Electronic structure and noncovalent interactions within ion-radical complexes of N-(2-furylmethyl)aniline molecular ions.

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Electronic Structure and Non-Covalent Interactions Within the IonRadical Complexes of the N-(2-Furylmethyl)Anilines Molecular Ions Wilmer Esteban Vallejo Narváez, and Tomas Rocha-Rinza J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp512355j • Publication Date (Web): 16 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Electronic Structure and Non-Covalent Interactions within Ion-Radical Complexes of N-(2-Furylmethyl)Anilines Molecular Ions Wilmer E. Vallejo Narváez∗ and Tomás Rocha-Rinza∗ Departamento de Fisicoquímica, Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Coyoacán 04510, Mexico City, México E-mail: [email protected]; [email protected]

Abstract We investigate the electronic structure and non-covalent interactions within cation radical complexes which are relevant in the electron impact mass spectrometry of N-(2-furylmethyl) anilines, 4–R−C6 H4 −NH−CH2 −C4 H3 O with (R = –H, –OCH3 , –CH3 , –F, –Cl, –Br). In particular, we consider the reactive intermediates which precede the final products of two previously suggested dissociation pathways for these systems, i.e., i) a direct cleavage of the NH−CH2 bond and ii) an isomerization/fragmentation mechanism. The study is performed by means of correlated calculations (UCCSD and UMP2) together with density functionals (UM06 an UM06-2x) along with the triple zeta quality basis set 6-311++G(2d,2p). In addition, we carried out a topological analysis of the electron density in accordance with the Quantum Theory of Atoms In Molecules (QTAIM) together with the examination of the NonCovalent Interaction (NCI) index. In contrast with previous studies based on the UB3LYP approximation, we could determine the transition states associated to both fragmentation pathways. The Rice-Ramsperger-Kassel-Marcus theory, used to determine the relative importance ∗ To

whom correspondence should be addressed

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of these dissociation mechanisms, indicates that while the direct cleavage and the isomerization/fragmentation reaction routes have similar constant rates at low energy, the former prevails when the energy of the system is increased. The QTAIM analysis reveals that the charge of the cation radical complex is mainly located on either a furfuryl (direct cleavage mechanism) or a pyrylium (isomerization/fragmentation pathway) ion and that these units interact with a neutral radical aniline moiety. The localization of the positive charge in either a furfuryl or pyrylium cation is in agreement with the preminecence of the m/z = 81 fragment in the mass spectrometry of N-(2-furylmethyl) anilines. Moreover, the QTAIM properties indicate that the α unpaired electron of the system is principally distributed over the nitrogen and the -ortho and -para carbon atoms with respect to the –NH group in the R−C6 H4 −NH unit. The investigation of the NCI index and the intermolecular bond critical points and bond paths gives an account of the NCIs linking the radical cation clusters under consideration. Finally, we found correlations which indicate that the concentration of the m/z = 81 fragment in a mass spectrum is reduced with the interaction energy of the radical complex from which it is originated. Altogether, this work shows how the combination of suitable electronic structure calculations along with post wavefunction analyses can yield important insights about the formation and properties of cation radical complexes relevant in mass spectrometry.

Introduction Mass Spectrometry (MS) has become a very important analytical tool in the study of organic and inorganic compounds. 1 In addition, this technique has often been instrumental in the detection of novel structures and processes underwent by ions and radical in gas phase. 2 For example, the fragmentation pattern analysis in MS indicates that many unimolecular dissociations do not occur directly from the ionized intact molecules, but involve several rearrangements to reactive intermediates as critical steps of the dissociation mechanism. 3–7 More specific to the broad field of MS, there has been a considerable interest in the study of the electronic impact mass spectra of substituted N-(2-furylmethyl)anilines 8,9 4 − R−C6 H4 −NH−CH2 – 2


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Figure 1: 4-substituted N-(2-furylmethyl)anilines considered in this work.

–C4 H3 O (R = –H, –CH3 , –OCH3 , –F, –Cl and –Br) shown in Figure 1. As reported in Table 1, the m/z = 81 ion is either the base peak, the most or the second most relevant fragmentation in the electron impact mass spectra of N-(2-furylmethyl)anilines. The exception to this statement occurs when R = –OCH3 and the energy of the ionizing electrons is 10 eV for which the m/z = 81 cation corresponds to the third most important fragmentation. In most impact mass spectra at low energies (videlicet at 8 and 10 eV) the base peak is the molecular ion rather than the m/z = 81 fragment. This is the case because the ionizing electrons do not have enough energy to divide the N-(2-furylmethyl)anilines to an appreciable extent. When the energy of the ionizing electrons increases the m/z = 81 cation becomes more important in the impact mass spectra. Due to the prominence of the m/z = 81 ion in the electron impact MS of N-(2furylmethyl) aniline and its derivatives, there have been different proposals about its mechanism of formation. 10–12 For instance, it has been put forward that this cation is generated as a consequence of the direct dissociation of the NH−CH2 bond. 11 More recently, 10 B3LYP calculations were used to develop a kinetic model to identify the most probable pathways of dissociation in the electron impact MS of N-(2-furylmethyl)anilines. The corresponding results indicate that the formation of the m/z = 81 fragment might take place by an isomerization of the 2-methylfuran ring prior to dissociation to yield the pyrilium cation besides the direct cleavage previously proposed (Scheme 1). The formation of the m/z = 81 fragment or its complementary M − 81 cation follow from the heterolytic or homolytic rupture of one of the two C−N bonds. Nonetheless, it was not possible to find the transition state of every reaction pathway using the B3LYP functional. Moreover, neither the electron structures nor the intermolecular interactions within the intermediate radical-ion complexes

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Table 1: Relative abundance percentage of the molecular ion and the m/z = 81 fragment cation in the electron impact mass spectra of N-(2-furylmethyl)anilines at different energies of the ionizing electrons. Ionizing electrons energy

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R

M+ ·

Br Cl F H CH3 OCH3

253–251† 209–207‡ 191 173 187 203

70 eV %RA %RA

40 eV %RA %RA

15 eV %RA %RA

10 eV %RA %RA

%RA

%RA

M+·

M+·

M+·

M+·

M+·

m/z = 81 95? 54? 40? 22§ 14§ 8§

36–37† 16–47‡ 65 81 88 100

m/z = 81 100 26–28† 100 13–40‡ 100 57 100 71 100 95 ? 69 96

m/z = 81 100 100 100 100 100 100

31–32† 17–52‡ 74 100 100 100

m/z = 81 100 55–55† 100 30–90‡ 100 100 ? 83 100 ? 66 100 § 42 100

%RA M+· : Percentage of relative abundance of the molecular ion. %RA m/z = 81: Percentage of relative abundance of the m/z = 81 cation. † Mass and percentage of relative abundance of the isotopes 79 Br and 81 Br. ‡ Mass and percentage of relative abundance of the isotopes 35 Cl and 37 Cl. ? Most important fragmentation. § Second most important fragmentation. ¶ Third most important fragmentation.


8 eV

m/z = 81 100 96–10† 100 32–100‡ ? 72 100 ? 44 100 § 30 100 ¶ 17 100

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leading to the dissociation products were characterized.

Scheme 1: Scheme for the gas-phase unimolecular rearrangements of the 4-substituted N-(2furylmethyl) anilines based on density functional theory combined with Rice-Ramsperger-KasselMarcus theory calculations 10 Given this overview, the aim of this paper is twofold i) to revisit the dissociation mechanisms of N-(2-furylmethyl)aniline and five different substituted analogs and ii) to analize the electronic structure and the non-covalent interactions within the radical ion complexes that appear as reactive intermediates in the different fragmentation pathways. For this purpose, we combined RiceRamsperger-Kassel-Marcus (RRKM) theory with correlated calculations (UMP2 and UCCSD) and the functionals UM06 and UM06-2x to further examine the dissociation reaction mechanisms of the systems of interest for this work (Figure 1). These electronic structure methods have proved to give accurate results in the study of open shell species. 13 Likewise, we used the Quantum Theory of Atoms In Molecules (QTAIM) and the Non-Covalent Interactions (NCI) index to examine the electronic structure and the intermolecular interactions present in the radical ion complexes formed in the dissociation pathway processes. Hence, the paper is organized as follows. First, we present the computational details of the calculations performed throughout this investigation. Afterwards, we report and discuss the main results of this work. Finally, we provide a few concluding remarks. Overall, we show how detailed electronic structure calculations together with post wavefunction

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analyses might reveal important insights about the radical-ion complexes generated in mass spectrometry.

Computational details Every electronic structure calculation was carried out using the G AUSSIAN 09 program. 14 The ab initio and DFT methods mentioned below were used together with the basis set 6-311++G(2d,2p) which is adequate for the study of non-covalent interactions. 15,16 In particular, the inclusion of diffuse functions reduces the basis set superposition error. 17,18 We carried out geometry optimizations of the N-(2-furylmethyl)anilines of interest (Figure 1) together with the relevant points throughout their dissociation profiles using the the UM06-2x and UMP2 approximations. Each stationary structure was characterized as a minimum or a saddle point of the first order by means of the calculation of the corresponding harmonic frequencies. The obtained geometries with the UMP2 approximation were used to perform single point calculations with i) the methods PMP2, ROMP2 and ii) UCCSD and UCCSD(T) to assess the effects of spin contamination and a better description of electron correlation upon the obtained results. Similarly, the structures obtained with the M06-2x functional were utilized for M06 and ROMP2 single point calculations and compared with UCCSD and UCCSD(T) results. We checked on the convergence of the basis sets for the UCCSD calculations by comparing the results obtained with the basis set 6-311++G(2d,2p) with those of double zeta quality basis sets, i.e. 6-31++G(d,p) and aug-cc-pVDZ. We did not consider quadruple zeta basis sets containing diffuse functions e.g. aug-cc-pVQZ, for this examination, because UCCSD calculations involving such basis for the systems considered in this work are unfortunately not feasible. In addition, we checked the multiconfigurational character of every stationary point over the calculated dissociation profiles with the UMP2 approximation and the M06-2x functional, via the T1 diagnostic obtained with single point UCCSD calculations. 19,20 RRKM microcanonical rate coefficients, k(E), for the reaction limiting steps of the determined fragmentation pathways were calculated. All RRKM computations were performed us-

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ing the M ASS K INETICS package, version 1.14.3.533 Beta. 21 The characterization of the interactions present in the radical-ion complexes concerned in this work was performed by the analysis of the topological properties of the electron density in the intermolecular region in accordance with the quantum theory of atoms in molecules. We also considered the integration of the spin density ρS (r), over the atoms defined by QTAIM together with the electron delocalization index δ (Ω, Ω0 ) 22,23 which has been successfully used for the characterization of non covalent interactions. 24 These QTAIM-derived properties were computed with the aid of the AIMA LL program. 25 Finally, an examination of the Non-Covalent Index 26 was made with the NCIPLOT program. 27 Every post wavefunction analysis was performed with the electron density calculated with the UM06-2x/6-311++G(2d,2p) approximation.

Results and discussion Conformers and potential energy profile of N-(2-furylmethyl) aniline First, we carried out a search of conformers by varying the dihedral angles φa and φb shown in Figure 2 (a) with the UM06-2x functional. The angle φa was also varied to carry out calculations using the approximations UMP2 and PMP2//UMP2. The corresponding potential energy surface and curves displayed in Figure 2 (b) and (c) permitted us to identify three different conformational minima 1a, 1b and 1c. Table 2 shows that the UMP2 method and the UM06-2x functional lead to very similar structural parameters for each of the three most stable conformations found. We note that the number of conformers obtained by UMP2 and UM06-2x methods is different from those reported with the UB3LYP approximation, 10 which revealed the existence of two conformers (similar to 1c and 1b). Table 2 also indicates that the UMP2 conveys a significant spin contamination (hS2 i = 0.75 for a triplet) which is reduced by the spin projection PMP2 (Table 2). Subsequently, we proceeded to compute the reaction profile for the two fragmentation pathways shown in Scheme 1. Initially, the calculations to obtain the potential energy surface for the direct cleavage of the CH2 −NH bond leading to the formation of the m/z = 81 fragment were 7



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Figure 2: (a) Dihedral angles which were varied in the search of conformational minima of N-(2-furylmethyl) aniline. (b) Potential energy surface of the C6 H5 −NH−CH2 −C4 H3 O aniline as a function of the dihedral angles φa and φb using the UM06-2x functional. (c) Potential energy curve of the same molecule for the rotation of the dihedral angle φa using different approximations. ACS Paragon Plus Environment

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Table 2: Root mean-square deviation (in angstroms) of the bond lengths of the conformers considered in this work of N-(2-furylmethyl) aniline calculated with the UMP2 approximation and the UM06-2x functionals. The φa and φb (Figure 2(a)) dihedral angles together with the average value hS2 i computed with the UMP2, PMP2//UMP2 and UM06-2x approximations are presented as well.

0.009 Method UMP2 UM06-2x Method UMP2 UM06-2x UMP2 PMP2//UMP2 UM06-2x

176.5 174.4 290.0 294.0 1.163 0.888 0.778

Root mean square deviation 0.009 0.009 φa 296.5 86.5 285.3 85.3 φb 308.0 293.6 301.6 293.3 2 hS i 1.150 1.145 0.874 0.877 0.777 0.776

Figure 3: Structures of the cation-radical complexes Cx_1 and Cx_2, the intermediates preceding the last steps of the direct dissociation and the isomerization/fragmentation pathways respectively.

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carried out with the UM06-2x functional (see Figure S1 in the Supporting Information) because it yields similar profiles of the potential energy curves than those obtained with the UCCSD approximation of open shell systems 13 and it has been successfully used in the study of complexes formed by radicals with closed shell molecules. 28 The same methodology was used to examine the degradation pathway via isomerization/fragmentation, 10 characterized by the rearrangement of the 2-methylfuran moiety to the pyrylium cation (see Figure S1 in the Supporting Information). The radical ion complexes which lead to the fragmentation products i)A+ + B· and ii) C+ + B· (Scheme 1) are denoted as Cx_1 and Cx_2 respectively: the structure of these reaction intermediates is shown in Figure 3. As opposed to conformations 1b and 1c, only conformer 1a leads to the formation of cation radical clusters associated with the dissociation of the molecular ion to C5 H5 O+ (in the form of pyrylium or furfuryl) and the corresponding complementary neutral radical. The assessment of the multiconfigurational character of the N-(2-furylmethyl) aniline molecular ion and Cx_1 and Cx_2 is reported in Table 3. The T1 diagnostic must be smaller than 0.0450 for an open shell species to be regarded as monoconfigurational. 19,20 Based on these results, we can consider that the single reference calculations made in this work, such as UCCSD, UMP2, UM06-2x are valid and lead to reliable results about the process of dissociation of the N-(2-furylmethyl) aniline molecular ions.

Table 3: T1 diagnostic values calculated using the UCCSD approximation and optimized geometries with the UMP2 and UM06-2x approximations. The values computed with the UM06-2x structures are reported in parentheses. Species 1a Cx_1 Cx_2

T1 diagnostic 0.0261 0.0299 (0.0308) 0.0295 (0.0304)

The reaction profiles obtained for the direct cleavage of the CH2 −NH bond as well as the iso-

merization/fragmentation mechanism are similar to those reported in the literature 10 (Scheme 1). 10


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However, in contrast with previous work, we were able to find the transition state (TS) conecting every given pair of intermediates throughout the reaction mechanism. The xyz coordinates of every TS in Figure 4 are reported in Tables S2–S9 in the Supporting Information. According to Figure 4 and Table 4, the UMP2 approximation leads to different energy barriers for the rate-limiting steps 1a → Cx_1 and 1a → 2 in comparison with i) PMP2, ROMP2, UCCSD(T), UCCSD methods (all of them based on UMP2 geometries) and ii) the DFT methodologies employed in the study.∗ In fact, the UMP2 level of theory presents the largest deviations with respect to the UCCSD//UMP2 results which were taken as a reference. These discrepancies are attributed to the considerable spin contamination accompanying the UMP2 computations. The situation is particularly important for the energy associated to the 1a → A+ + B· process. In contrast, single point calculations using the levels of theory ROMP2 and PMP2 (the former having no spin contamination and the latter presenting a value of hS2 i closer to 0.75 as shown in Table 2) together with UMP2 optimized geometries, lead to closer results to those obtained with the UCCSD approximation. This suggests that the important problems of spin contamination inherent to the UMP2 approximation make the energies obtained with this approximation unreliable. Moreover, the values in Table 4 show that the UM06//UM06-2x and UM06-2x energetic differences are very similar to those computed with the UCCSD//UMP2 and UCCSD(T)//UMP2 approaches which are taken as a reference. The PMP2//UMP2 results are next closest to the coupled cluster energy differences, and the UMP2 energy based calculations have the largest deviations with respect to the interaction energies of the cation radical complexes Cx_1 and Cx_2. Hence, we carried out the calculations of the RRKM constants and the post wavefunction analyses relying upon UM06 and UM06-2x computations. ∗ For the sake of completeness we present the Gibbs energy profiles as well in Figure S2 of the Supporting Information.

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Figure 4: Energy profile of the two routes of decomposition of the molecular ion of the N-(2-furylmethyl)aniline (1a) obtained with different approaches. The cation-radical complexes Cx_1 and Cx_2 (Figure 3) are indicated in the energy diagram. The energy values include Zero-Point Vibrational Energy (ZPVE).


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Table 4: Energies (in kcal/mol) associated to different processes and transition barriers throughout the reaction profile of Figure 4. The interaction energies of Cx_1 and Cx_2 are presented as well. The differences with respect to the UCCSD//UMP2 results are reported in parentheses.

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Process 1a → Cx_1 Eact (1a → Cx_1) Cx_1 → A + B· Eact (1a → 2) Eact (7 → Cx_2) 1a → Cx_2 1a → A+ + B· 1a → C+ + B· Eact (1a → Cx_1) − Eact (1a → 2) Cx_1 Cx_2 % error IE Cx_1? % error IE Cx_2?

UMP2 41.93 45.19 16.62 26.87 17.33 25.27 58.55 41.66

(8.80) (8.63) (1.81) (−9.28) (3.31) (6.23) (10.61) (8.47)

18.33

(17.92)

16.62 16.39

PMP2// UMP2 36.35 (3.22) 39.42 (2.86) 15.54 (0.73) 35.64 (−0.51) 10.93 (−3.10) 19.36 (0.32) 51.89 (3.95) 35.00 (1.81) 3.78

(3.37)

ROMP2// UCCSD(T)// UMP2 UMP2 38.63 (5.50) 42.19 (5.63) 18.62 (3.81) 35.36 (−0.79) 9.12 (−4.91) 22.55 (3.51) 57.25 (9.31) 47.80 (−0.14) 40.35 (7.16) 33.34 (0.15) 6.63

(6.22)

Interaction Energy (IE) (1.81) 15.54 (0.73) 18.62 (3.81) (2.24) 15.64 (1.49) 17.80 (3.65) 12.3 5.0 25.7 15.9 10.6 25.9

UCCSD// UMP2 33.13 36.56 14.81 36.15 14.03 19.04 47.94 33.19

UM06-2x 37.89 40.49 15.25 33.79 12.92 21.95 53.14 36.94

(4.76) (3.93) (0.44) (−2.36) (−1.11) (2.91) (5.20) (3.75)

0.41

6.70

(6.29)

14.81 14.15

15.25 14.99

? The relative errors in the interaction energies were computed by taking the UCCSD//UMP2 results as reference.


UM06// UM06-2x 32.16 (−0.97) 34.20 (−2.36) 15.81 (1.00) 31.68 (−4.47) 9.45 (−4.58) 19.98 (0.94) 47.96 (0.02) 34.41 (1.22) 2.53

(2.12)

(0.44) 15.81 (0.84) 14.43 3.0 5.9

6.8 2.0

ROMP2// UM06-2x 41.21 (8.08) 44.28 (7.72) 17.59 (2.78) 38.52 (2.37) 12.04 (−1.99) 24.40 (5.36) 58.80 (10.86) 41.76 (8.57) 5.76

(5.35)

(1.00) 17.59 (0.28) 17.36

(2.78) (3.21) 18.8 22.7


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Rice-Ramsperger-Kassel-Marcus rate constants calculation As previously stated, we used the RRKM theory to calculate the rate constants of the two reaction pathways of interest and thereby determining their relative importance. The calculation of the RRKM constants in this work is based on UM06//UM06-2x and UCCSD//UMP2 results. By taking into account the barrier heights in Figure 4 and the limiting step approximation, 29 we considered that the determining steps for the two decomposition channels are: 1a → Cx_1 for the direct dissociation, and the 1a → 2 for the isomerization/fragmentation pathway (Figure 4).

Figure 5: RRKM rate constants, k(E), of 1a → Cx_1 (blue colour) and 1a → 2 (red colour) steps from the molecular ion N-(2-furylmethyl)aniline, based on UM06//UM06-2x (solid line) and UCCSD//UMP2 (dashed line) calculations.

Figure 5 indicates that the direct dissociation and the isomerization/fragmentation processes have similar rate constants at low internal energies of the molecular ion (about 2 eV). Therefore, the formation of both complexes, Cx_1 and Cx_2, in this energy range is equally likely. However, the direct dissociation appears to be the predominant process when the internal energy of the ions is larger than 3 eV and k(E) > 106 s−1 , which are the conditions that best reproduce those given in the ionization source in the mass spectrometer. 30 This implies that an increase in the internal energy of 14


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the molecular ion, would lead to a preference of the 1a → Cx_1 process over the 1a → 2 reaction. This is observed in both methodologies, UM06//UM06-2x and UCCSD//UMP2. The results presented in Figure 5 are in agreement with the observation that rates of rearrangement/fragmentation processes approach their limit values at high energies more quickly than bond cleavage reactions, in a way that direct dissociation processes prevail at high internal energies. 31 The relative magnitudes of the rates of the direct dissociation and the isomerization/fragmentation pathways has been rationalized considering that the transition state of the latter mechanism involves the attainment of a specific molecular conformation. Hence, any excess energy of the system cannot be used to carry out the rearrangement/dissociation process unless the required conformation is reached. On the other hand, the direct dissociation process has a loose transition state 31 which means that no particular conformation of the system is needed for the reaction to occur. The kinetic results described here, differ from those obtained at UB3LYP level of theory, 10 which suggest that the two decomposition pathways mentioned are equally likely in a wide range of internal energy.

Non-covalent interactions in the ion-radical complexes Cx_1 and Cx_2 As Figure 3 shows, equilibrium geometries of the ion-radical complexes Cx_1 and Cx_2 show a structural difference in the C5 H5 O unit, while the C6 H4 NH moiety is the same in both systems. The dissimilarity of the C5 H5 O fragment in the Cx_1 and Cx_2 clusters might entail a significant difference in the non-covalent interactions within these complexes. We proceed to analyze these interactions through the topology of the electron density in the intermolecular region and the examination of the NCI index. Figure 6 displays the molecular graphs of the cation radical complexes Cx_1 and Cx_2 while Table 5 reports some selected topological properties of the intermolecular bond critical points of these systems. The molecular graphs show that the C5 H5 O+ and C6 H5 NH · units in the system Cx_1 are linked by two Hydrogen Bonds (HB) N· · · H(I) and N· · · H(II). These HBs conduce to the formation of a six-membered ring, evidenced by the corresponding ring critical point. The 15



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Figure 6: Molecular graphs of the cation radical complexes Cx_1 and Cx_2. Bond and ring critical points are represented with green and red spheres respectively. The bond lengths N· · · H (O· · · H and O· · · N) within Cx_1 (Cx_2) are shown as well. The analysis was performed with the electron density computed with the UM06-2x/6-311++G(2d,2p) approximation.

In the case of Cx2, positive values of H PC and —2 rPC hows that N· · · H (I) and N· · · H (II) are closed shell interactions due to H PC and —2 rPC O· · · H and O· · · N interactions (Table 6) indicate that the Table 5: Selected topological properties of the intermolecular bond critical points of the charge within the cation radical complexes Cx_1 and Cx_2. The critical of ρ(r) undercylindrical ve values. distribution In addition, the electron density distribution is notpoints completely consideration are shown in hydrogen Figure 6. The computed properties include values of the electron bond O· · · H and antheunusual interaction betwe density ρ(rbcp ), kinetic energy density G(rbcp ), potential energy density V (rbcp ), total energy denhydrogen shown in Table C > 0 in the sitytwo H(rbcp ), Laplacian bonds of the electron density ∇2 ρ(rbcp5. ) and the ellipticity εbcp . The electron the molecular graph of Cx2 (Table 6), it observes the f delocalization index and N· · · H (O· · · H and O· · · N) in Cx_1 (Cx_2) are also presented. Atomic throughout. The analysis was are carried out with the electron density obtained withcharacterized the · · H (I) andunits N·are· ·used H (II) hydrogen bonds formed a six membered ring, UM06-2x/6-311++G(2d,2p)interactions approximation. O· · · N and O· · · H are present. In this ring esponding ring critical point. According to the electron delocalization index, d (W0 , W), Cx_1 the ring critical Cx_2 point makes magnitude of (O· · · H) with N· · · H (I) N· · · H (II) O· · · H O· · · N (I) and (II) exhibit a similar covalent character, whose results are consistent with the ρ(rbcp ) 0.015 0.015 0.005 0.012 ellipticity in the critical point0.010 of O· · · H significantly inc G(rbcp ) 0.010 0.010 0.003 hs N· · · H (I) ⇡ N· · · H (II). V (rbcp ) −0.009 −0.008 −0.003 −0.008 N· · · H0.002 (I) and N· · · H0.001 (II) (in the complex Cx1). T H(rbcpO· ) · · N, 0.002 0.001 ∇2 ρ(rbcp ) 0.048 0.047 13 εbcp 0.099 0.126 of the complex Cx2 0 δ (Ω, Ω ) 0.062 0.060

0.016 0.044 3.223 2.264 is0.013 topologically 0.059

unstable due to sma

induce changes in the molecular graph of the system. Th

in Figure 2), 16 the system goes from a structural regions t ACS Paragon Plus Environment

by catastrophe theory. 44

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positive values of H(rbcp ) and ∇2 ρ(rbcp ) at the bond critical points associated to the N· · · H (I) and N· · · H (II) HBs indicate, as expected, that these contacts are closed shell interactions. The computation of the electron delocalization index, δ (Ω, Ω0 ), points out that the N· · · H (I) and N· · · H (II) interactions exhibit a similar covalent character, in accordance with the similar lengths of these hydrogen bonds. Figure 6 also shows that the intermolecular interactions within the complex Cx_2 are an O· · · H hydrogen bond and an unusual interaction between the oxygen and the nitrogen atoms of the C5 H5 O and C6 H4 NH units. The associated bond paths of the intermolecular interaction lead to the generation of a five-membered ring. Within this cyclic structure, the proximity of the corresponding ring critical point (rcp) with the O· · · H bond critical point (bcp) diminishes drastically the absolute value of the less negative curvature of this latter critical point. Thereby, the ellipticity of the O· · · H bcp is considerably larger than that of the O· · · N interaction and those NCIs in Cx_1. The large ellipticity of the O· · · H bond critical point is indicative of the topological instability of the molecular graph of Cx_2. In a similar way that for Cx_1, the positive values of H(rbcp ) and ∇2 ρ(rbcp ) at the bond critical points at the intermolecular interactions of Cx_2, i.e., O· · · H and O· · · N (Figure 6) indicate that these are also closed-shell contacts. The electron delocalization index in O· · · H is significantly less important than those present in the N· · · H (I) and N· · · H (II) interactions in Cx_1 (Table 5), indicating that it is a weaker hydrogen bond, with a correspondingly larger bond length (see Figure 6). Furthermore, the value of δ (Ω0 , Ω) associated with the O· · · N interaction is i) greater than the electron delocalization related to the H· · · O HB and ii) comparable with the corresponding values of the hydrogen bonds N· · · H(I) and N· · · H(II). The computation of the delocalization index implies that electron sharing is more important in NCIs of Cx_1 than it is in those in Cx_2. This statement is based on the equation

N(Ω) = λ (Ω) +

1 δ (Ω, Ω0 ) 2 Ω6∑ 0 =Ω

(1)

and the fact that the electron populations of the atoms involved in these interactions are such that N(O) ≈ N(N) >> N(H). Hence, the nitrogen and hydrogen in the interactions N· · · H(I) and (II) 17



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share a larger fraction of the electrons in the corresponding basins than the atoms in the rest of NCIs under examination. Once considered the topological properties of the electron density in the intermolecular regions of the cation-radical complexes Cx_1 and Cx_2, we proceed to analyze the QTAIM atomic charges and the integral of the spin density over the different atomic basins. Figure 7 shows that the C5 H5 O ring (C1-C5, O13 and H14-H18) has 96% of the charge excess of the system (the total charge of the complex being equal to 1) while the rest is contained in the C6 H4 NH unit. This result indicates that i) the fragment ion m/z = 81 has indeed the formula C5 H5 O+ with the furfuryl structure and ii) this species form a cation radical complex with the neutral C6 H4 NH · residue before its detection in mass spectrometry. The integral of the spin density, which represents the disposition of unpaired electrons, is preferably distributed among the -ortho and -para carbons with respect to the –NH group (C7, C9 and C10: see Figure 7) and the nitrogen of the aniline moiety. The sum of the values of N(s) throughout the atoms forming the complex Cx_1 indicates that the C6 H4 NH unit contains 99% of the α unpaired electron of the system. Concerning the Cx_2 complex (Figure 8), the grouping of both the atomic charges and the integrals of the spin density leads to similar results than those described for the system Cx_1, namely, the C5 H5 O unit (C1-C5, O13 and H14-H18) in Cx_2 has 97% of the total excess charge, while the C6 H4 NH · ring has the same distribution of the unpaired electron observed in Cx_1. Importantly, the results of Figure 7 and Figure 8 agree with the experimental observation of the fragment m/z = 81: the former (latter) suggests that this species corresponds to the furfuryl (pyrylium) ion. In both cases, the cation forms an ion-radical complex with the C6 H4 NH · radical complex. We compare now the distribution of the spin density obtained with the UM06-2x functional and the UMP2 approximation. It was found that the UMP2 calculations conduce to a different unpaired electron distribution to that observed with the UM06-2x method (see Figure S3 in the Supporting Information). The UMP2 results indicate that the α unpaired electron is mainly distributed over the -ipso and -meta carbons with respect to the –NH group. However, the high spin contamination of the UMP2 level of theory suggests that the disposition of unpaired electrons based on this

18


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methodology might be erroneous. Finally, we point out that the topological analysis of the electron density performed with the electron density computed with the UM06/6-311++G(2d,2p)//UM062x/6-311++G(2d,2p) approximation (Table S10 and Figure S4 in the Supporting Information) conduces to similar results to those reported in this section.

Figure 7: Atomic charges q(Ω) (pink) and the integral of the spin density N(s) (blue), in Cx_1 complex atoms. The QTAIM analysis was carried out with the electron density computed with the UM06-2x/6-311++G(2d,2p) approximation.

Non-covalent interactions index analysis Apart from the previously described topological analysis, we also examined the non-covalent interactions within Cx_1 and Cx_2 using the reduced gradient of the electron density. 27 The NCI index analysis of complex Cx_1 (Figure 9 (a)) shows a peak of the reduced gradient around sgn(λ2 )ρ(r) = −0.02 a.u. represented by two isosurfaces of s(ρ(r)) corresponding to the hydrogen bonds N· · · H (I) and N· · · H (II) (Figure 9 (b)). These interactions were formerly characterized by the properties of ρ(r) at the associated bond critical points (Table 5). Figure 9 also indicates that there are van der Waals contacts and steric repulsions among the atoms that form the C5 H5 O+ and C6 H4 NH · rings. 19



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Figure 8: Atomic charges q(Ω) (pink) and the integral of the spin density N(s) (blue) in Cx_2 complex atoms. The QTAIM analysis was performed with the electron density calculated with the UM06-2x/6-311++G(2d,2p) level of theory.

A similar inspection for the Cx_2 complex reveals five different types of noncovalent interactions in this system: • The first corresponds to the interaction O· · · N presenting peaks at sgn(λ2 )ρ(r) = −0.02 • The hydrogen bonding O· · · H at sgn(λ2 )ρ(r) = −0.01 constitute the second type of interaction. This HB and the former intermolecular contact had been previously identified with the topological analysis of ρ(r). The NCI analysis indicates that the O· · · N interaction has an attractive nature greater than that observed for O· · · H, agreeing with the QTAIM results in Table 5. • The third sort of interaction (sgn(λ2 )ρ(r) = 0.05) occurs between the nitrogen and the C3 carbon (the atom numbering is shown in Figure 8). The fact that the involved atoms have opposite electric charges (see Figure 7) suggests that electrostatics might play an important role in these intermolecular contacts.

20


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• The peaks of Figure 10 (a) around sgn(λ2 )ρ(r) = 0.01 a.u. are related to the fourth kind of interaction, i.e. van der Waals contacts between the aniline nitrogen atom and the C3 carbon atom. • The fifth kind of NCI entails repulsion interactions among the atoms comprising the aniline and the pyrylium rings as reflected by the NCI isosurfaces wherein sgn(λ2 )ρ(r) > 0.02. Figures S5 and S6 in the Supporting Information show that the NCI index analysis based upon the electron density computed with the UM06/6-311++G(2d,2p)//UM06-2x/6-311++G(2d,2p) method reveals the same kind of interactions.

(a)

(b)

Figure 9: NCI index examination of the cation radical complex Cx_1. (a) Relation between s(ρ(r)) and sgn(λ2 )ρ(r). (b) NCI index isosurfaces (s = 0.4 a.u.) The value of sgn(λ2 )ρ(r) is indicated by means of the scale colour at the bottom of the Figure. The N· · · H hydrogen bonds and van der Waals contacts are indicated by arrows. The analysis was made with the electron density computed with the UM06-2x/6-311++G(2d,2p) method.

4-substituted N-(2-furylmethyl)anilines The methodology described previously for the N-(2-furylmethyl) aniline molecular ion was applied to the five substituted analogs, 4 − R−C6 H4 NH−CH2 C4 H3 O (R = –CH3 , –OCH3 , –F, –Cl and –Br). Comparison of the results shows that all substituted species lead to similar reaction pathways to those obtained for compound 4 − C6 H5 NH−CH2 C4 H3 O with the UM06//UM06-2x 21



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(a)

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(b)

Figure 10: NCI index examination of the cation radical complex Cx_2. (a) Relation between s(ρ(r)) and sgn(λ2 )ρ(r). (b) NCI index isosurfaces (s = 0.4 a.u.) The value of sgn(λ2 )ρ(r) is indicated by means of the scale colour at the bottom of the Figure. The O· · · N, O· · · H and van der Waals contacts are pointed with arrows. The analysis was made with the electron density computed with the UM06-2x/6-311++G(2d,2p) approximation.

approximation (Figures S7 and S8 in the Supporting Information). The calculations of the rate constants by means of RRKM theory for the molecular ions of the substituted N-(2-furylmethyl) anilines follow the same trends as those of 4 − C6 H5 NH−CH2 C4 H3 O as can be observed in Figure S9. Once optimized the structures of the cation radical complexes, Cx_1 and Cx_2, we considered the topology of the electron density in the intermolecular region of the different substituted systems. Every radical complex Cx_1 has the same interactions that the corresponding system derived from the N-(2-furylmethyl)aniline, i.e., N· · · H (I) and N· · · H (II) (Figure S10 in the Supporting Information). Table 6 shows that the topological properties of the electron density at the bond critical points N· · · H (I) and N· · · H (II) are very similar regardless of the substituent R. The number of electrons shared by the aniline nitrogen and the hydrogen involved in the H-bond is slightly increased with substitution as indicated by the values of δ (Ω, Ω0 ) in Table 6. This augmentation might be related to the electron donating character of CH3 and OCH3 which increases the charge distribution around the nitrogen atom, thereby making it a better proton acceptor. The resonance effect of the halogens, which acts in opposition to its inductive effect, has a similar influence over the hydrogen bonds N· · · H (I) and N· · · H (II). 22


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Table 7: Graph molecular of the radical-ion complex Cx2 (-OCH3 ) and values properties: kinetic energy density, GPC , potential energy density, V PC , total e Laplacian of the density, —2 r(r), ellipticity, ePC were calculated at the critical A similar situation is observed in the study of the cation radical complexes Cx_2, that is to interaction. The electron delocalization index and C· · · N length are also present say, the one formed between the pyrylium cation and the aniline radical. All substituted species were performed at UM06-2x/6-311++G(2d,2p) level of theory (with the exception of R = –OCH3 ) exhibit the same interactions and present similar values of the topological properties of the charge density at the intermolecular bond critical points of the unsubstituted species (Table 6 and Figure S11 of the Supporting Information). Nonetheless, the ellipticities of the O· · · H interactions are importantly decreased with the substitution of the N-(2furylmethyl) aniline and hence a structural change is less likely to occur.

C· · · N

ρ(rbcp ) G(rbcp ) V (rbcp ) 0.0145 0.0105 −0.0090

H(rbcp ) ∇2 ρ(rbcp ) εbcp 0.0014 0.0475 1.4158

δ (Ω, Ω0 ) 0.0572

Figure 11: Molecular graph of the cation radical complexes Cx_2 derived from 4 − H3 CO−C6 H4 −NH−CH2 −C4 H3 O. Bond and ring critical points are indicated with green and red spheres respectively. The bond length of the intermolecular interaction C· · · N is also presented. According to the results,properties positive indicateat the that H PC andbond GPC Finally, some selected topological of thevalues charge distribution intermolecular critical point (in atomic units) are indicated. The examined electron density was obtained with the UM06-2x/6-311++G(2d,2p) method.

O· · · N

like the other aforementioned intermolecular interactions (Tables 7 and 8) la

values Concerning d (W0 , W)theatCx_2 C· complex · · N are similar to those observed in N· · · H (Cx1) and O· · derived from N-(2-furylmethyl)-4-metoxianiline, it presents a

geometry analogous to those obtained with the other substituents. However, the analysis have of the a similar substituents), which allows inferring you’re three interactions topology of the electron density of this cation radical cluster presents only one intermolecular

which is higher than H· · · O (Table 4.10). This explains the vicinity of the bo C· · · N bond path between the C H NH · and C H O+ rings as shown in Figure 11. Similar to 6

4

5

5

C· · · N with the substituent -OCH3 O (Table 4.11) and O· · · N with the remainin 23


ure 4.11). By making a comparison Î¸tPC value in all cases (Tables 4.9 to 4


Table 6: Selected topological properties of the intermolecular bond critical points of the charge distribution within the cation radical complexes Cx_1 and Cx_2† with different substituents R = –Br, –Cl, –F, –CH3 and –OCH3 (the position of these substituents is indicated in Figure 1). The corresponding results for R = H are also presented for comparison. The critical points of ρ(r) under examination are shown in Figure 6. The computed properties include the values of the electron density ρ(r), kinetic energy density G(rbcp ), potential energy density V (rbcp ), total energy density H(rbcp ) and Laplacian of the electron density, ∇2 ρ(rbcp ) and the ellipticity, εbcp . The electron delocalization index N· · · H (I) and (II) (O· · · H and O· · · N) in Cx_1 (Cx_2) are also presented. Atomic units are used throughout. The analyzed electron densities were computed with the UM06-2x/6-311++G(2d,2p) approximation.

24

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R –Br –Cl –F –H –CH3 –OCH3

ρ(rbcp ) (I) (II) 0.015 0.015 0.015 0.014 0.015 0.014 0.015 0.015 0.016 0.015 0.016 0.016

G(rbcp ) (I) (II) 0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.011 0.010 0.011 0.011

R –Br –Cl –F –H –CH3

ρ(rbcp ) O· · · H O· · · N 0.005 0.012 0.005 0.012 0.004 0.012 0.004 0.012 0.005 0.013

G(rbcp ) O· · · H O· · · N 0.004 0.010 0.003 0.010 0.003 0.010 0.003 0.010 0.004 0.010

†

N· · · H interactions in Cx_1 V (rbcp ) H(rbcp ) ∇2 ρ(rbcp ) (I) (II) (I) (II) (I) (II) −0.008 −0.008 0.002 0.002 0.047 0.047 −0.009 −0.008 0.002 0.002 0.048 0.047 −0.009 −0.008 0.002 0.002 0.049 0.045 −0.009 −0.008 0.002 0.002 0.048 0.047 −0.009 −0.009 0.002 0.002 0.051 0.049 −0.010 −0.009 0.002 0.002 0.053 0.051 O· · · H and O· · · N interactions in Cx_2 V (rbcp ) H(rbcp ) ∇2 ρ(rbcp ) O· · · H O· · · N O· · · H O· · · N O· · · H O· · · N −0.003 −0.008 0.001 0.001 0.018 0.045 −0.003 −0.008 0.001 0.001 0.017 0.044 −0.003 −0.008 0.001 0.001 0.016 0.045 −0.003 −0.008 0.001 0.001 0.016 0.044 −0.003 −0.009 0.001 0.001 0.018 0.047

εbcp

(II) 0.125 0.130 0.130 0.126 0.132 0.131

δ (Ω, Ω0 ) (I) (II) 0.062 0.061 0.064 0.060 0.065 0.059 0.062 0.060 0.067 0.062 0.068 0.065

εbcp O· · · H O· · · N 0.570 1.871 0.838 2.042 1.380 2.193 3.223 2.264 0.543 2.165

δ (Ω, Ω0 ) O· · · H O· · · N 0.016 0.060 0.015 0.059 0.014 0.061 0.013 0.059 0.016 0.062

(I) 0.101 0.093 0.091 0.099 0.094 0.101

The data for –OCH3 are not presented because the cation radical complexes generated prior to the fragmentation of the molecular ion 4 − H3 CO−C6 H4 −NH−CH2 C4 H3 O present a different molecular graph to the rest of the considered systems.


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the previously discussed interactions, the values of H(rbcp ), G(rbcp ) and ∇2 ρ(rbcp ) indicate that the C· · · N contacts in Figure 11 are closed shell interactions. The electron delocalization index δ (Ω, Ω0 ) related to the C· · · N interaction is alike to those observed in the N· · · H (Cx_1) and O· · · N (Cx_2) contacts suggesting that the interaction of these pairs of atoms has a similar covalent character which is higher than that of the O· · · H hydrogen bond in the cation radical complex Cx_2. This comparison of the values of δ (Ω, Ω0 ) is in accordance with the similarity of bond lengths i) C· · · N in Cx_2 originated from 4 − H3 CO−C6 H4 −NH−CH2 −C4 H3 O and ii) O· · · N generated from N-(2-furylmethyl)aniline and the rest of the substituted systems. Regarding the analysis of the properties of atoms in molecules, Figure 12 and Figure 13 shows that the units C5 H5 O contain almost completely the total excess charge of the system (between 95 and 97% in all cases). Therefore, every cation radical complex Cx_1 (Cx_2) under examination is comprised by a furfuryl (pyrylium) cation interacting with a neutral aniline radical in a similar fashion to the unsubstituted 4 − H3 CO−C6 H4 −NH−CH2 −C4 H3 O considered beforehand. As expected, the carbon bonded to the substituents, i.e., the C9 atom, has a considerable positive charge when R = –OCH3 or F. Concomitantly, the oxygen or fluorine atoms of the substituent present a sizeable negative charge which is comparable to the N in the aniline or O in the pyrylium or furfuryl cation. The integral of the spin density over the QTAIM atoms (Figure 14) for the substituted N-(2furylmethyl) anilines shows that in a similar way to the unsubstituted compound, the unpaired α electron is almost completely (≈ 99%) located in the neutral aniline unit of the cation radical complex. In addition, it is observed that the integral of the spin density is negative for the aniline ipso carbons indicating an excess of spin β over α on these atoms. The results of the integration of the spin density shown in Figure 14 (A) suggest that the mesomeric structures that contribute the most to the hybrid of resonance of the neutral aniline radical are those displayed in Figure 14 (B). Such structures indicate that the unpaired α electron within the 4 − R−C6 H4 −NH · (R = – H, –F, –Cl, –Br, –CH3 and –OCH3 ) species is mainly distributed over the nitrogen and the ortho and para carbons with respect to this N atom. Finally, the NCI analysis at the Cx_1 and Cx_2

25



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Figure 12: Atomic charges, q(Ω) of the atoms of the radical-ion complex Cx_1 derived from 4 − R−C6 H4 −NH−CH2 −C4 H3 O with R = –H, –F, –Cl, –Br, –OCH3 and –CH3 . R24 indicates the charge of the substituent when it is a single atom i.e R = H, F, Cl or Br. When R = –OCH3 , R24 designates the oxygen of the substituent. The labels C–25, H–26, H–27 and H–28 represent the methyl atoms of R = –OCH3 , –CH3 . The analysis was performed with the electron density obtained with UM06-2x/6-311++G(2d,2p) level of theory.

26


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Figure 13: Atomic charges, q(Ω) of the atoms of the radical-ion complex Cx_2 originated from the molecular ion 4 − R−C6 H4 −NH−CH2 −C4 H3 O with R = –H, –F, –Cl, –Br, –OCH3 and –CH3 . The labels of the atoms are the same than in Figure 12. The analysis was performed with the electron density computed with the UM06-2x/6-311++G(2d,2p) method.

complexes of the substituted species, have the same non covalent interactions observed in the cation radical complexes derived from C6 H5 −NH−CH2 −C4 H3 O (Figures S12 and S13 in the Supporting Information).

Correlations among experimental and computational results Given the similarities of the six compounds under study, both in structure and in the pathways of fragmentation, we proceeded to identify some correlations among computational and experimental observations more specifically those between the results obtained in this investigation and the relative abundances of the fragment-ion [C5 H5 O]+ (m/z = 81) present in the mass spectra. Figure 15 shows linear correlations between the interaction energies of the two complexes Cx_1 and Cx_2 as a function of the concentration of the fragment-ion [C5 H5 O]+ in the electronic impact mass spectrum at 40 eV (R2 = 0.97 and 0.95 respectively). Overall, the stronger the binding energy of the cation radical complex, the lower the concentration of the ion C5 H5 O+ in the mass 27



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Figure 14: A) Integration of the spin density, N(s) over the topological atoms in the cation-radical complex Cx_1 originated from the molecular ions of 4 − R−C6 H4 −NH−CH2 −C4 H3 O with R = –H, –F, –Cl, –Br, –CH3 and –OCH3 . The same labelling of Figure 12 is used. B) Relevant resonance structures for the aniline radical showing the distribution of the unpaired electron. The unpaired α spin electron is mainly located over the nitrogen along with the ortho- and para- carbon atoms. Similar results for (A) and (B) are obtained with the complex Cx_2 for all the different substituents. The analysis was performed with the electron density obtained with the UM06-2x/6311++G(2d,2p) methodology.

28


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spectrum. Furthermore, the cation concentration [C5 H5 O+ ] increases with the electron withdrawing nature of the substituent (–Cl ≈ –Br > –F > –H > –CH3 > –OCH3 ). These trends extend to concentrations obtained from the mass spectra to 8, 10, 15 and 70 eV (See Figures S14 and S15 of the Supporting Information).

Figure 15: Interaction energy of the complex Cx_1 (left) and Cx_2 (right) as a function of the concentration of the fragment ion [C5 H5 O]+ at 40 eV.

Concluding remarks We have carried out ab initio and DFT calculations together with a topological analysis of the electron density in order to get insights about the electronic structure and the intermolecular interactions occurring in cation radical complexes relevant in the mass spectrometry fragmentation of N-(2-furylmethyl)anilines. Two different dissociation pathways leading to the formation of the m/z = 81 fragment were considered in this work, namely, i) direct cleavage and ii) isomerization/fragmentation mechanisms. In both cases, the division of the p-C4 H3 OCH2 NHC6 H4 -R systems follows the formation of complexes in which either furfuryl or pyrylium ions interact with neutral radical anilines. The calculated interaction energies of these cation radical clusters derived with the UM06-2x and UM06//UM06-2x functionals (the calculations were performed with 29



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the 6-311++G(2d,2p) basis set in every case) are similar to those computed with the methodology UCCSD//UMP2 (using the same basis functions). This concordance suggests that the UM06//UM062x methodoloy along with a triple zeta quality basis set supplemented with diffuse functions might be suitable for the determination of interaction energies in cation-radical clusters. The computation of the rate constants by means of RRKM theory indicates that the isomerization/fragmentation and the direct dissociation pathways are equally feasible at low internal energies of the system. The RRKM results also point out that the direct cleavage is the prevalent pathway of dissociation when the energy of the system is increased, leading thereby to a higher concentration of furfuryl over pyrylium cations under these circumstances. The topological analysis of the electron density as well as the examination of the NCI index reveal a number of closed shell interactions (N· · · H, O· · · H, O· · · N and C· · · N) within the complexes Cx_1 and Cx_2, the intermediates preceding the final steps of the direct cleavage and the isomerization/fragmentation mechanisms respectively. The interactions within Cx_1 and Cx_2 do not change importantly with the substituent with the exception of the complex between the pyrylium cation with the neutral aniline when R = OCH3 . The calculation of the QTAIM properties revealed that the charge of the studied radical-ion complexes is almost completely contained in the furfuryl or pyrylium rings. This distribution of charge is in agreement with the prominence of the m/z = 81 peak observed in the electronic impact spectra of the N-(2-furylmethyl)anilines and indicates that prior to the detection of the furfuryl or pyrylium cations, these species form a cluster with the remanent neutral aniline fragment. The integration of the spin density points out that the α unpaired electron over the pNH-C6 H4 R moiety is mainly distributed in the nitrogen in addition to the ortho- and para- carbon atoms with respect to the group -NH. Finally, we found a linear correlation between the interaction energy of the complexes Cx_1 and Cx_2 and the concentration of the fragment-ion [C5 H5 O]+ in the mass spectra at 40 eV). This relationship indicates that an increase in interaction energy of complex is accompanied by a diminution in the concentration of the fragment-ion [C5 H5 O]+ in the mass spectra. Overall, this study shows how detailed electronic structure studies accompanied by post wavefunction analysis such as the computation of QTAIM properties and the investigation of the NCI index may reveal

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important insights about molecular clusters formed in mass spectrometry.

Acknowledgements The authors gratefully acknowledge computer time from DGTIC/UNAM (project SC14-1-I-80) and financial support from PAPIIT/UNAM (grant IN209715). The authors are also thankful to Víctor Duarte Alaniz for insightul discussion. W.E.V.N also express his gratitude to CONACYT/México for the M. Sc. scholarship number 273438. Supporting Information Available: Comparison of the relative energies obtained with different basis sets in UCCSD/UMP2 calculations of intermediates and transition states for the direct dissociation and isomerization/fragmentation pathways of the C4 H3 O−CH2 −NH−C6 H4+· molecular ion. XYZ coordinates of the transition states throughout the same reaction mechanisms. Energy profiles of the C–N bond breaking for the two cation radical complexes, Cx_1 and Cx_2, involved in the aforementioned reaction pathways. Selected topological properties of ρ(r) at the intermolecular bond critical points, molecular graphs and NCI analysis of Cx_1 and Cx_2 obtained with the UM06/6-311++G(2d,2p)//UM06-2x/6-311++G(2d,2p) approximation. Free energy profiles for the direct division and the isomerization/dissociation pathways of C4 H3 O−CH2 −NH−C6 H4+· . Distribution of the unpaired electron within the Cx_1 adduct and the neutral C6 H5 NH · unit obtained with the UM06-2x/6-311++G(2d,2p) and UMP2/6-311++G(2d,2p) levels of theory. Energy profiles of the two routes of decomposition, RRKM rate constants, molecular graphs of Cx_1 and Cx_2 and NCI index analysis corresponding to the substituted N-(2-furylmethyl)anilines. Correlation between the concentration of the ion [C5 H5 O+ ] and the interaction energies of Cx_1 and Cx_2 in mass spectra at different energies of the ionizing electrons. This material is available free of charge via the Internet at http://pubs.acs.org.

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Graphical TOC Entry We investigate the electronic structure and non-covalent interactions within cation radical complexes formed in mass spectrometry (MS) of N -(2-furylmethyl) anilines. The previously suggested dissociation pathways i) direct cleavage of the NH−CH2 bond and ii) isomerization/fragmentation were analyzed with the a) UCCSD and UMP2 methods and b) UM06 and UM06-2x functionals along with Rice-Ramsperger-Kassel-Marcus theory. Bader analysis together with the examination of the NCI index reveal the nature of the non-covalent interactions and the most important mesomeric structures of the resonance hydrids of the systems of interest. Finally, a few correlations relating the concentration of the m/z = 81 ion, a preeminent fragment in the MS of N -(2-furylmethyl) anilines are presented. Altogether, we present how suitable quantum chemical calculations and post wavefunction analysis can yield insights about systems relevant in mass spectrometry.

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Control of metal catalyst selectivity through specific noncovalent molecular interactions.

Investigation of VO-salophen complexes electronic structure.

Fingerprinting Electronic Molecular Complexes in Liquid.

conductivity relationships.

Electronic structure and hydration of tetramine cobalt hydride complexes.

Synthesis and electronic structure of the first cyaphide-alkynyl complexes.

Molecular and electronic structures of donor-functionalized dysprosium pentadienyl complexes.

Hydrophobic noncovalent interactions of inosine-phenylalanine: a theoretical model for investigating the molecular recognition of nucleobases.

Analysis of protein--RNA interactions within Ro ribonucleoprotein complexes.

Paper spray ionization of noncovalent protein complexes.

Spin-orbit TDDFT electronic structure of diplatinum(II,II) complexes.

How to Make Weak Noncovalent Interactions Stronger.

nES GEMMA Analysis of Lectins and Their Interactions with Glycoproteins - Separation, Detection, and Sampling of Noncovalent Biospecific Complexes.

Matrix-isolation studies of noncovalent interactions: more sophisticated approaches.

Molecular connectivity: treatment of the electronic structure of amino acids.

Gas-phase anionic complexes of alkali metal ions and peptides: Structure and collision activated decompositions.

Protein: nucleic acid interactions. I. Electronic structures of cytosine, indole, and guanine complexes.

Theory and Modeling of RNA Structure and Interactions with Metal Ions and Small Molecules.

Role of noncovalent interactions in vanadium tellurite chain connectivities.

Pursuit of Noncovalent Interactions for Strategic Site-Selective Catalysis.

Properties of ammonium ion-water clusters: analyses of structure evolution, noncovalent interactions, and temperature and humidity effects.

Benchmark Calculations of Noncovalent Interactions of Halogenated Molecules.

Magnetic properties and electronic structure of neptunyl(VI) complexes: wavefunctions, orbitals, and crystal-field models.

Noncovalent interactions determine the conformation of aurophilic complexes with 2-mercapto-4-methyl-5-thiazoleacetic acid ligands.