Theoretical Study of Long-Distance Electronic Coupling in H₂C(CH₂)_n-2CH₂Chains, n=3-16

The long-range electronic coupling in model chain alkyls H2C(CH2)n-2CH2, n = 3-16, and 1,4-dimethylenecyclohexane has been investigated using ab initio molecular orbital theory to assess dependence of the results on basis set and method of calculation. Both anion and cation pi couplings were examined. Basis sets ranging from minimal STO-3G to triple-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 basis sets. Couplings were calculated from differences in Hartree-Fock energies 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 of calculation, especially at distances greater than 7 angstrom. In most cases the distance dependence was not purely exponential. Small basis sets performed very poorly in calculating long-range direct ("through-space") interactions but showed dramatic improvement when augmented by "ghost" basis functions located between the interacting groups. Agreement between the 3-21G (+ghost) results and those from larger basis sets provides evidence that direct, through-space interactions are reliably calculated. The direct interactions decrease much more rapidly with distance (beta almost-equal-to 3.0 angstrom-1) than the couplings for the chain alkyls (beta < 1 angstrom-1), demonstrating the importance of superexchange via through-bond interactions. We speculate that 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 basis functions (ghost orbitals) on the intermediate atoms assist in computation of superexchange interactions even when the intermediate atoms themselves are not involved. Finally, electronic couplings in 1,3-dimethylenecyclohexane, 1,4-dimethylenecyclohexane, and 2,6-dimethylenedecalin are calculated and compared with experiments on molecules with the same spacer groups.