Article pubs.acs.org/JPCA
Threshold Photoelectron Spectra of Combustion Relevant C4H5 and C4H7 Isomers Melanie Lang,† Fabian Holzmeier,† Patrick Hemberger,*,‡ and Ingo Fischer*,† †
Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg Molecular Dynamics Group, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
‡
S Supporting Information *
ABSTRACT: Threshold photoelectron spectra of combustion relevant C4H5 isomers, 2-butyn-1-yl and 1-butyn-3-yl, and C4H7 isomers, 1methylallyl and 2-methylallyl, have been recorded using vacuum ultraviolet synchrotron radiation. Adiabatic ionization energies (IEad) have been determined by assigning spectroscopic transitions in mass-selected threshold photoelectron spectra aided by Franck−Condon simulations. The following values were obtained: (7.97 ± 0.02) eV (1-butyn-3-yl), (7.94 ± 0.02) eV (2butyn-1-yl), (7.48 ± 0.01) eV (1-E-methylallyl), (7.59 ± 0.01) eV (1-Zmethylallyl), and (7.88 ± 0.01) eV (2-methylallyl). Good agreement with CBS-QB3 calculations and simulations could be achieved.
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INTRODUCTION The spectroscopic investigation of small hydrocarbon radicals is motivated by their role as intermediates, for example, in combustion processes.1−3 Photoionization studies on isolated radicals yield information that is necessary for their identification in reactive environments. It was recently shown that especially coincidence techniques such as photoelectron photoion coincidence (PEPICO) coupled with vacuum ultraviolet synchrotron radiation are powerful analytical tools for the isomer-selective identification of reactive intermediates in flames.4−7 Especially mass-selected threshold photoelectron spectra deliver the spectroscopic fingerprints needed for the assignment of each individual species.8,9 Since flame sampling coupled with PEPICO became state-of-the-art in the combustion community and photoelectron spectra in the literature often suffer from insufficient signal/noise, there is a need to measure clean spectra of radicals and carbenes for comparison with combustion data. C4H5 and C4H7, on which we focus in this study, play a dominant role in the pyrolysis of methyl tert-butyl ether,10 in the unimolecular decomposition of dimethylfuran,11 in the combustion of methyl methacrylate flames,12 in fuel-rich flames2,13 and biodiesel.14,15 The latter contains, e.g., fatty acid esters, compounds with double bonds that preferentially lose H atoms in the α-position to the CC double bonds, forming intermediates that can be described as alkylated allyl radicals. The smallest alkylated allyl radicals are the methylallyl isomers. The influence of an adjacent methyl group on several properties of the neutral and cationic allyl moiety is also of interest. The radicals presented in this report are two C4H7 isomers, 2-methylallyl (2-MA) and 1-methylallyl (1-MA), as well as the C4H5 isomers, 2-butyn-1-yl and 1-butyn-3-yl. © 2015 American Chemical Society
The MA-radicals have been studied before by conventional photoelectron spectroscopy,16 high-resolution zero kinetic energy (ZEKE) photoelectron spectroscopy,17 in UV absorption studies18 and by resonance-enhanced multiphoton ionization (REMPI) experiments.19,20 The photodissociation dynamics have been explored by H atom photofragment Doppler spectroscopy.21,22 In addition, our group recently investigated the excited state dynamics in picosecond timeresolved experiments.23 1-Butyn-3-yl and the E- and Z-conformers of methylallyl possess Cs symmetry, whereas 2-butyn-1-yl slightly deviates from Cs symmetry, since the CH3 group is rotated out of the C−CCH2 plane. 2-MA is expected to be of C2v symmetry. The methyl group can be treated as a free internal rotor, as observed before.17,23 The purpose of the present work was to produce C4H5 and C4H7 radicals cleanly in a tubular reactor and derive adiabatic ionization energies (IEad) as well as spectroscopic information on the respective cationic structures by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy.24−26 This technique has been applied before to the investigation of hydrocarbon radicals relevant in combustion processes, ranging from small radicals like allyl, C3H5,27 and ethyl, C2H5,28 but also larger radicals, like the three C9H7 isomers,29,30 the isomers of xylyl, C8H9,31 as well as the fluorenyl and benzhydryl radicals C13H9 and C13H11.32 Received: March 5, 2015 Revised: April 7, 2015 Published: April 7, 2015 3995
DOI: 10.1021/acs.jpca.5b02153 J. Phys. Chem. A 2015, 119, 3995−4000
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The Journal of Physical Chemistry A
Scheme 1. Investigated Isomers of C4H5, 2-Butyn-1-yl and 1-Butyn-3-yl, and C4H7, E-, Z-1-Methylallyl, and 2-Methylallyl, Their Symmetries, and the Precursors Used in the Present Experiment
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EXPERIMENT AND THEORETICAL METHODS The C4H5 and C4H7 radical isomers were investigated by using the iPEPICO (imaging PEPICO) setup of the VUV X04DB beamline at the SLS (Swiss Light Source) storage ring. The details of the beamline layout33 and the iPEPICO apparatus25,34 have been described in the literature, thus only a brief overview is given. Vacuum ultraviolet (VUV) synchrotron radiation is provided by a bending magnet and collimated onto a plane grating (150 grooves/mm and 600 grooves/mm were used) of a normal incidence monochromator. The maximum resolution achieved thereby is E/ΔE = 104. A rare gas filter is used to suppress higher order radiation and is either used with a mixture of Ne/Ar/Kr for photon energies below 14.0 eV. Energy was calibrated by using the 11s′−14s′ autoionization states of Argon, both in the first and the second order. All radicals were produced from nitrite and bromine precursors by pyrolysis according to Scheme 1. The nitrite precursors for the generation of 1-MA and 2-MA were synthesized according to literature procedures.22,35 The C4H5Br isomers were purchased from Sigma-Aldrich and used without further purification. The nitrite precursors were seeded in 2 bar of Argon, the bromobutynes in 1 bar of Argon and passed through an electrically heated silicon carbide (SiC) tube that was mounted onto a 100 μm orifice. The details of the pyrolysis source were described by Kohn et al.36 The pyrolysis power was chosen to yield temperatures between 450 and 550 °C. A Type C thermocouple was used to measure the temperature outside the pyrolysis tube with an estimated accuracy of ±100 °C. Photoions were detected in a Wiley−McLaren time-of-flight mass spectrometer in coincidence with photoelectrons. Photoelectrons were collected by a position sensitive detector. All charged particles were accelerated by a 120 V cm−1 extraction field. Threshold electrons were considered by selecting the central part of the photoelectron image, which corresponds to photoelectrons with a kinetic energy of less than 5 meV. Contributions of hot electrons were subtracted according to a procedure described elsewhere.37 Due to the almost constant photon flux over the investigated energy range, the signal was not corrected for flux. Typically the spectra have been recorded with a step size of 5 meV and an acquisition time of 120 s per data point, and for 2-methylallyl an acquisition time of 300 s per data point was employed. Ionization energies and vibrational frequencies were computed with the Gaussian 09 program.38 The B3LYP/6-311g(d,p) and M06L/6-311g(d,p) level of theory was chosen for the computation of all molecular
geometries, harmonic frequencies, and rotational constants. The CBS-QB3 method was used to obtain accurate ionization energies and the reaction pathway for the rearrangement of the butynyl radicals.39,40 All energies were corrected for zero point vibrational energies and harmonic Franck−Condon simulations were performed with the program FCFit version 2.8.20.41 Rotational barriers and barriers of rearrangements were determined by relaxed potential energy surface scans and subsequent transition state geometry optimization.
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RESULTS AND DISCUSSION (a). Mass Spectra. Figure 1 shows the mass spectra of 2butyn-1-yl (upper panel) and 1-butyn-3-yl (lower panel) from pyrolysis of their respective brominated precursors.
Figure 1. Mass spectra of 1-bromo-2-butyne (upper panel) and 3bromo-1-butyne (lower panel) at 8.0 eV with a pyrolysis temperature of 550 °C.
At 8.0 eV and pyrolysis temperatures of 550 °C 3-bromo-1butyne and 1-bromo-2-butyne are cleanly converted to the C4H5 radicals (m/z = 53) and no further mass signals are visible in the energy range around the ionization threshold. As the ionization energies of both bromides are above 9.5 eV, the conversion of the precursors was deduced from mass spectra recorded at 10 eV (not shown here). The dissociative photoionization of the bromo-butyne isomers has already been examined by Bodi et al., who reported the 0 K appearance energies for 2-butyn-1-ylium (AE0K = 10.375 eV) and for 1butyn-2-ylium cation (AE0K = 10.284 eV), respectively.42 Figure 2 shows the mass spectra of 3-penten-1-yl nitrite (upper panel) and 3-methyl-3-buten-1-yl nitrite (lower panel). 3996
DOI: 10.1021/acs.jpca.5b02153 J. Phys. Chem. A 2015, 119, 3995−4000
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ionization energy of 2-butyn-1-yl. The spectrum of the second C4H5 isomer 1-butyn-3-yl, recorded from 7.85 to 8.5 eV does not appear to be very different from the first one. The first peak, assigned to the ionization energy of 1-butyn-3-yl, is found at 7.97 eV. This peak is also accompanied by a shoulder at +0.1 eV. As stated above, dissociative photoionization of the precursors is not relevant for either butynyl isomer within the investigated energy range, however both spectra exhibit contributions of hot- and sequence bands at around 7.9 eV. The two spectra show some vibrational structure, which can tentatively be assigned with the help of the Franck−Condon (FC) simulations (red line in Figure 3). The simulation of 2-butyn-1-yl (upper panel) shows a band around +0.1 eV, which is mainly caused by mode ν16+ (computed 0.10 eV/806 cm−1), a stretch of the two C−C single bonds, and the combination of ν16+ with ν21+ (computed 0.012 eV/97 cm−1). The band in the range of +0.3 eV is in the FC simulation caused by ν6+ (computed 0.28 eV/2243 cm−1), the C2−C3 triple bond stretching vibration (for labeling of atoms, see Scheme 1). Additionally, there is a band simulated at around +0.4 eV (3200 cm−1), which is not distinguishable in the experimental spectrum. The relative positions of the simulated bands of 1-butyn-3-yl also match the experimental spectrum quite well. The band around +0.1 eV is assigned to ν15+ (computed to 0.11 eV/867 cm−1), which corresponds to the H3C4−C1 single bond stretching vibration. ν15+ is in addition computed to contribute to the spectrum with the first overtone at around +0.22 eV, which is barely visible as a shoulder in the spectrum. Both Franck−Condon stick spectra were convoluted with a Gaussian function of 25 meV full width at half-maximum. In both molecules, the internal rotations of the methyl groups are expected to cause broadening of the bands in the spectra. Since the harmonic approximation often fails for internal rotations, torsional modes are not always well represented in FCsimulations. Here only in the simulation of 2-butyn-1-yl the contribution of the methyl torsional mode ν21+ is predicted correctly, while it does not appear in the simulated spectrum of 1-butyn-3-yl. However, the similar shape of the spectra leads us to consider isomerization in the pyrolysis as a possible side reaction. The calculated rearrangement pathway from 2-butyn-1-yl to 1butyn-3-yl is depicted in Scheme 2. The isomerization of 2butyn-1-ylium to 1-butyl-3-ylium cations has been computed before, and a reaction sequence was found.43 This work guided our computations on the CBS-QB3 level, which also predict a four step rearrangement on the neutral potential surface of C4H5. As the highest barrier is computed to be only 2.19 eV (212 kJ/mol), at pyrolysis temperatures of 550 °C a small degree of isomerization cannot be excluded and might explain the broadening in the spectrum of 1-butyn-3-yl. Since 2-butyn1-yl and 1-butyn-3-yl differ only by 13 meV in their absolute energies, the rearrangement is not supposed to produce a dominant product if there is enough reaction time to reach the chemical equilibrium in the reactor. However, not only does the rearrangement of the produced radicals have to be considered, but the rearrangement of the precursor molecules does as well. In previous TPEPICO studies, a similar isomerization has been observed for a doubly brominated propyne that was used as a radical precursor.44 The barrier of the isomerization of 3-bromo-1-butyne to 1-bromo-1methyl-allene has been computed to 3.69 eV/356 kJ/mol (CBS-QB3) and the respective rearrangement of 1-bromo-2-
Figure 2. Mass spectra of 3-penten-1-yl nitrite (upper panel) and 3methyl-3-buten-1-yl (lower panel) at 8.0 eV with a pyrolysis temperature of 450 °C.
At 450 °C a large signal at m/z = 55 is visible, corresponding to the C4H7 radicals. Both precursors are not visible at any photon energy, indicating either dissociative ground states for the two nitrite precursor cations or large geometry changes upon ionization. Therefore, the dissociative photoionization (DPI) of the nitrites was not elucidated further. Nevertheless, there are several indications that we can exclude contributions to the m/z = 55 radical signals from dissociative photoionization of the precursors: without pyrolysis m/z = 55 is just observed from 3penten-1-yl nitrite beyond energies of 9 eV and from 3-methyl3-buten-1-yl nitrite beyond 9.25 eV, respectively. In addition, the major fragment visible from the precursors without pyrolysis at around 9.1 eV is m/z = 85, corresponding to the loss of NO (not shown here). Under the pyrolysis conditions applied in the experiment (≈450 °C) m/z = 85 is absent, which can be seen as evidence for complete precursor conversion. (b). Threshold Photoelectron Spectra of C4H5: 2Butyn-1-yl and 1-Butyn-3-yl. The threshold photoelectron spectra (TPES) and the Franck−Condon simulations of 2butyn-1-yl (upper trace) and 1-butyn-3-yl (lower trace) are depicted in Figure 3. Since there are no other masses visible in
Figure 3. TPES of 2-butyn-1-yl (upper panel) and 1-butyn-3-yl (lower panel) with the respective Franck−Condon simulations.
the investigated energy range, all threshold photoelectrons without selection of the coincident ions were analyzed. The comparably small signal originates from low vapor pressure of the precursors. The TPE spectrum of 2-butyn-1-yl was recorded from 7.8 to 8.6 eV and shows a peak at 7.94 eV followed by a shoulder at 8.04 eV. A second less intense band is visible around 8.2 eV. The signal at 7.94 eV is assigned to the 3997
DOI: 10.1021/acs.jpca.5b02153 J. Phys. Chem. A 2015, 119, 3995−4000
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The Journal of Physical Chemistry A Scheme 2. Rearrangement of the Radicals upon Pyrolysisa
a
Energies are given in eV (kJ/mol) with respect to the more stable radical isomer 2-butyn-1-yl. The transition state structures (TS1-TS4) are not shown.
butyne to 1-bromo-3-methylallene has a barrier of 3.88 eV/374 kJ/mol (CBS-QB3). Since the barriers for precursor rearrangement are significantly higher than those for the radical rearrangement and the formation of the radical itself (bromine loss at 2.76 eV (267 kJ/mol) and 2.67 eV (258 kJ/mol) from 1bromo-2-butyne and 3-bromo-1-butyne, respectively), the precursor isomerization can be ignored. (c). Threshold Photoelectron Spectra of C4H7: 1Methylallyl and 2-Methylallyl. The precursor 3-penten-1yl nitrite used to generate 1-MA seems to preferentially form the Z-conformation. However, the absolute energy difference between the two E- and Z-conformations of the formed 1-MA radical is marginal (3 meV/0.3 kJ/mol), and the rotational barrier for isomerization is only 0.67 eV (65 kJ/mol). Thus, under our pyrolysis conditions, both the E- and the Zconformer are formed, as it has been observed before.16 The mass-selected TPE spectrum (black line) of the two 1MA radicals is depicted in Figure 4. At around 7.25 eV the TPE
Table 1 present work molecule 1-butyn-3-yl 2-butyn-1-yl 1-E-methylallyl 1-Z-methylallyl 2-methylallyl
literature
IEad/eV (exp.)
IEad /eV (CBS-QB3)
IEad/eV (exp.,Lit.)
IE/eV (calc. CBSTQ5)45
7.97 7.94 7.48 7.59 7.88(s) 9.85(t)
8.02 7.99 7.54 7.65 7.93 (s) 9.93 (t)
7.97a 7.95a 7.49b 7.67c 7.9016 7.87717,d
7.433 7.540 7.827 -
a Derived values. 46 b Adiabatic value, no specification of the conformation of 1-MA.16 cVertical value, no specification of the conformation of 1-MA.16 dValue determined for C4D7.
cm−1) and the ν24+ (computed 0.063 eV/514 cm−1) vibration of the E-isomer. ν25+ is assigned to the C−C−C bending motion and ν24+ to the C−C stretching vibration (bonds C3− C1 and C3−C8). The band around +0.18 at 7.66 eV reveals the mode ν23+ (computed 0.071 eV/572 cm−1) of 1-Z-MA, a wagging mode of C3 and C1. There is a discrepancy between the experimental and simulated spectra, which might be attributed to activity in torsional modes of the CH3 groups. The participation of autoionizing states can be neglected since the conventional PES study shows very similar relative signal levels and shape. In addition, no excited states on the singlet and triplet surface were found in the same energy range. Figure 5 shows the ms-TPES of 2-MA (black) in the range from 7.70 to 12.0 eV. The Franck−Condon simulations (red) of the transitions to the singlet cation X+ 1A1 and to the triplet cation A+ 3A are also displayed in the graph. A first peak can be observed at 7.83 eV, followed by the most intense peak in the
Figure 4. Ms-TPE spectrum of the E-1-methylallyl and Z-1-methylallyl radicals with the convolution (fwhm = 25 meV) of the FC simulations for both isomers.
signal starts to rise, reaching the first peak in the spectrum at 7.48 eV, assigned to the IEad of 1-E-methylallyl. The computed IE for 1-E-methylallyl is 7.54 eV, 0.11 eV lower than for 1-Zmethylallyl (7.65 eV). A comparison of the experimental and computed IEs is given in Table 1. Three further peaks are observed, showing increasingly higher intensities. The most intense peak in the spectrum appears at 7.59 eV and is assigned to the IEad of 1-Z-MA, based on the best FC-simulation. In previous work the adiabatic and a vertical ionization energy of 1-MA were determined instead of the IEs for the individual conformers.16 The peaks at 7.52 eV (+0.04 eV) and 7.55 eV (+0.06 eV) correspond to the ν25+ (computed 0.037 eV/299
Figure 5. Mass-selected TPE spectrum of 2-methylallyl radical. The FC simulation (fwhm = 25 meV) is shown in red. 3998
DOI: 10.1021/acs.jpca.5b02153 J. Phys. Chem. A 2015, 119, 3995−4000
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spectrum at 7.88 eV, which is assigned to the IEad of 2-MA. This value matches the computed IE of 7.93 eV quite well. The vibrational progression with spacing of 0.055 eV is assigned to the totally symmetric bending mode ν 24 + (computed 0.056 eV/453 cm−1) Gasser et al.17 derived from their ZEKE-spectra a value of 450 cm−1 for this mode. The ν24+ is broadened by a combination with even quanta of the methyl torsion ν27+ that is computed with a wavenumber of 33 cm−1. The peak at 7.83 eV is assigned to hot band 2410 with a computed wavenumber of 484 cm−1 for the respective bending mode in the neutral radical. It shows a fwhm of 30 meV, 10 meV larger than the 0−0 transition, and might contain contributions of other hot and sequence bands. Note that the program employed for the FC simulation does not include hot bands. The band at 9.85 eV is assigned to the transition to the triplet cation. Its shape is represented quite well by the FC simulation. The geometry obtained for the triplet cation changes significantly in comparison to the neutral molecule and to the singlet cation. The symmetry is reduced from C2v in the neutral and in the singlet cation to C2 in the triplet cation. The CH2 moieties move out of the molecular plane leading to an increase of the dihedral angle d(H3C1C4C5) by about 65°. In the ZEKE-spectra of Gasser et al.17 the excitation of two further non totally symmetric modes in the 2-MA cation were observed up to 64700 cm−1 (8.02 eV), but an ionization energy was only determined for the d7-isotopologue. However, a value of 63530 cm−1 was reported, corresponding to 7.877 eV, very similar to the 7.88 eV obtained here. Since a [1 + 1′] scheme via intermediate Rydberg-states was employed, only the fundamental of the CCC bending mode was observed in the ZEKE spectra.
Article
ASSOCIATED CONTENT
S Supporting Information *
The computed frequencies of all neutral and cationic species and full ref 38 are given in the Supporting Information. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: ingo.fi[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the German Science Foundation, contract FI575/7-3. The experimental work was carried out at the VUV Beamline of the Swiss Light Source, Paul Scherrer Institute. The work was financially supported by the Swiss Federal Office for Energy (BFE Contract Number 101969/152433). We would like to thank Kathrin Weiland and Florian Scharnagl for their contributions to the experiments on 1-methylallyl (K.W.) and 2-methylallyl (F.S.), respectively.
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REFERENCES
(1) Huang, C.; Wei, L.; Yang, B.; Wang, J.; Li, Y.; Sheng, L.; Zhang, Y.; Qi, F. Lean Premixed Gasoline/Oxygen Flame Studied with Tunable Synchrotron Vacuum UV Photoionization. Energy Fuels 2006, 20, 1505−1513. (2) Hansen, N.; Klippenstein, S. J.; Taatjes, C. A.; Miller, J. A.; Wang, J.; Cool, T. A.; Yang, B.; Yang, R.; Wei, L.; Huang, C.; et al. Identification and Chemistry of C4H3 and C4H5 Isomers in Fuel-Rich Flames. J. Phys. Chem. A 2006, 110, 3670−3678. (3) Yang, B.; Osswald, P.; Li, Y. Y.; Wang, J.; Wei, L. X.; Tian, Z. Y.; Qi, F.; Kohse-Hoinghaus, K. Identification of Combustion Intermediates in Isomeric Fuel-Rich Premixed Butanol-Oxygen Flames at Low Pressure. Combust. Flame 2007, 148, 198−209. (4) Bierkandt, T.; Kasper, T.; Akyildiz, E.; Lucassen, A.; Oßwald, P.; Köhler, M.; Hemberger, P. Flame Structure of a Low-Pressure Laminar Premixed and Lightly Sooting Acetylene Flame and the Effect of Ethanol Addition. Proc. Combust. Inst. 2015, 35, 803−811. (5) Felsmann, D.; Moshammer, K.; Krüger, J.; Lackner, A.; Brockhinke, A.; Kasper, T.; Bierkandt, T.; Akyildiz, E.; Hansen, N.; Lucassen, A.; et al. Electron Ionization, Photoionization and Photoelectron/Photoion Coincidence Spectroscopy in Mass-Spectrometric Investigations of a Low-Pressure Ethylene/Oxygen Flame. Proc. Combust. Inst. 2015, 35, 779−786. (6) Krüger, J.; Garcia, G. A.; Felsmann, D.; Moshammer, K.; Lackner, A.; Brockhinke, A.; Nahon, L.; Kohse-Höinghaus, K. PhotoelectronPhotoion Coincidence Spectroscopy for Multiplexed Detection of Intermediate Species in a Flame. Phys. Chem. Chem. Phys. 2014, 16, 22791−22804. (7) Oßwald, P.; Hemberger, P.; Bierkandt, T.; Akyildiz, E.; Köhler, M.; Bodi, A.; Gerber, T.; Kasper, T. In Situ Flame Chemistry Tracing by Imaging Photoelectron Photoion Coincidence Spectroscopy. Rev. Sci. Instrum. 2014, 85, 025101. (8) Bodi, A.; Hemberger, P.; Osborn, D. L.; Sztáray, B. MassResolved Isomer-Selective Chemical Analysis with Imaging Photoelectron Photoion Coincidence Spectroscopy. J. Phys. Chem. Lett. 2013, 4, 2948−2952. (9) Hemberger, P.; Trevitt, A. J.; Ross, E.; da Silva, G. Direct Observation of para-Xylylene as the Decomposition Product of the meta-Xylyl Radical Using VUV Synchrotron Radiation. J. Phys. Chem. Lett. 2013, 4, 2546−2550. (10) Zhang, T.; Wang, J.; Yuan, T.; Hong, X.; Zhang, L.; Qi, F. Pyrolysis of Methyl tert-Butyl Ether (MTBE). 1. Experimental Study
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CONCLUSION C4H5 and C4H7 radicals were investigated by threshold photoelectron photoion coincidence spectroscopy. The ionization energies of 2-butyn-1-yl, C4H5, and 1-butyn-3-yl, C4H5, were obtained from the TPE spectra to be 7.94 ± 0.02 eV and 7.97 ± 0.02 eV, respectively. As shown in Table 1, the experimental IEs are in good agreement with the computed values of 7.99 and 8.02 eV. The Franck−Condon simulations represent the spectra of both isomers quite well. Since the spectra have a similar appearance, isomerization between the two C4H5 isomers in the pyrolysis was investigated and a barrier of 2.19 eV (212 kJ/mol) was computed. As the ionization energies of the isomers differ slightly, we assume contributions from the second isomer to be small. From the TPE spectra of 1-methylallyl the ionization energies of 7.48 eV and at 7.59 eV were derived for the 1-E- and 1-Z- isomers of methylallyl. The computed values agree well with this assignment, as the IE of 1-E-MA is predicted at 7.54 eV and the IE of 1-Z-MA is predicted at 7.65 eV, respectively. Further vibrational bands were assigned with the help of Franck− Condon simulations. Separate IEs for the two conformers have not been obtained in earlier experiments.16 From the TPES of 2-methylallyl, the ionization energy of 2-MA to the singlet cation X+ 1A1 ←X 2A2 is determined to be 7.88 ± 0.01 eV. Furthermore, we assign an extensive vibrational progression of the C−C−C bending mode ν24+ with a spacing of 0.055 eV. In addition, the transition to the triplet cation A+ 3A ← X 2A2 was observed, and an excitation energy of 9.85 eV was determined. The assignments are again in good agreement with computations, which yield the adiabatic IE at 7.93 eV and the transition into the lowest triplet state at 9.93 eV. 3999
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The Journal of Physical Chemistry A with Molecular-Beam Mass Spectrometry and Tunable Synchrotron VUV Photoionization. J. Phys. Chem. A 2008, 112, 10487−10494. (11) Sirjean, B.; Fournet, R. Unimolecular Decomposition of 2,5Dimethylfuran: A Theoretical Chemical Kinetic Study. Phys. Chem. Chem. Phys. 2013, 15, 596−611. (12) Lin, Z.; Wang, T.; Han, D.; Han, X.; Li, S.; Li, Y.; Tian, Z. Study of Combustion Intermediates in Fuel-Rich Methyl Methacrylate Flame with Tunable Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry. Rapid Commun. Mass Spectrom. 2009, 23, 85−92. (13) Wheeler, S. E.; Allen, W. D.; Schaefer, H. F. Thermochemistry of Disputed Soot Formation Intermediates C4H3 and C4H5. J. Chem. Phys. 2004, 121, 8800−8813. (14) Westbrook, C. K. Biofuels Combustion. Annu. Rev. Phys. Chem. 2013, 64, 201−219. (15) Kohse-Höinghaus, K.; Oßwald, P.; Cool, T. A.; Kasper, T.; Hansen, N.; Qi, F.; Westbrook, C. K.; Westmoreland, P. R. Biofuel Combustion Chemistry: From Ethanol to Biodiesel. Angew. Chem., Int. Ed. 2010, 49, 3572−3597. (16) Schultz, J. C.; Houle, F. A.; Beauchamp, J. L. Photoelectron Spectroscopy of Isomeric C4H7 Radicals. Implications for the Thermochemistry and Structures of the Radicals and Their Corresponding Carbonium Ions. J. Am. Chem. Soc. 1984, 106, 7336−7347. (17) Gasser, M.; Frey, J. A.; Hostettler, J. M.; Bach, A. Vibronic Structure of the 3s Rydberg State of the 2-Methylallyl Radical. J. Mol. Spectrosc. 2010, 263, 93−100. (18) Callear, A. B.; Lee, H. K. Free Radical Spectra in the Flash Photolysis of Olefins. Trans. Faraday Soc. 1968, 64, 2017−2022. (19) Hudgens, J. W.; Dulcey, C. S. Observation of the 3s 2A1 Rydberg States of Allyl and 2-Methylallyl Radicals with Multiphoton Ionization Spectroscopy. J. Phys. Chem. 1985, 89, 1505−1509. (20) Chen, C.-C.; Wu, H.-C.; Tseng, C.-M.; Yang, Y.-H.; Chen, Y.-T. One- and Two-Photon Excitation Vibronic Spectra of 2-Methylallyl Radical at 4.6−5.6 eV. J. Chem. Phys. 2003, 119, 241−250. (21) Gasser, M.; Bach, A.; Chen, P. Photodissociation Dynamics of the 2-Methylallyl Radical. Phys. Chem. Chem. Phys. 2008, 10, 1133− 1138. (22) Gasser, M.; Frey, J. A.; Hostettler, J. M.; Bach, A. Probing for Non-Statistical Effects in Dissociation of the 1-Methylallyl Radical. Chem. Commun. 2011, 47, 301−303. (23) Herterich, J.; Gerbich, T.; Fischer, I. Excited-State Dynamics of the 2-Methylallyl Radical. ChemPhysChem 2013, 14, 3906−3908. (24) Baer, T.; Guyon, P.-M. An Historical Introduction to Threshold Photoionization. In High Resolution Laser Photoionisation and Photoelectron Studies; Ng, C. Y., Baer, T., Powis, I., Eds.; Wiley: New York, 1995. (25) Bodi, A.; Johnson, M.; Gerber, T.; Gengeliczki, Z.; Sztáray, B.; Baer, T. Imaging Photoelectron Photoion Coincidence Spectroscopy with Velocity Focusing Electron Optics. Rev. Sci. Instrum. 2009, 80, 034101. (26) Garcia, G. A.; Soldi-Lose, H.; Nahon, L. A Versatile ElectronIon Coincidence Spectrometer for Photoelectron Momentum Imaging and Threshold Spectroscopy on Mass Selected Ions Using Synchrotron Radiation. Rev. Sci. Instrum. 2009, 80, 023102. (27) Schüßler, T.; Deyerl, H.-J.; Dummler, S.; Fischer, I.; Alcaraz, C.; Elhanine, M. The Vacuum Ultraviolet Photochemistry of the Allyl Radical Investigated Using Synchrotron Radiation. J. Chem. Phys. 2003, 118, 9077−9080. (28) Schüßler, T.; Roth, W.; Gerber, T.; Alcaraz, C.; Fischer, I. The VUV Photochemistry of Radicals: C3H3 and C2H5. Phys. Chem. Chem. Phys. 2005, 7, 819−825. (29) Hemberger, P.; Steinbauer, M.; Schneider, M.; Fischer, I.; Johnson, M.; Bodi, A.; Gerber, T. Photoionization of Three Isomers of the C9H7 Radical. J. Phys. Chem. A 2010, 114, 4698−4703. (30) Holzmeier, F.; Lang, M.; Hemberger, P.; Fischer, I. Improved Ionization Energies for the Two Isomers of Phenylpropargyl Radical. ChemPhysChem 2014, 15, 3489−3492. (31) Hemberger, P.; Trevitt, A. J.; Gerber, T.; Ross, E.; da Silva, G. Isomer-Specific Product Detection of Gas-Phase Xylyl Radical
Rearrangement and Decomposition Using VUV Synchrotron Photoionization. J. Phys. Chem. A 2014, 118, 3593. (32) Lang, M.; Holzmeier, F.; Fischer, I.; Hemberger, P. Threshold Photoionization of Fluorenyl, Benzhydryl, Diphenylmethylene, and Their Dimers. J. Phys. Chem. A 2013, 117, 5260−5268. (33) Johnson, M.; Bodi, A.; Schulz, L.; Gerber, T. Vacuum Ultraviolet Beamline at the Swiss Light Source for Chemical Dynamics Studies. Nucl. Instrum. Methods A 2009, 610, 597−603. (34) Bodi, A.; Hemberger, P.; Gerber, T.; Sztáray, B. A New Double Imaging Velocity Focusing Coincidence Experiment: i2PEPICO. Rev. Sci. Instrum. 2012, 83, 083105. (35) Zabarnick, S.; Heicklen, J. Reactions of Alkoxy Radicals with O2. I. C2H5O Radicals. Int. J. Chem. Kinet. 1985, 17, 455−476. (36) Kohn, D. W.; Clauberg, H.; Chen, P. Flash Pyrolysis Nozzle for Generation of Radicals in a Supersonic Jet Expansion. Rev. Sci. Instrum. 1992, 63, 4003−4005. (37) Sztáray, B.; Baer, T. Suppression of Hot Electrons in Threshold Photoelectron Photoion Coincidence Spectroscopy Using Velocity Focusing Optics. Rev. Sci. Instrum. 2003, 74, 3763−3768. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, revision B.1; Gaussian, Inc.: Wallingford, CT, 2009. (39) Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. A Complete Basis Set Model Chemistry. VI. Use of Density Functional Geometries and Frequencies. J. Chem. Phys. 1999, 110, 2822−2827. (40) Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. A Complete Basis Set Model Chemistry. VII. Use of the Minimum Population Localization Method. J. Chem. Phys. 2000, 112, 6532− 6542. (41) Spangenberg, D.; Imhof, P.; Kleinermanns, K. The S1 State Geometry of Phenol Determined by Simultaneous Franck−Condon and Rotational Constants Fits. Phys. Chem. Chem. Phys. 2003, 5, 2505−2514. (42) Bodi, A.; Hemberger, P. Imaging Breakdown Diagrams for Bromobutyne Isomers with Photoelectron−Photoion Coincidence. Phys. Chem. Chem. Phys. 2014, 16, 505−515. (43) Cunje, A.; Lien, M. H.; Hopkinson, A. C. The C4H5+ Potential Energy Surface. Structure, Relative Energies, and Enthalpies of Formation of Isomers of C4H5+. J. Org. Chem. 1996, 61, 5212−5220. (44) Hemberger, P.; Lang, M.; Noller, B.; Fischer, I.; Alcaraz, C.; Cunha de Miranda, B. K.; Garcia, G. A.; Soldi-Lose, H. Photoionization of Propargyl and Bromopropargyl Radicals: A Threshold Photoelectron Spectroscopic Study. J. Phys. Chem. A 2011, 115, 2225− 2230. (45) Lau, K.-C.; Zheng, W.; Wong, N.-B.; Li, W.-K. Theoretical Prediction of the Ionization Energies of the C4H7 Radicals: 1Methylallyl, 2-Methylallyl, Cyclopropylmethyl, and Cyclobutyl Radicals. J. Chem. Phys. 2007, 127, 154302. (46) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Gas-Phase Ion and Neutral Thermochemistry. J. Phys. Chem. Ref. Data 1988, No. Suppl.1, 1−861.
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DOI: 10.1021/acs.jpca.5b02153 J. Phys. Chem. A 2015, 119, 3995−4000
Threshold Photoelectron Spectra of Combustion Relevant C4H5 and C4H7 Isomers Melanie Lang,† Fabian Holzmeier,† Patrick Hemberger,*,‡ and Ingo Fischer*,† †
Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg Molecular Dynamics Group, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
‡
S Supporting Information *
ABSTRACT: Threshold photoelectron spectra of combustion relevant C4H5 isomers, 2-butyn-1-yl and 1-butyn-3-yl, and C4H7 isomers, 1methylallyl and 2-methylallyl, have been recorded using vacuum ultraviolet synchrotron radiation. Adiabatic ionization energies (IEad) have been determined by assigning spectroscopic transitions in mass-selected threshold photoelectron spectra aided by Franck−Condon simulations. The following values were obtained: (7.97 ± 0.02) eV (1-butyn-3-yl), (7.94 ± 0.02) eV (2butyn-1-yl), (7.48 ± 0.01) eV (1-E-methylallyl), (7.59 ± 0.01) eV (1-Zmethylallyl), and (7.88 ± 0.01) eV (2-methylallyl). Good agreement with CBS-QB3 calculations and simulations could be achieved.
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INTRODUCTION The spectroscopic investigation of small hydrocarbon radicals is motivated by their role as intermediates, for example, in combustion processes.1−3 Photoionization studies on isolated radicals yield information that is necessary for their identification in reactive environments. It was recently shown that especially coincidence techniques such as photoelectron photoion coincidence (PEPICO) coupled with vacuum ultraviolet synchrotron radiation are powerful analytical tools for the isomer-selective identification of reactive intermediates in flames.4−7 Especially mass-selected threshold photoelectron spectra deliver the spectroscopic fingerprints needed for the assignment of each individual species.8,9 Since flame sampling coupled with PEPICO became state-of-the-art in the combustion community and photoelectron spectra in the literature often suffer from insufficient signal/noise, there is a need to measure clean spectra of radicals and carbenes for comparison with combustion data. C4H5 and C4H7, on which we focus in this study, play a dominant role in the pyrolysis of methyl tert-butyl ether,10 in the unimolecular decomposition of dimethylfuran,11 in the combustion of methyl methacrylate flames,12 in fuel-rich flames2,13 and biodiesel.14,15 The latter contains, e.g., fatty acid esters, compounds with double bonds that preferentially lose H atoms in the α-position to the CC double bonds, forming intermediates that can be described as alkylated allyl radicals. The smallest alkylated allyl radicals are the methylallyl isomers. The influence of an adjacent methyl group on several properties of the neutral and cationic allyl moiety is also of interest. The radicals presented in this report are two C4H7 isomers, 2-methylallyl (2-MA) and 1-methylallyl (1-MA), as well as the C4H5 isomers, 2-butyn-1-yl and 1-butyn-3-yl. © 2015 American Chemical Society
The MA-radicals have been studied before by conventional photoelectron spectroscopy,16 high-resolution zero kinetic energy (ZEKE) photoelectron spectroscopy,17 in UV absorption studies18 and by resonance-enhanced multiphoton ionization (REMPI) experiments.19,20 The photodissociation dynamics have been explored by H atom photofragment Doppler spectroscopy.21,22 In addition, our group recently investigated the excited state dynamics in picosecond timeresolved experiments.23 1-Butyn-3-yl and the E- and Z-conformers of methylallyl possess Cs symmetry, whereas 2-butyn-1-yl slightly deviates from Cs symmetry, since the CH3 group is rotated out of the C−CCH2 plane. 2-MA is expected to be of C2v symmetry. The methyl group can be treated as a free internal rotor, as observed before.17,23 The purpose of the present work was to produce C4H5 and C4H7 radicals cleanly in a tubular reactor and derive adiabatic ionization energies (IEad) as well as spectroscopic information on the respective cationic structures by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy.24−26 This technique has been applied before to the investigation of hydrocarbon radicals relevant in combustion processes, ranging from small radicals like allyl, C3H5,27 and ethyl, C2H5,28 but also larger radicals, like the three C9H7 isomers,29,30 the isomers of xylyl, C8H9,31 as well as the fluorenyl and benzhydryl radicals C13H9 and C13H11.32 Received: March 5, 2015 Revised: April 7, 2015 Published: April 7, 2015 3995
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Scheme 1. Investigated Isomers of C4H5, 2-Butyn-1-yl and 1-Butyn-3-yl, and C4H7, E-, Z-1-Methylallyl, and 2-Methylallyl, Their Symmetries, and the Precursors Used in the Present Experiment
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EXPERIMENT AND THEORETICAL METHODS The C4H5 and C4H7 radical isomers were investigated by using the iPEPICO (imaging PEPICO) setup of the VUV X04DB beamline at the SLS (Swiss Light Source) storage ring. The details of the beamline layout33 and the iPEPICO apparatus25,34 have been described in the literature, thus only a brief overview is given. Vacuum ultraviolet (VUV) synchrotron radiation is provided by a bending magnet and collimated onto a plane grating (150 grooves/mm and 600 grooves/mm were used) of a normal incidence monochromator. The maximum resolution achieved thereby is E/ΔE = 104. A rare gas filter is used to suppress higher order radiation and is either used with a mixture of Ne/Ar/Kr for photon energies below 14.0 eV. Energy was calibrated by using the 11s′−14s′ autoionization states of Argon, both in the first and the second order. All radicals were produced from nitrite and bromine precursors by pyrolysis according to Scheme 1. The nitrite precursors for the generation of 1-MA and 2-MA were synthesized according to literature procedures.22,35 The C4H5Br isomers were purchased from Sigma-Aldrich and used without further purification. The nitrite precursors were seeded in 2 bar of Argon, the bromobutynes in 1 bar of Argon and passed through an electrically heated silicon carbide (SiC) tube that was mounted onto a 100 μm orifice. The details of the pyrolysis source were described by Kohn et al.36 The pyrolysis power was chosen to yield temperatures between 450 and 550 °C. A Type C thermocouple was used to measure the temperature outside the pyrolysis tube with an estimated accuracy of ±100 °C. Photoions were detected in a Wiley−McLaren time-of-flight mass spectrometer in coincidence with photoelectrons. Photoelectrons were collected by a position sensitive detector. All charged particles were accelerated by a 120 V cm−1 extraction field. Threshold electrons were considered by selecting the central part of the photoelectron image, which corresponds to photoelectrons with a kinetic energy of less than 5 meV. Contributions of hot electrons were subtracted according to a procedure described elsewhere.37 Due to the almost constant photon flux over the investigated energy range, the signal was not corrected for flux. Typically the spectra have been recorded with a step size of 5 meV and an acquisition time of 120 s per data point, and for 2-methylallyl an acquisition time of 300 s per data point was employed. Ionization energies and vibrational frequencies were computed with the Gaussian 09 program.38 The B3LYP/6-311g(d,p) and M06L/6-311g(d,p) level of theory was chosen for the computation of all molecular
geometries, harmonic frequencies, and rotational constants. The CBS-QB3 method was used to obtain accurate ionization energies and the reaction pathway for the rearrangement of the butynyl radicals.39,40 All energies were corrected for zero point vibrational energies and harmonic Franck−Condon simulations were performed with the program FCFit version 2.8.20.41 Rotational barriers and barriers of rearrangements were determined by relaxed potential energy surface scans and subsequent transition state geometry optimization.
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RESULTS AND DISCUSSION (a). Mass Spectra. Figure 1 shows the mass spectra of 2butyn-1-yl (upper panel) and 1-butyn-3-yl (lower panel) from pyrolysis of their respective brominated precursors.
Figure 1. Mass spectra of 1-bromo-2-butyne (upper panel) and 3bromo-1-butyne (lower panel) at 8.0 eV with a pyrolysis temperature of 550 °C.
At 8.0 eV and pyrolysis temperatures of 550 °C 3-bromo-1butyne and 1-bromo-2-butyne are cleanly converted to the C4H5 radicals (m/z = 53) and no further mass signals are visible in the energy range around the ionization threshold. As the ionization energies of both bromides are above 9.5 eV, the conversion of the precursors was deduced from mass spectra recorded at 10 eV (not shown here). The dissociative photoionization of the bromo-butyne isomers has already been examined by Bodi et al., who reported the 0 K appearance energies for 2-butyn-1-ylium (AE0K = 10.375 eV) and for 1butyn-2-ylium cation (AE0K = 10.284 eV), respectively.42 Figure 2 shows the mass spectra of 3-penten-1-yl nitrite (upper panel) and 3-methyl-3-buten-1-yl nitrite (lower panel). 3996
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ionization energy of 2-butyn-1-yl. The spectrum of the second C4H5 isomer 1-butyn-3-yl, recorded from 7.85 to 8.5 eV does not appear to be very different from the first one. The first peak, assigned to the ionization energy of 1-butyn-3-yl, is found at 7.97 eV. This peak is also accompanied by a shoulder at +0.1 eV. As stated above, dissociative photoionization of the precursors is not relevant for either butynyl isomer within the investigated energy range, however both spectra exhibit contributions of hot- and sequence bands at around 7.9 eV. The two spectra show some vibrational structure, which can tentatively be assigned with the help of the Franck−Condon (FC) simulations (red line in Figure 3). The simulation of 2-butyn-1-yl (upper panel) shows a band around +0.1 eV, which is mainly caused by mode ν16+ (computed 0.10 eV/806 cm−1), a stretch of the two C−C single bonds, and the combination of ν16+ with ν21+ (computed 0.012 eV/97 cm−1). The band in the range of +0.3 eV is in the FC simulation caused by ν6+ (computed 0.28 eV/2243 cm−1), the C2−C3 triple bond stretching vibration (for labeling of atoms, see Scheme 1). Additionally, there is a band simulated at around +0.4 eV (3200 cm−1), which is not distinguishable in the experimental spectrum. The relative positions of the simulated bands of 1-butyn-3-yl also match the experimental spectrum quite well. The band around +0.1 eV is assigned to ν15+ (computed to 0.11 eV/867 cm−1), which corresponds to the H3C4−C1 single bond stretching vibration. ν15+ is in addition computed to contribute to the spectrum with the first overtone at around +0.22 eV, which is barely visible as a shoulder in the spectrum. Both Franck−Condon stick spectra were convoluted with a Gaussian function of 25 meV full width at half-maximum. In both molecules, the internal rotations of the methyl groups are expected to cause broadening of the bands in the spectra. Since the harmonic approximation often fails for internal rotations, torsional modes are not always well represented in FCsimulations. Here only in the simulation of 2-butyn-1-yl the contribution of the methyl torsional mode ν21+ is predicted correctly, while it does not appear in the simulated spectrum of 1-butyn-3-yl. However, the similar shape of the spectra leads us to consider isomerization in the pyrolysis as a possible side reaction. The calculated rearrangement pathway from 2-butyn-1-yl to 1butyn-3-yl is depicted in Scheme 2. The isomerization of 2butyn-1-ylium to 1-butyl-3-ylium cations has been computed before, and a reaction sequence was found.43 This work guided our computations on the CBS-QB3 level, which also predict a four step rearrangement on the neutral potential surface of C4H5. As the highest barrier is computed to be only 2.19 eV (212 kJ/mol), at pyrolysis temperatures of 550 °C a small degree of isomerization cannot be excluded and might explain the broadening in the spectrum of 1-butyn-3-yl. Since 2-butyn1-yl and 1-butyn-3-yl differ only by 13 meV in their absolute energies, the rearrangement is not supposed to produce a dominant product if there is enough reaction time to reach the chemical equilibrium in the reactor. However, not only does the rearrangement of the produced radicals have to be considered, but the rearrangement of the precursor molecules does as well. In previous TPEPICO studies, a similar isomerization has been observed for a doubly brominated propyne that was used as a radical precursor.44 The barrier of the isomerization of 3-bromo-1-butyne to 1-bromo-1methyl-allene has been computed to 3.69 eV/356 kJ/mol (CBS-QB3) and the respective rearrangement of 1-bromo-2-
Figure 2. Mass spectra of 3-penten-1-yl nitrite (upper panel) and 3methyl-3-buten-1-yl (lower panel) at 8.0 eV with a pyrolysis temperature of 450 °C.
At 450 °C a large signal at m/z = 55 is visible, corresponding to the C4H7 radicals. Both precursors are not visible at any photon energy, indicating either dissociative ground states for the two nitrite precursor cations or large geometry changes upon ionization. Therefore, the dissociative photoionization (DPI) of the nitrites was not elucidated further. Nevertheless, there are several indications that we can exclude contributions to the m/z = 55 radical signals from dissociative photoionization of the precursors: without pyrolysis m/z = 55 is just observed from 3penten-1-yl nitrite beyond energies of 9 eV and from 3-methyl3-buten-1-yl nitrite beyond 9.25 eV, respectively. In addition, the major fragment visible from the precursors without pyrolysis at around 9.1 eV is m/z = 85, corresponding to the loss of NO (not shown here). Under the pyrolysis conditions applied in the experiment (≈450 °C) m/z = 85 is absent, which can be seen as evidence for complete precursor conversion. (b). Threshold Photoelectron Spectra of C4H5: 2Butyn-1-yl and 1-Butyn-3-yl. The threshold photoelectron spectra (TPES) and the Franck−Condon simulations of 2butyn-1-yl (upper trace) and 1-butyn-3-yl (lower trace) are depicted in Figure 3. Since there are no other masses visible in
Figure 3. TPES of 2-butyn-1-yl (upper panel) and 1-butyn-3-yl (lower panel) with the respective Franck−Condon simulations.
the investigated energy range, all threshold photoelectrons without selection of the coincident ions were analyzed. The comparably small signal originates from low vapor pressure of the precursors. The TPE spectrum of 2-butyn-1-yl was recorded from 7.8 to 8.6 eV and shows a peak at 7.94 eV followed by a shoulder at 8.04 eV. A second less intense band is visible around 8.2 eV. The signal at 7.94 eV is assigned to the 3997
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The Journal of Physical Chemistry A Scheme 2. Rearrangement of the Radicals upon Pyrolysisa
a
Energies are given in eV (kJ/mol) with respect to the more stable radical isomer 2-butyn-1-yl. The transition state structures (TS1-TS4) are not shown.
butyne to 1-bromo-3-methylallene has a barrier of 3.88 eV/374 kJ/mol (CBS-QB3). Since the barriers for precursor rearrangement are significantly higher than those for the radical rearrangement and the formation of the radical itself (bromine loss at 2.76 eV (267 kJ/mol) and 2.67 eV (258 kJ/mol) from 1bromo-2-butyne and 3-bromo-1-butyne, respectively), the precursor isomerization can be ignored. (c). Threshold Photoelectron Spectra of C4H7: 1Methylallyl and 2-Methylallyl. The precursor 3-penten-1yl nitrite used to generate 1-MA seems to preferentially form the Z-conformation. However, the absolute energy difference between the two E- and Z-conformations of the formed 1-MA radical is marginal (3 meV/0.3 kJ/mol), and the rotational barrier for isomerization is only 0.67 eV (65 kJ/mol). Thus, under our pyrolysis conditions, both the E- and the Zconformer are formed, as it has been observed before.16 The mass-selected TPE spectrum (black line) of the two 1MA radicals is depicted in Figure 4. At around 7.25 eV the TPE
Table 1 present work molecule 1-butyn-3-yl 2-butyn-1-yl 1-E-methylallyl 1-Z-methylallyl 2-methylallyl
literature
IEad/eV (exp.)
IEad /eV (CBS-QB3)
IEad/eV (exp.,Lit.)
IE/eV (calc. CBSTQ5)45
7.97 7.94 7.48 7.59 7.88(s) 9.85(t)
8.02 7.99 7.54 7.65 7.93 (s) 9.93 (t)
7.97a 7.95a 7.49b 7.67c 7.9016 7.87717,d
7.433 7.540 7.827 -
a Derived values. 46 b Adiabatic value, no specification of the conformation of 1-MA.16 cVertical value, no specification of the conformation of 1-MA.16 dValue determined for C4D7.
cm−1) and the ν24+ (computed 0.063 eV/514 cm−1) vibration of the E-isomer. ν25+ is assigned to the C−C−C bending motion and ν24+ to the C−C stretching vibration (bonds C3− C1 and C3−C8). The band around +0.18 at 7.66 eV reveals the mode ν23+ (computed 0.071 eV/572 cm−1) of 1-Z-MA, a wagging mode of C3 and C1. There is a discrepancy between the experimental and simulated spectra, which might be attributed to activity in torsional modes of the CH3 groups. The participation of autoionizing states can be neglected since the conventional PES study shows very similar relative signal levels and shape. In addition, no excited states on the singlet and triplet surface were found in the same energy range. Figure 5 shows the ms-TPES of 2-MA (black) in the range from 7.70 to 12.0 eV. The Franck−Condon simulations (red) of the transitions to the singlet cation X+ 1A1 and to the triplet cation A+ 3A are also displayed in the graph. A first peak can be observed at 7.83 eV, followed by the most intense peak in the
Figure 4. Ms-TPE spectrum of the E-1-methylallyl and Z-1-methylallyl radicals with the convolution (fwhm = 25 meV) of the FC simulations for both isomers.
signal starts to rise, reaching the first peak in the spectrum at 7.48 eV, assigned to the IEad of 1-E-methylallyl. The computed IE for 1-E-methylallyl is 7.54 eV, 0.11 eV lower than for 1-Zmethylallyl (7.65 eV). A comparison of the experimental and computed IEs is given in Table 1. Three further peaks are observed, showing increasingly higher intensities. The most intense peak in the spectrum appears at 7.59 eV and is assigned to the IEad of 1-Z-MA, based on the best FC-simulation. In previous work the adiabatic and a vertical ionization energy of 1-MA were determined instead of the IEs for the individual conformers.16 The peaks at 7.52 eV (+0.04 eV) and 7.55 eV (+0.06 eV) correspond to the ν25+ (computed 0.037 eV/299
Figure 5. Mass-selected TPE spectrum of 2-methylallyl radical. The FC simulation (fwhm = 25 meV) is shown in red. 3998
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spectrum at 7.88 eV, which is assigned to the IEad of 2-MA. This value matches the computed IE of 7.93 eV quite well. The vibrational progression with spacing of 0.055 eV is assigned to the totally symmetric bending mode ν 24 + (computed 0.056 eV/453 cm−1) Gasser et al.17 derived from their ZEKE-spectra a value of 450 cm−1 for this mode. The ν24+ is broadened by a combination with even quanta of the methyl torsion ν27+ that is computed with a wavenumber of 33 cm−1. The peak at 7.83 eV is assigned to hot band 2410 with a computed wavenumber of 484 cm−1 for the respective bending mode in the neutral radical. It shows a fwhm of 30 meV, 10 meV larger than the 0−0 transition, and might contain contributions of other hot and sequence bands. Note that the program employed for the FC simulation does not include hot bands. The band at 9.85 eV is assigned to the transition to the triplet cation. Its shape is represented quite well by the FC simulation. The geometry obtained for the triplet cation changes significantly in comparison to the neutral molecule and to the singlet cation. The symmetry is reduced from C2v in the neutral and in the singlet cation to C2 in the triplet cation. The CH2 moieties move out of the molecular plane leading to an increase of the dihedral angle d(H3C1C4C5) by about 65°. In the ZEKE-spectra of Gasser et al.17 the excitation of two further non totally symmetric modes in the 2-MA cation were observed up to 64700 cm−1 (8.02 eV), but an ionization energy was only determined for the d7-isotopologue. However, a value of 63530 cm−1 was reported, corresponding to 7.877 eV, very similar to the 7.88 eV obtained here. Since a [1 + 1′] scheme via intermediate Rydberg-states was employed, only the fundamental of the CCC bending mode was observed in the ZEKE spectra.
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ASSOCIATED CONTENT
S Supporting Information *
The computed frequencies of all neutral and cationic species and full ref 38 are given in the Supporting Information. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: ingo.fi[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the German Science Foundation, contract FI575/7-3. The experimental work was carried out at the VUV Beamline of the Swiss Light Source, Paul Scherrer Institute. The work was financially supported by the Swiss Federal Office for Energy (BFE Contract Number 101969/152433). We would like to thank Kathrin Weiland and Florian Scharnagl for their contributions to the experiments on 1-methylallyl (K.W.) and 2-methylallyl (F.S.), respectively.
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CONCLUSION C4H5 and C4H7 radicals were investigated by threshold photoelectron photoion coincidence spectroscopy. The ionization energies of 2-butyn-1-yl, C4H5, and 1-butyn-3-yl, C4H5, were obtained from the TPE spectra to be 7.94 ± 0.02 eV and 7.97 ± 0.02 eV, respectively. As shown in Table 1, the experimental IEs are in good agreement with the computed values of 7.99 and 8.02 eV. The Franck−Condon simulations represent the spectra of both isomers quite well. Since the spectra have a similar appearance, isomerization between the two C4H5 isomers in the pyrolysis was investigated and a barrier of 2.19 eV (212 kJ/mol) was computed. As the ionization energies of the isomers differ slightly, we assume contributions from the second isomer to be small. From the TPE spectra of 1-methylallyl the ionization energies of 7.48 eV and at 7.59 eV were derived for the 1-E- and 1-Z- isomers of methylallyl. The computed values agree well with this assignment, as the IE of 1-E-MA is predicted at 7.54 eV and the IE of 1-Z-MA is predicted at 7.65 eV, respectively. Further vibrational bands were assigned with the help of Franck− Condon simulations. Separate IEs for the two conformers have not been obtained in earlier experiments.16 From the TPES of 2-methylallyl, the ionization energy of 2-MA to the singlet cation X+ 1A1 ←X 2A2 is determined to be 7.88 ± 0.01 eV. Furthermore, we assign an extensive vibrational progression of the C−C−C bending mode ν24+ with a spacing of 0.055 eV. In addition, the transition to the triplet cation A+ 3A ← X 2A2 was observed, and an excitation energy of 9.85 eV was determined. The assignments are again in good agreement with computations, which yield the adiabatic IE at 7.93 eV and the transition into the lowest triplet state at 9.93 eV. 3999
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