technique. J. Phys. Chem. A 2008, 112, 1363−1379. (46) Li, Y.; Liu, H.-L.; Huang, X.-R.; Sun, Y.-B.; Li, Z.; Sun, C.-C. Radical−Molecule Reaction C(3P) + C3H6: Mechanistic Study. J. Chem. Phys. A 2009, 113, 10577−10587. (47) Li, Y.; Liu, H.-L.; Huang, X.-R.; Wang, D.-Q.; Sun, C.-C. Theoretical Study of the C(3P) + Trans-C4H8 Reaction. J. Phys. Chem. A 2009, 113, 6800−6811. (48) Parker, D. S. N.; Zhang, F.; Kim, Y. S.; Kaiser, R. I.; Mebel, A. M. On the Formation of Resonantly Stabilized C5H3 Radicals−A Crossed Beam and Ab Initio Study of the Reaction of Ground State Carbon Atoms with Vinylacetylene. J. Phys. Chem. A 2011, 115, 593− 601. (49) Balucani, N.; Lee, H. Y.; Mebel, A. M.; Lee, Y. T.; Kaiser, R. I. A Combined Crossed Beam and Ab Initio Investigation on the Reaction of Carbon Species with C4H6 Isomers. III. 1,2-Butadiene, H2CCCH(CH3) (X1A′)−a non-Rice−Ramsperger−Kassel−Marcus system? J. Chem. Phys. 2001, 115, 5107−5116. (50) Kaiser, R. I.; Nguyen, T. L.; Mebel, A. M.; Lee, Y. T. Stripping Dynamics in the Reactions of Electronically Excited Carbon Atoms, C(1D), with Ethylene and PropyleneProduction of Propargyl and Methylpropargyl Radicals. J. Chem. Phys. 2002, 116, 1318−1324. (51) Gaspar, P. P.; Berowitz, D. M.; Strongin, D. R.; Svoboda, D. L.; Tuchler, M. B.; Ferrieri, R. A.; Wolf, A. P. Reactions of Recoiling Carbon-11 Atoms with Toluene. J. Phys. Chem. 1986, 90, 4691. (52) Armstrong, B. M.; Zheng, F.; Shevlin, P. B. Mode of Attack of Atomic Carbon on Benzene Rings. J. Am. Chem. Soc. 1998, 120, 6007− 6011. (53) Matzinger, S.; Bally, T.; Patterson, E. V.; McMahon, R. J. The C7H6 Potential Energy Surface Revisited: Relative Energies and IR Assignment. J. Am. Chem. Soc. 1996, 118, 1535−1542. (54) Wong, M. W.; Wentrup, C. Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C7H6 Energy Surface. J. Org. Chem. 1996, 61, 7022−7039. (55) Schreiner, P. R.; Karney, W. L.; Schleyer, P. v. R.; Borden, W. T.; Hamilton, T. P.; Schaefer, H. F. Carbene Rearrangements Unsurpassed: Details of the C7H6 Potential Energy Surface Revealed. J. Org. Chem. 1996, 61, 7030−7039. (56) Reed, C. A.; Kim, K.-C.; Stoyanov, E. S.; Stasko, D.; Tham, F. S.; Mueller, L. J.; Boyd, P. D. W. Isolating Benzenium Ion Salts. J. Am. Chem. Soc. 2003, 125, 1796−1804. (57) Chiavarino, B.; Crestoni, M. E.; Di Rienzo, B.; Fornarini, S.; Lanucara, F. Site-Selectivity of Protonation in Gaseous Toluene. Phys. Chem. Chem. Phys. 2008, 10, 5507−5509. (58) Kaiser, R. I.; Vereecken, L.; Peeters, J.; Bettinger, H. F.; Schleyer, P. v. R.; Schaefer, H. F. Elementary Reactions of the Phenyl Radical, C6H5, with C3H4 Isomers, and of Benzene, C6H6, with Atomic Carbon in Extraterrestrial Environments. Astron. Astrophys. 2003, 406, 385−391. (59) Kaiser, R. I; Hahndorf, I.; Huang, L. C. L.; Lee, Y. T.; Bettinger, H. F.; Schleyer, P. v. R.; Schaefer, H. F.; Schreiner, P. R. Crossed Beams Reaction of Atomic Carbon, C(3Pj), with d6-benzene, C6D6(X1A1g): Observation of the Per-Deutero-1,2-Didehydro-Cycloheptatrienyl Radical, C7D5(X2B2). J. Chem. Phys. 1999, 110, 6091− 6094. (60) Kaiser, R. I. Experimental Investigation on the Formation of Carbon-Bearing Molecules in the Interstellar Medium via Neutral− Neutral Reactions. Chem. Rev. 2002, 102, 1309−1358. (61) Carpenter, B. K. Nonexponential Decay of Reactive Intermediates: New Challenges for Spectroscopic Observation, Kinetic Modeling and Mechanistic Interpretation. J. Phys. Org. Chem. 2003, 16, 858−868. (62) Carpenter, B. K. Energy Disposition in Reactive Intermediates. Chem. Rev. 2013, 113, 7265−7286.
(22) Shevlin, P. B.; Wolf, A. P. The Formation of Carbon Atoms in the Decomposition of a Carbene. Tetrahedron Lett. 1970, 3987−3990. (23) Ache, H. J.; Wolf, A. P. Reactions of Energetic Carbon Atoms with Nitrogen Molecules [1]. Radiochim. Acta 1966, 6, 33−39. (24) Gaspar, P. P.; Berowitz, D. M.; Strongin, D. R.; Svoboda, D. L.; Tuchler, M. B.; Ferrieri, R. A.; Wolf, A. P. Reactions of Recoiling Carbon-11 Atoms with Toluene. J. Phys. Chem. 1986, 90, 4691−4694. (25) Schreiner, P. R.; Reisenauer, H. P. The Non-reaction of Ground-State Triplet Carbon Atoms with Water Revisited. ChemPhysChem 2006, 7, 880−885. (26) Skell, P. S.; Plonka, J. H.; Engel, R. R. Deoxygenation by Carbon Atoms. J. Am. Chem. Soc. 1967, 89, 1748−1749. (27) Skell, P. S.; Villaume, J. E.; Plonka, J. H.; Fagone, F. A. Reactions of Metastable Singlet Carbon Atoms with Unsaturated Hydrocarbons. J. Am. Chem. Soc. 1971, 93, 2699−2702. (28) Skell, P. S.; Plonka, J. H. Deoxygenation by Atomic Carbon. II. Generation of Carbenes from the Reaction of Carbonyl Compounds with Metastable Singlet State Carbon Atoms. J. Am. Chem. Soc. 1970, 92, 836−839. (29) Skell, P. S.; Plonka, J. H. Deoxygenation by Atomic Carbon. III. Dichlorocarbene and Methoxycarbene. J. Am. Chem. Soc. 1970, 92, 2160−2161. (30) Skell, P. S.; Klabunde, K. J.; Plonka, J. H. Deoxygenation of Oxetans by Carbon Atoms. On the Intermediacy of Trimethylene. Chem. Commun. 1970, 1109−1110. (31) Skell, P. S.; Plonka, J. H. Deoxygenation of Cyclopropanecarboxaldehyde and Cyclopropylmethylketone: Cyclopropyl and Cyclopropylmethylcarbene. Tetrahedron 1972, 28, 3571−3582. (32) Welch, M. J.; Wolfe, A. P. Reaction Intermediates in the Chemistry of Recoil Carbon Atoms. Chem. Commun. 1968, 117−118. (33) Ferrieri, R. A.; Wolf, A. P.; Tang, Y.-N. Reaction Selectivity of Translationally and Electronically Excited Carbon-11 Atoms with Ethylene. J. Am. Chem. Soc. 1983, 105, 5428−5433. (34) Ferrieri, R. A.; Sharma, R. B.; Wolf, A. P. Structural Influences on the Chemical Reactivity of Hydrocarbons Toward Nucleogenic Carbon Atoms. Tetrahedron 1997, 53, 10155−10168. (35) Herrick, D. B.; Thamattoor, D. M.; Shevlin, P. B. Reactions of Atomic Carbon with Acyl Chlorides. Tetrahedron Lett. 2008, 49, 6036−6038. (36) Geise, C. M.; Hadad, C. M.; Zheng, F.; Shevlin, P. B. An Experimental and Computational Evaluation of the Energetics of the Isomeric Methoxyphenylcarbenes Generated in Carbon Atom Reactions. J. Am. Chem. Soc. 2002, 124, 355−364. (37) Armstrong, B. M.; Zheng, F.; Shevlin, P. B. Mode of Attack of Atomic Carbon on Benzene Rings. J. Am. Chem. Soc. 1998, 120, 6007− 6011. (38) Sevin, F.; Sökmen, I.̇ ; Düz Shevlin, P. B. Trapping of a Cycloheptatetraene in the Reaction of Atomic Carbon with Phenol. Terahedron Lett. 2003, 44, 3405−3407. (39) Sökmen, I.̇ ; Ece, A.; Düz, B.; Sevin, F. Reaction of Atomic Carbon with Isomeric Cresols. Lett. Org. Chem. 2009, 6, 650−665. (40) MacKay, C.; Nicholas, J.; Wolfgang, R. Reactions of Carbon Atoms and Methyne with Hydrogen and Ethylene. J. Am. Chem. Soc. 1967, 89, 5758−5766. (41) Casavecchia, P.; Balucani, N.; Cartechini, L.; Capozza, G.; Bergeat, A.; Volpi, G. G. Crossed Beam Studies of Elementary Reactions of N and C Atoms and CN Radicals of Importance in Combustion. Faraday Disc. 2001, 119, 27−49. (42) Nguyen, T. L.; Kim, G.-S.; Mebel, A. M.; Nguyen, M. T. A Theoretical Re-evaluation of the Heat of Formation of Phenylcarbene. Chem. Phys. Lett. 2001, 349, 571−577. (43) Park, W. K.; Park, J.; Park, S. C.; Braams, B. J.; Chen, C.; Bowman, J. M. Quasiclassical Trajectory Calculations of the Reaction C+C2H2→l-C3H, c-C3H+H, C3+H2 using Full-Dimensional Triplet and Singlet Potential Energy Surfaces. J. Chem. Phys. 2006, 125, 081101−1−081101−5. (44) Gu, X.; Zhang, F.; Kaiser, R. I. Investigating the Chemical Dynamics of the Reaction of Ground-State Carbon Atoms with 2808
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(63) Mikosch, J.; Zhang, J.; Trippel, S.; Eichhorn, C.; Otto, R.; Sun, R.; de Jong, W. A.; Weidemüller, M.; Hase, W. L.; Wester, R. Indirect Dynamics in a Highly Exoergic Substitution Reaction. J. Am. Chem. Soc. 2013, 135, 4250−4259. (64) Sun, L.; Hase, W. L. Born-Oppenheimer Direct Dynamics Classical Trajectory Simulations. Rev. Comput. Chem. 2003, 19, 79− 146. (65) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03 E.01; Gaussian, Inc.: Wallingford, CT, 2004 (see Supporting Information for full citation). (66) The 0.001 electrons/bohr3 contour is used by G03 to determine molecular volumes (keyword=VOLUME). (67) The 0.002 au contour is recommended by Bader and co-workers as the van der Waals envelope. Bader, R. F. W.; Carroll, M. T.; Cheeseman, J. R.; Chang, C. Properties of Atoms in Molecules: Atomic Volumes. J. Am. Chem. Soc. 1987, 109, 7968−7979. (68) Bondi, A. van der Waals Volumes and Radii. J. Phys. Chem. 1964, 68, 441−451. (69) Moore, C. E. Atomic Energy Levels; National Bureau of Standards Circular No. 467; National Bureau of Standards: Washington, DC, 1949, 1952, and 1958. (70) Herzberg, G. Atomic Spectra and Atomic Structure; Dover Books: New York, 1944. (71) gOpenMol is a molecular visualization program written by Leif Laaksonen and available from him at http://www.csc.fi/gopenmol (accessed Mar 13, 2014). (72) Molcas 4.1 (Current version 7.8): Karlström, G.; Lindh, R.; Malmqvist, P.-Å.; Roos, B. O.; Ryde, U.; Veryazov, V.; Widmark, P.-O.; Cossi, M.; Schimmelpfennig, B.; Neogrady, P.; Seijo, L. Molcas: a program package for computational chemistry. Comput. Mater. Sci. 2003, 28, 222−239. (73) Car, R.; Parrinello, M. Unified Approach for Molecular Dynamics and Density-Functional Theory. Phys. Rev. Lett. 1985, 55, 2471−2474. (74) CPMD V3.13.1, CPMD, http://www.cpmd.org (accessed Mar 13, 2014). (75) Marx, D.; Hutter, J. In Modern Methods and Algorithms of Quantum Chemistry; Grotendorst, J., Ed.; NIC Series; Forschungszentrum Jülich: Jülich, Germany, 2000; Vol. 1, pp 301−449. (76) Ceperley, D. M.; Alder, B. J. Ground State of the Electron Gas by a Stochastic Method. Phys. Rev. Lett. 1980, 45, 566−569. (77) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (78) Troullier, N.; Martins, J. L. Efficient Pseudopotentials for PlaneWave Calculations. Phys. Rev. B 1991, 43, 1993−2006. (79) Sander, W. Reaction of Phenylcarbene with Oxygen. Angew. Chem. Int. Edit. Engl. 1985, 24, 988−989. (80) Polino, D.; Famulari, A.; Cavallotti, C. Analysis of the Reactivity on the C7H6 Potential Energy Surface. J. Phys. Chem. A 2011, 115, 7928−7936. (81) Polino, D.; Cavallotti, C. Fulvenallene Decomposition Kinetics. J. Phys. Chem. A 2011, 115, 10281−10289. (82) A bold notation without superscript refers to a singlet species whereas a superscript 3 refers to a triplet species (e.g., 2 and 32). (83) Janowski, T.; Pulay, P. High Accuracy Benchmark Calculations on the Benzene Dimer Potential Energy Surface. Chem. Phys. Lett. 2007, 447, 27−32.
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Combined ab Initio Molecular Dynamics and Experimental Studies of Carbon Atom Addition to Benzene Michael L. McKee,*,‡ Hans Peter Reisenauer,§ and Peter R. Schreiner*,§ ‡
Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, United States Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, 35392 Giessen, Germany
§
S Supporting Information *
ABSTRACT: Car−Parrinello molecular dynamics was used to explore the reactions between triplet and singlet carbon atoms with benzene. The computations reveal that, in the singlet C atom reaction, products are very exothermic where nearly every collision yields a product that is determined by the initial encounter geometry. The singlet C atom reaction does not follow the minimum energy path because the bimolecular reaction is controlled by dynamics (i.e., initial orientation of encounter). On the other hand, in a 10 K solid Ar matrix, ground state C(3P) atoms do tend to follow RRKM kinetics. Thus, ab initio molecular dynamics (AIMD) results indicate that a significant fraction of C−H insertion occurs to form phenylcarbene whereas, in marked contrast to previous theoretical and experimental conclusions, the Ar matrix isolation studies indicate a large fraction of direct cycloheptatetraene formation, without the intermediacy of phenylcarbene. The AIMD calculations are more consistent with vaporized carbon atom experiments where labeling studies indicate the initial formation of phenylcarbene. This underlines that the availability of thermodynamic sinks can completely alter the observed reaction dynamics.
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INTRODUCTION The study of reactions with small or no barriers presents formidable challenges to experimental as well as computational methods, especially because such reactions are also subject to nonstatical dynamics kinetics.1−17 Carbon atom reactions are a particular example where most products are over 100 kcal mol−1 more stable than the reactants. In addition, a number of different products are possible and selectivity is expected to be low. Carbon atoms can be generated photolytically by vaporizing a graphite surface,18,19 electrically by striking a carbon arc,18,20 chemically by decomposing a suitable precursor,17,20−22 or in nucleogenic reactions.18,23,24 A mixture of electronic states (3P, 1D, and 1S) with relative energies of 0, 30, and 52 kcal mol−1, respectively, ensues. In arc generated carbon atom reactions, higher carbon clusters, i.e., C2, C3, etc. form.25 The initially formed products of C atom reactions are almost never observed. Rather, more stable products formed by rearrangement or by trapping are detected, and the primary reactions are inferred. Pioneering work by Wolf, Skell, Shevlin, and others, has led to a number of discoveries concerning the products of carbon atom reactions.26−39 There are four ways of studying atomic carbon reactions (1) carbon atom reactors,26−39 (2) crossed beams,40−50 (3) use of radioactive carbon atoms (11C or 14 C),23,24 and (4) matrix isolation of carbon atoms deposited in low-temperature matrices.25 In the last method, the initial kinetic energy and subsequent potential energy can dissipate into the matrix. In carbon-atom reactors, the temperature at © 2014 American Chemical Society
which the reaction takes place is very difficult to assess. However, a branching ratio is clearly observed. In the case of carbon atoms plus benzene derivatives, C−H insertion to form phenylcarbene has been suggested.18,36−39 This was first reported by Wolf and co-workers51 in the reaction of 11C with toluene where 43% of the benzocyclobutene arose from otolylcarbene formed by C−H insertion (eq 1).
More elaborate labeling studies were carried out on toluene and benzene (1) by Shevlin and co-workers using 13C rather than 11C.52 The reaction of 13C with C6D6 where HBF4 was added to the cold matrix, yielded only one H−C7D6+BF4− product (eq 2) labeled in the position expected from initial C− H insertion followed by the phenylcarbene ring-expansion to cycloheptatetrene.53−55 However, as benzene readily exchanges its protons for deuterons in the presence of strong acids,56,57 this conclusion is not entirely convincing. As we will show in the present work, there is a direct path to the final product cycloheptateraene (3) not involving phenylcarbene (2) as an intermediate. Received: January 31, 2014 Revised: March 22, 2014 Published: March 24, 2014 2801
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Due to the different goals of the present study and the fact that the present system is more complicated, the direct dynamics classical trajectory simulations applied in this study is simpler than that used for F− + CH3I. Specifically, the initial relative velocity of carbon and benzene is set to zero. At the starting coordinates of the trajectory, the interaction is attractive or repulsive depending on the position on the “reactivity surface” (see below).
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METHODS Computational Details. Gaussian0365 was used for the nondynamic electronic structure calculations. In the bimolecular reaction between a carbon atom and benzene, we assume that the initial trajectory of the atom will be random. As the atom approaches the benzene molecule, the electrons in the two fragments will begin to interact. The initial geometry for the trajectories were random positions on an isodensity surface at 0.0005 au at the B3LYP/6-31G(d) level. Values of 0.001 or 0.002 are sometimes used as estimates of molecular volume.66,67 Thus, the value of 0.0005 au encloses a volume somewhat larger than the van der Waals volume for individual atoms. However, carbon atoms placed on this contour level will interact with the substrate because the distance from an atom to the “reactivity surface” will be substantially smaller than the sum the van der Waals (vdW) radius of carbon (the vdW radius of C is 1.70 Å68) and the vdW radius of the substrate atom. The interaction energy will be either repulsive or attractive, depending on the donor−acceptor interactions at that position. The two carbon atom singlet states (1D and 1S) are known to be 29.1 and 61.8 kcal mol−1 above the C(3P) state.69 In the present AIMD computations the singlet wave function for a carbon atom is not properly symmetry adapted and the singlet state does not correspond to a pure 1D or 1S state. Therefore, it is not surprising that the singlet state (approximately the average of the 1D or 1S states) is 44.4 kcal mol−1 above the triplet state. The C(1D) state consists of five microstates with both closed-shell and open-shell character.70 At infinite distance all five microstates have the same energy. However, as the carbon atom approaches the molecule, the different configurations separate. As the carbon atom approaches benzene, the 5-fold degeneracy will split into different electronic states. In most cases the lowest energy state will have significant closed shell character because that state will have the strongest donor−acceptor interactions with the substrate. The gOpenMol program71 was used to extract a mesh of points on the surface of benzene (isodensity surface at 0.0005 au at the B3LYP/6-31G(d) level). The points were selected with positive x, y, and z values (8-fold reduction surface area). The mesh of points was further reduced to obtain a set of 53 equally spaced points for benzene. A series of input files was created with benzene plus a carbon atom at one of the mesh points. Preliminary computations were carried out at CASPT2/ CASSCF/6-31G(d) with Molcas4.172 where the lowest two states were calculated with an 8,6 active space. The computed surface represents a “reactivity surface” where the interaction energy is mapped onto the isodensity surface. The CASPT2 natural population indicates that at many points the wave function has substantial closed shell character on the singlet surface. Each point of the reactivity surface was used as a starting geometry for a Car−Parrinello molecular dynamics (CPMD) computation.73−75 We have used the generalized gradient approximation (GGA) using the Perdew−Burke−Ernzerhof
One way to distinguish between products from triplet carbon and singlet carbon is to run the reaction in the presence of molecular oxygen. The 3Σg− state of molecular oxygen reacts efficiently with C(3P) to produce CO plus O(3P). If a particular product remains constant in the presence of molecular oxygen, then it may be assumed that the reaction is between C(1D) and the substrate. −
C(3P) + O2 (3Σ g ) → CO + O(3P)
(3)
In a crossed beam experiment, reactant conditions can be more carefully controlled. The supersonic expansion ensures that reactions take place under single-collision conditions; i.e., all of the potential energy is converted into kinetic, vibrational, and rotational energy of the reaction complex. The observed product always results from a fragmentation accompanied with hydrogen atom loss. In the reaction of C(3P) + CnHm, the most common product is Cn+1Hm−1 + H.58−60 In contrast, in a matrix isolation experiment, a graphite surface can be vaporized with a short laser pulse. With a sufficient delay in introducing the substrate, the reactions of ground state carbon C(3P) can be selected; two of us used this method to demonstrate that C(3P) atoms do not react with water molecules.25 In the present work, we use ab initio molecular dynamics (AIMD) to demonstrate that the initial reaction between carbon atoms and benzene is controlled by dynamics. After the formation of the initial product, further unimolecular rearrangements appear to follow RRKM theory; i.e., the rate of rearrangement is slow compared to the rate of internal vibrational energy redistribution (IVR).61,62 The products are determined by the initial approach of the carbon atom to benzene. On the other hand, the experimental work in a lowtemperature matrix demonstrates that the reaction of C(3P) atoms with 1 does follow the minimum energy pathway, which does, however, not involve the intermediacy of 2. The purpose of the present AIMD calculations is not to determine rate constants, but rather to rationalize why phenylcarbene is the major product of the gas phase beam Catom reaction with benzene whereas the same reactants give mainly cycloheptatetraene in a low temperature matrix. In contrast to molecular beam studies, the experimental conditions in C-atom reactions are not well controlled. The interior walls of the reaction vessel are coated with substrate and cooled to liquid nitrogen temperature, a carbon arc is struck, and volatile products are extracted and analyzed. A reaction that has some similarity to the C+C6H6 reaction that has been studied by molecular beam is F− + H3CI where the major products are FCH3 + I−, but HF + H2CI− and CH3 + FI− also form.63 The reaction is barrierless and the major products form quite exothermically. The reaction was also studied by direct dynamics classical trajectory simulations,64 where collision energies and impact parameters were varied.63 2802
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Figure 1. Experimental setup A and B (schematic): horizontal cut through the lower part of the cryostats used in this study.
Figure 2. FTIR spectra of the matrix isolated (Ar, 10 K) reaction products of atomic carbon generated by laser ablation of graphite and (A) benzene (1) and (B) benzene-d6 (1-d6). Bands of unreacted 1 and 1-d6 have been removed by subtraction for clarity as far as possible.
(PBE)76,77 exchange−correlation functional. A plane-wave basis set with an energy cutoff of 70 Ry and the Martins−Trouiller PBE norm-conserving pseudopotentials78 have been used. The time step and the fictitious electronic mass were fixed at 0.12 fs and 600 au, respectively.
The dynamics runs were started with no initial excess kinetic energy and allowed to run for 2000 time steps where the atom movements were dramatic. After 2000 time steps, a thermostat was set to 500 K and the trajectory was allowed to run another 20 000 time steps. The thermostat removes most of the kinetic energy, and molecular species did not cross major barriers after 2803
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Figure 3. Spectrum A shows the 254 nm photolyses products of matrix isolated phenyldiazomethane (Ar, 10 K) with the IR bands of phenylcarbene (2) and cycloheptatetraene (3). Spectrum B: FTIR spectrum of the reactions products of atomic carbon with benzene: 3 forms predominantly.
1100 nm); ESR spectrometer Varian E4 (at X-band frequency, 0.1−6.5 mT).
the thermostat was turned on. This behavior resembles the usual carbon-arc matrix isolation experiments where excess energy effectively dissipates into the matrix material. In contrast, in experiments carried out in crossed-beam studies, the products do not dissipate excess kinetic energy to thirdbody collisions and eventually eject a “hot” hydrogen atom. Experimental Section. We used experimental setups A and B (Figure 1) for carbon atom generation.25 The excimer laser beam was focused on the surface of a rotating graphite disk, which was driven by a magnetically coupled motor. The main difference between device A and B is the distance between the graphite disk and the cold window (A, 4 cm; B, 8 cm). In setup A the collimated laser beam passes through a hole in the cold window before hitting the graphite target. The shorter distance allows a higher concentration of atomic carbon in the matrix. Matrix samples were prepared by codeposition of the products of the laser-ablated graphite plume with a reactant/Ar mixture on the surface of the cold window (KBr, 10−12 K). The gas mixture was stored in a 2 L glass bulb and entered the vacuum system through a stainless steel capillary. The gas deposition was performed either continuously via an adjustable needle valve during the laser ablation (1−3 Hz) or pulsed by a magnetic valve (pulse length in the order of several ms). In the latter case the gas pulse (0.5−2 Hz) triggers the laser pulse with a variable delay, which normally lies in the middle of the gas pulse. The substrate/argon concentrations were typically in the range 0.1−0.4%; depositon rates were in the range 1−2 mmol h−1. UV/vis and EPR (Figure S2, Supporting Information) spectra were taken directly after deposition and after subsequent annealing of the matrix for 10−30 min at 27−30 K. In doing so, matrix-isolated carbon atoms that have not yet reacted during the condensation period become mobile and react with the substrate molecules leading to a significant increase of the product bands. Technical Details (Figure 1). Cryostat: closed-cycle helium refrigerator system (APD cryogenics, HC-2/DE-202; Leybold, RW-2/RGD 210). Laser: excimer laser LPX 100 (Lambda Physik) with Kr/F (248 nm), maximum pulse energy approximately 100 mJ per pulse. Spectrometer: FTIR Bruker IFS 85, IFS 55 (4500−300 cm−1, resolution 0.7 cm−1); diodearray UV/vis spectrometer Hewlett-Packard HP8453 (190−
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RESULTS AND DISCUSSION As noted previously, the main reaction product of “hot” carbon atoms reacting with benzene (1) (Figure 2A) and hexadeuterobenzene (1-d6) (Figure 2B) in a low-temperature Ar matrix are cycloheptatetraene (3) and hexadeuterocycloheptatetraene (3-d6), respectively (eq 2). We have repeated these experiments with the key difference being the use of ground state triplet carbon atoms as briefly outlined above and in detail in ref 25. All compounds were identified by comparison of their IR bands taken from the literature79 or from an authentic sample, obtained from photolysis of matrix isolated phenyldiazomethane (Figure 3). This comparison also shows that the concentration of phenylcarbene (2), which is also a photoproduct of phenyldiazomethane, cannot be detected in our matrix sample using IR spectroscopy. EPR spectroscopy, which is much more sensitive and indicative for paramagnetic species, however, does show that traces of 2 also form in the reaction of ground state carbon atoms with benzene. However, the main product under matrix isolation conditions is 3. Highly exothermic reactions are expected to show nonArrhenius behavior.14 This is particularly true when there is no activation barrier and the internal modes are converted into vibrational modes of the product. The rate of IVR is slow compared to the reaction,61,62 and the products are determined by the initial orientations of the reactants. Thus, the reaction does not follow the mass-weighted intrinsic reaction pathway (MIRP) where all of the momentum is removed from the system as the system evolves in time; rather the trajectory evolves in time and the system can increase its vibrational and kinetic energy. However, when carbon atoms are deposited on a low-temperature matrix, the vibrational/kinetic energy can be rapidly dissipated into the matrix and the product distributions may be quite different; this is likely to be the case also for reactions in the solid state and at surfaces. When the “reactivity surface” between C(3P) and benzene was constructed at the 0.0005 au electron density contour, the computed interaction energies varied from +23.5 to −3.9 kcal mol−1 on the triplet surface and +18.8 to −8.3 kcal mol−1 on 2804
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CCSD(T)/CBS level. In this study, which also included 5 and 6, the largest deviation in relative energies from our G3B3 results was less than 1 kcal mol−1. From C(3P) plus benzene, the lowest energy pathway leads to a 1,2-addition complex (37) 16.0 kcal mol−1 more stable than the reactants (Figure 4) with a small barrier (2.6 kcal mol−1) to 3 3. A 1,3-addition complex (36) is 27.2 kcal mol−1 more favorable than the reactants and displays a 15.1 kcal mol−1 barrier to the 1,2-addition complex (37). The larger kinetic stability of the 1,3-complex (36) compared to 37 (ΔH‡ = 15.1 versus 2.6 kcal mol−1), probably accounts for the AIMD results (see below); five trajectories found the 1,3-addition complex (36) but no 1,2-addition complex (37) products were found (Figure 4). From an inspection of the activation barrier to 33 and 32 (Figure 4), the branching ratio from either triplet complex (1,2- or 1,3-) would strongly favor 33 over 32. Once 33 forms, rearrangement to 32 should be slow due to 3TS2, which is 47.1 kcal mol−1 higher than 33. Instead, intersystem crossing to 3 is expected to be fast as it is 26.7 kcal mol−1 more stable, and because the geometries of the two states are rather similar.55 On the C(1D) plus benzene potential energy hypersurface (Figure 5), two complexes were found, a 1,2-addition complex (7) 72.7 kcal mol−1 lower in enthalpy (30.0 + 42.7) than the reactants (C(1D) + C6H6) and a 1,4-addition complex (8), which is 86.4 kcal mol−1 lower in enthalpy (30.0 + 56.4). The first complex has a 1.9 kcal mol−1 barrier (42.7−40.8) for formation of 3 whereas the second complex has a 13.3 kcal mol−1 barrier (56.4−43.1) for formation of 3. In the AIMD results, no 1,2-addition complex (7) products were found whereas three trajectories led to the 1,4-addition complex 8 (Figure 5). In the construction of the PES at the G3B3 level, no direct low-energy pathway could be located from C(1D) plus benzene to 2. Instead, the initial low-energy product is 3, which has a 30.2 kcal mol−1 barrier to 2 (Figure 5). RRKM theory, which follows this PES, would indicate that only 33 or 3 should result from the addition of C(3P) or C(1D) carbon atoms to benzene.
the singlet surface (CASPT2(8,6)/6-31G(d)). As mentioned in the Methods, as the carbon atom position changes over the “reactivity surface”, the interactions with benzene are dominated by attractive donor−acceptor interactions or by Pauli repulsion. To compare the AIMD results (Table 1) with behavior expected for a reaction where excess internal energy is removed, Table 1. Number of Times a Product Was Formed out of 53 MD Runs of Triplet and Singlet Carbon Atoms Adding to Benzene
phenylcarbene (2) cycloheptatetraene (3) H2 loss benzocyclopropane (5) 7-norbornadienylidene (6) 1,2-complex (7) 1,4-complex (8) no reaction total
triplet (Figures 4 and 6A)
singlet (Figures 5 and 6B)
20 25 0 0 5 0
29 19 1 1 0
3 53
3 0 53
the PES of C7H6 was computed at the G3B3 level. Our results on both the triplet (Figure 4) and singlet (Figure 5) surfaces agree well with previous results.53−55,80,81 The singlet phenylcarbene rearrangement 2 → 3 is well-known48−50 and the current results are in good agreement with previous results. For example, the energies for 2/TS2/4/TS1/3 are (298 K) 17.4, 30.2, 14.1, 16.5, and 0.0 kcal mol−1 (Figure 5) versus 15.7, 30.2, 14.8, 17.5, and 0.0 kcal mol−1 at the CCSD(T)/cc-pVTZ// B3LYP/6-31G(d) level.55 In addition, bicyclo[4.1.0]hepta2,4,6-triene (4), benzocyclopropene (5), and 7-norbordienylidene (6) are relevant to early stages of the C addition to benzene. The computed 32/2 separation is 3.2 kcal mol−1 (triplet favored) and the 33/3 separation is −26.7 kcal mol−1 (singlet favored).82 In a recent study80,81 of the C7H6 potential energy surface, relative energies were computed at the
Figure 4. Computed G3B3 potential energy surface for triplet carbon atoms adding to benzene. The numbers in parentheses indicate the number of times that product was formed out of the 53 trajectories. 2805
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Figure 5. Computed G3B3 potential energy surface for singlet carbon atoms adding to benzene. The energy of C(1D) + C6H6 (shown in brackets) was taken from experiment (not computed). The numbers in parentheses indicate the number of times that product formed out of 53 trajectories (cf. Table 1).
6B). In two trajectories, the insertion of C into the C−C bond is accompanied by H-migration. In these two cases, 2 initially formed followed by ring-opening (2 → 4 → 3). In one example, 2 formed after 650 time steps. After 2300 time steps, 4 formed, which persisted for about 1150 time steps before collapsing again to 3. These results indicate that both 2 and 3 are important species expected in the addition of C(1D) to benzene. The AIMD results in this study suggest a direct route to phenylcarbene (2) in addition to a route to cycloheptatetraene (3) with a computed ratio of 60:40 2:3. In contrast, the conclusion from the PES would be that only 3, which is separated from 2 by a 30.2 kcal mol−1 barrier, must form first. The AIMD computations (formation of both 2 and 3) as opposed to the PES predictions (formation of only 3) are in better agreement with the experimental results of Shevlin and co-workers18 who found only C−H insertion to phenylcarbene (2) under carbon arc conditions. In the experimental system of Shevlin et al.,18 intersystem system crossing may occur because the reactive species are a mixture of C(1D) and C(3P) as well as carbon dimers and trimers. The crossing may take place before collision with benzene (C(1D) → C(3P)), after reaction (triplet → singlet phenylcarbene and triplet → singlet cycloheptatetraene), or during the reaction. However, in all scenarios, the predicted ratio of phenylcarbene:cycloheptatetraene is about 1:1. There appear to be three limiting behaviors: (a) carbon-arc generation of carbon atoms that insert into the C−H bond of benzene to give 2, (b) the C(3P) + benzene reaction where carbons atoms insert into the benzene CC bond to form 3, and (c) crossed beam reaction where a hydrogen atom is eliminated. Path (a) is controlled by chemical dynamics where the initial trajectory controls the product; path (b) is controlled by RRKM dynamics where the C(3P) atoms are thermalized in a low temperature matrix. In this matrix, the direct CC addition is clearly preferred over C−H insertion.
Indeed, this result is entirely consistent with our matrix isolation studies of C(3P) but not with the results from arcgenerated carbon atoms where the initial formation of 2 is indicated from labeling studies.18 In the matrix isolation study, only C(3P) atoms are present in the matrix (vide supra).25 This is a significant simplification over the carbon arc results where there is a mixture of C(3P), C(1D), and carbon clusters, C2, C3, etc. Of the 53 AIMD trajectories on the triplet surface (Figure 6A), 25 (47%)
Figure 6. “Reactivity surfaces” for triplet (A) and singlet (B) carbon plus benzene. The color indicates the product formed when a trajectory starts with a carbon atom at this position. A: C(3P) (A) with 3 2 = orange (25), 33 = brown (20), 36 = purple (5), no reaction = green (3). B: C(1D) with 2 = orange (29), 3 = brown (19), 8 = purple (3), and other = green (2). The numbers in parentheses indicate the number of times that product formed out of 53 trajectories.
produced 32 whereas 20 (38%) produced 33, 5 trajectories produced 36 (9%), and 3 trajectories were nonreactive. In contrast, all of the trajectories on the singlet surface were found to be reactive. Presumably, the greater stability of a triplet relative to a singlet carbon atom results in a reduced reactivity. Indeed, in the reaction of C(3P) with H2O, Schreiner and Reisenauer found that C(3P) was unreactive toward H2O.25 Twenty-nine trajectories on the singlet surface (out of 53) gave 2 (yellow) whereas 19 trajectories formed 3 (blue, Figure 2806
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Notes
The AIMD modeling studies suggest the formation of a mixture of 2 and 3 for both triplet and singlet carbon atom reactions with benzene whereas only C−H insertion is observed experimentally for C(1D).18 We do not have an explanation for the exclusive production the C−H insertion product with arc-generated carbon atoms. However, it is possible that the benzene molecules coated onto the walls of the vessel are orientated in the matrix in such a way as to favor C−H insertion. The parallel-displaced and T-shaped orientation of the benzene dimer are calculated to be nearly isoenergetic.83 However, if π-stacking is favored in the matrix, the exposure of the benzene π-system to incoming carbon atoms might be hindered which would favor in-plane approach.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.L.M. thanks Philip B. Shevlin, emeritus, for significant mentoring during his many years investigating carbon atom reactions. The Alabama Supercomputer Network is thanked for a generous allocation of computer time.
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CONCLUSIONS The reactions of triplet and singlet carbon atoms with benzene show significant dynamic behavior as almost all reaction channels proceed without activation. In the absence of fast energy dissipation (as in the condensed state, e.g., under matrix isolation conditions), the reaction path followed is controlled by the initial trajectories of reactants, and the reaction rates are faster than the internal vibrational energy redistribution. The potential energy surface would lead one to expect that the reactions start from an initial complex of the carbon atom with the unsaturated CC bond in benzene. However, the majority of trajectories are controlled by C-atom insertion into benzene C−H bonds forming 2 as an intermediate. Dynamic reactivity is likely to be common in carbon atom reactions where atoms are generated either thermally or photolytically and allowed to react with (and be stabilized by) a substrate. The initially formed products are quenched by the matrix and allowed to exist long enough to react with substrates. Previous experimental studies of carbon atoms reactions with benzene and other aromatic compounds are consistent with the formation of 2. The AIMD computations give a ratio of 55:35 for the C(1D) trajectories for the formation of 2 and 3, respectively, and a 47:38 ratio for the C(3P) trajectories. This is significantly different from a consideration of the potential energy hypersurfaces where only 3 is expected. However, the experimental results of Shevlin and co-workers18 indicate that only 2 initially forms. The present study suggests that the initial approach of the carbon atom is the decisive factor in determining the product. Thus, the reactivity surface can be divided into a surface fraction giving C−H insertion product and a surface area giving C−C addition.
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ASSOCIATED CONTENT
* Supporting Information S
Figures S1 and S2 with additional FTIR and EPR spectra. Table S1 gives total and relative energies of species in Figures 4 and 5. Table S2 gives the Cartesian coordinates of triplet and singlet species in Figures 4 and 5 optimized at the B3LYP/631G(2df,p) level. Full ref 65. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
(1) Bunker, D. L.; Hase, W. L. Non-RRKM [Rice-RamspergerKassel-Marcus] Unimolecular Kinetics. Molecules in General, and Acetonitrile in Particular. J. Chem. Phys. 1973, 59, 4621−4632. (2) Marcus, R. A.; Hase, W. L.; Swamy, K. N. RRKM and nonRRKM Behavior in Chemical Activation and Related Studies. J. Phys. Chem. 1984, 88, 6717−6720. (3) Borkovec, M.; Straub, J. E.; Berne, B. J. The Influence of Intramolecular Vibrational Relaxation on the Pressure Dependence of Unimolecular Rate Constants. J. Chem. Phys. 1986, 85, 146−149. (4) Carpenter, B. K. Dynamic Behavior of Organic Reactive Intermediates. Angew. Chem., Int. Ed. 1998, 37, 3340−3350. (5) Tse, J. E. Ab Initio Molecular Dynamics with Density Functional Theory. Annu. Rev. Phys. Chem. 2002, 53, 249−290. (6) Wang, Y.; Hase, W. L. Trajectory Studies of SN2 Nucleophilic Substitution. 9. Microscopic Reaction Pathways and Chemical Kinetics for Cl− + CH3Br. J. Chem. Phys. 2003, 118, 2688−2695. (7) Carpenter, B. K. Nonstatistical Dynamics In Thermal Reactions of Polyatomic Molecules. Annu. Rev. Phys. Chem. 2005, 56, 57−89. (8) Levine, R. D. Molecular Reaction Dynamics; Cambridge University Press: Cambridge, U.K., 2005. (9) Upadhyay, S. K. Chemical Kinetics and Reaction Dynamics; Springer: New York, 2006. (10) Litovitz, A. E.; Keresztes, I.; Carpenter, B. K. Evidence for Nonstatistical Dynamics in the Wolff Rearrangement of a Carbene. J. Am. Chem. Soc. 2008, 130, 12085−12094. (11) Lourderaj, U.; Park, L.; Hase, W. L. Classical Trajectory Simulations of Post-Transition State Dynamics. Int. Rev. Phys. Chem. 2008, 27, 361−403. (12) Lourderaj, U.; Hase, W. L. Theoretical and Computational Studies of Non-RRKM Unimolecular Dynamics. J. Phys. Chem. A 2009, 113, 2236−2253. (13) Glowacki, D. R.; Liang, C. H.; Marsden, S. P.; Harvey, J. N.; Pilling, M. J. Alkene Hydroboration: Hot Intermediates That React While They Are Cooling. J. Am. Chem. Soc. 2010, 132, 13621−13623. (14) Yamataka, H. Molecular Dynamics Simulations and Mechanism of Organic Reactions: Non-TST Behaviors. Adv. Phys. Org. Chem. 2010, 44, 173−222. (15) Berne, B. J.; Borkovec, M.; Straub, J. E. Classical and Modern Methods in Reaction Rate Theory. J. Phys. Chem. 1988, 92, 3711− 3725. (16) Manikandan, P.; Zhang, J.; Hase, W. L. Chemical Dynamics Simulations of X− + CH3Y− → XCH3 + Y− Gas-Phase SN2 Nucleophilic Substitution Reactions. Nonstatistical Dynamics and Nontraditional Reaction Mechanisms. J. Phys. Chem. A 2012, 116, 3061−3080. (17) Paranjothy, M.; Sun, R.; Zhuang, Y.; Hase, W. L. Direct Chemical Dynamics Simulations: Coupling of Classical and Quasiclassical Trajectories with Electronic Structure Theory. WIREs Comput. Mol. Sci. 2013, 3, 296−316. (18) Shevlin, P. B. Atomic Carbon. In Reactive Intermediate Chemistry; Moss, R. A., Platz, M. S., Jones, M., Eds.; Wiley: New York, 2004; pp 463−500. (19) Verdieck, J. F.; Mau, A. W. H. Some Laser-Induced Reactions in the Gas Phase. Chem. Commun. 1969, 226−226. (20) Skell, P. S.; Havel, J.; McGlinchey, M. J. Chemistry and the Carbon Arc. Acc. Chem. Res. 1973, 6, 97−105. (21) Moll, N. G.; Thompson, W. E. Reactions of Carbon Atoms with N2, H2, and D2 at 4.2 K. J. Chem. Phys. 1966, 44, 2684−2686.
AUTHOR INFORMATION
Corresponding Authors
*M. L. McKee: e-mail, [email protected]; telephone, +1-334-844-6953. *P. R. Schreiner: e-mail, [email protected]; telephone: +49641-9934300. 2807
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Acetylene and Its Isotopomers. J. Phys. Chem. A 2007, 111, 2980− 2992. (45) Leonori, F.; Petrucci, R.; Segoloni, E.; Bergeat, A.; Hickson, K. M.; Balucani, N.; Casavecchia, P. Unraveling the Dynamics of the C(3P,1D) + C2H2 Reactions by the Crossed Molecular Beam Scattering Technique. J. Phys. Chem. A 2008, 112, 1363−1379. (46) Li, Y.; Liu, H.-L.; Huang, X.-R.; Sun, Y.-B.; Li, Z.; Sun, C.-C. Radical−Molecule Reaction C(3P) + C3H6: Mechanistic Study. J. Chem. Phys. A 2009, 113, 10577−10587. (47) Li, Y.; Liu, H.-L.; Huang, X.-R.; Wang, D.-Q.; Sun, C.-C. Theoretical Study of the C(3P) + Trans-C4H8 Reaction. J. Phys. Chem. A 2009, 113, 6800−6811. (48) Parker, D. S. N.; Zhang, F.; Kim, Y. S.; Kaiser, R. I.; Mebel, A. M. On the Formation of Resonantly Stabilized C5H3 Radicals−A Crossed Beam and Ab Initio Study of the Reaction of Ground State Carbon Atoms with Vinylacetylene. J. Phys. Chem. A 2011, 115, 593− 601. (49) Balucani, N.; Lee, H. Y.; Mebel, A. M.; Lee, Y. T.; Kaiser, R. I. A Combined Crossed Beam and Ab Initio Investigation on the Reaction of Carbon Species with C4H6 Isomers. III. 1,2-Butadiene, H2CCCH(CH3) (X1A′)−a non-Rice−Ramsperger−Kassel−Marcus system? J. Chem. Phys. 2001, 115, 5107−5116. (50) Kaiser, R. I.; Nguyen, T. L.; Mebel, A. M.; Lee, Y. T. Stripping Dynamics in the Reactions of Electronically Excited Carbon Atoms, C(1D), with Ethylene and PropyleneProduction of Propargyl and Methylpropargyl Radicals. J. Chem. Phys. 2002, 116, 1318−1324. (51) Gaspar, P. P.; Berowitz, D. M.; Strongin, D. R.; Svoboda, D. L.; Tuchler, M. B.; Ferrieri, R. A.; Wolf, A. P. Reactions of Recoiling Carbon-11 Atoms with Toluene. J. Phys. Chem. 1986, 90, 4691. (52) Armstrong, B. M.; Zheng, F.; Shevlin, P. B. Mode of Attack of Atomic Carbon on Benzene Rings. J. Am. Chem. Soc. 1998, 120, 6007− 6011. (53) Matzinger, S.; Bally, T.; Patterson, E. V.; McMahon, R. J. The C7H6 Potential Energy Surface Revisited: Relative Energies and IR Assignment. J. Am. Chem. Soc. 1996, 118, 1535−1542. (54) Wong, M. W.; Wentrup, C. Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C7H6 Energy Surface. J. Org. Chem. 1996, 61, 7022−7039. (55) Schreiner, P. R.; Karney, W. L.; Schleyer, P. v. R.; Borden, W. T.; Hamilton, T. P.; Schaefer, H. F. Carbene Rearrangements Unsurpassed: Details of the C7H6 Potential Energy Surface Revealed. J. Org. Chem. 1996, 61, 7030−7039. (56) Reed, C. A.; Kim, K.-C.; Stoyanov, E. S.; Stasko, D.; Tham, F. S.; Mueller, L. J.; Boyd, P. D. W. Isolating Benzenium Ion Salts. J. Am. Chem. Soc. 2003, 125, 1796−1804. (57) Chiavarino, B.; Crestoni, M. E.; Di Rienzo, B.; Fornarini, S.; Lanucara, F. Site-Selectivity of Protonation in Gaseous Toluene. Phys. Chem. Chem. Phys. 2008, 10, 5507−5509. (58) Kaiser, R. I.; Vereecken, L.; Peeters, J.; Bettinger, H. F.; Schleyer, P. v. R.; Schaefer, H. F. Elementary Reactions of the Phenyl Radical, C6H5, with C3H4 Isomers, and of Benzene, C6H6, with Atomic Carbon in Extraterrestrial Environments. Astron. Astrophys. 2003, 406, 385−391. (59) Kaiser, R. I; Hahndorf, I.; Huang, L. C. L.; Lee, Y. T.; Bettinger, H. F.; Schleyer, P. v. R.; Schaefer, H. F.; Schreiner, P. R. Crossed Beams Reaction of Atomic Carbon, C(3Pj), with d6-benzene, C6D6(X1A1g): Observation of the Per-Deutero-1,2-Didehydro-Cycloheptatrienyl Radical, C7D5(X2B2). J. Chem. Phys. 1999, 110, 6091− 6094. (60) Kaiser, R. I. Experimental Investigation on the Formation of Carbon-Bearing Molecules in the Interstellar Medium via Neutral− Neutral Reactions. Chem. Rev. 2002, 102, 1309−1358. (61) Carpenter, B. K. Nonexponential Decay of Reactive Intermediates: New Challenges for Spectroscopic Observation, Kinetic Modeling and Mechanistic Interpretation. J. Phys. Org. Chem. 2003, 16, 858−868. (62) Carpenter, B. K. Energy Disposition in Reactive Intermediates. Chem. Rev. 2013, 113, 7265−7286.
(22) Shevlin, P. B.; Wolf, A. P. The Formation of Carbon Atoms in the Decomposition of a Carbene. Tetrahedron Lett. 1970, 3987−3990. (23) Ache, H. J.; Wolf, A. P. Reactions of Energetic Carbon Atoms with Nitrogen Molecules [1]. Radiochim. Acta 1966, 6, 33−39. (24) Gaspar, P. P.; Berowitz, D. M.; Strongin, D. R.; Svoboda, D. L.; Tuchler, M. B.; Ferrieri, R. A.; Wolf, A. P. Reactions of Recoiling Carbon-11 Atoms with Toluene. J. Phys. Chem. 1986, 90, 4691−4694. (25) Schreiner, P. R.; Reisenauer, H. P. The Non-reaction of Ground-State Triplet Carbon Atoms with Water Revisited. ChemPhysChem 2006, 7, 880−885. (26) Skell, P. S.; Plonka, J. H.; Engel, R. R. Deoxygenation by Carbon Atoms. J. Am. Chem. Soc. 1967, 89, 1748−1749. (27) Skell, P. S.; Villaume, J. E.; Plonka, J. H.; Fagone, F. A. Reactions of Metastable Singlet Carbon Atoms with Unsaturated Hydrocarbons. J. Am. Chem. Soc. 1971, 93, 2699−2702. (28) Skell, P. S.; Plonka, J. H. Deoxygenation by Atomic Carbon. II. Generation of Carbenes from the Reaction of Carbonyl Compounds with Metastable Singlet State Carbon Atoms. J. Am. Chem. Soc. 1970, 92, 836−839. (29) Skell, P. S.; Plonka, J. H. Deoxygenation by Atomic Carbon. III. Dichlorocarbene and Methoxycarbene. J. Am. Chem. Soc. 1970, 92, 2160−2161. (30) Skell, P. S.; Klabunde, K. J.; Plonka, J. H. Deoxygenation of Oxetans by Carbon Atoms. On the Intermediacy of Trimethylene. Chem. Commun. 1970, 1109−1110. (31) Skell, P. S.; Plonka, J. H. Deoxygenation of Cyclopropanecarboxaldehyde and Cyclopropylmethylketone: Cyclopropyl and Cyclopropylmethylcarbene. Tetrahedron 1972, 28, 3571−3582. (32) Welch, M. J.; Wolfe, A. P. Reaction Intermediates in the Chemistry of Recoil Carbon Atoms. Chem. Commun. 1968, 117−118. (33) Ferrieri, R. A.; Wolf, A. P.; Tang, Y.-N. Reaction Selectivity of Translationally and Electronically Excited Carbon-11 Atoms with Ethylene. J. Am. Chem. Soc. 1983, 105, 5428−5433. (34) Ferrieri, R. A.; Sharma, R. B.; Wolf, A. P. Structural Influences on the Chemical Reactivity of Hydrocarbons Toward Nucleogenic Carbon Atoms. Tetrahedron 1997, 53, 10155−10168. (35) Herrick, D. B.; Thamattoor, D. M.; Shevlin, P. B. Reactions of Atomic Carbon with Acyl Chlorides. Tetrahedron Lett. 2008, 49, 6036−6038. (36) Geise, C. M.; Hadad, C. M.; Zheng, F.; Shevlin, P. B. An Experimental and Computational Evaluation of the Energetics of the Isomeric Methoxyphenylcarbenes Generated in Carbon Atom Reactions. J. Am. Chem. Soc. 2002, 124, 355−364. (37) Armstrong, B. M.; Zheng, F.; Shevlin, P. B. Mode of Attack of Atomic Carbon on Benzene Rings. J. Am. Chem. Soc. 1998, 120, 6007− 6011. (38) Sevin, F.; Sökmen, I.̇ ; Düz Shevlin, P. B. Trapping of a Cycloheptatetraene in the Reaction of Atomic Carbon with Phenol. Terahedron Lett. 2003, 44, 3405−3407. (39) Sökmen, I.̇ ; Ece, A.; Düz, B.; Sevin, F. Reaction of Atomic Carbon with Isomeric Cresols. Lett. Org. Chem. 2009, 6, 650−665. (40) MacKay, C.; Nicholas, J.; Wolfgang, R. Reactions of Carbon Atoms and Methyne with Hydrogen and Ethylene. J. Am. Chem. Soc. 1967, 89, 5758−5766. (41) Casavecchia, P.; Balucani, N.; Cartechini, L.; Capozza, G.; Bergeat, A.; Volpi, G. G. Crossed Beam Studies of Elementary Reactions of N and C Atoms and CN Radicals of Importance in Combustion. Faraday Disc. 2001, 119, 27−49. (42) Nguyen, T. L.; Kim, G.-S.; Mebel, A. M.; Nguyen, M. T. A Theoretical Re-evaluation of the Heat of Formation of Phenylcarbene. Chem. Phys. Lett. 2001, 349, 571−577. (43) Park, W. K.; Park, J.; Park, S. C.; Braams, B. J.; Chen, C.; Bowman, J. M. Quasiclassical Trajectory Calculations of the Reaction C+C2H2→l-C3H, c-C3H+H, C3+H2 using Full-Dimensional Triplet and Singlet Potential Energy Surfaces. J. Chem. Phys. 2006, 125, 081101−1−081101−5. (44) Gu, X.; Zhang, F.; Kaiser, R. I. Investigating the Chemical Dynamics of the Reaction of Ground-State Carbon Atoms with 2808
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(63) Mikosch, J.; Zhang, J.; Trippel, S.; Eichhorn, C.; Otto, R.; Sun, R.; de Jong, W. A.; Weidemüller, M.; Hase, W. L.; Wester, R. Indirect Dynamics in a Highly Exoergic Substitution Reaction. J. Am. Chem. Soc. 2013, 135, 4250−4259. (64) Sun, L.; Hase, W. L. Born-Oppenheimer Direct Dynamics Classical Trajectory Simulations. Rev. Comput. Chem. 2003, 19, 79− 146. (65) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03 E.01; Gaussian, Inc.: Wallingford, CT, 2004 (see Supporting Information for full citation). (66) The 0.001 electrons/bohr3 contour is used by G03 to determine molecular volumes (keyword=VOLUME). (67) The 0.002 au contour is recommended by Bader and co-workers as the van der Waals envelope. Bader, R. F. W.; Carroll, M. T.; Cheeseman, J. R.; Chang, C. Properties of Atoms in Molecules: Atomic Volumes. J. Am. Chem. Soc. 1987, 109, 7968−7979. (68) Bondi, A. van der Waals Volumes and Radii. J. Phys. Chem. 1964, 68, 441−451. (69) Moore, C. E. Atomic Energy Levels; National Bureau of Standards Circular No. 467; National Bureau of Standards: Washington, DC, 1949, 1952, and 1958. (70) Herzberg, G. Atomic Spectra and Atomic Structure; Dover Books: New York, 1944. (71) gOpenMol is a molecular visualization program written by Leif Laaksonen and available from him at http://www.csc.fi/gopenmol (accessed Mar 13, 2014). (72) Molcas 4.1 (Current version 7.8): Karlström, G.; Lindh, R.; Malmqvist, P.-Å.; Roos, B. O.; Ryde, U.; Veryazov, V.; Widmark, P.-O.; Cossi, M.; Schimmelpfennig, B.; Neogrady, P.; Seijo, L. Molcas: a program package for computational chemistry. Comput. Mater. Sci. 2003, 28, 222−239. (73) Car, R.; Parrinello, M. Unified Approach for Molecular Dynamics and Density-Functional Theory. Phys. Rev. Lett. 1985, 55, 2471−2474. (74) CPMD V3.13.1, CPMD, http://www.cpmd.org (accessed Mar 13, 2014). (75) Marx, D.; Hutter, J. In Modern Methods and Algorithms of Quantum Chemistry; Grotendorst, J., Ed.; NIC Series; Forschungszentrum Jülich: Jülich, Germany, 2000; Vol. 1, pp 301−449. (76) Ceperley, D. M.; Alder, B. J. Ground State of the Electron Gas by a Stochastic Method. Phys. Rev. Lett. 1980, 45, 566−569. (77) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (78) Troullier, N.; Martins, J. L. Efficient Pseudopotentials for PlaneWave Calculations. Phys. Rev. B 1991, 43, 1993−2006. (79) Sander, W. Reaction of Phenylcarbene with Oxygen. Angew. Chem. Int. Edit. Engl. 1985, 24, 988−989. (80) Polino, D.; Famulari, A.; Cavallotti, C. Analysis of the Reactivity on the C7H6 Potential Energy Surface. J. Phys. Chem. A 2011, 115, 7928−7936. (81) Polino, D.; Cavallotti, C. Fulvenallene Decomposition Kinetics. J. Phys. Chem. A 2011, 115, 10281−10289. (82) A bold notation without superscript refers to a singlet species whereas a superscript 3 refers to a triplet species (e.g., 2 and 32). (83) Janowski, T.; Pulay, P. High Accuracy Benchmark Calculations on the Benzene Dimer Potential Energy Surface. Chem. Phys. Lett. 2007, 447, 27−32.
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