Resonance Raman spectra and quantum chemical vibrational analysis of the C₇H₇ ^. and C₇D₇ ^. benzyl radicals

Time-resolved resonance Raman (RR) spectra of the benzyl radical and its perdeutero isotopomer are presented. The radicals are created by laser flash photolysis (λ=266 nm) of benzylchloride in solution. The spectra are excited at a wavelength of 315 nm, in resonance with the intense D₃(2 ²A₂) → D₀(1 ²B₂) transition. Twenty Raman bands, both polarized and depolarized, are observed. The RR spectra are analyzed through quantum chemical force field for π electrons (QCFF/PI) and ab initio calculations of equilibrium geometries, vibrational frequencies, Franck-Condon factors, and vibronic interactions. Polarized intense bands are assigned to totally symmetric a1 modes, and a number of depolarized bands to fundamentals of b1 modes. The observed activity of b1 modes suggests vibronic coupling, which is confirmed theoretically by calculations of vibronic interactions between the D₃ (2 ²A₂) and D₅ (4 ²B₂) states. The results from semiempirical and ab initio calculations are compared with experiment. The contributions of the different internal coordinates to the normal modes are calculated together with the changes in the molecular structure upon electronic excitation from D₀ to D₃ and D₅.