Lone-Pair Orbital Interactions in Polythiaadamantanes

The electronic structures of a series of polythiaadamantanes from thiaadamantane through 2,4,6,8,9,10-hexathiaadamantane (HTA) have been analyzed using density functional theory calculations in conjunction
with Hückel and natural bond orbital analysis. The effects of multiple sulfur p-type lone-pair orbital interactions
on ionization potentials, hole mobilities, and electronic coupling have been determined. An overall increase
in the average energy of the lone-pair orbitals as the number of sulfur atoms increases is predicted, with the
exact positioning of the HOMO depending on specific lone-pair interactions. Separation of through-bond
(TB) and through-space (TS) interactions between intramolecular sulfur atoms has been performed using
localized molecular orbitals and model systems based on interacting hydrogen sulfide molecules. TB interations
were found to reduce orbital splitting, while TS interactions were found to increase orbital splitting. TS
interactions were more or less constant from one polythiaadamantane to the next, and the contributions of TB
effects to individual orbital energies vary depending on the relative orientation of sulfur atoms as determined
by the σ molecular framework. Electronic coupling between intermolecular sulfur lone-pair orbitals was
determined by investigating unique dimer pairs observed in the crystal structure of HTA. Electronic coupling
is not as strong as expected given the short intermolecular S−S distances observed in the crystal structure.
In general, B3LYP/6-31G(d) and B3LYP/6-311+G(d,p) give very similar orbital energies and splittings.