Vertical and relaxed structures of a reactive organosilane radical cation from CW and transient resonance Raman spectra

Resonance Raman spectra of two p-methoxybenzyltrialkylsilanes (alkyl = meth yl and ethyl) have been obtained both as their neutral charge-transfer comp lexes with tetracyanoethylene in steady-state cw experiments and as their r adical cations via two-color pump-probe transient measurements. The ground- state charge-transfer resonant spectra exhibit intensity predominantly in p henyl-localized modes, suggesting that vertical excitation to the contact i on-pair state involves little participation of the bond that is known to un dergo subsequent nucleophile-assisted cleavage in the separated radical cat ion. Quantitative modeling of the absolute cross sections for the methyl co mpound is used to determine the mode-specific reorganization energies accom panying vertical electron transfer. Transient spectra of the relaxed radica l cations show more than 20 resonance-enhanced modes, several of which have significant contributions from the C-Si stretching coordinate based on fre quency shifts between the natural abundance and benzyl C-13 labeled methyl compounds. These modes with significant benzyl C-Si stretching character ar e considerably lower in frequency in the radical cation than in the neutral , indicating weakening of this bond upon oxidation. The experimental freque ncies are reproduced quite well by density functional theory calculations a t the B3LYP/6-31g(d,p) level which give a C-Si bond length increase of 0.10 Angstrom upon oxidation.