Trapped Abstraction in the O(1D) + CHD3 → OH + CD3 Reaction

Letter pubs.acs.org/JPCL

Trapped Abstraction in the O(1D) + CHD3 → OH + CD3 Reaction Jiayue Yang,† Kejie Shao,† Dong Zhang, Quan Shuai, Bina Fu,* Dong H. Zhang,* and Xueming Yang* State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China S Supporting Information *

ABSTRACT: Despite significant progress made in past decades, it is still challenging to elucidate dynamics mechanisms for polyatomic reactions, in particular, involving complex formation. The reaction of O(1D) with methane has long been regarded as a prototypical polyatomic system of direct insertion reaction in which the O(1D) atom can insert into the C−H bond of methane to form a “hot” methanol intermediate before decomposition. Here, we report a combined theoretical and experimental study on the O(1D) + CHD3 reaction, on which good agreement between theory and experiment is achieved. Our study revealed that this complex-forming reaction actually proceeds via a trapped abstraction mechanism, rather than an insertion mechanism as has long been thought. We anticipate that this reaction mechanism should also be responsible for the reaction of O(1D) with ethane and propane, as well as many other chemical reactions with deep wells in the interaction region. SECTION: Kinetics and Dynamics O(1D) + HF → OH + F reaction.30 In later crossed molecular beam experimental studies on the O(1D) + HCl reaction, the measured differential cross sections exhibit forward and backward scattering peaks, with forward preferred at a high collision energy.28,29 An osculating complex model for chemical reactions was proposed to explain the pronounced forward scattering, without mentioning the underlying mechanism given above.29 This implies that the dynamical picture provided in ref 24 for the O(1D) + HCl reaction has not been widely accepted. Due to its significance in both the atmospheric and combustion chemistry, the O(1D) reaction with methane has attracted great attention both experimentally and theoretically.31−41 In 2000, Yang and co-workers investigated the O(1D) + CH4 reaction using the universal crossed molecular beam technique31 and found that OH + CH3 is the main reaction channel, with a branching ratio of roughly 70%. Furthermore, the observed angular distribution of the OH product relative to the incoming O atom beam direction showed both forward and backward peaks, with the forward scattering peak considerably higher than the backward peak. This is probably not consistent with the direct insertion reaction picture, which is more likely to exhibit forward− backward scattering symmetry. More interestingly, such a phenomenon was also observed in the O(1D) reactions with ethane and propane, in which the OH product angular distribution shows a strong forward scattering peak.41−44 Recently, Suzuki and co-workers performed crossed molecular beam ion imaging experiment on the O(1D) + CD4/CH4 → OD/OH + CD3/CH3 reaction.33−35 Two distinctive reaction mechanisms were observed, in which the

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any chemical reactions take place when an old chemical bond breaks and a new bond forms in a collision event. The processes of breaking an old chemical bond and forming a new bond occur simultaneously in a majority of chemical reactions that involve atoms or radicals with one unpaired electron. This type of reaction is known as the direct abstraction or exchange reaction, which has been studied extensively in the past decades.1−11 Another well-known class of reactions is the complex-forming reaction, in which a reaction intermediate exists.12,13 Such an intermediate is formed because of a potential well along the reaction path, and it eventually breaks apart to form the product species. The prototypes for complex-forming reactions are the insertion reactions, in which the excited atom such as O(1D) can directly insert into a chemical bond to form a long-lived reaction intermediate that eventually decomposes to products. In such reactions, two new chemical bonds are formed simultaneously via insertion. A classic example for this type of reactions is the O(1D) + H2 → OH + H reaction, in which the O(1D) atom inserts into the H−H bond to form a “hot” (H−O−H) intermediate with two O−H bonds formed simultaneously.14−20 The hot water intermediate lives for a period of time before falling apart, producing H and OH products with nearly forward and backward scattering symmetry. Another interesting triatomic reaction involving O(1D), the O(1D) + HCl → OH + Cl reaction, has also been extensively investigated both theoretically21−25 and experimentally.26−29 A quasiclassical calculation study on the reaction revealed that the O atom initially attaches to the H atom, instead of inserting into the Cl−H bond, as has been generally assumed, although at low impact parameters, a genuine insertion does occur.24 The potential energy surface (PES) used in that study, however, was not very accurate. More recently, the same mechanism was found in a quasiclassical trajectory study on the © 2014 American Chemical Society

Received: August 11, 2014 Accepted: August 27, 2014 Published: August 27, 2014 3106

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Figure 1. Detailed schematic of the ground-state PES employed in the present study. The fitted energies are in kcal/mol, relative to the reactants O(1D) + CH4, and those shown in parentheses are from direct ab initio calculations of MRCI(Q)/aug-cc-pVTZ theory. All of the energies are shown without vibrational zero-point energy correction.

supposed “insertion” reaction on the ground-state PES exhibits a strongly forward scattering angular distribution for the CD3/ CH3 product relative to the incoming CD4/CH4, while the abstraction channel shows a clear backward scattering product angular distribution with discrete structures and occurs most likely on the excited-state PES, as suggested by Hernando et al.39 More recently, we carried out a time-sliced velocity map imaging crossed beams experiment on the O(1D) + CD4 → OD + CD3 reaction and showed that the abstraction pathway is a minor process,36,37 in agreement with Suzuki’s recent experimental results.34 Theoretically, Yu and Muckerman performed a directdynamics study of the reaction on the ground-state PES and uncovered that the reaction occurs via both direct and longlived intermediate pathways.40 However, a clear dynamical picture for the ground-state reaction was not obtained because of the low-level electronic structure method used as well as limited numbers of trajectories calculated. Although extensive experimental and theoretical studies have been carried out for the title reaction, the dynamical origin of the pronounced forward scattering products in addition to the backward and sideways scattering products produced on the ground-state PES still remains unclear. Furthermore, this pronounced forward scattering feature exists in many polyatomic reactions involving O(1D). Therefore, the intriguing question is whether the reaction on the ground-state PES truly proceeds via the so-called direct insertion mechanism as for the triatomic O(1D) + H2 reaction, as has been anticipated, or via the mechanism observed in the O(1D) + HCl/HF reactions. In order to answer this question, we carried out a joint experimental and theoretical study on the O(1D) + CHD3 → OH + CD3 reaction. Great details have been unraveled for the dynamics of this reaction with comprehensive quasiclassical trajectory (QCT) calculations, based on an accurate, fulldimensional ab initio PES, which are able to produce dynamics

results in good agreement with the time-sliced velocity map imaging experiment at the collision energy (Ec) of 6.8 kcal/mol. Experimentally, the O(1D) + CHD3 → OH + CD3 reaction was studied by the crossed molecular beams method combined with time-sliced velocity map imaging technique.36,45 (Details of the experiment and Figure S1 are given in the Supporting Information.) Theoretically, a new global, full-dimensional ground-state PES of the O(1D) + CH4 reaction was developed based on ∼240 000 high-level ab initio energies using the permutationally invariant polynomial fitting method developed by Bowman and co-workers.46−48 A complete schematic of the PES given in Figure 1 shows in detail the stationary point structures and energies relative to the reagents O(1D) + CH4 (with vibrational zero-point energies not included) corresponding to the relevant product channels. As seen, the CH3OH molecule is located at −137.6 kcal/mol below the O(1D) + CH4 asymptote, indicating that the reaction can, in principle, evolve via this deep well on the PES and subsequently decompose to various products. Due to the complexity of the PES for this system, the fitting is challenging to describe accurately all of the stationary points and reaction channels. The comparisons made for the energies obtained from the current PES and MRCI(Q)/aug-cc-pVTZ calculations show good agreement among them. On the newly constructed PES, we have performed extensive QCT calculations on the title reaction to investigate its reaction mechanism. (Details of this PES and QCT calculations are given in the Supporting Information.) Figure 2a compares the product translational energy distributions, P(ET), for the O + CHD3 → OH + CD3(ν = 0) reaction between theory and experiment. Overall, the agreement between theory and experiment is good. The P(ET) have peaks around 7 kcal/mol with long tails up to the energy of about 43.5 kcal/mol for both theory and experiment. The average fraction of the total available energy (50.1 kcal/mol) 3107

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released as translational energy for CD3(ν = 0) + OH is 0.24 for theory, in good agreement with the experimental value of 0.23, indicating a very high internal (rovibrational) excitation of the OH product. Both the experimental and theoretical angular distributions of the CD3(ν = 0) product show a pronounced forward scattering peak with relatively small signals from the sideways and backward scattering (Figure 2b). The agreement between theory and experiment is satisfactory despite the fact that the theoretical distribution shows a more intense forward scattering peak than experimental one. Given that the theoretical simulation is capable of reproducing the translational energy distribution and the angular distribution for the product CD3(ν = 0), in particular, for the pronounced forward scattering feature, we are now confident of using theory to provide complete dynamical information for the reaction. The theoretical angular distribution with all CD3 rovibrational states included shown in Figure 3a has the same shape as that for CD3(ν = 0) shown in Figure 2a, except the small peak at the backward direction. This distribution closely resembles the early experimental results of O(1D) + CH4 → OH + CH3 by Yang and co-workers31 with a pronounced forward peak and a relatively small backward one, despite the fact that the direct abstraction reaction proceeding on the excited-state PES, which can also produce backward scattering peak, is ignored in the current theory due to its small contributions to the overall reactivity. In order to determine the physical origin of the pronounced forward scattering peak, we calculated the distribution of times for the reaction, defined as the duration time for a reactive trajectory proceeding from the reactant side to product side with a distance between two fragments of 6 bohr (Figure 3b). The reaction time distribution is noticeably peaked at around 97 fs, with a long but small tail up to the time of 6 ps. Further

Figure 2. Comparisons of product translational energy distributions and angular distributions of the O(1D) + CHD3 → OH + CD3 reaction between theory and experiment. (a) Center-of-mass (CM) translational energy distributions of OH + CD3, with CD3 in its ground vibrational state (ν = 0), as obtained by theory and experiment. (b) Angular distributions of CD3 (ν = 0) from theory and experiment. The forward direction (0°) corresponds to the direction of the CHD3 reagent.

Figure 3. Theoretical distributions of all CD3 rovibrational states included for O(1D) + CHD3 → OH + CD3 obtained by the current QCT calculations. (a) The angular distributions of CD3 with different reaction time intervals, together with the total angular distribution for this reaction. (b) The distribution of reaction time (fs) for all of the reactive trajectories. (c) The internal energy distributions of CD3 at different reaction time intervals (fs), together with the total internal energy distributions. (d) The corresponding internal energy distributions of OH. 3108

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Figure 4. Snapshots of one trajectory resulting in the forward scattering with short reaction time. (a−i) Each panel shows the geometry at the reaction time indicated (fs). The internuclear distances (Å) between the O and H atoms and between the H and C atoms are indicated by red and purple fonts, respectively.

CD3(ν = 0) product mainly originates from intermediates with short lifetimes, its distribution has no backward peak, as shown in Figure 2b, and the corresponding OH products are highly excited rovibrationally, resulting in the very low kinetic energy distribution, as shown in Figure 2a. To provide further insight into the associated dynamics, we analyzed all of the reactive trajectories carefully to investigate how the reaction occurs. It turned out that only ∼0.8% of trajectories actually proceed via the direct insertion of O(1D) into the C−H bond of CHD3, forming a long-lived CD3OH complex with an average lifetime of 2.4 ps and eventually producing CD3 with forward and backward scattering symmetry as in the O(1D) + H2 reaction (see the Supporting Information for an animation). A vast majority of the trajectories actually do not proceed via the direct insertion as has been expected. The snapshots of one typical forward scattering trajectory with a large initial impact parameter (b = 5.0 bohr) for O(1D) + CHD3 are depicted in Figure 4. It clearly shows that the O(1D) atom first attacks the H atom when approaching CHD3 and abstracts the H atom out from a big separation distance, forming a highly vibrationally excited OH with H pointing to CD3 (Figure 4a−c). Subsequently, the OH group rotates by a large angle due to the strong anisotropy of the interaction potential, leaving the H atom pointing away from CD3, accompanied by sliding of OH and CD3 into the deep CD3OH well to form a hot CD3OH complex with enormous internal energy (Figure 4d−f). For this trajectory, the hot CD3OH complex dissociates immediately with the OH and CD3 moieties oscillating only once to form CD3 and OH products with OH scattered at the forward direction (Figure 4g−i), apparently because the energy that the OH and CD3 moieties gained while sliding into the deep CD3OH well largely remains in the relative motion of these two moieties, the very reaction coordinate for the OH and CD3 dissociation process, rendering dissociation easy to occur. This is very different from

analysis revealed that those reactions completed in an extremely short time (200 fs with a contribution of 46.5%), the angular distribution of CD3 is almost symmetric with respect to the forward and backward directions, as expected for a long-time complex formation reaction. Therefore, the pronounced forward scattering peak for the reaction is contributed mainly to by trajectories with a [100, 200] fs reaction time, supporting the conjecture that the forward scattering occurs owing to an extremely short-lived CD3OH intermediate.33,41 Consequently, the total angular distribution exhibits a pronounced forward scattering peak and a small backward one, which is very different from the product angular distribution of a typical insertion reaction such as O(1D) + H2 with nearly the symmetric forward and backward scattering peaks but similar to that for the O(1D) + HCl/HF → OH + Cl/F reactions. The trajectories with different reaction times not only have different angular distributions but also have very different internal energy distributions for the CD3 (Figure 3c) and OH (Figure 3d) products. The internal energy of product CD3 is relatively low for the reactive trajectories with short reaction time and increases considerably with the increase of the reaction time, implying that more energy is relaxed into the rovibrational motion of CD3 due to the long lifetime of the CD3OH complex. In strong contrast, the reactive trajectories with short reaction times mainly produce OH with high internal energies, and the internal energy of OH decreases substantially with the increase of the reaction time. Because the 3109

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a genuine insertion process, for which the energy from the initial formation of the complex cannot highly localize in one reaction coordinate. Due to the short lifetime of the intermediate, the energy initially deposited in the OH bond cannot be channeled into the CD3 moiety, leaving the product OH high and product CD3 low in internal energy, consistent with the result shown in Figure 3. Furthermore, it can be observed from the complete animation of the trajectory (see the Supporting Information) that the reaction is very similar to a stripping process except that the OH moiety oscillated once in the direction perpendicular to the stripping path toward the CD3 moiety due to the deep well, producing forward-scattered OH. Overall, the trajectories with short lifetimes behave as an abstraction reaction, mainly producing backward-scattered product with initial small impact parameters and forwardscattered product with large impact parameters, as shown in Figure 3. In contrast to the reactive trajectories with short-lifetime intermediates, a significant portion of trajectories are trapped in the CD3OH well for a long period of time, although they also start with the abstraction of the H atom, as illustrated in Figure 4a−f (see the Supporting Information for the animation). The longer time that a trajectory is trapped in the well, the more energy that is channeled into the CD3 internal motion and the less energy that is left on the OH bond that was formed initially with very high internal energy. These kinds of trajectories manifest as those for an insertion reaction, producing products with forward and backward symmetry. Therefore, the O(1D) + CHD3 → OH + CD3 reaction mainly proceeds through the CD3OH intermediate via the trapped abstraction mechanism, starting with the abstraction of the hydrogen atom, with only a very small portion of trajectories going through the typical direct insertion pathway with the O(1D) atom directly inserting into the C−H bond of CHD3. The initial abstraction of the H atom causes the energy released from complex formation to be highly localized in the relative motion of OH and CD3, making complex dissociation easy to occur. As a result, although the lifetime of the CD3OH complex spans a wide range from less than 100 fs to 6 ps, more than half of the complex-forming trajectories have lifetimes less than 200 fs. The process with a short lifetime behaves like an abstraction reaction, producing a pronounced forward scattering peak by a stripping mechanism with large impact parameter, as observed for this and O(1D) + CH4/CD4 reactions, while the process with a long lifetime produces reaction products with forward and backward scattering symmetry, similar to an insertion reaction. As a result, we observed dual characters of an abstraction reaction and an insertion reaction in the title reaction. We anticipate that this reaction mechanism should also be responsible for the reaction of O(1D) with ethane and propane, where forward scattering OH products were clearly observed, as well as many other chemical reactions that have deep wells in the interaction region.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.F.). *E-mail: [email protected] (D.H.Z.). *E-mail: [email protected] (X.Y.). Author Contributions †

J.Y. and K.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Chinese Academy of Sciences, the National Natural Science Foundation of China, and the Ministry of Science and Technology.

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ASSOCIATED CONTENT

* Supporting Information S

Details of the experimental and theoretical methods and four animations of typical trajectories for the O(1D) + CHD3 → OH + CD3 reaction. This material is available free of charge via the Internet at http://pubs.acs.org. 3110

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