Role of Conjugation in the Stabilities and Rotational Barriers of Formamide and Thioformamide. An ab Initio Valence-Bond Study

Abstract

The validity of the traditional resonance model for the amide and thioamide functional groups is examined by calculating the planar and rotated conformations of formamide and thioformamide with and without allowing for the delocalization of the nitrogen's lone pair. The vertical delocalization energies are found to be quite significant, in support of the traditional view that resonance is an important feature of the electronic structure of amides and thioamides. The rotational barriers are however not solely due to conjugation; they also partly arise from the preferred orientation of the nitrogen's lone pair as perpendicular to the molecular plane, even in the absence of conjugation. The resonance stabilization of the planar conformers is responsible for about one-half of the rotational barrier of formamide and two-thirds of that of thioformamide. The larger rotational barrier of thioformamide is therefore due to a greater importance of conjugation effects relative to formamide.