Studies on the Electronic Structures of Hydrocarbon Diradicals by the Unrestricted Hartree-Fock Theory

Abstract

Abstract Electronic structures of low-lying states of cyclobutadiene, trimethylenemethane (2), and tetramethyleneethane (3) are studied by the unrestricted Hartree-Fock (UHF) theory. Characteristics of electronic correlations, called dynamical spin polarization effect, in these molecules are well described by corresponding orbitals and spin density structures of low-lying UHF solutions. The spin polarization of σ electrons is found to be relatively small but not negligible for quantitative discussion. The two low-lying singlet states of 2 are described by two UHF solutions with different spin density structures. The fact that the singlet UHF state of 3 lies a little below the triplet UHF state is explained by the difference in the spin polarization of the bonding π electrons between the singlet and triplet states. Two rules are given which predict the spin structure of low-lying UHF states and the spin multiplicity of ground states of hydrocarbon diradicals. These rules are applied to 1,1,2,3,3-pentamethylenepropane and 1,3-dimethylenecyclobutadiene, which confirms their validity.