Hydrogen Cyanide Could Be Efficiently Produced by the Sequential Reaction of CH ( ² Π) Radicals with N ₂ Molecules in the Atmosphere of Titan: Investigations by Quantum-Chemical and Statistical Rate Theories

In this theoretical study, the possibility of forming important prebiotic molecules such as hydrogen cyanide, acetylene, and nitrogen-containing heterocyclic compounds in Titan’s atmosphere via the sequential reactions of methylidyne radicals, denoted as 2CH, with N2 molecules is explored. The state-of-the-art quantum-chemical methods such as CCSD(T)-F12 and CCSDT(Q) are employed to calculate reliable molecular electronic energies. Next, the rate coefficients for the formation of products are computed by a transition-state and RRKM statistical rate theory. A master equation analysis is employed to calculate deactivation of energized intermediates. A special version of a RRKM theory, VRC-TST, is used for computing unimolecular dissociation of bond scission processes. It is found that first, the CH radical reacts with the N2 molecule to form the HCNN radical. The calculations reveal that HCNN is produced efficiently at pressures and temperatures corresponding to the atmosphere of Titan. Next, an HCNN radical could undergo self reaction or react with an additional CH radical. According to the present study, both latter reactions are fast barrierless processes and lead to hydrogen cyanide and acetylene molecules.