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

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.

Collision-driven state-changing efficiency of different buffer gases in cold traps: He(1S), Ar(1S) and p-H2(1Σ) on trapped CN−(1Σ)

“…Rotationally inelastic collision processes involving CN À with He atoms and p-H 2 ( j = 0) molecules have been already discussed in our earlier work, 16 where we highlighted that the interaction between H 2 and CN À also leads to Associative Detachment (AD) processes with the destruction of the anionic molecule. 34 This process, however, turns out to be rather slow under cold trap conditions. It thus follows that the use of p-H 2 as a partner in cold traps could also still be an interesting alternative for which one can assess through calculations how feasible would be to use it as a buffer gas at higher temperatures than the more usual He noble gas.…”

Vibrational quenching of CN− in collisions with He and Ar

“…25 Collisional processes of the anion with the astrochemically relevant He and H 2 species 33,34 for rotational transitions have recently been studied and we have also investigated the rotational cooling of this molecular anion with He, Ar, and H 2 as buffer gasses. 35 The CN − anion is also thought to be an important participant as well in reactions in the interstellar medium (ISM) [36][37][38][39] and in the atmosphere of Titan 40 where it has been detected. 41,42 We note in passing that the corresponding neutral species CN was one of the first molecules to be detected in space 43 and cross sections and rates for this species have been investigated and obtained for various rovibrational process in collisions with He and H 2 .…”

Cold reactive and nonreactive collisions of Li and Rb with C2− : Implications for hybrid-trap experiments

“…For example, trap losses have been experimentally observed for the Rb + OH − system 23 . We have previously shown that choosing another alkali or alkaline earth atomic species would not lead to detachment 24 since the vertical detachment energy of MOH − depends on M. An other example is the CN − anion for which the AED reaction can occur for some collision partners despite having a large electron affinity (3.862 eV) 25,26 . In conclusion, it is difficult to rely on general molecular properties to predict whether or not to expect AED reactions at low temperatures without explicitly calculating both the anion and neutral PES.…”