Improved bond-orbital calculations of rotation barriers in molecules containing conjugated double bonds and/or π lone pairs

Abstract

Ab initio bond-orbital calculations of the internal rotation in molecules containing conjugated double bonds (buta-1,3-diene and glyoxal) or one double bond and a π lone pair (formic acid and nitrous acid) have been carried out using an STO-3G basis set, experimental geometry and rigid rotation. Hybridization and polarity parameters were optimized variationally in first order (one-configuration approximation), and pair delocalization between the bond orbitals accounted for in second order, of perturbation theory in terms of all single excitations from bonding to antibonding orbitals. For all molecules, first-order Coulombic and penetration components account for the relative stability of the planar forms, whereas barriers separating stable forms appear to be due to small σσ* and large ππ* vicinal delocalizations shielded by σπ* and πσ* interactions. Besides these effects, nitrous acid exhibits a strong σ-geminal delocalization stabilizing the syn form and opposing the first-order contribution.