The interstellar gas-phase chemistry of HCN and HNC

TL;DR

This paper reviews the gas-phase reactions of HCN and HNC in dark molecular clouds, identifying key reactions and testing their effects on chemical models to explain observed HCN/HNC ratios.

Contribution

It introduces new reaction pathways and rate constants, improving the understanding of HCN and HNC chemistry in dark clouds.

Findings

C + HNC reaction maintains high HCN/HNC ratio with abundant carbon.

H + HCN reaction and subsequent dissociative recombination can convert HCN to HNC.

Model predictions align with observed ratios in TMC-1 and L134N.

Abstract

We review the reactions involving HCN and HNC in dark molecular clouds to elucidate new chemical sources and sinks of these isomers. We find that the most important reactions for the HCN-HNC system are Dissociative Recombination (DR) reactions of HCNH+ (HCNH+ + e-), the ionic CN + H3+, HCN + C+, HCN and HNC reactions with H+/He+/H3+/H3O+/HCO+, the N + CH2 reaction and two new reactions: H + CCN and C + HNC. We test the effect of the new rate constants and branching ratios on the predictions of gas-grain chemical models for dark cloud conditions. The rapid C + HNC reaction keeps the HCN/HNC ratio significantly above one as long as the carbon atom abundance remains high. However, the reaction of HCN with H3+ followed by DR of HCNH+ acts to isomerize HCN into HNC when carbon atoms and CO are depleted leading to a HCN/HNC ratio close to or slightly greater than 1. This agrees well with observations in TMC-1 and L134N taking into consideration the overestimation of HNC abundances through the use of the same rotational excitation rate constants for HNC as for HCN in many radiative transfer models.