A Complete Experimental Assessment of Franck-Condon Structural Effects for an Irreversible Outer-Sphere Electron-Transfer Reaction: Applications of Time-Dependent Raman Scattering Theory to the One-Electron Reduction of 4-Cyano-N-methylpyridinium

A complete experimental description of the vibrational Franck-Condon barrier to the one-electron reduction of 4-cyanoN-methylpyridinium cation (NC-py-CH,+) has been obtained. Because of the chemical irreversibility of the electron transfer, the reaction is not readily amenable to study by conventional structural methods. The approach successfully employed here, however, was a time-dependent analysis of near-resonant Raman scattering where resonance was achieved by pairing the cation to iodide anion; the pairing gives rise to a weak (e = 565 M-I cm-’) outer-sphere charge-transfer band centered at 428 nm in acetonitrile as solvent. The time-dependent analysis of scattering yields vibrational frequencies, coordinate displacements, and single-mode barrier contributions for each of 13 modes involved in outer-sphere electron transfer. Depolarization measurements, excitation profiles, and scattering studies with the nonchromophoric [NC-py-CH3+,CI-] ion pair confirm that the vibrational structural data pertain directly to the outer-sphere charge-transfer reaction rather than to higher lying electronic transitions.