Models for π-π Interactions in “Molecular Metals”. Ionization Energies, Electronic Structure, and Through-Space/Through-Bond Interactions in [2.2]Paracyclophane. A Hartree-Fock-Slater Study

This contribution reports a detailed study of the interactions involving the two cofacial π-electron systems in [2.2] paracyclophane, using the first principles discrete variational method local exchange (DV-Xα) technique. Excellent agreement with experimental PES-derived orbital energies and orderings is obtained. Through studies of carefully selected component model structures (cofacial benzene molecules, cofacial bent benzene molecules, p-xylene, bent p-xylene) it is possible to partition “through-space” (π-π) and “through-bond” (π-σ-π) interactions. The former interaction (ca. 1.3 eV) is larger than the latter and is also studied as a function of interplanar spacing; it is expected to be dominant in π-based molecular conductors. The through-bond interaction, which decreases the through-space splitting of one pair of occupied π levels, consists of two components. The highest occupied π level mixes with an unoccupied bridge orbital and is lowered in energy by 0.8 eV; the third highest occupied π level mixes with an occupied bridge level and is raised in energy by 0.2 eV. Thus, the total through-bond effect on the through-space splitting is ca. 1.0 eV, larger than previous studies have indicated. The importance of the separation of functional group interactions to studies of the bandwidths of organic metals is stressed as is the relevance of the through-space interaction to the understanding of molecular chain conductors.