Associative detachment (AD) paths for H and CN- in the gas-phase: astrophysical implications

TL;DR

This study uses high-level quantum chemical calculations to identify specific associative detachment pathways for H and CN- in cold interstellar environments, revealing preferred formation routes for HCN over HNC.

Contribution

The paper provides detailed potential energy surfaces for H + CN- leading to HCN and HNC, highlighting the preferred formation pathway for HCN at low temperatures in space.

Findings

AD energetics favor HCN formation at low temperatures

HNC formation pathway is energetically forbidden

Implications for observed HCN/HNC ratios in molecular clouds

Abstract

The direct dynamical paths leading to Associative Detachment (AD) in the gas-phase, and specifically in the low-temperature regions of the Dark Molecular Clouds (DMC) in the ISM, or in cold trap laboratory experiments, are investigated with quantum chemical methods by using a high-level multi-reference Configuration Interaction (CI) approach that employs single and double excitations plus Davidson perturbative correction [MRSDCI(Q)] and the d-aug-cc-pV5Z basis set. The potential energy curves for H + CN- are constructed for different directions of the H partner approaching the CN- anion within the framework of the Born-Oppenheimer approximation. The present calculations found that the AD energetics at low temperature becomes favorable only along a selected range of approaching directions, thus showing that there is a preferred path of forming HCN at low temperatures, while that of forming its HNC isomer is found to be energetically forbidden. Given the existence in the ISM of different HCN/HNC ratios in different environments, we discuss the implications of our findings for selective formation of either isomer in the low-temperature conditions of the Molecular Cloud Cores.