Vertical and Relaxed Structures of a Reactive Organosilane Radical Cation from CW and Transient Resonance Raman Spectra

Abstract

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 a...