ISOMERIZATION VERSUS HYDROGEN-EXCHANGE REACTION IN THE HNC-REVERSIBLE-ARROW-HCN CONVERSION

For the HNC/HCN interconversion we show that the push-pull hydrogen ex change reaction H + CNH reversible arrow HCNH reversible arrow HCN + H is favoured over internal isomerization; the formation of H2CN or CNH 2 followed by rearrangement to HCNH and subsequent elimination are mor e energy demanding processes. Both push-pull. forward and reverse reac tions present activation barriers. However, the activation energy on t he H + CNH entrance channel (4.2 +/- 1.0 kcal/mol) is four times small er than on the HCN + H path. As a consequence, it can be anticipated t hat there will be a range of temperatures where the H + CMI reaction w ill be efficient while the reverse HCN + H process is still inhibited. This process, much less endothermic than internal isomerization, shou ld become an important path for HNC/HCN conversion with increasing tem perature in star forming regions.