ABINITIO STUDIES OF ELECTRON-TRANSFER .2. PATHWAY ANALYSIS FOR HOMOLOGOUS ORGANIC SPACERS

Ab initio calculations of transfer integrals (T(DA)) for long-range pi- and sigma-type electron transfer through saturated spacers linking donor (D) and acceptor (A) groups in several radical anion and cation systems have been carried out and analyzed in terms of additive superexchange models. The sensitivity of calculated results to orbital basis and wave function type has been carefully examined. The one-electron Koopmans' theorem approach, based on the neutral triplet diradical parent state and employing the split valence 3-21 G basis, provides generally reliable results. The minimal STO-3G basis is quantitatively useful in some cases, where it facilitates a compact perturbative analysis of the coupling in terms of competing pathways. For trans alkyl spacers containing one to seven CC bonds, reasonably good global exponential fits are obtained for fall-off of T(DA) with D-A separation even though the relative importance of different pathway types (hole, electron, and hybrid) changes markedly over the range of spacers. The calculated decay coefficients beta(r) cover a range of approximately 0.5-0.9 angstrom-1, being systematically greater for radical anions than for cations (by a factor of 1. 1- 1.7, depending on wavefunction type), consistent with similar findings by Jordan and Paddon-Row for radical cation and anion systems involving norbornyl spacer groups. The behavior of calculated transfer integrals for spacers composed of bicyclo[1.1.1]pentane (bcp) and bicyclo[2.2.2]octane (bco) units is complex, including cases of nonmonotonic variation with number of units (1, 2, or 3), and global exponential fits were possible only for pi-transfer through the bcp spacers. The calculated lower limit Of beta(r) (approximately 1.6 angstrom-1, based on the first two members of the series) for pi transfer through the bco spacers is consistent with inferences from experimental fluorescence quenching data. The decay found in the intramolecular systems is similar to that (beta(r) approximately 0.9 angstrom-1) exhibited by non-covalently-linked spacers of methane molecules in van der Waals contact. While nearest-neighbor (NN) McConnell-type pathways based on local bonding or antibonding orbitals are found to be unimportant, a McConnell-type model was recovered at a more coarse-grained level based on larger spacer units and representable in a matrix form isomorphic to the standard scalar McConnell expression.