Spectroscopic Evidence of Vibronic Relaxation in Methyl Substituted Benzyl Radicals

Abstract

Energy transfer and redistribution processes are believed to play important roles in explaining the reaction dynamics of large aromatic compounds, and have been studied for many decades. The benzyl radical is known to have closelying 22B2(D2) and 1 2A2(D1) excited electronic states separated by about 22,000 cm−1 from the 12B2(D0) ground electronic state. Ring substitution is also expected to affect the energies of the two excited electronic states differently, which could be mixed by vibronic coupling. In methylsubstituted benzyl radicals, the electronic interaction between the methyl group and the benzene ring is undoubtedly of the second-order compared to that between the methylene group and the benzene ring, since the molecule has a planar structure of 7 delocalized π electrons in which the CH2 group contributes an electron. The weak visible emission from benzyl-type radicals is believed to arise from transitions from the close-lying D2 and D1 excited electronic states to the D0 ground state. Since the vibronic coupling between the two excited electronic states is believed to be very efficient in benzyl-type radicals, it might not be possible to directly observe the transition from the D2 state except for the p-chlorobenzyl radical, in which the energy separation between the two excited states is only 95.2 cm−1. In this paper, we report for the first time the observation of the transition arising from the D2 state of methyl substituted benzyl radicals and the spectroscopic evidence that the efficiency of vibronic relaxation is related to the energy difference between the two excited electronic states to be coupled as well as the vibrational mode frequencies of benzyl-type radicals. Vibronically excited but jet-cooled methyl substituted benzyl radicals were generated using a technique of corona excited supersonic expansion (CESE) 9 using a pinhole-type glass nozzle described elsewhere. For observation of the visible vibronic emission spectra of the methyl substituted benzyl radicals, the precursors (reagent grade from the SigmaAldrich) toluene, 1,2,3-trimethylbenzene, and 1,2,4-trimethylbenzene were used to produce benzyl, 2,6-dimethylbenzyl, and 3,4-dimethylbenzyl radicals, respectively. The spectral region from 20000 to 23000 cm−1 was scanned at increments of 2.0 cm−1 over 2 hrs for the vibronic emission spectra in Figure 1. The wavenumber of the spectra was calibrated using the He atomic lines observed in the same spectral region, and is believed to be accurate within ± 1.0 cm−1. Although the mechanism for generation and excitation of benzyl-type radicals is not exactly known, it is quite possible that electrons in a corona discharge excite the most abundant species present, the inert carrier gas He, and subsequently the excited atoms collide with the precursor, resulting in the formation of the radical over a wide range of vibronic states by extracting a hydrogen atom from the methyl group. The processes could be summarized for the case of the benzyl radical as follows: