Theoretical Kinetics Study of the F( 2 P) + NH 3 Hydrogen Abstraction Reaction J. Espinosa-Garcia,* ,† A. Fernandez-Ramos, ‡ Y. V. Suleimanov, §,∥ and J. C. Corchado † † Departamento de Química Física, Universidad de Extremadura, 06071 Badajoz, Spain ‡ Departamento de Química Física y Centro Singular de Investigació n en Química Bioló gica y Materiales Moleculares (CIQUS), Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain § Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ∥ Department of Mechanical and Aerospace Engineering, Combustion Energy Frontier Research Center, Princeton University, Olden Street, Princeton, New Jersey 08544, United States ABSTRACT: The hydrogen abstraction reaction of fluorine with ammonia represents a true chemical challenge because it is very fast, is followed by secondary abstraction reactions, which are also extremely fast, and presents an experimental/theoretical controversy about rate coefficients. Using a previously developed full-dimensional analytical potential energy surface, we found that the F + NH 3 → HF + NH 2 system is a barrierless reaction with intermediate complexes in the entry and exit channels. In order to understand the reactivity of the title reaction, thermal rate coefficidents were calculated using two approaches: ring polymer molecular dynamics and quasi-classical trajectory calculations, and these were compared with available experimental data for the common temperature range 276−327 K. The theoretical results obtained show behavior practically independ- ent of temperature, reproducing Walther−Wagner’s experiment, but in contrast with Persky’s more recent experiment. However, quantitatively, our results are 1 order of magnitude larger than those of Walther−Wagner and reasonably agree with the Persky at the lowest temperature, questioning so Walther−Wagner’s older data. At present, the reason for this discrepancy is not clear, although we point out some possible reasons in the light of current theoretical calculations. 1. INTRODUCTION The F + NH 3 reaction is difficult to study experimentally because it is very fast, followed by extremely fast secondary atom/radical reactions, F + NH 2 → HF + NH. It is also difficult to study theoretically because the accurate description of low- energy barriers requires a very high level of quantum chemistry theory. The title reaction has been studied theoretically in the past a few times, 1−6 although recently there has been renewed interest in this reaction. 4−6 These theoretical studies contrast with the extensive experimental literature, 1,4,7−18 both kinetics and dynamics. From the dynamics point of view, Sloan et al. 1,12,14 and Wategaonkar and Setser 13 reported an inverted vibrational distribution of the HF product. These authors found theoretically a hydrogen-bonded FH···NH 2 complex in the exit channel, which causes a randomization of the reaction exoergicity among all available product degrees of freedom. In addition, Goddard et al. 1 also found this complex theoretically using high-level ab initio calculations with energy 8.1 kcal mol −1 lower than the products. More recently, Misochko et al. 16,17 in their infrared and EPR spectroscopic studies observed for the first time this intermediate complex, FH···NH 2 . From the kinetics point of view, there is a controversy about the positive/ negative activation energy for the forward reaction. Thus, while the experiment of Walther and Wagner 18 reported conventional temperature dependence, and consequently positive activation energy, Persky’s more recent experiment 15 in the temperature range 276−327 K reported inverse temperature dependence, and these values are 1 order of magnitude larger. To explain this behavior, Persky suggested the existence of an intermediate complex in the entry channel. Therefore, intermediate complexes in the entry and exit channels may affect the reaction kinetics and dynamics, and whether they have an influence becomes an important issue. Recently, Xiao et al. 4 and Feng et al. 5 using very high-level ab initio calculations investigated some complexes in the entry and exit channels for the title reaction. However, the multiple electronic states due to the open-shell character of the system were not taken into account. Received: December 3, 2013 Revised: January 2, 2014 Published: January 2, 2014 Article pubs.acs.org/JPCA © 2014 American Chemical Society 554 dx.doi.org/10.1021/jp4118453 | J. Phys. Chem. A 2014, 118, 554−560