Criegee Intermediate Reaction with CO: Mechanism, Barriers

Feb 14, 2014 - Density functional theory and transition state theory rate constant calculations have been performed to gain insight into the bimolecul...
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Criegee Intermediate Reaction with CO: Mechanism, Barriers, Conformer-Dependence, and Implications for Ozonolysis Chemistry Manoj Kumar,†,‡ Daryle H. Busch,†,‡ Bala Subramaniam,‡,§ and Ward H. Thompson*,†,‡ †

Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States Center for Environmentally Beneficial Catalysis, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States § Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, Kansas 66045, United States ‡

S Supporting Information *

ABSTRACT: Density functional theory and transition state theory rate constant calculations have been performed to gain insight into the bimolecular reaction of the Criegee intermediate (CI) with carbon monoxide (CO) that is proposed to be important in both atmospheric and industrial chemistry. A new mechanism is suggested in which the CI acts as an oxidant by transferring an oxygen atom to the CO, resulting in the formation of a carbonyl compound (aldehyde or ketone depending upon the CI) and carbon dioxide. Fourteen different CIs, including ones resulting from biogenic ozonolysis, are considered. Consistent with previous reports for other CI bimolecular reactions, the anti conformers are found to react faster than the syn conformers. However, this can be attributed to steric effects and not hyperconjugation as generally invoked. The oxidation reaction is slow, with barrier heights between 6.3 and 14.7 kcal/ mol and estimated reaction rate constants 6−12 orders-of-magnitude smaller than previously reported literature estimates. The reaction is thus expected to be unimportant in the context of tropospheric oxidation chemistry. However, the reaction mechanism suggests that CO could be exploited in ozonolysis to selectively obtain industrially important carbonyl compounds. tions19,20 are also contributing to the growing interest in Criegee chemistry. Su et al.18 recently measured the infrared spectrum of the simplest CI (H2COO) in the gas phase and their spectral interpretations favor a zwitterionic CI rather than a biradical one. Welz et al.19 have synthesized and detected gas phase H2COO species using a photoinization technique that allowed the direct kinetic measurements of its reactions with NO, NO2, H2O, and SO2. In a recent study, they applied this technique to the conformer-dependent reactions of the next larger CI (CH3CHOO) with SO2, NO2, and H2O.20 In all the studied reactions, anti-CH3CHOO is found to be more reactive than syn-CH3CHOO, a result they attribute to hyperconjugation. In the condensed phase, the dominant reaction of CI is the recombination reaction with the carbonyl compound due to the cage effect.21,22 However, in the gas phase, a significant fraction of CI is vibrationally stabilized and can take part in a variety of unimolecular or bimolecular reactions.23 The scope of a bimolecular Criegee reaction depends upon several factors: the nature of the olefin, reaction conditions, and the availability of potent coreactants. As the second most abundant atmospheric coreactant (behind water) in forested regions as well as urban and polluted environments,24 carbon monoxide (CO) is a potential coreactant for study in bimolecular CI

1. INTRODUCTION Criegee intermediates (CIs) are highly reactive carbonyl oxides that play a key role in atmospheric and condensed-phase ozonolysis of unsaturated hydrocarbons.1 Importantly, a CI formed during ozonolysis can engage in bimolecular chemistry if appropriate reaction partners are available.2 Under atmospheric conditions, the reaction of CIs with water is considered to be a major CI decay process,3 though other reactions can also be relevant.4−11 The fate of the CI is critical in tropospheric oxidation chemistry, particularly aerosol formation.12,13 In synthetic applications of ozonolysis, where a cleaner and greener approach for producing functionalized organics (e.g., aldehydes, ketones, and carboxylic acids) is sought, the oxidation products may be controlled by reacting the CI with well-designed partners.14,15 CIs have also been implicated as the key intermediates in the catalytic cycles of flavin-dependent Baeyer−Villiger monooxygenases (BVMOs)16 that provide an environmentally benign alternative to the conventional Baeyer−Villiger reaction based on enhanced enantio- and regioselectivity. Baeyer−Villiger reactions17 are of significant synthetic value because ketones are converted into esters or lactones using stoichiometric amounts of hydrogen peroxide, peracids, or alkylhydroperoxides. A clear understanding of Criegee chemistry is thus crucial in understanding (or manipulating) the outcomes of these widely varying chemical processes. Recent spectroscopic characterization of the CI18 and the direct kinetic measurements of bimolecular Criegee reac© 2014 American Chemical Society

Received: January 9, 2014 Revised: February 14, 2014 Published: February 14, 2014 1887

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reactions. The average concentration of CO has been estimated to be on the order of 1012−1013 molecules/cm3 in these regions.25 Moreover, in synthetic ozonolysis, the reaction partners of the CI can be selected and potentially designed to optimize the reaction product distribution. This provides a significant impetus to widely explore bimolecular CI chemistry and ultimately determine the general principles of CI reactivity. In this article, we report the results of an electronic structure investigation of the bimolecular CI reaction with CO, which has not been extensively discussed in the literature. The aim is to understand the mechanism along with the associated reaction barrier heights and estimated rate constants. In the following, the mechanism is discussed in the context of the reaction involving the simplest CI, H2COO, in section 2.1. The conformer dependence discussed above is then examined in section 2.2 for the next simplest CIs, syn- and anti-CH3CHOO, and its origin is investigated. The role of conjugation is investigated in section 2.3. The reactions of several biogenic terpenoids relevant to atmospheric chemistry, shown in Scheme 1, are discussed in section 2.4. Estimated rate constants Scheme 1. Biogenic Criegee Intermediates (CIs) Investigated in the Current Study

Figure 1. M06-2X/aug-cc-PVTZ-calculated profile of H2COO + CO reaction. The numbers in parentheses correspond to single-point CCSD(T)/aug-cc-PVTZ energies.

only slightly, and thus in the following only results obtained using the M06-2X/aug-cc-PVTZ level of theory are presented. The corresponding reaction energies, barrier heights, enthalpies, and free energies are given in Table S1, Supporting Information. The H2COO + CO reaction first proceeds by formation of a weakly bound, ΔE = −2.2 kcal/mol, H2COO···CO reactant complex (Int1) with R(OCO···CCI) = 2.84 Å. The involvement of analogous weak complexes in other CI reactions has previously been reported.26 Then, Int1 evolves via the transition state (TS) to form a formaldehyde and CO2 complex. The imaginary frequency vibrational mode of TS (ω‡ = 442.4i cm−1) corresponds to the transfer of the terminal oxygen of the CI to the CO carbon; for TS, R(CCO···OCI) = 1.77 Å, and the O−O bond distance in the CI is 1.38 Å. The structures show that the OCO···CCI distance in TS (2.08 Å) is shorter by ∼0.76 Å compared to Int1, suggesting that strengthening of the CI···C interaction is key for facilitating O-atom transfer. The TS leads to a H2CO···CO2 product complex that is highly exoergic compared to the reactants (ΔE = −125.0 kcal/mol); this complex is bound by weak dipolar and van der Waals intermolecular interactions between CO2 and formaldehyde and has a binding energy of 2.4 kcal/mol relative to isolated products. The calculated exothermicity, ΔHrxn = −122.6 kcal/ mol, is in reasonable agreement with the empirically estimated heat of reaction27 of −120.6 kcal/mol. Overall, this oxidation reaction involves an activation barrier of 11.2 kcal/mol (13.4 kcal/mol) relative to the separated reactants (reactant complex). It is interesting to compare this mechanism with the H2COO + NO reaction, which has been previously reported.24 The

from transition state theory are presented in section 2.5 before concluding remarks in section 3. The details of the computational approach, which is based on density functional theory calculations, are described in section 4.

2. RESULTS AND DISCUSSION 2.1. H2COO + CO Reaction. The calculated zero-point energy (ZPE) corrected electronic energy profile for the reaction of the simplest CI, H2COO, with CO is presented in Figure 1 along with the optimized geometries of the stationary points along the reaction pathway. The calculated IRC path for the reaction is shown in Figure S1, Supporting Information. In order to verify the accuracy of our theoretical method, CCSD(T)/aug-cc-PVTZ//M06-2X/aug-cc-PVTZ calculated energies are also shown in parentheses. The energies calculated using the M06-2X and CCSD(T)//M06-2X methods differ 1888

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reaction with NO proceeds through two additional intermediates before the release of the aldehyde and NO2;24 in the case of CO, these intermediates are not stable and the reaction is direct. In order to facilitate the comparison with the previous results that were calculated using a different method (CCSD(T)/aug-cc-PVTZ//M05-2X/aug-cc-PVDZ), we have estimated the barrier height of the preliminary step of CI + NO reaction (5.3 kcal/mol), which is only 0.5 kcal/mol different from the previously reported value of 5.8 kcal/mol indicating that the accuracy of our selected theoretical method (M06-2X/ aug-cc-PVTZ) is comparable to that of CCSD(T)/aug-ccPVTZ//M05-2X/aug-cc-PVDZ. Thus, it is reasonable to directly compare the results of our calculations with the earlier reported data on the CI + NO reaction. The prereaction complex involved in the CI + NO reaction has a binding energy of 2.6 kcal/mol, which is quite similar to that of CI···CO (2.2 kcal/mol). Interestingly, the prereaction complex in the CI + NO reaction has an access to multiple paths of decomposition, whereas the Int1 of CI + CO reaction decomposes by only a single route. Moreover, the two decomposition routes for the CI + NO reaction characterized so far involve activation barriers of 5.8 and 8.1 kcal/mol, which are appreciably lower than that calculated for the CI + CO reaction (11.2 kcal/mol). This indicates that the reaction of CI with NO is significantly faster than with CO. The only two experimental studies of the CI + CO reaction reported in the literature are indirect ones. Su et al.28 found that the addition of CO to the reaction mixture in ethene ozonolysis accelerated the formation of formic acid anhydride (FAA). They attributed this to the CI + CO reaction leading directly to FAA, in contrast to the present findings that CO2 and aldehyde are the reaction products. They found the reaction to be relatively slow, estimating the relative rate constant to be 570 times slower than CI + SO2 and 140 times slower than CI + H2CO. This led Vereecken et al.24 to estimate the CI + CO rate constant as 3.6 × 10−14 cm3 molecule−1 s−1 based on established rate constants for CI reactions with aldehydes and ketones. This rate constant is significantly larger than that estimated in this work, as is perhaps evident from the significant barriers found for the CI + CO reaction. Gutbrod et al.29 examined the effect of CO addition in isoprene ozonolysis with in the context of OH radical generation. Their results, which were interpreted assuming that CI + CO produces an aldehyde or ketone and CO2, indicated that the reaction is slow relative to the CO reaction with OH, i.e., k ≪ 1 × 10−13 cm3 molecule−1 s−1; they did not estimate the CI + CO rate constant. This issue is discussed in more detail in section 2.5. 2.2. CH 3CHOO + CO Reaction and Conformer Dependence. We next investigated the same reaction for a larger CI, CH3CHOO, which has two conformers that interconvert slowly (theoretical calculations give a barrier of 20−25 kcal/mol for this conversion29) and thus react independently with CO. The reaction, irrespective of the CI conformation considered (syn or anti), follows the mechanistic pathway of the H2COO + CO reaction, as shown in Figure 2 and Tables S2−S3, Supporting Information. An interesting feature of the CH3CHOO + CO reaction is the discriminatory reactivity of the anti and syn conformers of CH3CHOO. Specifically, anti-CH3CHOO is 3.1 kcal/mol less stable than its syn analogue, but its reactant complex (Int1) is slightly better bound (by 0.4 kcal/mol) than that for syn-CH3CHOO, and its reaction barrier is significantly smaller, 9.8 versus 14.7 kcal/mol (relative to the separated reactants).

Figure 2. Same as that in Figure 1 but for the CH3CHOO + CO reaction involving both syn and anti forms of the CI. The zero of energy is chosen to be separated CO and syn-CH3CHOO.

Similar conformational dependence has been observed experimentally by Taatjes et al.,20 who found that antiCH3CHOO reacts faster than syn-CH3CHOO with both H2O and SO2. Though hyperconjugation has been used to explain this difference in anti and syn reactivity,20,30 the present calculations suggest that a strong contribution to this effect comes from the steric congestion around the CI carbon in the syn-CH3CHOO···CO complex that restricts the approach of the CO molecule even in the reactant complex, where R(CCI··· OCO) = 3.04 Å for syn-CH3CHOO compared to R(CCI···OCO) = 2.79 Å for anti-CH3CHOO. The steric interaction is also evident in the calculated transition state structure. In the calculated syn transition state R(CCI···OCO) = 2.15 Å, which is 0.05 Å longer than that in the anti transition state. Indication of this steric effect can also be seen by taking the anti-CH3CHOO TS geometry and switching the H and methyl group positions to examine how the syn-CH3CHOO would interact differently. The close approach of CO in the anti-CH3CHOO TS leads to an OCO···Hmethyl distance of 1.33 Å (Figure 3) when the synCH3CHOO conformation is superimposed in this way, well within the van der Waals radii. Similar effects have been observed in the DFT calculations on the reaction of

Figure 3. Transition state geometry for the reaction of antiCH3CHOO with CO when the H and methyl group positions are switched. 1889

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CH3CHOO with O3 (unpublished results). This insight into the conformer dependence of CH3CHOO + CO reaction could contribute to a better understanding of reactivity of other CIs, many of which involve significant steric bulk. It is worth noting that the atomic partial charges do not provide an explanation for the reactivity trends (see Table S5, Supporting Information). As noted above, Anglada et al. ascribed the difference in syn and anti CI reactivity to hyperconjugation effects,30 a factor also invoked in previous studies on isoprene ozonolysis.29,31 Anglada et al. used a natural bond orbital-based analysis to estimate the nπX → π*CO stabilization energies, where X represents one of the Criegee substituents, as an indication of the hyperconjugation effect.32 They found stabilization energies of 6.01 and 5.58 kcal/mol for the syn- and anti-methyl substituents, respectively, suggestive of lower reactivity of the syn conformer due to its greater stabilization. We note that this difference is small compared to that between the barrier heights (4.9 kcal/mol as shown in Figure 2) but is consistent with that expected for methyl substituents.32 Thus, it suggests a role for other interactions, such as steric effects, as noted by Anglada et al.30 The stabilization energies increase significantly for substituents with lone pairs,30,32 suggesting that hyperconjugation may play a larger role in those systems. To further support our steric viewpoint, next, we studied the reaction of (CH3)2COO with CO. The comparison of the reactions of (CH3)2COO and CH3CHOO with CO provide useful insights into the relative contributions of steric and hyperconjugation effects on the reaction barrier. In particular, if steric effects are dominant, similar barrier heights would be expected for (CH3)2COO and syn-CH3CHOO. However, the natural bond orbital calculations of Anglada et al. suggest that the stabilization energies for the two methyl substituents in (CH3)2COO are the same as each methyl in syn- and antiCH3CHOO. This indicates that the hyperconjugation effects in (CH3)2COO should be additive, stabilizing the molecule by ∼6 kcal/mol more than syn-CH3CHOO and making it significantly less reactive than even this conformer. Our results indicate this is not the case. Interestingly, the (CH3)2COO + CO reaction has a barrier, ΔE‡ = 13.5 kcal/mol, similar to, but smaller than, that for the syn-CH3CHOO reaction, ΔE‡ = 14.7 kcal/mol, and significantly higher than for the anti-CH3CHOO case, ΔE‡ = 9.8 kcal/mol. While these results do not eliminate a role for hyperconjugation, they implicate steric effects as the key driver of the conformer-dependent barriers and indicate that a methyl group in a syn position disfavors the attack of an incoming CO moiety on the carbonyl carbon of CI and raises the barrier height of the reaction by ∼4.0 kcal/mol (Figure 4). The slight difference between the syn-CH3CHOO and (CH3)2COO reaction barriers may be due to other interactions involving the anti-methyl group with the carbonyl oxygen of CI. It is also interesting to note that another Criegee intermediate reaction is consistent with the trends found in the present results. Anglada et al. found that the synCH3CHOO reaction with H2O has a barrier that is 6.5 kcal/ mol higher than the corresponding anti-CH3CHOO reaction. However, the same reaction with (CH3)2COO has an intermediate barrier that is only ∼1.8 kcal/mol smaller than for the syn-CH3CHOO case.30 2.3. Effect of Conjugation in the CI + CO Reaction. We also investigated the CI + CO reaction for simple conjugated CIs, (CH3)(CH2)C(CHOO). These CIs are important for nucleation in polluted atmospheric conditions33 as well as

Figure 4. Same as that in Figure 2 but for the (CH3)2COO + CO reaction.

synthesis, e.g., of D,L-camptothecin, a highly conjugated alkaloid that has high antitumor activity in cell lines and animal screens, that is produced in an inexpensive manner by ozonolysis.34 We considered three possible conformations for (CH3)(CH2)C(CHOO) including two rotameric forms (see Figure S2 and the Supporting Information for details). The reaction profiles are shown in Figures 5 and S3−S5 with energies given in Tables S6−S9, Supporting Information. The calculated data suggest that conjugation in the CI does not strongly affect the reaction barriers relative to the separated reactants, ΔE‡anti = 10.2 kcal/ mol, ΔE‡syn2 = 11.3 kcal/mol, and ΔE‡syn1 = 14.7 kcal/mol, comparable to the CH3CHOO case. These conjugated CIs exhibit the same conformation-dependent barrier as well, which can be attributed to steric effects analogous to those for CH3CHOO. If hyperconjugation were the primary origin of the differences in reactivity, the syn1 and syn2 rotameric forms would be expected to have similar barriers, which is not the case. The relatively higher reactivity of the syn2 CI with CO compared to the syn1 conformer can be attributed to the differences in interaction between the terminal O of the CI and the methyl or methylene group in the syn substituent. This interaction is stronger for the syn1 CI (Figure S2, Supporting Information), and thus, the O-atom transfer reaction with the syn2 CI is favored. 2.4. Biogenic-CI + CO Reactions. Finally, we examined the CI + CO reaction for the larger biogenic CIs that result from the olefinic cleavage of α-pinene, β-pinene, limonene, and 3-carene (Scheme 1 and Figure 6). These substrates constitute an important subclass of monoterpenes that have high emission rates into the atmosphere35−37 and a better knowledge of their chemistry may improve our mechanistic understanding of biogenic aerosol formation. Moreover, the terpenoids are used extensively in the fragrance and perfume industries. For 1890

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of biogenic CIs with CO is being reported here for the first time. We studied the reaction for all the possible CIs that are generated by the olefinic cleavage of a given terpene and observed the same reaction mechanism with CO, leading to CO2 and an aldehyde or ketone, in all cases. The ZPE-corrected barrier heights for the CIs investigated are summarized in Figure 6; reaction profiles and complete thermodynamic data are given in Figures S6−S14 and Tables S10−S17, Supporting Information. The behavior of these monoterpenes is consistent with that of the smaller CIs. The reactant complex of the CIs with CO is slightly more stabilized, by 2.5 to 4.5 kcal/mol, relative to the separated reactants compared to the smaller CIs in Figures 1, 2, and 4. The barriers are also on the same order as for the simpler CIs, ranging from 6.3 to 13.3 kcal/mol, as shown in Figure 6. The reactivity trends cannot be solely explained on the basis of steric arguments, though they appear to play a role. Of the eight monoterpene-derived CIs, five have a barrier of approximately 13 kcal/mol (within 0.3 kcal/mol). These five can be seen to have similar steric bulk based on the transition state structures (see Figures S7, S9−S11, and S13, Supporting Information). It is interesting then to consider the origin of the lower barriers for the other three biogenic CIs. The 3-carene case, CI13‑carene, has an exceptionally low barrier compared to the rest of the series. For this CI, the CO molecule can approach the central carbon from two different sides resulting in two possible Int1 conformations (Figure S12, Supporting Information). However, Int1 is found to be 1.6 kcal/mol more stable than Int1′ because the CO is stabilized by a favorable interaction with the terminal methyl group of the CI. This interaction also stabilizes the transition state, resulting in a significant lowering of the barrier height (ΔE‡ = 6.3 kcal/ mol). The next lowest barrier, ΔE‡ = 8.5 kcal/mol, is observed for the case of CI1α‑pinene, which exhibits little steric bulk around the central carbon as indicated by the relatively short R(CCI··· OCO) of 2.77 Å in Int1 and 2.11 Å in TS. Generally, CIs with hydrogen in the syn position have lower barriers, a key exception being CI1limonene, due to its three-dimensional structure. Finally, we note that there are two CIs that result from β-pinene cleavage: H2COO (see section 2.1) and CIβ‑pinene. The reaction of CIβ‑pinene is found to have a comparatively low barrier (ΔE1‡ = 9.5 kcal/mol; Figure S8 and Table S12, Supporting Information) despite the fact that it is disubstituted. Indeed, the transition state structure does not appear to differ in steric congestion significantly from the higher barrier biogenic CIs. This suggests an electronic effect for the barrier lowering for this cyclic CI that is not present for the other CI cases; this effect is smaller than those associated with the reduction of the steric congestion (CI1α‑pinene) or stabilization of the CO (CI13‑carene). 2.5. Rate Constant Estimations. It is instructive to consider the present results in the context of atmospheric and synthetic ozonolysis chemistry by estimating reaction rate constants using transition state theory (TST). As noted in section 2.1, the rate constant for the CI + CO reaction was previously estimated11 as kest(298 K) = 3.6 × 10−14 cm3 molecule−1 s−1 based on work by Su et al.28 This value arises from rate constants for CI reactions with aldehydes and ketones combined with the estimate of Su et al. that CO reacts 140 times slower than aldehyde.43 The TST calculations for the reaction pathway presented here suggest the reaction is much slower. For the simplest CI, H2COO, the rate constant is

Figure 5. Same as that in Figure 2 but for the (CH3)(CH2)C(CHOO) + CO reaction. The zero of energy is chosen to be separated CO and syn1-(CH3)(CH2)C(CHOO). Here, syn1 and syn2 refer to the CI conformations when the methylene and methyl hydrogen, respectively, are syn relative to the terminal CI oxygen.

Figure 6. Barrier heights for the CI + CO reaction examined here for the 14 CIs studied, from left to right: H2COO, syn-CH3CHOO, antiCH3CHOO, syn1-(CH3)(CH2)C(CHOO), syn2-(CH3)(CH2)C(CHOO), anti-(CH 3 )(CH2 )C(CHOO), CI1 α‑pinene, CI2α‑pinene, CIβ‑pinene, CI13‑carene, CI23‑carene, CI1limonene, CI2limonene, and CI3limonene (see Scheme 1).

example, (−)-3-isopropyl-6-oxoheptanal, a diketo compound, which is produced by the ozonolysis of the terpene, (+)-pmenth-1-ene, is used to make a variety of fragrant molecules.38 3-Hydroxycephem, an important intermediate for producing cephalosporin antibiotics (effective against infections) is synthesized by the ozonolysis of 3-exomethylene cephalosporin.39 The ozonolysis of terpenes has been extensively studied by theoretical means.12,40−42 However, the bimolecular chemistry 1891

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estimated to be kTST(298 K) = 6.4 × 10−23 cm3 molecule−1 s−1, roughly nine orders-of-magnitude smaller than the previous estimate. The reaction is predicted to be slow for all the CIs studied, with kTST(298 K) varying from 2.4 × 10−20 to 7.3 × 10−26 cm3 molecule−1 s−1 (Table S18, Supporting Information). The small rate constants are the result of the consequential barrier height (cf. for the barrierless CI + SO2 reaction,9 k = 3.9 × 10−11 cm3 molecule−1 s−1 was measured) as well as a nonnegligible, and negative, entropy of activation. For the simplest CI, the former lowers the rate constant by a factor of ∼108 relative to a barrierless reaction, while the latter lessens k by another order-of-magnitude. The consequence is that these results thus suggest that the CI + CO reaction is likely not a significant part of CI pathways in tropospheric chemistry, even after taking into account the comparatively high concentration of CO in some forested and urban settings. The story may be quite different in synthetic ozonolysis where the concentrations of reactants with the CI can be controlled, and CO may be useful in directing the product distributions to aldehydes or ketones rather than carboxylic acids. The activation energies for these CI + CO reactions also indicate that this pathway would increase dramatically in importance as the reaction temperature is elevated. One caveat is the potential effect of reaction of the CI with carbonyl compounds (aldehyde or ketone) or carboxylic acids produced in the ozonolysis. These reactions have been predicted24 to be faster than those with CO, but control of concentrations and temperature, informed by the present results for reaction barriers and rate constants, may provide routes for using CO to direct the chemistry. The present results do not directly address why the rate constant estimates based on the indirect measurements of Su et al.28 yield rate constants that are apparently significantly too large. It may be that CO plays other roles in ethene ozonolysis beyond reacting with the CI or that the CO reacts with the nascent energetic CI before it is thermalized. It is interesting to note that, in contrast to Su et al., the calculations here do not find formic acid anhydride as the product of the CI + CO reaction despite the fact that the approach for finding the transition state (see section 4) should reveal such a pathway. Moreover, our calculations indicate that the formation of FAA is 7.1 kcal/mol less-favored over that of CO2 and aldehyde. Our results are generally in accord with those of Gutbrod et al. who inferred a slow reaction for CI + CO based on studies of isoprene ozonolysis using CO as an OH scavenger.29 However, resolution of this issue will likely require additional theoretical and experimental investigations.

4. COMPUTATIONAL DETAILS All electronic structure calculations reported in the present work were carried out using the Gaussian09 quantum chemistry program.44 To explore the scope of the reaction, we have examined a variety of CIs ranging from the simplest ones, CH2OO and CH3CHOO, to those formed in the ozonolysis of biogenic terpenes (Scheme 1). Despite the fact that the terpenoids, with high emission rates into the troposphere,35 are important in the nucleation of aerosol particles36 and CO is a major tropospheric species, the reactions of biogenic CIs with CO have not been discussed in the literature to the best of our knowledge. Because CH3CHOO has two conformations (syn and anti) that act as distinct chemical entities at moderate temperatures,20 and the oxidative fission of the olefinic bond in terpenes can result in the formation of multiple, structurally different CIs, we have considered all of these possibilities. The stationary points involved in the CI + CO reactions, except for the biogenic CIs, have been calculated for the singlet state at the M06-2X/aug-cc-pVTZ level of theory. To probe the reaction mechanism, we first fully optimized the CI···CO complex. In the optimized complex, the oxygen in CO was directed toward the CI carbonyl carbon, while the terminal oxygen atom in the CI was proximal to the CO carbon. This viewpoint is also supported by a recent study of Vereecken et al.,24 who suggested that the reaction of CI with CO would most likely lead to a carbonyl compound and carbon dioxide. It is important to mention here that an alternate decomposition pathway for the reaction of CI with CO can lead to the formation of FAA. Though, we have not probed that mechanistic possibility since the formation of FAA would require the cleavage of a strong C−H bond, and thus, this reaction route is expected to be significantly slower than the one involving the formation of a carbonyl compound and CO2. We also optimized all the species involved along the reaction path of the CH2OO + CO reaction using RM06-2X/aug-ccpVTZ and UM06-2X/aug-cc-pVTZ levels of theory. However, the same final geometries were obtained in both cases, indicating that spin contamination is not an issue. For that reason, we have performed the entire computational analysis using RM06-2X/aug-cc-pVTZ level of theory. The authenticity of our chosen theoretical method has also been verified by calculating single-point energies for the stationary points of the CH2OO + CO reaction at the CCSD(T)/aug-cc-pVTZ level (see section 2.1 and Figure 1). For the reactions involving biogenic CIs with greater structural complexity, the slightly smaller aug-cc-pVDZ basis set was used. This is done to strike a balance between the computational cost and the accuracy of the method; single-point calculations with the larger aug-cc-pVTZ basis set found no significant changes in the calculated energetics. Harmonic vibrational frequencies were calculated for all the optimized structures to verify that each structure is either a global minima or a first order saddle point on the calculated potential energy surface. The electronic energies reported here are corrected for zero-point vibrational energy. The reaction rate constants were estimated using the standard transition state theory approximation:

3. CONCLUSIONS In summary, electronic−structure calculations suggest that the Criegee intermediate reaction with CO involves the transfer of an oxygen atom from the CI to form CO2 and an aldehyde or ketone. The reaction is estimated to be 6−12 orders-ofmagnitude slower than previously estimated24 and is likely too slow to compete with others, e.g., CI + H2O, in the troposphere but might be exploited to direct the reaction products of olefin ozonolysis toward aldehydes and ketones rather than carboxylic acids. The dependence of the reaction barrier on Criegee intermediate conformation, syn versus anti, that has been observed for other reactions,19,20,24 is also observed for the CO reaction. However, the origin is attributable primarily to steric effects rather than hyperconjugation, as previously proposed.

k TST(T ) =

kBT (Q*/V ) e−β ΔE * h (Q CI/V )(Q CO/V )

where kB is Boltzmann’s constant, T is temperature, β = 1/kBT, h is Planck’s constant, ΔE‡ is the zero-point corrected barrier 1892

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height (relative to the separated reactants), and Q‡/V, QCI/V, and QCO/V are the partition functions per unit volume for the transition state, Criegee intermediate, and CO molecule, respectively, using a harmonic approximation for the vibrational degrees-of-freedom. Note that the rate constants were estimated assuming a bimolecular reaction, i.e., the free reactants, rather than the Int1 structures being considered in these estimates. Anharmonicity associated with low frequency modes may be significant for the larger CIs considered but is expected to be a smaller factor than that associated with typical errors in the electronic structure estimates of ΔE‡.45

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ASSOCIATED CONTENT

S Supporting Information *

Reaction profiles, optimized conformations, thermodynamic data, and estimated rate constants. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(W.H.T.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Dr. Michael Lundin and Dr. Andrew Danby for many useful discussions. This work is funded by USDA grant no. 2011-10006-30362.

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