Is There an Entrance Complex for the F+NH3 Reaction?

Abstract

Challenges associated with the theoretical and experimental kinetics of the F+NH(3)→HF+NH(2) reaction suggest the need for a more-precise potential surface. We have investigated the reactants and the products of the reaction, as well as the transition state and two complexes, with rather rigorous ab initio methods. The F·····NH(3) complex existing in the entrance valley is predicted to lie 13.7 kcal mol(-1) below the reactants. A small classical barrier of 2.0 kcal mol(-1) separates this entrance well from products HF+NH(2). These results explain the observation by Persky of unprecedented inverse temperature dependence for the F+NH(3) rate constants. The strong hydrogen-bonded complex FH·····NH(2) exists in the exit valley, and with a binding energy of 9.9 kcal mol(-1) relative to separated products. The vibrational frequencies of all stationary points are predicted with the CCSD(T)/aug-cc-pVQZ method.