Abstract

Both HCN and HNC are prominent in the interstellar medium and may have significant roles in prebiotic chemistry. Considerable attention has, therefore, been accorded to the HCN ↔ CNH isomerisation, sometimes viewed as a prototypical unimolecular process. However, detailed analysis of the potential energy of the HCN/CNH system along its intrinsic reaction coordinate, in terms of the first and second derivatives of the energy, shows that this is not a straightforward proton transfer. It appears to involve two distinct transition regions, one in which the C–H bond breaks and the other in which the N–H forms. Between these regions is a transitory state, with all vibrational frequencies being real, in which the hydrogen is situated above the C–N bond and not directly associated with either the carbon or the nitrogen. In this state, the vibrational modes of the hydrogen are, respectively, approximately parallel and perpendicular to the C–N bond.