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