criegee intermediate reaction with co: mechanism, barriers, conformer-dependence, and implications...

Criegee Intermediate Reaction with CO: Mechanism, Barriers,Conformer-Dependence, and Implications for Ozonolysis ChemistryManoj 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 constantcalculations have been performed to gain insight into the bimolecular reaction of theCriegee intermediate (CI) with carbon monoxide (CO) that is proposed to beimportant in both atmospheric and industrial chemistry. A new mechanism is suggestedin which the CI acts as an oxidant by transferring an oxygen atom to the CO, resulting inthe formation of a carbonyl compound (aldehyde or ketone depending upon the CI)and carbon dioxide. Fourteen different CIs, including ones resulting from biogenicozonolysis, are considered. Consistent with previous reports for other CI bimolecularreactions, the anti conformers are found to react faster than the syn conformers.However, this can be attributed to steric effects and not hyperconjugation as generallyinvoked. 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 thanpreviously reported literature estimates. The reaction is thus expected to be unimportantin the context of tropospheric oxidation chemistry. However, the reaction mechanismsuggests that CO could be exploited in ozonolysis to selectively obtain industrially important carbonyl compounds.

1. INTRODUCTION

Criegee intermediates (CIs) are highly reactive carbonyl oxidesthat play a key role in atmospheric and condensed-phaseozonolysis of unsaturated hydrocarbons.1 Importantly, a CIformed during ozonolysis can engage in bimolecular chemistryif appropriate reaction partners are available.2 Under atmos-pheric conditions, the reaction of CIs with water is consideredto be a major CI decay process,3 though other reactions canalso be relevant.4−11 The fate of the CI is critical intropospheric oxidation chemistry, particularly aerosol forma-tion.12,13 In synthetic applications of ozonolysis, where acleaner and greener approach for producing functionalizedorganics (e.g., aldehydes, ketones, and carboxylic acids) issought, the oxidation products may be controlled by reactingthe CI with well-designed partners.14,15 CIs have also beenimplicated as the key intermediates in the catalytic cycles offlavin-dependent Baeyer−Villiger monooxygenases (BVMOs)16

that provide an environmentally benign alternative to theconventional Baeyer−Villiger reaction based on enhancedenantio- and regioselectivity. Baeyer−Villiger reactions17 areof significant synthetic value because ketones are converted intoesters or lactones using stoichiometric amounts of hydrogenperoxide, peracids, or alkylhydroperoxides. A clear under-standing of Criegee chemistry is thus crucial in understanding(or manipulating) the outcomes of these widely varyingchemical processes.Recent spectroscopic characterization of the CI18 and the

direct kinetic measurements of bimolecular Criegee reac-

tions19,20 are also contributing to the growing interest inCriegee chemistry. Su et al.18 recently measured the infraredspectrum of the simplest CI (H2COO) in the gas phase andtheir spectral interpretations favor a zwitterionic CI rather thana biradical one. Welz et al.19 have synthesized and detected gasphase H2COO species using a photoinization technique thatallowed the direct kinetic measurements of its reactions withNO, NO2, H2O, and SO2. In a recent study, they applied thistechnique to the conformer-dependent reactions of the nextlarger CI (CH3CHOO) with SO2, NO2, and H2O.

20 In all thestudied reactions, anti-CH3CHOO is found to be more reactivethan syn-CH3CHOO, a result they attribute to hyper-conjugation.In the condensed phase, the dominant reaction of CI is the

recombination reaction with the carbonyl compound due to thecage effect.21,22 However, in the gas phase, a significant fractionof CI is vibrationally stabilized and can take part in a variety ofunimolecular or bimolecular reactions.23 The scope of abimolecular Criegee reaction depends upon several factors:the nature of the olefin, reaction conditions, and the availabilityof potent coreactants. As the second most abundantatmospheric coreactant (behind water) in forested regions aswell as urban and polluted environments,24 carbon monoxide(CO) is a potential coreactant for study in bimolecular CI

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

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© 2014 American Chemical Society 1887 dx.doi.org/10.1021/jp500258h | J. Phys. Chem. A 2014, 118, 1887−1894

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reactions. The average concentration of CO has been estimatedto be on the order of 1012−1013 molecules/cm3 in theseregions.25 Moreover, in synthetic ozonolysis, the reactionpartners of the CI can be selected and potentially designed tooptimize the reaction product distribution. This provides asignificant impetus to widely explore bimolecular CI chemistryand 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 hasnot been extensively discussed in the literature. The aim is tounderstand the mechanism along with the associated reactionbarrier heights and estimated rate constants. In the following,the mechanism is discussed in the context of the reactioninvolving the simplest CI, H2COO, in section 2.1. Theconformer dependence discussed above is then examined insection 2.2 for the next simplest CIs, syn- and anti-CH3CHOO,and its origin is investigated. The role of conjugation isinvestigated in section 2.3. The reactions of several biogenicterpenoids relevant to atmospheric chemistry, shown inScheme 1, are discussed in section 2.4. Estimated rate constants

from transition state theory are presented in section 2.5 beforeconcluding remarks in section 3. The details of the computa-tional approach, which is based on density functional theorycalculations, are described in section 4.

2. RESULTS AND DISCUSSION2.1. H2COO + CO Reaction. The calculated zero-point

energy (ZPE) corrected electronic energy profile for thereaction of the simplest CI, H2COO, with CO is presented inFigure 1 along with the optimized geometries of the stationarypoints along the reaction pathway. The calculated IRC path forthe reaction is shown in Figure S1, Supporting Information. Inorder to verify the accuracy of our theoretical method,CCSD(T)/aug-cc-PVTZ//M06-2X/aug-cc-PVTZ calculatedenergies are also shown in parentheses. The energies calculatedusing the M06-2X and CCSD(T)//M06-2X methods differ

only slightly, and thus in the following only results obtainedusing the M06-2X/aug-cc-PVTZ level of theory are presented.The corresponding reaction energies, barrier heights, en-thalpies, and free energies are given in Table S1, SupportingInformation.The H2COO + CO reaction first proceeds by formation of a

weakly bound, ΔE = −2.2 kcal/mol, H2COO···CO reactantcomplex (Int1) with R(OCO···CCI) = 2.84 Å. The involvementof analogous weak complexes in other CI reactions haspreviously been reported.26 Then, Int1 evolves via the transitionstate (TS) to form a formaldehyde and CO2 complex. Theimaginary frequency vibrational mode of TS (ω‡ = 442.4icm−1) corresponds to the transfer of the terminal oxygen of theCI to the CO carbon; for TS, R(CCO···OCI) = 1.77 Å, and theO−O bond distance in the CI is 1.38 Å. The structures showthat the OCO···CCI distance in TS (2.08 Å) is shorter by ∼0.76Å compared to Int1, suggesting that strengthening of the CI···Cinteraction is key for facilitating O-atom transfer. The TS leadsto a H2CO···CO2 product complex that is highly exoergiccompared to the reactants (ΔE = −125.0 kcal/mol); thiscomplex is bound by weak dipolar and van der Waalsintermolecular interactions between CO2 and formaldehydeand has a binding energy of 2.4 kcal/mol relative to isolatedproducts. The calculated exothermicity, ΔHrxn = −122.6 kcal/mol, is in reasonable agreement with the empirically estimatedheat of reaction27 of −120.6 kcal/mol. Overall, this oxidationreaction involves an activation barrier of 11.2 kcal/mol (13.4kcal/mol) relative to the separated reactants (reactantcomplex).It is interesting to compare this mechanism with the H2COO

+ NO reaction, which has been previously reported.24 The

Scheme 1. Biogenic Criegee Intermediates (CIs)Investigated in the Current Study

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

The Journal of Physical Chemistry A Article

dx.doi.org/10.1021/jp500258h | J. Phys. Chem. A 2014, 118, 1887−18941888

reaction with NO proceeds through two additional inter-mediates before the release of the aldehyde and NO2;

24 in thecase of CO, these intermediates are not stable and the reactionis direct. In order to facilitate the comparison with the previousresults that were calculated using a different method (CCSD-(T)/aug-cc-PVTZ//M05-2X/aug-cc-PVDZ), we have esti-mated the barrier height of the preliminary step of CI + NOreaction (5.3 kcal/mol), which is only 0.5 kcal/mol differentfrom the previously reported value of 5.8 kcal/mol indicatingthat the accuracy of our selected theoretical method (M06-2X/aug-cc-PVTZ) is comparable to that of CCSD(T)/aug-cc-PVTZ//M05-2X/aug-cc-PVDZ. Thus, it is reasonable todirectly compare the results of our calculations with the earlierreported data on the CI + NO reaction. The prereactioncomplex involved in the CI + NO reaction has a binding energyof 2.6 kcal/mol, which is quite similar to that of CI···CO (2.2kcal/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 asingle route. Moreover, the two decomposition routes for theCI + NO reaction characterized so far involve activationbarriers of 5.8 and 8.1 kcal/mol, which are appreciably lowerthan that calculated for the CI + CO reaction (11.2 kcal/mol).This indicates that the reaction of CI with NO is significantlyfaster 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 thatthe addition of CO to the reaction mixture in ethene ozonolysisaccelerated the formation of formic acid anhydride (FAA).They attributed this to the CI + CO reaction leading directly toFAA, in contrast to the present findings that CO2 and aldehydeare the reaction products. They found the reaction to berelatively slow, estimating the relative rate constant to be 570times slower than CI + SO2 and 140 times slower than CI +H2CO. This led Vereecken et al.

24 to estimate the CI + CO rateconstant as 3.6 × 10−14 cm3 molecule−1 s−1 based onestablished rate constants for CI reactions with aldehydes andketones. This rate constant is significantly larger than thatestimated in this work, as is perhaps evident from the significantbarriers found for the CI + CO reaction. Gutbrod et al.29

examined the effect of CO addition in isoprene ozonolysis within the context of OH radical generation. Their results, whichwere interpreted assuming that CI + CO produces an aldehydeor ketone and CO2, indicated that the reaction is slow relativeto the CO reaction with OH, i.e., k ≪ 1 × 10−13 cm3

molecule−1 s−1; they did not estimate the CI + CO rateconstant. This issue is discussed in more detail in section 2.5.2.2. CH3CHOO + CO Reaction and Conformer

Dependence. We next investigated the same reaction for alarger CI, CH3CHOO, which has two conformers thatinterconvert slowly (theoretical calculations give a barrier of20−25 kcal/mol for this conversion29) and thus reactindependently with CO. The reaction, irrespective of the CIconformation considered (syn or anti), follows the mechanisticpathway of the H2COO + CO reaction, as shown in Figure 2and Tables S2−S3, Supporting Information. An interestingfeature of the CH3CHOO + CO reaction is the discriminatoryreactivity of the anti and syn conformers of CH3CHOO.Specifically, anti-CH3CHOO is 3.1 kcal/mol less stable than itssyn analogue, but its reactant complex (Int1) is slightly betterbound (by 0.4 kcal/mol) than that for syn-CH3CHOO, and itsreaction barrier is significantly smaller, 9.8 versus 14.7 kcal/mol(relative to the separated reactants).

Similar conformational dependence has been observedexperimentally by Taatjes et al.,20 who found that anti-CH3CHOO reacts faster than syn-CH3CHOO with bothH2O and SO2. Though hyperconjugation has been used toexplain this difference in anti and syn reactivity,20,30 the presentcalculations suggest that a strong contribution to this effectcomes from the steric congestion around the CI carbon in thesyn-CH3CHOO···CO complex that restricts the approach ofthe 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 alsoevident in the calculated transition state structure. In thecalculated syn transition state R(CCI···OCO) = 2.15 Å, which is0.05 Å longer than that in the anti transition state. Indication ofthis steric effect can also be seen by taking the anti-CH3CHOOTS geometry and switching the H and methyl group positionsto examine how the syn-CH3CHOO would interact differently.The close approach of CO in the anti-CH3CHOO TS leads toan OCO···Hmethyl distance of 1.33 Å (Figure 3) when the syn-CH3CHOO conformation is superimposed in this way, wellwithin the van der Waals radii. Similar effects have beenobserved in the DFT calculations on the reaction of

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

Figure 3. Transition state geometry for the reaction of anti-CH3CHOO with CO when the H and methyl group positions areswitched.



CH3CHOO with O3 (unpublished results). This insight intothe conformer dependence of CH3CHOO + CO reaction couldcontribute to a better understanding of reactivity of other CIs,many of which involve significant steric bulk. It is worth notingthat the atomic partial charges do not provide an explanationfor the reactivity trends (see Table S5, Supporting Informa-tion).As noted above, Anglada et al. ascribed the difference in syn

and anti CI reactivity to hyperconjugation effects,30 a factor alsoinvoked in previous studies on isoprene ozonolysis.29,31

Anglada et al. used a natural bond orbital-based analysis toestimate the nπX → π*CO stabilization energies, where Xrepresents one of the Criegee substituents, as an indication ofthe hyperconjugation effect.32 They found stabilization energiesof 6.01 and 5.58 kcal/mol for the syn- and anti-methylsubstituents, respectively, suggestive of lower reactivity of thesyn conformer due to its greater stabilization. We note that thisdifference is small compared to that between the barrier heights(4.9 kcal/mol as shown in Figure 2) but is consistent with thatexpected for methyl substituents.32 Thus, it suggests a role forother interactions, such as steric effects, as noted by Anglada etal.30 The stabilization energies increase significantly forsubstituents with lone pairs,30,32 suggesting that hyper-conjugation 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 thereactions of (CH3)2COO and CH3CHOO with CO provideuseful insights into the relative contributions of steric andhyperconjugation effects on the reaction barrier. In particular, ifsteric effects are dominant, similar barrier heights would beexpected for (CH3)2COO and syn-CH3CHOO. However, thenatural bond orbital calculations of Anglada et al. suggest thatthe stabilization energies for the two methyl substituents in(CH3)2COO are the same as each methyl in syn- and anti-CH3CHOO. This indicates that the hyperconjugation effects in(CH3)2COO should be additive, stabilizing the molecule by ∼6kcal/mol more than syn-CH3CHOO and making it significantlyless reactive than even this conformer. Our results indicate thisis not the case. Interestingly, the (CH3)2COO + CO reactionhas a barrier, ΔE‡ = 13.5 kcal/mol, similar to, but smaller than,that for the syn-CH3CHOO reaction, ΔE‡ = 14.7 kcal/mol, andsignificantly higher than for the anti-CH3CHOO case, ΔE‡ =9.8 kcal/mol. While these results do not eliminate a role forhyperconjugation, they implicate steric effects as the key driverof the conformer-dependent barriers and indicate that a methylgroup in a syn position disfavors the attack of an incoming COmoiety on the carbonyl carbon of CI and raises the barrierheight of the reaction by ∼4.0 kcal/mol (Figure 4). The slightdifference between the syn-CH3CHOO and (CH3)2COOreaction barriers may be due to other interactions involvingthe 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 inthe present results. Anglada et al. found that the syn-CH3CHOO 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 anintermediate barrier that is only ∼1.8 kcal/mol smaller thanfor the syn-CH3CHOO case.30

2.3. Effect of Conjugation in the CI + CO Reaction. Wealso investigated the CI + CO reaction for simple conjugatedCIs, (CH3)(CH2)C(CHOO). These CIs are important fornucleation in polluted atmospheric conditions33 as well as

synthesis, e.g., of D,L-camptothecin, a highly conjugated alkaloidthat has high antitumor activity in cell lines and animal screens,that is produced in an inexpensive manner by ozonolysis.34 Weconsidered three possible conformations for (CH3)(CH2)C-(CHOO) including two rotameric forms (see Figure S2 and theSupporting Information for details). The reaction profiles areshown in Figures 5 and S3−S5 with energies given in TablesS6−S9, Supporting Information. The calculated data suggestthat conjugation in the CI does not strongly affect the reactionbarriers 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 CIsexhibit the same conformation-dependent barrier as well, whichcan be attributed to steric effects analogous to those forCH3CHOO. If hyperconjugation were the primary origin of thedifferences in reactivity, the syn1 and syn2 rotameric formswould be expected to have similar barriers, which is not thecase. The relatively higher reactivity of the syn2 CI with COcompared to the syn1 conformer can be attributed to thedifferences in interaction between the terminal O of the CI andthe methyl or methylene group in the syn substituent. Thisinteraction is stronger for the syn1 CI (Figure S2, SupportingInformation), and thus, the O-atom transfer reaction with thesyn2 CI is favored.

2.4. Biogenic-CI + CO Reactions. Finally, we examinedthe CI + CO reaction for the larger biogenic CIs that resultfrom the olefinic cleavage of α-pinene, β-pinene, limonene, and3-carene (Scheme 1 and Figure 6). These substrates constitutean important subclass of monoterpenes that have high emissionrates into the atmosphere35−37 and a better knowledge of theirchemistry may improve our mechanistic understanding ofbiogenic aerosol formation. Moreover, the terpenoids are usedextensively in the fragrance and perfume industries. For

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



example, (−)-3-isopropyl-6-oxoheptanal, a diketo compound,which is produced by the ozonolysis of the terpene, (+)-p-menth-1-ene, is used to make a variety of fragrant molecules.38

3-Hydroxycephem, an important intermediate for producingcephalosporin antibiotics (effective against infections) issynthesized by the ozonolysis of 3-exomethylene cephalospor-in.39 The ozonolysis of terpenes has been extensively studied bytheoretical means.12,40−42 However, the bimolecular chemistry

of biogenic CIs with CO is being reported here for the firsttime. We studied the reaction for all the possible CIs that aregenerated by the olefinic cleavage of a given terpene andobserved the same reaction mechanism with CO, leading toCO2 and an aldehyde or ketone, in all cases. The ZPE-correctedbarrier heights for the CIs investigated are summarized inFigure 6; reaction profiles and complete thermodynamic dataare given in Figures S6−S14 and Tables S10−S17, SupportingInformation.The behavior of these monoterpenes is consistent with that

of the smaller CIs. The reactant complex of the CIs with CO isslightly more stabilized, by 2.5 to 4.5 kcal/mol, relative to theseparated reactants compared to the smaller CIs in Figures 1, 2,and 4. The barriers are also on the same order as for the simplerCIs, ranging from 6.3 to 13.3 kcal/mol, as shown in Figure 6.The reactivity trends cannot be solely explained on the basis ofsteric arguments, though they appear to play a role. Of the eightmonoterpene-derived CIs, five have a barrier of approximately13 kcal/mol (within 0.3 kcal/mol). These five can be seen tohave similar steric bulk based on the transition state structures(see Figures S7, S9−S11, and S13, Supporting Information). Itis interesting then to consider the origin of the lower barriersfor 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 COmolecule can approach the central carbon from two differentsides resulting in two possible Int1 conformations (Figure S12,Supporting Information). However, Int1 is found to be 1.6kcal/mol more stable than Int1′ because the CO is stabilized bya favorable interaction with the terminal methyl group of theCI. This interaction also stabilizes the transition state, resultingin 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 aroundthe 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 withhydrogen in the syn position have lower barriers, a keyexception being CI1limonene, due to its three-dimensionalstructure. Finally, we note that there are two CIs that resultfrom β-pinene cleavage: H2COO (see section 2.1) andCIβ‑pinene. The reaction of CIβ‑pinene is found to have acomparatively low barrier (ΔE1‡ = 9.5 kcal/mol; Figure S8and Table S12, Supporting Information) despite the fact that itis disubstituted. Indeed, the transition state structure does notappear to differ in steric congestion significantly from thehigher barrier biogenic CIs. This suggests an electronic effectfor the barrier lowering for this cyclic CI that is not present forthe other CI cases; this effect is smaller than those associatedwith the reduction of the steric congestion (CI1α‑pinene) orstabilization of the CO (CI13‑carene).

2.5. Rate Constant Estimations. It is instructive toconsider the present results in the context of atmospheric andsynthetic ozonolysis chemistry by estimating reaction rateconstants using transition state theory (TST). As noted insection 2.1, the rate constant for the CI + CO reaction waspreviously estimated11 as kest(298 K) = 3.6 × 10−14 cm3

molecule−1 s−1 based on work by Su et al.28 This value arisesfrom rate constants for CI reactions with aldehydes and ketonescombined with the estimate of Su et al. that CO reacts 140times slower than aldehyde.43 The TST calculations for thereaction pathway presented here suggest the reaction is muchslower. 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 andsyn1-(CH3)(CH2)C(CHOO). Here, syn1 and syn2 refer to the CIconformations 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 forthe 14 CIs studied, from left to right: H2COO, syn-CH3CHOO, anti-CH3CHOO, syn1-(CH3)(CH2)C(CHOO), syn2-(CH3)(CH2)C-(CHOO), anti-(CH3)(CH2)C(CHOO), CI1α‑pinene, CI2α‑pinene,CIβ‑pinene, CI13‑carene, CI23‑carene, CI1limonene, CI2limonene, and CI3limonene(see Scheme 1).



estimated to be kTST(298 K) = 6.4 × 10−23 cm3 molecule−1 s−1,roughly nine orders-of-magnitude smaller than the previousestimate. The reaction is predicted to be slow for all the CIsstudied, 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 non-negligible, and negative, entropy of activation. For the simplestCI, the former lowers the rate constant by a factor of ∼108relative to a barrierless reaction, while the latter lessens k byanother order-of-magnitude. The consequence is that theseresults thus suggest that the CI + CO reaction is likely not asignificant part of CI pathways in tropospheric chemistry, evenafter taking into account the comparatively high concentrationof CO in some forested and urban settings. The story may bequite different in synthetic ozonolysis where the concentrationsof reactants with the CI can be controlled, and CO may beuseful in directing the product distributions to aldehydes orketones rather than carboxylic acids. The activation energies forthese CI + CO reactions also indicate that this pathway wouldincrease dramatically in importance as the reaction temperatureis elevated. One caveat is the potential effect of reaction of theCI with carbonyl compounds (aldehyde or ketone) orcarboxylic acids produced in the ozonolysis. These reactionshave been predicted24 to be faster than those with CO, butcontrol of concentrations and temperature, informed by thepresent results for reaction barriers and rate constants, mayprovide 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 etal.28 yield rate constants that are apparently significantly toolarge. It may be that CO plays other roles in ethene ozonolysisbeyond reacting with the CI or that the CO reacts with thenascent energetic CI before it is thermalized. It is interesting tonote that, in contrast to Su et al., the calculations here do notfind formic acid anhydride as the product of the CI + COreaction despite the fact that the approach for finding thetransition state (see section 4) should reveal such a pathway.Moreover, our calculations indicate that the formation of FAAis 7.1 kcal/mol less-favored over that of CO2 and aldehyde. Ourresults are generally in accord with those of Gutbrod et al. whoinferred a slow reaction for CI + CO based on studies ofisoprene ozonolysis using CO as an OH scavenger.29 However,resolution of this issue will likely require additional theoreticaland experimental investigations.

3. CONCLUSIONS

In summary, electronic−structure calculations suggest that theCriegee intermediate reaction with CO involves the transfer ofan oxygen atom from the CI to form CO2 and an aldehyde orketone. The reaction is estimated to be 6−12 orders-of-magnitude slower than previously estimated24 and is likely tooslow to compete with others, e.g., CI + H2O, in the tropospherebut might be exploited to direct the reaction products of olefinozonolysis toward aldehydes and ketones rather than carboxylicacids.The dependence of the reaction barrier on Criegee

intermediate conformation, syn versus anti, that has beenobserved for other reactions,19,20,24 is also observed for the COreaction. However, the origin is attributable primarily to stericeffects rather than hyperconjugation, as previously proposed.

4. COMPUTATIONAL DETAILSAll electronic structure calculations reported in the presentwork were carried out using the Gaussian09 quantum chemistryprogram.44 To explore the scope of the reaction, we haveexamined a variety of CIs ranging from the simplest ones,CH2OO and CH3CHOO, to those formed in the ozonolysis ofbiogenic terpenes (Scheme 1). Despite the fact that theterpenoids, with high emission rates into the troposphere,35 areimportant in the nucleation of aerosol particles36 and CO is amajor tropospheric species, the reactions of biogenic CIs withCO have not been discussed in the literature to the best of ourknowledge. Because CH3CHOO has two conformations (synand anti) that act as distinct chemical entities at moderatetemperatures,20 and the oxidative fission of the olefinic bond interpenes can result in the formation of multiple, structurallydifferent 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 singletstate at the M06-2X/aug-cc-pVTZ level of theory. To probe thereaction mechanism, we first fully optimized the CI···COcomplex. In the optimized complex, the oxygen in CO wasdirected toward the CI carbonyl carbon, while the terminaloxygen atom in the CI was proximal to the CO carbon. Thisviewpoint is also supported by a recent study of Vereecken etal.,24 who suggested that the reaction of CI with CO wouldmost likely lead to a carbonyl compound and carbon dioxide. Itis important to mention here that an alternate decompositionpathway for the reaction of CI with CO can lead to theformation of FAA. Though, we have not probed thatmechanistic possibility since the formation of FAA wouldrequire the cleavage of a strong C−H bond, and thus, thisreaction route is expected to be significantly slower than theone 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-cc-pVTZ 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 thatreason, we have performed the entire computational analysisusing RM06-2X/aug-cc-pVTZ level of theory. The authenticityof our chosen theoretical method has also been verified bycalculating single-point energies for the stationary points of theCH2OO + CO reaction at the CCSD(T)/aug-cc-pVTZ level(see section 2.1 and Figure 1). For the reactions involvingbiogenic CIs with greater structural complexity, the slightlysmaller aug-cc-pVDZ basis set was used. This is done to strike abalance between the computational cost and the accuracy of themethod; single-point calculations with the larger aug-cc-pVTZbasis set found no significant changes in the calculatedenergetics.Harmonic vibrational frequencies were calculated for all the

optimized structures to verify that each structure is either aglobal minima or a first order saddle point on the calculatedpotential energy surface. The electronic energies reported hereare corrected for zero-point vibrational energy. The reactionrate constants were estimated using the standard transitionstate theory approximation:

=* β− Δ *k T

k Th

Q VQ V Q V

( )( / )

( / )( / )e E

TSTB

CI CO

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



height (relative to the separated reactants), and Q‡/V, QCI/V,and QCO/V are the partition functions per unit volume for thetransition state, Criegee intermediate, and CO molecule,respectively, using a harmonic approximation for the vibrationaldegrees-of-freedom. Note that the rate constants wereestimated assuming a bimolecular reaction, i.e., the freereactants, rather than the Int1 structures being considered inthese estimates. Anharmonicity associated with low frequencymodes may be significant for the larger CIs considered but isexpected to be a smaller factor than that associated with typicalerrors in the electronic structure estimates of ΔE‡.45

■ ASSOCIATED CONTENT

*S Supporting InformationReaction profiles, optimized conformations, thermodynamicdata, and estimated rate constants. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

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

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

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

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