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 = methyl and ethyl) have been obtained both as their neutral charge-transfer complexes with tetracyanoethylene in steady-state cw experiments and as their radical cations via two-color pump-probe transient measurements. The ground-state charge-transfer resonant spectra exhibit intensity predominantly in phenyl-localized modes, suggesting that vertical excitation to the contact ion-pair state involves little participation of the bond that is known to undergo subsequent nucleophile-assisted cleavage in the separated radical cation. Quantitative modeling of the absolute cross sections for the methyl compound is used to determine the mode-specific reorganization energies accompanying vertical electron transfer. Transient spectra of the relaxed radical cations show more than 20 resonance-enhanced modes, several of which have significant contributions from the C-Si stretching coordinate based on frequency shifts between the natural abundance and benzyl C-13 labeled methyl compounds. These modes with significant benzyl C-Si stretching character are considerably lower in frequency in the radical cation than in the neutral, indicating weakening of this bond upon oxidation. The experimental frequencies are reproduced quite well by density functional theory calculations at the B3LYP/6-31g(d,p) level which give a C-Si bond length increase of 0.10 Angstrom upon oxidation.