Mode-specific quantum rate effects for interfacial electron transfer: Computational case studies based upon 4-cyano-N-methylpyridinium reduction

Multi-mode quantum rate theory is used to examine the interfacial kinetics of a prototypical organic-radical-generating reaction: the reduction of the 4-cyano-N-methylpyridinium cation. Charge-transfer-enhanced Raman scattering experiments (R. L. Blackbourn, et al., J. Phys. Chem., 1991, 95, 10535) previously showed that 13 vibrational modes are coupled to the reduction process. From a time-dependent analysis of the scattering spectrum, precise coordinate displacement and reorganizational parameters for each of the modes have now been determined. These parameters, when incorporated into the rate theory, have yielded an extremely detailed - perhaps unprecedented - description of electrochemical reaction kinetics, including complete descriptions of: (a) mode-specific barrier effects, (b) mode-specific tunnelling effects, (c) non-classical activation effects and (d) sum-over states driving force effects.