Temperature dependence for the CN+NH3 reaction under interstellar conditions: beyond capture theories?

The reaction between the radical CN and NH3 is representative of the important class of radical-neutral reactions that proceed rapidly at low temperature, due to strong inverse temperature dependences. Such reactions are assumed to be of great importance in the gas-phase chemistry of dense interstellar clouds; unfortunately, their temperature dependence is poorly understood theoretically. We estimated the rate constant for the CN+NH3 reaction as a function of temperature in the range 25–300 K, using the classical trajectory Monte Carlo approach. Employing an accurate ab initio potential energy surface, we confirm that capture approximations lead to a flat temperature dependence for the rate constant, in severe disagreement with the strong inverse temperature dependence observed experimentally. In order to understand the cause of this severe discrepancy, we investigated the possible influence of the rotation of reactants on the short-range dynamics. We present a crude illustrative model which reproduces both magnitude and temperature dependence of the experimental rate constant. This study emphasizes the importance of the formation conditions of the reaction complex and constitutes a first attempt to evaluate the relevance of processes subsequent to capture.