A theoretical analysis of the reaction between CN radicals and NH3

The reaction between CN radicals and NH3 molecules has been studied experimentally over an unusually wide range of temperature (25–716 K). Below 295 K, the rate constant exhibits a strong negative dependence on temperature; that is, it increases sharply as the temperature is lowered. The present work analyses the kinetics of this reaction theoretically, both to explain this unusual temperature-dependence and to identify the major products of the reaction—which have not been well established by experiment. Quantum chemical calculations at the CCSD(T) theoretical level show that the minimum energy path for reaction proceeds: (a) first, via a potential well, which is 39.3 kJ mol−1 below the energy of the separated reactants, when allowance is made for zero-point energies, corresponding to a quite strongly bound NC–NH3 complex, and (ii) then over a ‘submerged’ barrier with a crest 10.9 kJ mol−1 below the energy of the reactants to the products HCN + NH2. These ab initio calculations also demonstrate that there is no low energy path to the products NCNH2 + H. The dynamics of the main reaction have been further investigated using the two transition state model of Klippenstein and co-workers, in which transition state theory is applied at the selected E, J microcanonical level. The rate constants calculated for temperatures between 25 and 200 K are in excellent agreement with the experimental values.