Barrier to Rotation in Thioformamide: Implications for Amide Resonance

We investigate the energetics of rotation about the C−N bond in thioformamide at the molecular and atomic levels using the HF/6-311++G**//HF/6-311++G** level of theory. The barrier to rotation is 19.9 kcal mol -1 and is dominated by the increase in the C−N distance and the consequent loss in attractive energies between carbon and nitrogen. The origin of the barrier to rotation is shown to be the same as that found in formamide. There is a large transfer of charge from nitrogen to carbon as the system moves away from planarity, but, unlike the essentially unchanged oxygen in formamide, sulfur also transfers charge to carbon upon rotation. It is the preference of the amide nitrogen for planarity, making it more electronegative and better able to stabilized itself by withdrawing charge from its bonded neighbors, that dictates the barrier to rotation about the C−N bond. The Fermi hole is used to quantitatively demonstrate that there is little delocalization of the π charge density from nitrogen to sulfur. The larger barrier to rotation in thioamides and the negligible delocalization of π charge from nitrogen to sulfur is inconsistent with the expectations of the resonance model but is consistent with a the view that (thio)amides behave as ‘(thio)formylamines'.