The ab initio charge deformation density of bicyclobutane from an extended gaussian basis

The charge deformation density of bicyclobutane has been derived from SCF wavefunctions computed with two contracted gaussian bases, of double-zeta and double-zeta-plus-polarization quality. Carbon—carbon bond peaks are displaced to the outside of the three-carbon rings, the peak in the bridge bond being flatter and further from the CC line than those in the non-fused bonds. Inclusion of polarization functions broadens these peaks so that they merge in a flat plateau inside each cyclopropane triangle. The side bonds are also bent slightly out of the ring plane, evidently by non-bonded repulsion between the methylene carbon atoms. Partitioning into atomic fragments produces almost neutral atoms but the CH dipoles add up to give a molecular moment of 0.685 D, in excellent agreement with experiment. They also contribute to a net charge contraction towards the center of mass, whose calculated anisotropy agrees satisfactorily with the measured quadrupole moments. Multipole moments of the atomic deformation densities and inner moments of the total molecular charge about the atomic centers provide a detailed quantitative characterization of the charge distribution.