Theoretical study of long-distance electronic coupling in H[sub 2]C(CH[sub 2])[sub n[minus]2]CH[sub 2] chains, n = 3--16

The long-range electronic coupling in model chain alkyls H[sub 2]C(CH[sub 2])[sub n[minus]2]CH[sub 2], 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 bases 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. 16 refs., 11 figs., 4 tabs.