Improved bond-orbital calculations of rotation barriers in molecules containing lone pairs of electrons
Abstract
Improved bond-orbital calculations at the ab initio level using a STO–3G basis and assuming rigid rotation and experimental geometries have been performed for the eclipsed and staggered forms of CH3NH2 and CH3OH, for 11 conformations of N2H4 and 12 of NH2 OH. The one-configuration wavefunction of strictly localized non-orthogonal bond-orbitals is improved in second-order perturbation theory by admitting CI with non-orthogonal configurations arising from single excitations to antibonding orbitals. The results obtained using bond orbitals variationally optimized with respect to hybridization and polarity parameters show that in the cases of lower local symmetry (N2H4 and NH2OH) second-order delocalization is essential for a correct description of the dependence of molecular energy on the dihedral angle. All conformational parameters obtained in second order are in good agreement with the corresponding MO–SCF quantities in the same basis.