Solvent dynamical effects in electron transfer: molecular dynamics simulations of reactions in methanol

Abstract

Molecular-dynamics simulations of solvation dynamics accompanying activated electron-transfer (ET) reactions in methanol have been undertaken in order to explore the physical nature of the solvent dynamics coupled to ET barrier crossing, and to probe some underlying reasons for the facile kinetics observed in this solvent. The reactant is modeled by a pair of Lennard-Jones (LJ) spheres in contact, of varying diameter (4 or 5 Å), and containing a univalent charge (cation or anion) on one site so to probe possible effects of the ionic charge sign. Following equilibration, the collective solvent response to a sudden charge transfer between the spherical sites is followed, and described in terms of the response function C( t), describing the difference in the solvent-induced electrostatic potential between the initial and final solute states. In all cases, the C( t) curves exhibit a very rapid (50–100 fs) initial decay component associated with hydroxyl inertial motion, followed by components arising from hydrogen-bond librational and diffusive motions. Interestingly, the dynamics and relative importance of these relaxation modes are dependent on the charge sign as well as size of the solute pair. The molecular-level origins of these sensitivities are explored by examining time-dependent radial distribution functions, which implicate the dominance of short-range solvation in the rapid relaxation dynamics. In particular, the initial very rapid C( t) component for the smaller anion-neutral reactant pair is seen to arise chiefly from dissipation of the hydroxyl solvent polarization around the newly formed neutral site following ET, being accompanied by a slower build up of solvent structuring around the adjacent anionic site. The simulated ET reorganization energies are also shown to be dependent upon the reactant size and charge type. Some more general implications of these and other MD simulation results to the elucidation of dynamical solvent effects in activated ET processes are also noted.