title: Theoretical analysis of intermolecular covalent pi-pi bonding and magnetic properties of phenalenyl and spiro-biphenalenyl radical pi-dimers.
abstract: Singlet-triplet splittings DeltaEST and intermolecular covalent pi-pi bonding characteristics of the prototypical phenalenyl pi-dimer and eight spiro-biphenalenyl radical pi-dimer structures are analyzed with the aid of restricted and unrestricted density functional theory calculations and paramagnetic susceptibility data fitted using the Bleaney-Bowers dimer model and the Curie-Weiss model. Single determinant approximations for DeltaEST as a function of transfer integrals and on-site Coulomb repulsion energy are presented for the two-electron two-site pi-dimers of phenalenyls and the two-electron four-site pi-dimers of spiro-biphenalenyl radicals. Within the range of intermolecular separation of 3.12<D<3.51 A, for the shorter separations, restricted theory works quite well and indicates the presence of a relatively strong intermolecular covalent pi-pi bonding interaction. For the longer separations, the singlet-triplet splittings are small; electron correlation plays a significant role, and only the unrestricted theory provides results that are in qualitative agreement with experiments. The bonding interactions in the pi-dimers are gradually weakened with increasing D, showing a transition from low D values with significant intermolecular pi-pi bonding and electron delocalization to high D values with localized spins and a biradicaloid character.
authors.affiliation: Department of Chemistry, Georgetown University, 37th and O Street, Washington, DC 20057-1227, USA. huangj3@ornl.gov
authors.forename: Jingsong
authors.initials: J
authors.lastname: Huang
authors.forename: Miklos
authors.initials: M
authors.lastname: Kertesz