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.