Two Pathways for Dissociation of Highly Energized syn-CH3CHOO to

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Two Pathways for Dissociation of Highly Energized syn-CH3CHOO to OH plus vinoxy Xiaohong Wang, and Joel M Bowman J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01392 • Publication Date (Web): 11 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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The Journal of Physical Chemistry Letters

Two Pathways for Dissociation of Highly Energized syn-CH3CHOO to OH Plus Vinoxy Xiaohong Wang and Joel M. Bowman∗ Department of Chemistry, Emory University, Atlanta GA 30322; E-mail: [email protected]

August 11, 2016

∗

To whom correspondence should be addressed

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Abstract Ozonolysis of alkenes is an important non-photolytic source of hydroxl radicals in the troposphere. The reaction proceeds through cycloaddition and subsequent decomposition to a carbonyl oxide, known as Criegee intermediates. Ozonolysis of alkene releases about 50 kcal/mol excess energy to form highly energized Criegee molecules, which can be stabilized and undergo further reaction or dissociate to OH+vinoxy products. The dissociation dynamics of partially stabilized Criegee (syn-CH3 CHOO) has been thoroughly studied recently, in which the molecules dissociate by first isomerizing to vinyl hydroperoxide (VHP). Here we examine the dissociation dynamics of highly energized syn-CH3 CHOO (42 kcal/mol), and a second, prompt dissociation path is discovered. The dissociation dynamics of these two paths are carefully examined through the animation of trajectories and the energy distributions of products. The new prompt path reveals a distinctly different translational energy and internal energy distributions of products compared to the known path through VHP.

TOC Figure


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Hydroxyl (OH) radical is an important species in the troposphere, because it initiates the oxidative reaction of many trace species. In the daytime, the photolytic process of O3 dominates the production of OH radical. 1 During the night, the ozonolysis of alkenes is the main non-photolytic source of OH radical in the troposphere. 1–3 The ozonolysis is believed to proceed through a 1,3-cycloaddition of O3 to the double bond of alkenes to produce a primary ozonide, which decomposes to a carbonyl compound and energized carbonyl oxide, known as Criegee intermediates (CI). 4 The activated CI can react with other tropospheric species, such as SO2 , NO2 and H2 O. 5–8 Alternatively, the energized CI can undergo unimolecular dissociation, yielding the OH radical. 8,9 The OH yield of ozonolysis highly depends on the alkene structure. For the ozonolysis with trans-2-butene, which proceeds through the methylsubstituted CI CH3 CHOO, the OH yield is quite high with more than 60%. 10 In the present study, we focus on the syn-CH3 CHOO. Experimentally, OH radicals resulting from the unimolecular decay of Criegee syn-CH3 CHOO, were first directly detected in an action spectrum reported by the Lester group. 11 The synCH3 CHOO was excited at around 6000 cm−1 , which is near the top of the barrier for isomerization of syn-CH3 CHOO to vinyl hydroperoxide (VHP), roughly 17 kcal/mol above the syn-CH3 CHOO zero-point energy (ZPE), and determined the internal state distribution of the OH radicals. The theoretical study of the unimolecular decay of syn-CH3 CHOO is challenging due to the high dimensionality and electronic structure calculations, which must consider the multi-reference region of dissociation exit channel. Two full-dimensional, but local, potential energy surfaces (PES) were reported for the syn- and anti- conformers of CH3 CHOO in full dimensionality. These were used (with dipole moment surfaces) to calculate the infrared (IR) spectra in joint experimental/theoretical analysis of these spectra. 12 In more recent studies, a semi-global PES that describes the dissociation of syn-CH3 CHOO to OH+vinoxy was reported in a joint experimental/theoretical dynamics study. 13 The dissociation dynamics of syn-CH3 CHOO excited to around 6000 cm−1 (17 kcal/mol) above the ZPE were studied using



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tens of thousands quasiclassical trajectory calculations and the translational and internal energy distributions of the OH agreed well with experiment. In addition, the rotational and vibrational energy distributions of the vinoxy products were obtained from the trajectory calculations. 13 These studies confirmed that the dissociation of the syn-CH3 CHOO involves the isomerization to vinyl hydroperoxide (VHP, H2 C=CHOOH) via a transition state (TS) with a five-member ring structure. Then the energized VHP undergoes dissociation, forming OH and vinoxy radicals; however, that dissociation process is not a simple bond-fission process but involves transiting several exit channel features in the PES. 13 Nevertheless, the rate-determining step for the dissociation at this energy is isomerization from syn-CH3 CHOO to VHP. The lifetime for this was recently measured and calculated, using a sophisticated version of RRKM theory, to be in range of nanoseconds. 14 The subsequent time to dissociate from the energized VHP to the products is in the range of a few picoseconds. 13 As noted, the previous experimental and theoretical dynamics studies were done with the internal energy of syn-CH3 CHOO lower than the initial energy released in the ozonolysis in the troposphere. In fact, ozonolysis of trans-2-butene releases about 50 kcal/mol energy to form internally excited syn-CH3 CHOO along with H2 CO. 9 A portion of the CI will be collisionally stabilized under atmospheric conditions, and the resultant thermalized distribution of syn-CH3 CHOO will slowly isomerize to VHP and dissociate to OH and vinoxy products. 15 However, a significant portion of the energized CI may undergo prompt dissociation to products due to the high internal energy. The dynamics of dissociation of syn-CH3 CHOO at the upper end of internal energy has not been investigated before and this is the focus of current study. In this paper, we report the dissociation dynamics of syn-CH3 CHOO with high internal energy, using quasi-classical trajectories (QCT) simulations. The internal energy is chosen to be 42 kcal/mol, relative to the ZPE of syn-CH3 CHOO. This choice is reasonable based on the following simple statistical estimate of the energy partitioning given a total available energy of 50 kcal/mol: a ratio of 3:1 of vibrational modes for syn-CH3 CHOO (18) versus H2 CO (6) and more low-frequency modes in the former than the latter. So a energy


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partitioning ratio of 4:1 (or higher) is more realistic. The 4:1 ratio leads roughly to the chosen internal energy of syn-CH3 CHOO. The calculations are performed with an in-house code. 16,17 and computational details are given at the end of the paper. These calculations reveal an additional dissociation pathway that bypasses the VHP minimum and which shows different energy distributions compared with the pathway passing through VHP. As before, 13 ensembles of thousands (in this case roughly 30,000) of trajectories were initiated from the isomerization TS and from the VHP minimum, and propagated to the products, OH and vinoxy. (A plot of the potential along a schematic “reaction path” is shown in the TOC graphic, where the isomerization TS and VHP minimum are depicted and a figure showing relevant stationary points is given in the Supporting Information (SI).) The choice of these initial configurations was made in the previous work as a diagnostic for the microscopic mechanism of the reaction dynamics. If isomerization does in fact precede dissociation, then results from these two initial configurations should be very similar, and indeed they were at the energy of that earlier study. Thus, we run the same diagnostic here. Details of the calculations are given below; however, we note that given the high internal energy, classical microcanonical sampling of initial conditions was done for all trajectories. The classical vibrational energies of the fragments were determined to check for ZPE violation. The anharmonic ZPE of OH from the PES was calculated numerically to be 1865 cm−1 , using the Colbert-Miller discrete variable representation (DVR). 18 The anharmonic ZPE of vinoxy was determined to be 9279 cm−1 from the PES and using the MULTIMODE program. 19 Roughly 75% of trajectories resulted in either product with less than their respective ZPEs. This percentage is smaller than the one (roughly 90%) in the calculations run at lower total energy. 13 ZPE “violation” of products is a common issue of the QCT approach, which can have significant consequences on the translational energy distribution of products. Therefore, as before, 13 we applied the hard-ZPE constraint and excluded the trajectories with ZPE violation of either product. Finally, 7,447 trajectories from the isomerization TS and 7,283 trajectories from VHP were included in the following analysis. Using



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these sets of trajectories, we calculated the translational kinetic energy release (TKER) to the products, and the vibrational and rotational distributions of OH and vinoxy radicals. First, consider the TKER distributions starting from the isomerization TS and the VHP minimum (with the same total energy). These are given in Figure 1, where significant differences are seen, and which indicate non-RRKM behavior of the TS-initiated trajectories. The distribution using trajectories initiated at the TS is broader in the high-energy range. The calculated average kinetic energy release is given in Table 1; trajectories starting from the TS have more than 600 cm−1 kinetic energy release than those from VHP. This observation is quite different from previous studies at the lower energy (17 kcal/mol excitation), in which essentially the same results were observed for the trajectories starting from the isomerization TS and VHP, and the average kinetic energy releases using two sets of trajectories agreed very well with only a 90 cm−1 difference, as seen in Table 1. 13


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Figure 1: Translational kinetic energy release (TKER) of the OH and vinoxy products using the QCT trajectories starting from the isomerization transition state (black) and the VHP (red) for dissociation of syn-CH3 CHOO with 42 kcal/mol internal energy. The maximum probability in each distribution is set to unity.

Table 1: Results of dissociation trajectories of syn-CH3 CHOO with 42 kcal/mol internal energy starting from the isomerization TS and from vinyl hydroperoxide (VHP): percentage of vibrationally excited OH radicals, the average translational kinetic energy release, Etrans , and the average vibrational and rotational energies of vinoxy, Evib and Erot , respectively (cm−1 ) P(νoh = 1) . TS 8.8% VHP 6.0% a

Etrans 4771 4165

Evib 5129 6255

Erot 1615 1565

Etrans a 1811 1902

Ref. 13 Average relative kinetic energy using trajectories initiated with 17 kcal/mol internal energy

Besides the different TKER distributions, trajectories starting from the TS and VHP also give different internal energy distributions of the OH and vinoxy products. A small fraction of the OH radicals are vibrationally excited, in contrast to the previous results at lower energy where no vibrational excitation was calculated, in agreement with experiment. 7 ACS Paragon Plus Environment


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The percentage of OH radicals in the first excited state is shown in Table 1. (Standard histogram binning was done to obtain this percentage.) As seen in the table, OH radicals are more likely to be vibrationally excited starting from the TS compared with trajectories from VHP. Due to the higher kinetic energy release and vibrational energy of OH radical of trajectories starting from TS, it is obvious that vinoxy is formed “colder”. Table 1 shows that average internal energy of vinoxy radical formed from TS-initiated trajectories is ∼1100 cm−1 lower than trajectories at initiated at VHP. The present results show significant differences between the two sets of trajectories, in contrast to the previous study at the excitation energy of 17 kcal/mol, where virtually the trajectories dissociate by passing the VHP well. To investigate this further, trajectories initiated at the isomerization TS were examined and a new prompt dissociation pathway, denoted as TS-Prompt path, was discovered. In this pathway, CH3 CHOO dissociates directly and thus “promptly” to OH and vinoxy without visiting the VHP minimum. Animations of several trajectories illustrating this pathway are provided in the SI. Significant insight with respect to the prompt dissociation dynamics is gained from these animations. The bridging hydrogen atom quickly transfers to the terminal oxygen atom, instead of isomerizing to VHP. In addition, the O-O bond directly breaks and the complex quickly dissociates forming the OH and vinoxy radicals. In the prompt path, the reaction occurs right after surmounting the transition state and there is virtually no time for intramolecular vibrational redistribution of the large amount of internal energy. Thus, we expect that most of the available energy would be released as translational energy of fragments. 20 In this case, the barriers and complexes in exit channel of VHP dissociation become irrelevant. In addition, since the dissociation occur right after the hydrogen atom transfers to the O, we would anticipate that the forming OH radical is more likely to be vibrationally excited. These are indeed the results seen in Figure 1 and Table 1. By inspection of several hundred direct trajectories, a lifetime of 0.2 ps or less was determined for these trajectories. Thus, that value of the lifetime was selected as the upper limit


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for characterizing TS-initiated trajectories as “prompt”. This value is further justified in two ways. First, lifetime distribution plots from TS and VHP-initiated trajectories, given in Figure S2 of the SI, show a prominent spike at 0.2 ps, embedded in an otherwise-exponentiallooking distribution, for TS-initiated trajectories. For VHP-initiated trajectories, the distribution shows the expected exponential behavior. Based on this criterion, roughly 14% of trajectories starting from the isomerization TS are characterized as prompt. The second justification is given below where TKER distributions are shown for the two paths assigned to the TS-initiated trajectories. In order to investigate the signatures of the prompt path in more detail, TKER distributions were calculated for the 14% of trajectories assigned to that path and the remaining 86% that were assigned to the isomerizing path, which we denote as TS-VHP. The results are shown in Figure 2. As seen in Figure 2a, the two different paths starting from TS show very different relative translation energy distributions. The TS-VHP distribution is much colder than the TS-Prompt path one, as expected. The TS-Prompt path distribution peaks at about 8000 cm−1 , which is 4400 cm−1 higher than the TS-VHP one. Also, and very significant, the TS-VHP distribution is almost identical with the one obtained with trajectories initiated from VHP. This indicates that the lifetime criterion used to partition the trajectories produces consistent results for the translational energy distribution, as it should. Note that all the distributions shown in Figure 2a are assigned the value 1.0 at their maximum for display purposes. Using these separate TKER distributions from TS trajectories, the total TKER is decomposed into two parts, as shown in Figure 2b. It is obvious that the main differences of the distributions starting from TS and VHP is from the contribution of prompt path, which releases much more energy to the kinetic energies of the products.



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Figure 2: Translational kinetic energy release (TKER) distribution of the OH and vinoxy products (a) QCT trajectories starting from VHP (red), and the two different decay paths starting from TS, regular path via VHP (solid black) and prompt path (dashed black). The maximum probability is set to unity. (b) Decomposition of the TKER distributions of trajectories starting from TS of the two different reaction paths. These results are for syn-CH3 CHOO with 42 kcal/mol internal energy. More detailed analysis was performed for the two paths. The average translational energies, percentage of OH formed in the vibrationally excited state, and the vibrational and 10 ACS Paragon Plus Environment

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rotational energies of vinoxy from both paths are shown in Table 2. The products have about 14,250 cm−1 available energy given the excitation energy of 42 kcal/mol. For the TS-VHP path, the translation energy release is ∼30% of the available energy, and most energy release as the internal excitation of vinoxy, which agrees with previous study. 13 In contrast, the TS-Prompt shows totally different energy distributions of products, where more than half (∼ 51%) energy releases to kinetic energy of products, and the OH and vinoxy radicals are relatively less internally excited. The rotational and vibrational energy distributions of vinoxy from the two paths starting from TS are shown in Figure 3. As seen, the vinoxy radicals were highly vibrationally excited for the TS-VHP path, with a peak vibrational energy of more than 6,000 cm−1 . In contrast, vinoxy produced via the prompt path has much less vibrational excitation. Detailed final vibrational-state analysis of vinoxy is very challenging because of the high dimensionality and coupling between vibrational modes. However, determining this experimentally will hopefully be examined in the future. The rotational energy distributions of vinoxy from the two paths are quite different as well, as seen in Figure 3b, and the distribution from TS-Prompt path is much narrower than that from TS-VHP path. The average rotational energies are 1759 cm−1 and 702 cm−1 for TS-VHP and TS-Prompt path, respectively. From the comparison, it can be seen that the vinoxy radicals forming from the prompt reaction path are both vibrationally and rotationally less excited, with most of the excess energy released into relative translational energy and also some vibrationally excited OH.



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Table 2: The analysis results of two dissociation paths of syn-CH3 CHOO with 42 kcal/mol internal energy starting from the isomerization TS, including the percentage of vibrationally excited OH radicals, the average translational kinetic energy, Etrans , and the average vibrational and rotational energies of vinoxy, Evib and Erot , respectively. (cm−1 ) TS-Prompt TS-VHP VHP

P(νoh = 1) 13.1% 8.1% 6.0%

Etrans Evib Erot 8026 2342 702 4261 5568 1759 4165 6255 1565


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TS-Total TS-VHP Path TS-Prompt Path

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Figure 3: Vibrational (a) and Rotational (b) energy distributions of vinoxy radicals from dissociation of syn-CH3 CHOO with 42 kcal/mol internal energy, using trajectories starting from isomerization TS (solid blue), including TS-VHP path (solid black) and TS-Prompt path (dashed black) with 42 kcal/mol internal energy. The maximum probability is set to unity.

The OH rotational distributions from the two different paths starting at the isomerization TS, are given in Figure 4. Note that only the OH radicals formed in the ground vibrational



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state are included in Figure 4. The OH rotational distribution using trajectories starting from VHP is essentially identical to TS-VHP path, which is not shown here. In contrast to the large differences of kinetic energy distribution and internal energy distributions of vinoxy, the two paths showed quite similar rotational distributions of OH radicals. In each case, the OH radicals were not highly rotationally excited, with distribution peaks at N = 6 and N = 8 for TS-VHP and TS-Prompt paths respectively. The average OH rotational energy was calculated to be ∼1225 cm−1 , which accounts for less than 10% of the available energy.

Figure 4: Rotational energy distribution of hydroxyl radicals from dissociation of synCH3 CHOO with 42 kcal/mol internal energy, using trajectories starting from isomerization TS (solid blue), including TS-VHP path (solid black) and TS-Prompt path (dashed black). The maximum probability is set to unity.

In summary, we studied the unimolecular dissociation dynamics of syn-CH3 CHOO with 14 ACS Paragon Plus Environment

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42 kcal/mol internal energy, relative to the zero-point energy of syn-CH3 CHOO. This was done by running thousands of trajectories initiated at the vinyl hydroperoxide (VHP) isomerization TS and, for diagnostic purposes, from the VHP minimum. Two reaction paths were uncovered for the set of trajectories initiated at the TS. One is via isomerization to VHP, followed by dissociation to products. This was the pathway found in previous studies at lower syn-CH3 CHOO internal energy. A second, prompt dissociation pathway without visiting VHP was observed and reported for the first time. This path was calculated to be roughly 14% of all the trajectories initiated at the isomerization TS. It is worth noting that this percentage is perhaps sensitive to the sampling method of initial conditions and also the hard-ZPE constraint applied to the products. Future theoretical work can be done, with the availability of the potential energy surface, to investigate this. The two reaction paths were investigated by analyzing translational and product internal energy distributions. The prompt path shows different energy distributions of products with the known path through VHP. For the prompt path, more than half of the available energy is released as the kinetic energy of products, and as a result the vinoxy radicals are produced with much less internal energy. Finally, it is likely that in general unimolecular dissociations, a direct, prompt pathway will grow in importance relative to isomerization/dissociation, as the total energy increases.

Computational Methods The QCT simulations were performed on the global potential energy surface (PES), which accurately describes the dissociation channel of syn-CH3 CHOO. The PES was fitted in full dimensionality using permutationally invariant method 21 based on the energies from high level ab initio calculations. The configurations of stationary points on the PES are shown in SI. A wide energy range is covered by the PES with energy up to 90 kcal/mol with respect to syn-CH3 CHOO, which enables the simulations at high energies. More details of the PES



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can be found in Ref 13. In order to reduce the computational expense, the simulations were initiated at the isomerization transition state (TS) and VHP minimum structure instead of the syn-CH3 CHOO minimum. The initial energy of QCT simulations was chosen to be 42 kcal/mol relative to the ZPE of syn-CH3 CHOO. This amounts to a total energy of 79 kcal/mol above the syn-CH3 CHOO minimum. Relative to the isomerization TS, where one set of trajectories were initiated, the energy is roughly 61 kcal/mol, and relative to the VHP minimum, where a second set of trajectories were initiated, the energy is 98 kcal/mol. At such high energies for the two sets of trajectories, the vibrational density of states is large, and so classical microcanonical sampling of initial conditions was done. 22 Specifically, initial coordinates were fixed at the isomerization TS or VHP minimum configuration and initial momenta were selected randomly, subject to the constraint of fixed total energy (potential+kinetic), zero total angular momentum, and zero center-of-mass kinetic energy. Each trajectory was propagated for maximum 5 ps with step size equal to 0.1 fs. More than 70% trajectories could dissociate within in 5 ps. As noted the total angular momentum is equal to zero for all trajectories.

Acknowledgments Financial support from the Army Research Office (W911NF-14-1-0208) is gratefully acknowledged. Also, we thank Marsha I. Lester for stimulating this work and useful discussions.

Supporting Information Available A PES schematic showing relevant stationary points, lifetime distributions of syn-CH3 CHOO, and the animation of three prompt trajectories dissociating from syn-CH3 CHOO with 42 16 ACS Paragon Plus Environment

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kcal/mol internal energy, relative to the zero-point energy.



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Two Pathways for Dissociation of Highly Energized syn-CH3CHOO to

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