ABINITIO STUDIES OF ELECTRON-TRANSFER - PATHWAY ANALYSIS OF EFFECTIVE TRANSFER INTEGRALS

Effective transfer integrals (T) have been evaluated for sigma- and pi-type electron and hole transfer in radical ion systems comprising methylene donor/acceptor groups linked by various saturated organic spacer groups. The T values have been calculated on the basis of ab initio self-consistent-field wave functions for the radical ion states, obtained either directly for the system of interest (T(SCF)) or from the associated neutral state via Koopmans' theorem (T(KT)), and employing either minimal (STO-3G) or split-valence (3-21G) basis sets. The T(KT) values have been decomposed into additive contributions from individual pathways, both through-space (T(KT)TS) and through-bond (T(KT)TB), using perturbation theory as formulated by Ratner together with a localized orbital basis represented by natural bond orbitals (NBO's) as defined by Weinhold et al. The overall coupling has been shown to arise from interference among a large number of competing pathways, none of which is strongly dominant. Nearest-neighbor pathways of the McConnell type are significant for transfer in some radical cation systems, but are frequently of very minor significance in comparison with lower-order superexchange pathways, often with contributions differing in sign from the overall T(KT) values. These latter conclusions are generally consistent with those based on studies involving different saturated spacer groups by Naleway et al. (for radical anions) and Jordan and Paddon-Row (for radical anions and cations). We find that transfer in radical anions and cations is generally dominated, respectively, by "electron" and "hole" pathways, but both mechanisms are found to be significant in both types of transfer. A number of transferability relationships have been identified for generic pathway types, and the important influence of stereochemistry on coupling has been illustrated, with regard to both orientation of donor/acceptor groups relative to the spacer and internal conformation of the spacer, showing the competition between coupling via the carbon framework and via CH bonds (i.e., hyperconjugation).