Ab initio determination of the CN–NH3 capture potential energy surface Original Research Article

Recent comparisons between low temperature measurements on radical–neutral systems and long-range capture theories revealed a strong discrepancy as to the temperature dependence of the reaction rate constant. The most striking example is the reaction of CN with NH3, which proceeds rapidly below 300 K with a rate constant increasing as T−1.14. We present here detailed ab initio quantum chemical calculations to determine the CN–NH3 capture potential energy surface. Although the dispersion term is larger than expected and competes with electrostatic interactions, the intermediate- and long-range potential presents no unusual behaviour that could be responsible for the anomalous temperature dependence on the reactivity. We suggest that the capture process may influence the subsequent evolution of the short-range reactive complex.