SUPEREXCHANGE PATHWAY CALCULATION OF LONG-DISTANCE ELECTRONIC COUPLING IN H2C(CH2)M-2CH2 CHAINS

In a recent communication (C.A. Naleway, L.A. Curtiss and J.R. Miller, J. Phys. Chem. 95 (1991) 8434) we outlined a method for calculation and analysis of long-distance couplings in electron transfer reactions making use of McConnell's superexchange formalism with matrix elements from ab initio molecular orbital wave functions transformed to localized bond orbitals using Weinhold's natural bond orbital method. In this paper we present a detailed investigation into several aspects of the superexchange (SE) pathways method for calculation of anion and cation pi couplings in H2C(CH2)m-2CH2, m=4-8, trans alkyls. The convergence of the couplings calculated from this method is examined as a function of energy threshold. Good convergence is found for wave functions (neutral triplet beta, dication) for which paths through unoccupied orbitals dominate paths through occupied orbitals; convergence is not as good for wave functions (neutral triplet a, anion) for which occupied paths are dominant. Reasons for these results are explored. The couplings calculated from the SE method are in reasonable agreement with Koopman's theorem when convergence of the couplings with threshold is attained and the 3-2 1 G basis set did well compared to larger basis sets. In agreement with previous studies, pathways with hops over bonds make up the bulk of the interaction and most of these involve CC bonds or CC* antibonds. The SE pathways calculations, some of which included more than 10 million different paths, indicate that contributions from long paths tend to cancel so that the shorter paths (2-5 jumps) are largely responsible for determining the magnitude of the coupling.