Configuration interaction calculations for 1E′ trimethylenemethane

Abstract
Complete pi‐space CI calculations have been performed for the 1E′ state of trimethylenemethane in a planar D3h conformation using an STO‐3G basis. Molecular orbitals were defined by SCF calculations with C2v symmetry constraints for the lowest 3B2 and 1B2 states. As expected, the 3B2 orbitals had nearly D3h symmetry while the 1B2 pi orbitals were severely distorted. Full pi‐space configuration interaction calculations for both components of the 1E′ state using each set of molecular orbitals gave four nearly identical energies. This verified that the sigma cores from the two SCF calculations were essentially identical. The 1B2 component of the 1E′ wavefunction resembles an allyl radical plus an electron in a p orbital localized on one methylene group. Hence rotation of this methylene group into a plane orthogonal to the allyl radical should produce only a small change in energy.