Calculations on the rates, mechanisms, and interstellar importance of the reactions between C and NH2 and between N and CH2

Abstract

The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long standing and indecisive. In this paper we report quantum chemical and dynamical studies of two neutral—neutral reactions thought to be important in the formation of HNC and HCN, respectively — C + NH2 → HNC + H, and N + CH2 → HCN + H. We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production. The isomerization leads to near equal production rates of the two isomers, with HNC slightly favoured if there is sufficient rotational excitation. This result has been incorporated into our latest chemical model network of dense interstellar clouds.