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

Article abstract of DOI:10.1021/j100179a004

A complete experimental description of the vibrational Franck-Condon barrier to the one-electron reduction of 4-cyano-N-methylpyridinium cation (NC-py-Ch3⁺) 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 (ε = 565 M^-1cm^-1) 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 +, Cl^-> ion pair confirm that the vibrational structural data pertain directly to the outer-sphere charge-transfer reaction rather than to higher lying electronic transitions.