Electron transfer reactivity of O-2+O-2(-) system in low-spin coupling: Abinitio study at electron correlation level

The electron transfer reactivity of the O-2, + O-2(-); system in low-spin c oupling is studied at the second-order unrestricted M phi ller-Plesset (ful l)/6311+ G* basis set level by using different transition state structures. The properties and stabilities of the encounter complexes are compared for the five selected coupling structures: two T type, collinear, parallel, an d crossing. The activation barriers and the coupling matrix elements are al so calculated. The results indicate that the structures of the encounter co mplexes directly affect the electron transfer mechanism and rate. These enc ounter complexes are structurally unstable, the contact distances between t he acceptor O-2, and the donor O-2(-), are generally large, the interaction is weak, and the structures are floppy. The electronic transmission factor for the reacting system, O-2, + O-2(-),, is less than unity; thus, the ele ctron transfer reaction is nonadiabatic in nature. Analysis of the dependen ce of relevant kinetic parameters on various influencing factors has shown that the effect of the solvent medium on the coupling matrix element is sma ll but that on the electron transfer rate is very large. Among the five sel ected transition stale structures, the electron transfer is more Likely to take place via T,T-1-type and P-type structures. in the low-spin coupling t he favorable electronic states for two reacting species are (1)Sigma(g)(+)( O-2) and X(2)Pi(g)(O-2(-)) instead of X(3)Sigma(g)(-)(O-2) w and X(2)pi(g)( O-2(-)), which are favorable for the high-spin (quartet state) coupling mec hanism. (C) 1999 John Wiley & Sons, Inc.