Quasiclassical Trajectory Study on the Role of CH-Stretching

Dec 2, 2015 - energy surface at different collision energies were performed. The extra energy of the CH excitation had almost no effect on the reactiv...
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Quasiclassical Trajectory Study on the Role of CH-Stretching Vibrational Excitation in the F(2P) + CHD3(v1=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(2P) + CHD3(v1=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 + CHD2 channel, although it increased the reactivity for the HF + CD3 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 + CD3 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(2P) + CHD3(v1=0,1) reaction, which proceeds through two channels, HF + CD3 and DF + CHD2. Experimentally, two groups1−4 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 + CD3 channel, in favor of the DF + CHD2 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 + CD3 and DF + CHD2 channels, respectively. This unexpected result, namely, HF < DF, was explained2 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 Bowman7,8 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 © XXXX American Chemical Society

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 authors9 performed QCT calculations on a new spin−orbit- (SO-) corrected PES, CSBB-SO, reporting that the DF/HF ratio at low energies (Ecoll = 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 Manthe10 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 authors,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

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electron correlation and SO interactions play important roles in the entrance channel. Recently, three PESs were developed by different groups using different strategies. In 2006, our group developed a fulldimensional analytical valence bond (VB)/molecular mechanics- (MM-) based surface, PES-2006,12 that is semiempirical in nature, that is, it combines theoretical and experimental kinetics information in the fitting procedure. This surface was based on previous surfaces from our group, namely, PES-1996,18 which was the first full-dimensional surface for the title reaction, and PES-2005,19 which we developed in two versions, without and with SO corrections. However, both surfaces presented serious drawbacks that were corrected with PES-2006. This surface was developed without considering SO corrections, thus describing a single surface within the Born−Oppenheimer approximation, and it has been used in the kinetics and dynamics study of the F + CH4 reaction and isotope analogues,5,6,12−17 where the agreement with experiment has been always qualitative and sometimes quantitative. Note that, in the kinetics calculations, the SO correction is included in the reactant electronic partition function (2P1/2 excited state of F, with an excitation energy of 404 cm−1). In this regard, it is worth noting that the resonance signature predicted when performing 5D quantum mechanical studies based on the PES-2006 is more pronounced when the CH4 reactant is excited in the bending mode by one quantum;14 this was experimentally confirmed some years later.20,21 In 2009, Czakó et al.11 developed a full-dimensional ab initio-based PES, CSBB, using permutational invariant polynomials to fit 19384 ab initio points of UCCSD(T)/augcc-pVTZ quality. This version was later improved by the same group with an ab initio SO-corrected PES,9 CSBB-SO. Recently, Manthe’s group developed a series of PESs for this system.10,22,23 First, they constructed vibrationally SO-coupled diabatic PESs for the entrance channel and, so, were able to explain the photodetachment spectra of FCH4− and FCD4−. The latest surface from this group, PWEM,10 combines the lowest adiabatic surface from the previous diabatic model with the full-dimensional CSBB surface. Thus, both CSBB and PWEM are ab initio-based surfaces. Finally, two main differences between PES-2006 and the other two surfaces, CSBB and PWEM, can be observed. First, whereas PES-2006 is a valence-bond- (VB-) based surface (specifically, a semiempirical surface with a VB/MM functional form), the other two surfaces are molecular-orbital- (MO-) based surfaces that require many electronic structure calculations at high ab initio levels, usually on the order of 30000−50000 for polyatomic systems, which is very expensive and prohibitively computationally expensive when the molecular size increases. As will be seen in the present article, use of a very high number of high-level ab initio calculations does not ensure an accurate global potential energy surface. In addition, as noted recently by Warshel and co-workers,24 in the study of chemical reactions, “it is more physical to calibrate surfaces that reflect bond properties (i.e. valence-bond (VB) based surfaces) than to calibrate surfaces that reflect atomic properties (e.g. MO based surfaces)”. Second, it should be noted that, whereas the PES-2006 surface contains analytical first derivatives of the energy, the CSBB and PWEM surfaces use numerical gradients. Therefore, the times required to run trajectories are quite different. For instance, for a batch of 2500 trajectories at a collision energy of 9.0 kcal mol−1, PES-2006 takes roughly 1 CPU hour (1 h and 8 min on a single-core Intel Pentium D 3.2 GHz processor), whereas the CSBB takes 6 CPU hours, and

related to the classical nature of QCT calculations, where quantum effects such as tunnelling and zero-point energy (ZPE) are not considered and with limitations of PES descriptions, especially important at the entrance channel and the transition-state zone, and, in the second case, related to the experimental detection efficiencies of the two channels and the measures of the CHD2 product rotational distributions for the excited- and ground-state reactions. These previous studies focused on the entrance channel. However, there are other dynamics properties for which theory/experiment agreement is better. The energy initially deposited in the CH excited bond is channelled largely into the vibrational energy of the HF or DF products, where the HF product from the excited reaction is one vibrational quantum hotter than that from the ground-state reaction.3 In contrast, for the DF + CHD2 channel, reactant CH vibrational excitation barely affects the DF vibrational distribution.4 Finally, the product scattering angle distribution is forward in both excitedand ground-state reactions.2−4 This better agreement with experiment is related to the exit channel, which is relatively well-described by all potential energy surfaces. In previous works, we studied the F + CH4 (v=0) reaction12−14 and the isotopic analogues F + CD4(v=0),15 F + CH2D2(v=0),16,17 and F + CHD3(v=0),5,6 with the main aim of deepening our understanding of the dynamics of this reaction, analyzing effects such as bond selectivity and quantum resonances. In all cases, only the ground-state reaction was analyzed, and the reasonable agreement with the available experimental data increased our confidence in the potential energy surface developed by our group, PES-2006.12 Motivated by the controversies found for the title reaction when the reactant CH stretch is vibrationally excited by one quantum, in the present article, we contribute a theoretical study based on the PES-2006 surface,12 which presents a different development from the previous CSBB and PWEM surfaces, as shown in the next section. The article is organized as follows: In section 2, we present the recently developed potential energy surfaces and the computational details of the QCT calculations. The QCT results are presented and compared with previous theoretical and experimental values in section 3. Finally, section 4 summarizes the conclusions.

2. POTENTIAL ENERGY SURFACES AND COMPUTATIONAL DETAILS In general, the construction of potential energy surfaces is a very difficult task that is compounded, first, by molecular size, because of the greater number of degrees of freedom, and, second, by the presence of wells in the entrance and exit channels. The latter difficulty is especially important in highly exothermic reactions, with a very low barrier height and a very flat entrance channel. Therefore, given the energy differences in the entrance and exit channels, the fitting process of the global PES is very complicated. This is the case for the title reaction, F(2P) + CHD3, and possibly the cause of the theoretical discrepancies found when using different surfaces. In this sense, small topographic changes (especially in the entrance channel and the transition-state zone) can substantially modify the dynamics. Note that, when ab initio calculations are used in the description of the reaction system, these computations are also challenging in the entrance channel, and one might need multireference methods, because the single-configuration Hartree−Fock method might fail in this region. Furthermore, B

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Figure 1. Energy profile for the F + CHD3 reaction using the three PESs PES-2006 (top, black), CSBB (middle, red), and PWEM (bottom, blue). The values in parentheses include SO corrections. RC, SP, and PC represent the reactant complex, saddle point, and product complex, respectively.

trends.8 Given that all strategies are arbitrary to some extent and that we were interested in comparisons with other PESs, where quantization was avoided,10 in the present article, all trajectories were considered in the final analysis. Finally, note that, given the very high reactivity of this reaction, the number of trajectories run at each collision energy and reactant vibrational state was sufficient to ensure low statistical errors. Thus, the error bars on the reaction cross sections were always less than 6%. For instance, for the HF channel in the ground-state reaction, F + CHD3(v1=0), the cross section varied between 4.32 ± 0.26 and 10.42 ± 0.30 b2 (b = bohr) in the energy range of 0.5−9.0 kcal mol−1. With respect to the statistical errors for the HF/DF ratio, we also found small values. For instance, for the F + CHD3(v1=0) reaction, in the same energy range, the ratio varied between 2.35 ± 0.23 and 2.96 ± 0.13, diminishing with collision energy, whereas for the CH-stretch-excited reaction, the statistical error was practically constant, about ±0.10: 1.18 ± 0.11 and 2.37 ± 0.10, respectively. Therefore, for clarity of readership, these error bars are not represented in this article.

the PWEM takes roughly 32 CPU hours (these last two running on a single core of an eight-core Intel Xeon X5482 or X5550 CPU). I this work, the dynamics of the ground-state and CHstretch-excited reactions, F + CHD3(v1=0,1), have been studied by performing QCT calculations on the PES-2006 surface at six collision energies in the range 0.5−9.0 kcal mol−1 for both reactions. The aim of the low energies, Ecoll ≤ 3 kcal mol−1, is the analysis of the influence of the reactant well in the entrance channel, whereas the aim of the highest energy, 9.0 kcal mol−1, is comparison with the most recent experiments,3,4 performed at this energy. For each collision energy and reactant vibrational state, 50000 trajectories were run, where the maximum impact parameter, bmax, was calculated using batches of 5000 trajectories, increasing the value of b until no reactive trajectories were obtained. For the ground-state reaction, F + CHD3(v1=0), the bmax values ranged from 3.8 and 2.9 Å with the increase in the collision energy, whereas for the CH-stretchexcited reaction, F + CHD3(v1=1), this range was slightly larger, 4.0−3.1 Å, so the CH excitation opens the cone of acceptance. The initial and final C−F separations in each trajectory were 10.0 and 12.0 Å, sufficient to ensure there was no interaction in the reactant and product channels. For the ground-state reaction, the CHD3 vibrational energy was fixed at its ZPE value, 22.34 kcal mol−1, whereas for the CH-excited reaction, an extra energy of 8.88 kcal mol−1 (3106 cm−1) was added. In both reactions, rotational energy was selected by thermal sampling at 20 K. The propagation time step was selected to ensure energy conservation, whereas other scattering parameters (impact parameter, vibrational phases, and spatial orientations of the reactants) were chosen by a Monte Carlo approach as implemented in VENUS96.25,26 In the analysis of final QCT results, a related problem is ZPE violation. To correct this problem, different quantization criteria have been proposed, although satisfactory (and unambiguous) solutions have not emerged. For the title reaction, Czakó and Bowman7−9 performed an exhaustive analysis of different strategies, concluding that, although they might afford quantitative differences, they predict similar

3. RESULTS AND DISCUSSION 3.1. Stationary Points. An energy profile of the three PESs, PES-2006, CSBB, and PWEM, is shown in Figure 1. In this highly exothermic reaction, we focused attention on the barrier height and the reactant van der Waals complex in the entrance channel. It is well-known that the ab initio description of these stationary points is very difficult because of the very flat entrance channel, and this effect is reflected in the three PESs. Thus, the barrier height varies from 320 cm−1 (PWEM-SO) to 122 cm−1 (PES-2006), giving a difference of 198 cm−1, within the chemical accuracy of ±1 kcal mol−1. Given that PES-2006 presents the lowest barrier, this surface is obviously expected to predict a larger reactivity. An interesting theoretical issue is the depth of the reactant well, because it is intimately related to the DF/HF ratio upon CH vibrational excitation, which presents a serious controversy between theory and experiment, as discussed in the Introduction. It has been argued7 that the counterintuitive C

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The Journal of Physical Chemistry A finding of Zhang et al.2 about the DF/HF ratio at low collision energies, ≤4 kcal mol−1, could be explained by the presence of a deep valley in the reactant channel. Figure 1 shows that this depth varies significantly with the PES: being 45, 167, and 363 cm−1 for PES-2006, PWEM, and CSBB, respectively. Therefore, the topology of the surface in the entrance channel will condition the DF/HF ratio. At this point, it is important to note that, in our previous article,12 this reactant van der Waals complex was not reported. This was due to the fact that we incorrectly investigated the only C−Hb−F linear approach. Now, as a result of a more thorough search, we found a van der Waals complex in which the fluorine atom approaches three hydrogen atoms with C3v symmetry, H−CH3−F, stabilized by 45 cm−1 with respect to the reactants. Finally, note that the three PESs present similar reactant complex geometries, with the H−CH3−F structure, C3v symmetry, and a C−F bond distance of 2.959 Å (PES-2006) and 2.940 ± 0.105 Å (CSBB). 3.2. Excitation Function: Role of CH-Stretching Excitation. We begin by analyzing the effect of CH excitation on the total reactivity. The excitation functions (i.e., reaction cross section versus collision energy) are plotted in Figure 2 for the three PESs. Experimentally,2−4 it has been reported that CH excitation decreases reactivity, especially at low collision energies. PES-2006 predicts that CH excitation barely affects reactivity with respect to the reactant ground-state reaction, whereas the CSBB and PWEM surfaces predict an increase in reactivity. Therefore, the results obtained using the three PESs strongly contradict the experimental evidence, with this contradictory behavior being more pronounced in the CSBB and PWEM surfaces. The behavior found with PES-2006 can be explained by the mode−mode couplings in the reaction-path Hamiltonian. We found12 no coupling in the entrance channel, ̈ mode-selective picture, we would expect that so with this naive CH-mode excitation would not influence reactivity. This point has been also confirmed using the sudden vector projection (SVP) model.27−29 In general, the vibrational excitation of the CH mode presents two opposite effects. On one hand, excitation produces weakening of the C−H bond to be broken, favoring the route toward the transition-state region, and on the other hand, given that, in this highly exothermic reaction the transition state is early (it appears early on the reaction coordinate and is reactant-like), the vibrational motion (which is orthogonal to the reaction coordinate) produces a steric effect (it creates a bottleneck in the entrance channel) and disfavors the reaction. The final result is a delicate balance between these two effects that depends on the specific reaction system studied.30 So, whereas in the OH + NH3 reaction, which is also an “early”barrier reaction, the first effect is the important one,31 in the title reaction, opposite theoretical and experimental results are obtained. If the experiments are right, the second effect is most important, but if the theory is right, then the first effect is more important. Obviously, to reconcile these two positions, further studies are needed. Next, we analyzed the effects of CH excitation on the two channels independently. The effects obtained using the three PESs are plotted in Figure 3 for both channels, HF + CD3 and DF + CHD2, and for the reactant ground-state and CH-stretchexcited reactions, F + CHD3(v1=0,1). For a clearer comparison, the same range was used for the y axis in all cases. Using PES2006, whereas the reactant CH-stretching excitation slightly decreased reactivity for the DF + CHD2 channel, by factors between 0.83 and 0.97 in the energy range of 0.5−9.0 kcal

Figure 2. Excitation function [b2/(kcal mol−1)] for the overall reactivity of F + CHD3(v1=0,1) using the three surfaces. Solid line, reactant ground-state reaction; dotted line, CH-stretch-excited reaction. See the axis labels and legend in the top panel.

mol−1, it increased reactivity for the HF + CD3 channel, by factors between 1.66 and 1.20 in the same energy range. The first result agrees with recent experiments,4 although the decrease is larger here, 66% at 9 kcal mol−1, whereas the second contradicts the experimental evidence,2,3 where the excitedD

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Figure 3. Excitation functions [b2/(kcal mol−1)] for the two reaction channels, DF + CHD2 (left) and HF + CD3 (right) using the three surfaces. Solid line, reactant ground-state reaction; dotted line, CH-stretch-excited reaction. See the axis labels and legend in the upper left panel.

state reaction inhibits reactivity. For the DF + CHD2 channel, the other surfaces, CSBB and PWEM, predict an increase in reactivity with CH excitation, larger for the CSBB surface and practically negligible for the PWEM surface. Therefore, taking into account the respective statistical uncertainties, we conclude that CH-stretching excitation barely affects reactivity, except for the CSBB surface; this issue will be discussed further below. With respect to the HF + CD3 channel, all surfaces predict an increase in reactivity upon CH excitation, contrary to the experimental information. This discrepancy could be due to several theoretical/experimental sources: Theoretically, problems associated with the classical nature of the QCT calculations, which do not consider quantum effects, and the assumption that the reaction occurs on a single PES cannot be

ignored, although the fact that three very different surfaces obtain similar results seems to indicate some problems associated with the experiments. Another possible source of error is the SO coupling in the asymptotic region. Thus, the starting point for the trajectory has to be located at a point where F and CHD3 do not interact, but all of the dynamical information is collected at points where the interaction between them is strong enough to assume that the two F states are fully quenched and the trajectory proceeds on the lowest PES. Consequently, at a certain point in the early steps of the trajectory, there are strong nonadiabatic effects that are simply neglected by QCT calculations, and these effects could change the whole picture. A similar problem was analyzed in detail by Werner and co-workers32−34 for the F(2P) + H2 reaction. E

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that is, the vibrational excitation barely affected the overall reactivity. For the HF channel, the values were 5.50 ± 0.28 and 7.64 ± 0.34 b2, respectively (i.e., the CH excitation increased the reactivity of the HF channel by 39%), whereas for the DF channel, the values were 14.50 ± 0.43 and 12.51 ± 0.43 b2, respectively (i.e., the CH excitation diminished the reactivity by 14%). Only this second result reproduces experiment. Therefore, even under these extreme conditions, the PES-2006 theoretical results do not change, indicating that the role of the well in the entrance channel is small or negligible. Similar JKspecific rotational effects on the dynamics have recently been reported for related systems, namely, H, Cl, and O(3P) + CHD3.36−39 Furthermore, Wang and Czakó36 investigated the reactant rotational effects using a 4D quantum dynamics method for the F + CH4(J) reaction, finding results similar to the present ones for the F + CHD3(T) reaction. 3.4. DF/HF Ratio: Role of the CH-Stretching Excitation. The DF/HF branching ratios for the ground-state and CHstretch-excited reactions obtained using the three PESs are shown in Figure 5, where, for the F + CHD3 reaction, the

Reflecting further, significant differences between the PESs are observed. CSBB and PWEM-NOSO present atypical behavior at low collision energies, where the reaction cross section increases as the energy is reduced below 1 kcal mol−1. This behavior is especially important for the CSBB surface and is possibly related to the deeper well in the entrance channel obtained with this PES (Figure 1). This atypical behavior was corrected in a subsequent article by the same group9 using an SO-corrected PES. Therefore, the CSBB surface overestimates the depth of the well, and so, the steering effect argued7 to explain the experimental counterintuitive finding of Zhang et al.2 could be questioned. In the same way, in the case of the PWEM surfaces, this atypical behavior is corrected when the SO-corrected PES is used. A more typical behavior, namely, that the reaction cross section increases with increasing collision energy, is obtained with the PWEM-SO and PES2006 surfaces, although the reactivity is higher in this latter case because of the lower barrier height (Figure 1). Obviously, this behavior with different PESs, associated with the topology of the surface in the entrance channel and different for the ground-state and CH-stretch-excited reactions and for the two channels, will have an impact on the DF/HF branching ratio. 3.3. Effect of CHD3 Reactant Rotational Excitation. As a simple test of the effect of the CHD3 reactant rotational ̈ model where the energy on the reactivity, we used a naive rotational influence was studied at three temperatures, namely, 2, 20, and 200 K, with a collision energy of 3.0 kcal mol−1 using PES-2006. In sum, 200000 new trajectories were performed. As a Maxwell−Boltzmann distribution was used, it was expected that higher rotational states would be populated at higher temperatures. The results for the F + CHD3(v1=0,1) reactions are plotted in Figure 4. It can be seen that reactant rotational

Figure 4. Effects of the CHD3 rotational excitation on reactivity using PES-2006, at Ecoll = 3.0 kcal mol−1 and temperatures of 2, 20, and 200 K: overall (black) and DF (red) and HF (blue) channels. Figure 5. Effects of the C−H stretching excitation on the DF/HF branching ratio using different PESs.

excitation does not affect the previous conclusions on reactivity (taking into account the respective errors bars), either for overall reactivity (Figure 2) or for each channel (Figure 3). Note that reactant rotational excitation slightly increased overall reactivity, and this result reproduced the experimental evidence for the isotopic analogue F + CH4 (v=0,j) reaction,35 which is a severe test for the quality of the PES-2006 used. To more deeply analyze the role of the well in the entrance channel, an additional test on PES-2006 was performed under even more extreme conditions, Ecoll = 1.0 kcal mol−1 and T = 2 K, where it was expected that trajectories would “visit” the well. The cross sections for the ground-state and CH-stretch-excited reactions were 20.00 ± 0.50 and 20.15 ± 0.54 b2, respectively;

statistical value is 3. PES-2006 predicts DF/HF ratios close to the statistical value for the F + CHD3(v1=0) reaction, 2.3−2.9, in the 0.5−9.0 kcal mol−1 energy range, whereas CH-stretching excitation noticeably reduces this ratio, 1.2−2.4, in the same range. This result contradicts the experimental evidence.2 The CSBB surface gives similar results for the ground-state reaction, 2.6−2.8 in this range, but for the F + CHD3(v1=1) reaction, it shows DF/HF ratios larger than 3 at low energies. This result was used by these authors7 to explain the experimental results,2 although as was shown in the previous section, the reactivity at low energies with this surface is artificial due to the excessive depth of the reactant well. Finally, F

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The Journal of Physical Chemistry A the PWEM surface shows a DF/HF ratio close to the statistical value for the F + CHD3(v1=0) reaction (with a surprising peak larger than 3 at Ecoll = 2 kcal mol−1 when the SO version was used) and a significant decrease when the CH stretch was excited, although at low energies (≤1 kcal mol−1), the DF/HF ratio presented atypical behavior. In sum, taking into account the respective uncertainties, it can be observed that the three surfaces present similar behaviors for the ground-state reaction, with ratios not far from the statistical value. For the F + CHD3(v1=1) reaction, however, the CSBB surface increased this ratio at low energies, whereas the PES-2006 and PWEM surfaces significantly decreased it, in contradiction to experiment.2 Therefore, given the good agreement between these two surfaces and the problems associated with CSBB surface, this result seems to indicate some problems associated with the experiments. 3.5. Comparison with Other Hydrogen-Abstraction Reactions. A similar theoretical/experimental controversy was also reported recently by Zhang et al.40 for the O(3P) + CHD3(v1=0) reaction. Whereas the OH + CD3 channel is preferred (i.e., OD/OH ratio < 1) experimentally, theoretically, the other channel, OD + CHD2, is preferred (i.e., OD/OH ratio > 1). The theoretical study was performed using QCT calculations based on the Czakó and Bowman (CB) surface for this reaction system,41 which presents a construction similar to that reported for the title reaction. Motivated by this controversy, we recently reported a similar QCT study42 based on a VB/MM-based surface for this system developed in our group, PES-2014.43 The results of this study agree with the previous theoretical results and contradict the experimental results. In this case, the two surfaces, CB and PES-2014, present similar stabilities for the reactant complex, −150 and −140 cm −1 , respectively, and given the barrier height, 5116 and 4931 cm−1, respectively, the effect of this reactant complex on the dynamics will be small or negligible. (Note that the PES-2014 barrier better reproduces the benchmark classical barrier for this reaction,41 4925 cm−1). Zhang et al.40 concluded that the discrepancy arises most likely from experiment, although the fact that the physical quantities measured in the two cases are different cannot be ruled out. The recent QCT calculations based on PES-2014 seem to confirm this finding. 3.6. Other Product Dynamics Properties: Role of CHStretching Excitation. As was mentioned in the Introduction, the main objective of the present article was to shed light on the theoretical/experimental controversy about the effect of CH-stretching-mode excitation on the reactivity of the F + CHD3 reaction, as developed in the previous sections. For completeness, in this section, other dynamics properties (vibrational distributions of the HF and DF products and product scattering distributions) were analyzed using PES-2006 and compared with previous results. Given that the largest differences among the three surfaces appear to be in the entrance channel, whereas all three surfaces provide similar descriptions of the product channel, it can be expected that the differences in these product dynamics properties will be smaller. The HF and DF product vibrational distributions for the F + CHD3(v1=0,1) reactions are shown in Figure 6 using PES-2006 for a collision energy of 9.0 kcal mol−1, for a direct comparison with the experiments3,4 and other PESs.8,10 Experimentally,3,4 it was reported that the extra energy in the CH-stretch-excited reaction is mostly channelled into the HF and DF products and that the HF product is hotter by one quantum from the CHstretch-excited reaction than from the ground-state reaction.

Figure 6. Vibrational state distributions for HF (top panel) and DF (bottom panel) products in the F + CHD3(v1=0,1) reactions at Ecoll = 9.0 kcal mol−1. These vibrational distributions were obtained for the PES-2006 surface using all reactive trajectories and rounding the vibrational actions to the closer total value.

Using PES-2006, we obtained a HF vibrational energy fraction of about 60% and a shift of the vibrational peak from v = 2 to v = 3 for the excited-state reaction. These results reproduce the experimental finding3 and the theoretical results from the CSBB8 and PWEM10 surfaces. The DF product vibrational distributions are also plotted in Figure 6 (bottom panel). PES2006 gave DF vibrational energy fractions of 67% and 55% for the ground-state and CH excited reactions, respectively, thus reproducing the experimental findings.4 The maximum value was v(DF) = 3 for both F + CHD3(v 1=0,1) reactions; that is, the CH excitation had almost not effect in the DF vibrational distribution. This result agrees with that obtained with the CSBB surface,8 but both are one quantum hotter than that obtained using the PWEM surface,10 v(DF) = 2. However, all three surfaces showed that the effect of the CH excitation is practically negligible in this channel. Unfortunately, no quantitative experimental data on this parameter are available for comparison, and the reason for this difference between the surfaces is unclear, given that their topographies in the product channel are similar (exactly the same in this region for CSBB and PWEM surfaces, by construction10). Finally, the product scattering distributions were analyzed. The differential cross sections (DCSs) for the HF and DF products in the F + CHD3(v1=0,1) reactions are plotted in Figure 7 using PES-2006. The DCSs were obtained by the Legendre moment method44 at Ecoll = 9.0 kcal mol−1 for a direct comparison with experiment3,4 and other theoretical results.10 Using PES-2006, a forward distribution was obtained for both channels, HF + CD3 and DF + CHD2, that was practically independent of CH vibrational excitation. These results agree with recent experiments3,4 and QCT calculations G

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The Journal of Physical Chemistry A

the CSBB-NOSO surface has been shown with time to be artificial because of the excessively deep well in the entrance channel. (3) Other dynamics properties, such as the HF and DF product vibrational distributions and scattering distributions, present good agreement among different surfaces and experiments, which is the expected behavior given that these dynamics properties depend on the topology of the product channel, which is similarly described by the three surfaces. Furthermore, these properties are less sensitive to the entrance channel, which seems be the key in this highly exothermic reaction. (4) Finally, the sources of this discrepancy could be theoretical and/or experimental and/or the theoretical and experimental values might refer to different physical measurements. The theoretical/experimental contradictions are not resolved, but the present work represents a step forward that can motivate further studies. For instance, in the well-studied Cl + HD reaction,45,46 which also presents a van der Waals complex in the entrance channel, the effect of the HD-reactant rotational energy is decisive in the HCl/DCl branching ratio. Thus, whereas DCl is the experimentally preferred channel at all collision energies at low rotational quantum numbers, HD(j=0,1), when higher j values are considered, the two channels show similar reactivities. Interestingly, QCT calculations reproduce this experimental tendency and the results of quantum mechanics calculations when an accurate PES is used.46 Therefore, the stereodynamics steering effect due to the presence of a reactant well in the atom−diatom Cl + HD reaction is observed only under very specific conditions, and its generalization to polyatomic systems under any conditions should be done judiciously.

Figure 7. Scattering angle distributions for HF (top panel) and DF (bottom panel) products for the F + CHD3(v1=0,1) reactions using PES-2006 at a collision energy of 9.0 kcal mol−1.

using the PWEM surface.10 Thus, a stripping mechanism associated with large impact parameters is suggested.

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4. CONCLUSIONS To study the effects of CH-stretching excitation in the F + CHD3 reaction, QCT results based on the PES-2006 surface were analyzed and compared with recent experiments and other potential energy surfaces, and the main conclusions are summarized as follows: (1) The excitation function obtained with PES-2006 presents a typical behavior where the reaction cross section increases with the collision energy. This behavior agrees with the most recent CSBB-SO and PWEM-SO surfaces and noticeably improves the results from the CSBB-NOSO and PWEMNOSO surfaces, which present an artificial tendency at low energies. This behavior at low energies is directly related to the topology in the entrance channel, namely, the depth of the reactant well, which is deeper and wider for the CSBB surface. This deep well favors the trapping of reactive trajectories, and later surfaces have shown that it is artificial. According to PES2006, CH vibrational excitation practically does not modify overall reactivity, it slightly decreases the excitation function for the DF + CHD2 channel, and it increases reactivity for the HF + CD3 channel. This result partially agrees with other theoretical results obtained using different PESs but strongly contrasts with experiment. (2) The DF/HF branching ratio obtained with PES-2006 is close to the statistical value of 3 for the ground-state reaction, in agreement with experiments and other PESs. However, when the CH-stretch-excited reaction is analyzed controversial results are found. Specifically, the PES-2006 and PWEM surfaces significantly decrease the DF/HF ratio, contradicting the experimental results. The agreement initially reported with

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The author thanks Gabor Czakó and Juliana Palma for sending energy data for the CSBB and PWEM surfaces to plot Figure 1.

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DOI: 10.1021/acs.jpca.5b10399 J. Phys. Chem. A XXXX, XXX, XXX−XXX