Revisiting the seemingly straightforward hydrogen cyanide/hydrogen isocyanide isomerisation

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.

Gas phase basicities of polyfunctional molecules. Part 6: Cyanides and isocyanides

“…A recent theoretical study shows that this process involves a complex transition region characterized by a transitory system in which the proton is interacting with the πelectrons of the CN moiety. 34,35 A high critical energy barrier of 200-220 kJ/mol has been calculated for this reaction. An indirect mechanism involving a concerted proton transfer through three molecules of water has been shown to reduce drastically the barrier for HCN ↔ HNC isomerization.…”

“…For a long time, isomerization of neutral hydrogen cyanide and isocyanide, HCN ↔ HNC, has been considered as an archetypical case of 1,2‐proton migration. A recent theoretical study shows that this process involves a complex transition region characterized by a transitory system in which the proton is interacting with the π‐electrons of the CN moiety . A high critical energy barrier of 200‐220 kJ/mol has been calculated for this reaction.…”

Importance of the Hydrogen Isocyanide Isomer in Modeling Hydrogen Cyanide Oxidation in Combustion

“…10,11,32,34,35,38,42,49,52,53,64 Estimates of the barrier for the isomerization range from 44.7 to 48.2 kcal mol −1 . 10,11,32,34,35,38,42 With a barrier of this magnitude, isomerization (reaction R2) is much faster than thermal dissociation of HCN (reaction R1) and HCN will isomerize fairly easily to HNC at medium to high temperatures.…”

Dynamical Effects and Product Distributions in Simulated CN + Methane Reactions

“…Throughout this work we ignore HNC production, and doing so has little effect on our dynamics because the barriers to direct production by N-end approach and isomerization from HCN [33][34] are much higher than the energies we consider here.…”