THEORETICAL-STUDY OF LONG-DISTANCE ELECTRONIC COUPLING IN H2C(CH2)N-2CH2 CHAINS, N = 3-16

The long-range electronic coupling in model chain alkyls H2C(CH2)n-2CH 2, n = 3-16, and 1,4-dimethylenecyclohexane has been investigated usin g ab initio molecular orbital theory to assess dependence of the resul ts on basis set and method of calculation. Both anion and cation pi co uplings were examined. Basis sets ranging from minimal STO-3G to tripl e-zeta plus polarization were used, and diffuse orbitals were added to some of the basis sets. Small basis sets such as the split-valence 3- 21G generally gave results in reasonable agreement with the larger bas is sets. Couplings were calculated from differences in Hartree-Fock en ergies or from lower level ''Koopmans' theorem'' approximations based on orbital energies of the dianion, anion, neutral triplet diradical, monocation, and dication of the donor-acceptor molecules. The distance dependence is found in some cases to vary significantly with method o f calculation, especially at distances greater than 7 angstrom. In mos t cases the distance dependence was not purely exponential. Small basi s sets performed very poorly in calculating long-range direct (''throu gh-space'') interactions but showed dramatic improvement when augmente d by ''ghost'' basis functions located between the interacting groups. Agreement between the 3-21G (+ghost) results and those from larger ba sis sets provides evidence that direct, through-space interactions are reliably calculated. The direct interactions decrease much more rapid ly with distance (beta almost-equal-to 3.0 angstrom-1) than the coupli ngs for the chain alkyls (beta < 1 angstrom-1), demonstrating the impo rtance of superexchange via through-bond interactions. We speculate th at the reason that a small basis set such as 3-21G generally does well in the calculation of long-distance couplings in molecules is that ba sis functions (ghost orbitals) on the intermediate atoms assist in com putation of superexchange interactions even when the intermediate atom s themselves are not involved. Finally, electronic couplings in 1,3-di methylenecyclohexane, 1,4-dimethylenecyclohexane, and 2,6-dimethylened ecalin are calculated and compared with experiments on molecules with the same spacer groups.