Ab initio valence bond studies for formamide

Abstract

The results of some ab initio valence bond calculations, with STO-5G and STO-6G basis sets, are reported for the ground states of the planar and perpendicular conformers of formamide. Structures that differ in the locations of the four π electrons of the planar conformer are considered, and polarization of the CH and NH σ bonds is allowed for in some of the calculations. For either conformer, the dominant Lewis structure is H 2N̈CHO. The next largest contributor is H 2N̈ +CHÖ (−). In contrast to what is usually assumed, the H 2 +NCHÖ (−) structure is calculated to make only a small contribution to the resonance scheme for the planar conformer, and usually its weight is smaller than is that for the long bond structure ▪. The latter two structures are essentially absent from the resonance scheme for the perpendicular conformer. The primary origin of the rotation barrier is calculated to arise from resonance between both of these structures with H 2N̈CHO in the planar conformer, rather than from the presence of a CN π bond in planar H 2 +CHÖ (−). Some comments are made on qualitative valence bond descriptions of formamide and nitrosyl halides. It is shown that one formulation of the wave function for the increased-valence structure H 2N̈ - ̈ CHO can generate almost a minimum energy for the π electrons.