Theoretical Considerations in the NH2 + NO Reaction

We have studied the NH 2 + NO reaction theoretically in order to try to deduce a theoretical “model” that will accurately reproduce both the total rate coefficient k T ( T ) and the branching fraction α( T ) of the reaction NH 2 + NO → N 2 + H 2 O (a), NH 2 + NO → NNH + OH (b), and NH 2 + NO → N 2 O + H 2 (c), where k T = k a + k b + k c and α = k b / k T . The analysis, which makes the RRKM assumption and utilizes conventional transition-state theory for the internal-rearrangement transition states and microcanconical/fixed- J variational transition-state theory for the bond fissions, is discussed at length. The results of the analysis show clearly that k T ( T ) is determined almost exclusively by the transition state for the 1,3 hydrogen transfer connecting the initial NH 2 NO complex to HNNOH. The branching fraction is sensitive to several features of the potential energy surface, most of them associated with the fragmentation of the various HNNOH complexes into NNH + OH. By adjusting properties of the potential energy surface, we have constructed a theoretical model that predicts results for both k T ( T ) and α( T ) that are in good agreement with experiment. A variety of sensitivity analyses for the branching fraction indicate that reaction b is most likely thermoneutral to within ±1 kcal/mol. Our prediction of k c ( T ) is between 2 and 3 orders of magnitude smaller than values deduced from experiment, suggesting that the experiment may have detected the existence of a fourth channel, HNNO + H, or may have been contaminated by secondary reactions.