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.