The Reaction of the Methylidyne Radical (CH X2Π) with the Hydrogen Cyanide (HCN X1∑+) Molecule in Cold Molecular Clouds and Planetary Atmospheres

Abstract

The reaction of the methylidyne (CH; X2Π) radical with hydrogen cyanide (HCN; X1∑) molecule was studied at a collision energy of 4.0 kJ/mol with ab initio calculations of the potential energy surface (PES). Geometries and potential energies of reactants, products, intermediates and transition states (TS) for the reaction were found by means of ab initio quantum chemical method ωB97xd/cc-pVTZ and the higher-level corrections were evaluated at the CCSD(T)-F12 level of theory with the cc-pVQZ-f12 (E1) and cc-pVTZ-f12 (E2) basis sets. The calculated values then were used for extrapolation to the complete basis set (CBS) limit using the two-point expression E(CBS) = E1 + 0.69377 × (E1 – E2). Analysis of the found energies, structural and kinetic characteristics of the involved compounds allowed us to determine the reaction paths leading to the formation of linear and cyclic intermediates, as well as to the formation of atomic and molecular hydrogen. Those results were utilized in Rice−Ramsperger−Kassel−Marcus calculations of the product branching ratios at the zero pressure limit – common approach in modelling of the cold molecular clouds chemistry. Mechanism identified emphasizes importance of the CH+HCN reaction as an important supplier of the initial bricks for building heterocyclic hydrocarbons in extreme environments.