LETTER pubs.acs.org/JPCL
Quasiclassical Trajectory Calculations of the Dissociation Dynamics of CH3CHO at High Energy Yield Many Products Yong-Chang Han, Benjamin C. Shepler,† and Joel M. Bowman* Department of Chemistry and Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, Georgia 30322, United States ABSTRACT: We present a quasiclassical trajectory study of the photodissociation of CH3CHO using a global ab initio-based potential energy surface. Calculations are performed at a total energy of 160 kcal/mol, corresponding to a photolysis wavelength of 230 nm, and with all trajectories initiated from the acetaldehyde global minimum. Many product channels are energetically accessible and observed at this energy, and the branching ratios to these are presented. We identify a minor channel giving H2O plus vinylidene or acetylene. Mechanisms are identified for these products, both of which originate from dissociation of vinyl alcohol an isomer of acetaldehyde. The channel CH2dCHOH f HCCH + H2O produces acetylene with relatively low internal energy, while the acetylene which arises from the subsequent isomerization of vinylidene via the channel CH2dCHOH f [CH2dC + H2O] fHCCH + H2O, is highly excited. The dissociation of CD3CHO is also studied, and both HOD+DCCD (D2CC) and D2O+DCCH products are formed from the same mechanisms as in the CH3CHO dissociation. SECTION: Dynamics, Clusters, Excited States
T
he photochemistry and thermal decomposition of acetaldehyde is of fundamental interest as well practical importance in the natural and polluted troposphere and also in hydrocarbon combustion chemistry. Many products are possible from this dissociation. In particular, the photochemistry to produce molecular products CH4 + CO and radical products, CH3 + HCO has been the subject of many experimental investigations.113 Recent studies at a high photolysis energy11 and high temperature12 have detected numerous other products as well. Theoretical work has also been extensive.1324 Recently, calculations of the unimolecular dissociations dynamics on S0 aimed at simulating the photodissociation dynamics have been reported. In particular, Kurosaki and Yokoyama reported directdynamics quasiclassical trajectory (QCT) calculations of this photodissociation by running trajectories initiated at the molecular saddle point transition state (TS) and in the direction of dissociation to the CH4 + CO products.17,18 More recently, direct-dynamics calculations of other channels have also been reported.19,20 Stimulated by an experimental report of “roaming” in the CH4 + CO channel,7 we performed extensive QCT calculations on a global potential energy surface (PES) and concluded that roaming is the dominant pathway to these products.21,22 Roaming dynamics, first uncovered in joint theoretical/experimental work in formaldehyde dissociation,2325 describes a new pathway to the molecular products. In brief, this pathway can be described as a self-reaction of incipient radicals. In the case of H2CO, these are H and HCO, and the molecular products are H2 and CO. For acetaldehyde, the roaming pathway to CH4 + CO involves the radicals CH3 and HCO. In joint r 2011 American Chemical Society
theoretical and experimental work the importance of the roaming pathway was confirmed.26 Recent high-energy photolysis experiments at 157 nm by Lee10 reported numerous products with CH3 + CO + H accounting for approximately half of the total branching. Very recent experiments by Vasiliou et al.12 on the thermal decomposition of CH3CHO at 1700 K reported decomposition products including CH3, CO, H, H2, CH2CO, CH2CHOH, and H2O + C2H2. They proposed that the water plus acetylene products are formed via two pathways: one that produces these products directly, and a very interesting one that first produces water plus vinylidene. These proposed pathways are analogous to ones reported in recent dynamics calculations of the unimolecular dissociation of the allyl radical to form CH3 + C2H2.27,28 These experiments, and in particular the proposed water plus vinylidene channel, stimulated us to investigate the unimolecular decomposition of CH3CHO at higher energies than we had previously done. Here we consider a total energy of 160 kcal/mol relative to the CH3CHO global minimum. This energy is the sum of the CH3CHO harmonic zero-point energy plus the energy of a 230 nm photon. It is high enough so that many of the channels reported in those experiments are open, but not so high as to create even greater challenges for developing a PES. And, in the case of the 157 nm experiment, it would also be necessary to address the coupled multielectronic state dynamics, which is Received: May 27, 2011 Accepted: June 26, 2011 Published: June 26, 2011 1715
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Figure 1. Schematic of stationary points of relevance to the present dynamics calculations of dissociation of CH3CHO. The fitted energies are in kcal/ mol, relative to the global minimum acetaldehyde, and those shown in parentheses are from CCSD(T) /AVTZ calculations. The black lines denote the isomerization pathways, and the blue lines represent the dissociation pathways. See text for discussion of the barriers separating the vinyl alcohol to the H2O + C2H2 and H2CC channels.
beyond the aim and scope of this paper. Thus, comparisons to results of this higher-energy experiment will be made carefully and kept qualitative instead of quantitative. Comparison to the thermal dissociation experiment is again qualitative and is focused on the interesting speculation about the formation of vinylidene plus water. We do that here by building on previous calculations, which employed PESs, which were fits to roughly 170 000 ab initio energies computed at the CCSD(T)/aug-cc-pVTZ (AVTZ) level of theory and basis. The data set consists of electronic energies for complex regions and fragments of all possible channels. To improve the accuracy of the PES and to extend it to higher energies, we performed MRCI/cc-pVTZ (VTZ) calculations of roughly 27 000 energies in the open-shell complex regions: CH3 3 3 3 HCO, CH3CO 3 3 3 H, and MRCI/AVTZ calculations of roughly 3000 energies in the biradical (CH2CH2O) region. Also, in addition, CCSD(T)/AVTZ calculations of roughly 5000 energies were added in the vinyl alcohol and hydroxyethylidene regions. Both the MRCI/VTZ and MRCI/ AVTZ energies are shifted slightly (by roughly 13.6 and 7.9 kcal/mol, respectively) to be compatible with CCSD(T)/AVTZ energies before being added to the large CCSD(T)/AVTZ database. All the new added energies were obtained with the MOLPRO suite of electronic structure programs.29 Currently, the entire data set of roughly 200 000 energies was fit, as before, using a basis of permutationally invariant polynomials in Morse-like variables in all the internuclear distances.30 This choice of basis leads to the important property of invariance with respect to all permutations of like nuclei. The fit is of maximum fifth order and has a rootmean-square (rms) fitting error of 2.9 kcal/mol, for energies up to 160 kcal/mol above the CH3CHO minimum. The electronic energies of stationary points and fragment channels are shown in Figure 1. As seen, there is very good agreement between the PES and direct ab initio values, with differences ranging from 1 to 5 kcal/mol. For the important barriers, the differences are generally smaller, since we calculated more electronic energies in the
regions of the PES surrounding them. We note that the pathways indicated from vinyl alcohol to the H2O + C2H2 and H2O + H2CC channels refer to the known two isomers of CH2CHOH. The energy given in Figure 1 is for the lower-energy cis-isomer, which correlates via TS6 to H2O + H2CC (and also correlates with CH3CHO), whereas the slightly higher energy trans-isomer (1.3 kcal/mol higher) correlates via TS7 to H2O + C2H2. The barrier separating these two isomers is only 4.7 kcal/mol relative to the lower energy isomer. These results are in complete accord with previous calculations of these barriers by Ohno and coworkers.31 Clearly, at the total energy of 160 kcal/mol this barrier is insignificant, and so both isomers of CH2CHOH would be expected to be almost equally accessible in the dynamics calculations presented here. A detailed description of the new global ab initio PES has been reported elsewhere.22 The present QCT calculations were carried out starting at the CH3CHO equilibrium global minimum with, as noted already, a total energy of 160.0 kcal/mol. The total angular momentum was fixed at zero, and random microcanonical sampling of the Cartesian momenta in all degrees of freedom was done to generate initial conditions, as described in detail previously.21,22 The time step is 0.12 fs, and trajectories were integrated for a maximum of 10 million time steps (1.2 ns). 24800 trajectories were run, and roughly 90% dissociated properly; the remaining ones visited problematic regions of the PES and were discarded. (These are high-energy regions where ab initio data are sparse and thus the PES is not reliable.) Many products were observed in these calculations, and Table 1 summarizes these and gives their branching ratios. As seen, the major products are the radicals CH3 + HCO or H + CH3 + CO (roughly 60%). We observed production of CH3 from three processes. One is from direct CC bond cleavage to give CH3 + HCO. A second one is from the CH3 + H + CO channel, and the third process is from CH4* + CO, where highly excited CH4* dissociates to CH3 + H. Furthermore, examination of many trajectories indicates that the second process is mainly a 1716
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Table 1. CH3CHO Dissociation and Branching Percentages product channel
branching ratio (%)
1. CH3 + HCO
45.4
2. CH4 + CO
19.5
3. CH3 + CO + H
14.6
4. CH3CO + H
9.9
5. CH2CHO + H
5.1
6. CH2CO + H2
1.2
7. H2CC + H2O
0.4
8. C2H2 + H2O
0.2
sequential one, i.e., where H + CO is formed by the subsequent dissociation of HCO*. This is not surprising given the weak binding of HCO. This result is also in qualitative accord with the 157 nm experiments, which determined the time-of-flight of products and where it was stated that “...most of the CH3 and CO products originate from ternary dissociations.” Thus, the products CH3 + H + CO come from two distinct processes. The third process appears to be novel, and so it is worth focusing a bit on it. First, we see a significant amount of CH4 + CO, and the expectation, based on research reported previously,26 is that very highly vibrationally excited CH4 will be formed via the roaming pathway.21,26 This is indeed the case, as shown in Figure 2, where the CH4 internal energy distribution is plotted. Recalling that De is roughly 113 kcal/mol, it is clear from Figure 2 that roughly 70% of the methane formed in this dissociation has internal energies exceeding the dissociation energy to CH3 + H. Thus, the actual branching ratio for total CH3 formation is almost certainly underestimated in Table 1, perhaps by as much as 14%, and the more correct value for the branching percentage is probably closer to 75%. Also there would be a corresponding increase in the branching ratio to form H. In addition to the two major fragment channels of CH3 + HCO, and CH4* + CO, we also obtained H-fragment channels (channels 3, 4, and 5), one channel producing H2 (channel 6), and two channels producing the H2O fragment (channels 7 and 8), as shown in Table 1. The three channels, 35, producing H fragments are all barrierless. The calculated branching ratio of CH3CO + H is higher than that of CH2CHO + H, presumably because the dissociation limit of CH3CO + H (96.3 kcal/mol) is lower than that of CH2CHO + H (104.6 kcal/mol). In addition to the hydrogen atom fragment, we also observe the H2 + CH2CO products. As shown in Figure 1, these result from the dissociation of acetaldehyde via TS5 (∼92.1 kcal/mol). The branching ratio to form these products is smaller than that for CH3 + HCO and CH4 + CO. This is not unexpected for the radical channel, which is barrierless; however, it is a bit surprising for the comparison with the CH4 + CO, which has a TS (TS4) only a few kilocalories per mole less than TS5. But recalling that CH4 + CO is formed substantially from a roaming pathway provides a plausible explanation for the enhanced production of these products over CH2CO + H2 Next we consider the (minor) pathways to give H2O + acetylene either directly or via isomerization of vinylidene. Examination of trajectories indicates that both pathways originate from the dissociation of the vinyl alcohol (CH2CHOH), which is formed via isomerization from acetaldehyde. As shown in Figure 1, there are two pathways from vinyl alcohol to dissociate into water. One is via TS6 (∼95.6 kcal/mol) to form vinylidene and water, and the other one is via TS7 (∼93.5 kcal/mol) to form
Figure 2. The internal energy distribution of methane.
acetylene and water; these are expressed by eqs 1 and 2, respectively. CH3 CHO f ½cis-CH2 ¼ CHOH f H2 CC + H2 O ðTS6Þ ð1Þ CH3 CHO f ½trans-CH2 ¼ CHOH f HCCH + H2 O ðTS7Þ ð2Þ In order to form vinylidene via TS6, the OH group and the H atom eliminate from the same carbon, which can be called (1,1)elimination, while to form acetylene via TS7, the OH group and the H atom eliminate from the different carbon atoms and will be called (1,2)-elimination. A more detailed analysis of this branching can be seen in Figure 3, which shows snapshots of the relevant part of two trajectories leading to channels 1 and 2. The first frame in each column clearly represents the two isomers of vinyl alcohol. As is well-known, there is a small barrier (less than 4 kcal/mol) for the isomerization of vinylidene to acetylene.32 Thus, as one continues to propagate the trajectories that initially produce vinylidene, they do eventually isomerize into acetylene. The dissociation from vinyl alcohol directly to acetylene is shown in the left panels of Figure 3, and the dissociation from vinyl alcohol to vinylidene (H2CC) followed by the isomerization to acetylene is shown in the right panels of Figure 3. It can be expected that the acetylene formed by the isomerization of vinylidene are highly excited, because of the high internal energy of vinylidene. As shown in Figure 1, the dissociation limit for H2CC + H2O is about 87 kcal/mol, while that for HCCH + H2O is about 42.1 kcal/mol. For these two trajectories we show in Figure 3, the final internal energy of acetylene that was produced via H2CC is roughly 72 kcal/mol, which is much higher than the internal energy of the acetylene produced directly from vinyl alcohol dissociation, 26.8 kcal/mol. QCT calculations were also carried out on CD3CHO. For these trajectories a total energy of roughly 155 kcal/mol was used, which equals the energy of 230 nm photon plus the lower zero-point energy of CD3CHO. Both HOD and D2O are found, and these result from the same dissociation/isomerization mechanisms discussed above for CH3CHO. The vinyl alcohol isomer for the deuterated acetaldehyde is more likely to be CD2CHOD. If the CD2CHOD eliminates the OD and H from the same carbon atom, which is via TS6, the HOD and D2CC 1717
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one channel producing H2 and ketene: CH3 CHO f CH2 CO + H2
ð8Þ
and two channels producing water accompanied by acetylene or vinylidene with the intermediate vinyl alcohol: CH3 CHO f ½CH2 CHOH f ½H2 CC + H2 O f HCCH + H2 O ð9Þ CH3 CHO f ½CH2 CHOH f HCCH + H2 O
ð10Þ
The acetylene that arises from reaction 9 is highly excited, whereas the acetylene produced via reaction 10 has lower internal energy. The dissociation of the isotopologue CD3CHO via the same mechanisms into both HOD + DCCD (D2CC) and D2O + DCCH have also been studied.
’ AUTHOR INFORMATION Corresponding Author
*E-mail address:
[email protected]. Present Addresses †
Georgia Gwinett College, 1000 University Center Lane, Lawrenceville, GA 30043.
Figure 3. Snapshots of the two trajectories producing the acetylene plus water via two different mechanisms. Left panels 15 show the direct dissociation from vinyl alcohol to acetylene and water corresponding to eq 1, and right panels 15 show the dissociation from vinyl alcohol first to vinylidene and water and the later isomerization from vinylidene to acetylene according to eq 2. The internal energy of acetylene that is produced via H2CC is roughly 72 kcal/mol, and that directly produced via acetylene directly is 27 kcal/mol.
would be formed. If the OD and a D atom on the other carbon eliminate, which is via TS7, D2O and HCCD would be formed. As discussed above, it would be easier to form HOD and D2CC, which can later isomerize to DCCD, than it is to form D2O and HCCD. In our QCT calculations, the branching ratio for HOD channel is about 0.6%, and that for the D2O channel is about 0.2%. In summary, we studied the dissociation of CH3CHO at a high energy, corresponding to 230 nm wavelength, by QCT calculations on an improved global ab initio-based PES. Various product channels have been found at this high energy. The products obtained include: CH3, CO, H, H2, CH2CO, H2O, and C2H2, which are in agreement with the experimental thermal decomposition findings at 1700 K. We have also identified the products HCO, CH4, CH3CO, CH2CHO, and H2CC. In addition to the two major product channels: CH3 CHO f CH3 + HCO
ð3Þ
CH3 CHO f CH4 + CO
ð4Þ
we obtained three channels producing the H atom: CH3 CHO f ½CH3 + HCO f CH3 + CO + H
ð5Þ
CH3 CHO f CH3 CO + H
ð6Þ
CH3 CHO f CH2 CHO + H
ð7Þ
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