Quasiclassical Trajectory Study on the Role of CH-Stretching Vibrational Excitation in the F( 2 P) + CHD 3 (v 1 =0,1) Reactions J. Espinosa-Garcia* Departamento de Quimica Fisica, Universidad de Extremadura, 06071 Badajoz, Spain ABSTRACT: To analyze the role of CH-stretching vibrational excitation on the reactivity and dynamics of the F( 2 P) + CHD 3 (v 1 =0,1) reactions, quasiclassical trajectory calculations using a full-dimensional analytical potential energy surface at different collision energies were performed. The extra energy of the CH excitation had almost no effect on the reactivity for the DF + CHD 2 channel, although it increased the reactivity for the HF + CD 3 channel, with the net effect being that CH excitation barely modified overall reactivity. In addition, the DF/HF branching ratio was not far from the statistical value for the ground-state reaction, whereas CH excitation decreased this ratio. These results, intimately related to the topology of the entrance channel, agree with recent theoretical results obtained using different surfaces (although some differences even persist among them) but strongly contradict recent experiments. This controversy will doubtless guarantee more accurate experiments and theoretical calculations in the future. However, properties related to the exit channel show reasonable theoretical/experimental agreement. Thus, the extra energy of CH excitation is mainly channelled into the HF and DF products for the HF + CD 3 and DF + CHD 2 channels, respectively, and the product scattering distributions are forward in both channels, where CH excitation has almost no effect on them. 1. INTRODUCTION Controversies between theoretical models and experimental results have always represented stimulating scientific challenges. A recent paradigmatic example is the dynamics study of the F( 2 P) + CHD 3 (v 1 =0,1) reaction, which proceeds through two channels, HF + CD 3 and DF + CHD 2 . Experimentally, two groups 1-4 have studied the effects of CH-stretching vibrational excitation on reactivity and dynamics. In 2009, Zhang et al. 2 performed crossed molecular beam experiments at collision energies of ≤4.0 kcal mol -1 and found that reactant CH excitation diminishes the overall reactivity by about 10-fold, especially for the HF + CD 3 channel, in favor of the DF + CHD 2 channel. Later, Yang et al. 3,4 carried out a similar study at a collision energy of 9.0 kcal mol -1 , also reporting that the overall reactivity was suppressed by CH excitation, although the effect was somewhat smaller. Thus, with respect to the ground-state reaction, overall reactivity was found to be suppressed by (74 ± 4)% and (66.6 ± 4)% for the HF + CD 3 and DF + CHD 2 channels, respectively. This unexpected result, namely, HF < DF, was explained 2 because the reactant CH excitation steered the fluorine atom toward the C-D bond, practically shutting down the C-H bond scission channel, especially at low energies. Theoretically, the effect of CH excitation on this reaction has also aroused great interest. 5-10 Czakó and Bowman 7, 8 performed quasiclassical trajectory (QCT) calculations on a proper full-dimensional potential energy surface (PES), denoted CSBB. 11 They reported that, first, CH-stretching excitation increases the overall reactivity by factors of about 1.2-1.4 at collision energies of >3 kcal mol -1 for both channels and by larger factors, of 2-4, at lower energies, in direct contrast with experiment, 2-4 and, second, CH excitation increases the DF/HF branching ratio (above the statistical value of 3) at low energies, < 1.5 kcal mol -1 , reproducing the experimental results. 2 Later, however, these same authors 9 performed QCT calculations on a new spin-orbit- (SO-) corrected PES, CSBB-SO, reporting that the DF/HF ratio at low energies (E coll = 1.0 kcal mol -1 ) is dampened, that is, this ratio is only slightly larger for the CH-stretch-excited reaction than for the ground-state reaction. Recently, Palma and Manthe 10 also studied the role of CH-stretching excitation on reactivity. Performing QCT calculations on a new PES (with two versions, without and with SO corrections, PWEM-NOSO and PWEM-SO, respectively), they found, first, that total reactivity increases upon excitation for both channels, which agrees with previous QCT calculations but again strongly contradicts the experiments, and, second, that the DF/HF ratio is less than 2 when the CH-stretching mode is excited throughout the entire energy range (0.5-9.0 kcal mol -1 ), which differs from Czakó and Bowman’s results. 7-9 Palma and Manthe concluded that the discrepancies in DF/HF ratio could be due to small variations in the topography of the entrance channel, related to the depth of the reactant well. As was recognized by the experimental and theoretical au- thors, 2-4,8-10 these discrepancies could be due to both theoretical and experimental difficulties: in the first case, Received: October 23, 2015 Revised: November 30, 2015 Article pubs.acs.org/JPCA © XXXX American Chemical Society A DOI: 10.1021/acs.jpca.5b10399 J. Phys. Chem. A XXXX, XXX, XXX-XXX