Improved bond-orbital calculations of rotation barriers and geometrical isomerism

Abstract

Rotational barriers in 19 molecules possessing a single internal rotation angle around a B-N, C-C, C-N, C-O, N-N, N-O, O-O central bond and geometrical isomerism in 3 molecules possessing a N=N double bond have been studied ab initio by the improved bond-orbital method. The first approximation, where the chemical groups occurring in these molecules are described in terms of non-orthogonal SCF bond-orbitals constructed from energy-optimized bond hybrids and polarities, is improved in second order of perturbation theory by admitting single excitations from bonding to antibonding orbitals and accounting for induction including exchange (polarization and delocalization). The molecules studied possess 16 to 34 electrons and a variety of functional groups differing in their chemical structure (CH₃, NH₂, OH, NO, CHO, CH=CH₂, NH= and some of their F-derivatives). The overall results obtained using a STO-3G basis, rigid rotation and experimental geometries, are close to experiment and to the corresponding MO-SCF calculations in the same basis, but individual energy components allow us to establish a clear correlation between barriers and chemical structure, grouping the 22 molecules into 4 classes. In the first class (CH₃-X molecules and 1,2-difluoroethane) barriers are dominated by steric interactions (Pauli repulsions) which are sufficiently well described in first order. In the second class (N₂H₄, NH₂OH, NH=NH and its fluoroderivatives, molecules all possessing lone pairs adjacent to the central bond) barriers are due to competition between first-order Pauli repulsion and characteristic geminal σ-σ* delocalization occurring in second order. In the third class (1,3-butadiene, glyoxal, formamide and formic acid, molecules possessing double bonds and/or π-lone pairs at both ends of the rotation axis) barriers are dominated by large π-π* vicinal delocalization. In the fourth class (HNO₂, H₂O₂ and its fluoroderivatives, molecules presenting both previous structural features) barriers result from competition between all preceding effects.