Evaluation of the origin of rotational barrier in NH2X (XÂ =Â NO, NS)

The origin of rotational barrier in N-thionitrosamine NH2NS (TNA) has been examined with the aid of topological theory of atoms in molecules (AIM) and natural bond orbital (NBO) analyses and has been compared with N-nitrosamine NH2NO (NA). Frequency calculations show that the rotational barrier for TNA is greater than NA. This can be attributed to the more charge transfer from nitrogen of amino group to sulfur in TNA than from nitrogen to oxygen in NA. NBO analysis reveals pyramidalization of nitrogen atom of NH2 group leads to decrease of delocalization energy contribution and increase of Lewis energy contribution on total energy.