Criegee Intermediates React with Ozone - The ... - ACS Publications

Jul 18, 2013 - Absolute energies and structures; relative energies; IRC curves; reaction kinetics; energies and structure of SP1; harmonic and anharmo...
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Criegee Intermediates React with Ozone Henrik G. Kjaergaard,*,† Theo Kurtén,‡ Lasse B. Nielsen,† Solvejg Jørgensen,† and Paul O. Wennberg§,∥ †

Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark Laboratory of Physical Chemistry, Department of Chemistry, P.O. BOX 55, University of Helsinki, FI-00014, Finland § Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, United States ∥ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, United States ‡

S Supporting Information *

ABSTRACT: We have investigated the reaction of the one-carbon stabilized Criegee intermediate (H2COO, formaldehyde oxide) with ozone, theoretically, using high level coupled cluster ab initio methods. Key to the reactivity of the Criegee intermediate with ozone is the strongly exothermic formation of an intermediate consisting of five oxygen and one carbon atoms (H2CO5) in a six-membered ring structure. This intermediate proceeds via a spin-allowed route over two transition states with low energy barriers to form molecular oxygen and formaldehyde. The reaction may contribute to the loss of these biradicals in the atmosphere.

SECTION: Environmental and Atmospheric Chemistry, Aerosol Processes, Geochemistry, and Astrochemistry

O

of SO2 with several alkene derived sCI, at 1 atm. pressure and 293 K. The earlier study by Johnson et al.10 reported an even lower value of 4 × 10−15 molecule−1 cm3 s−1 for the rate constant of the reaction of SO2 with sCI formed in the ozonolysis of 2-methylbut-2-ene at 295 K and 1 atm, measured by monitoring the concentration of a tracer that reacted with the OH produced in unimolecular decomposition of the CI and sCI.10 The Welz et al. measurement is a direct measurement of the sCI + SO2 reaction rate, whereas the other measurements are indirect in either measuring H2SO4 formation or OH formation. The experiment of Welz et al. yielded higher rate constants than previous experimental studies, not only for the sCI + SO2 reaction, but also for the reactions of sCI with NO2 and carbonyl compounds.9,12−14 For the reaction of sCI with H2O, so far only upper limits of the rate constant have been determined experimentally.5,10 The bimolecular reaction mechanisms and energetics are different for various sCI. Recently, the rate constants for the reaction of syn- and antiCH3CHCOO (anti is faster) with SO2 were measured to differ by a factor of about 3, with the rate constant of the H2COO + SO2 reaction between the other two rate constants.5,14 Thus, the large difference in rates between, for example, the Welz et al. and Johnson et al. studies are likely explained by the difference in measuring technique rather than the precise nature of the sCI used.5,10 Berndt et al.11 and Mauldin et al.3 measured channel specific rate coefficients for the SO3 + carbonyl channel, via detection

ur understanding of the complexity of atmospheric oxidation chemistry is undergoing a major transformation. Historically, it was thought that almost all oxidation reactions are initiated by a very small number of chemical species: OH radicals, NO3 radicals, O3, and halogen atoms such as Cl or Br. Recently, laboratory and field studies as well as theoretical calculations have suggested that a new member should be added to the list: the Criegee intermediate (CI).1−6 In the atmosphere, CIs are formed in the ozonolysis of alkenes:7 R1R 2C=CR3R 4 + O3 → → R1R 2COO + R3R 4CO

where R1...R4 denote arbitrary functional groups, R1R2COO is a CI, and R3R4CO is an aldehyde or a ketone. The CIs are formed with a broad distribution of excess vibrational energy, and may either undergo unimolecular isomerization or decomposition reactions often leading to the formation of OH radicals, or be collisionally stabilized, forming stabilized Criegee intermediates (sCIs).8,9 Even sCIs may still react via unimolecular channels, but their lifetimes are long enough for bimolecular reactions with trace gases such as H2O, SO2, NO2 or carbonyl compounds to be competitive. Several recent studies have suggested that the reaction of sCI with SO2 is faster than previously expected, and may therefore contribute to the formation of H2SO4 in the atmosphere, thus influencing atmospheric aerosol production.3,5,10,11 However, rate constants reported by these different studies differ significantly. Welz et al.5 measured a rate constant of 3.9 × 10−11 molecule−1 cm3 s−1 for the H2COO + SO2 reaction at 4 Torr total pressure and 298 K, while Berndt et al.11 and Mauldin et al.3 estimate rate constants about 2 orders of magnitude lower for the formation of H2SO4 from the reaction © 2013 American Chemical Society

Received: June 11, 2013 Accepted: July 18, 2013 Published: July 18, 2013 2525

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of H2SO4 that is rapidly formed from SO3 with excess H2O. This is not necessarily the total rate. If, as suggested by Veerecken et al., a significant percentage of the H2COO + SO2 reactions lead to other stable products (denoted secondary ozonides by them, and cyclic adducts in our earlier study), then these experiments would underestimate the total rate.2,4 In addition, the lower pressures in the Welz et al. experiment mean that their value is a lower limit of the rate constant at the atmospheric pressure of the other experiments.5 One additional possible reason for the difference in the measured rate constants for ozone-free experiments of Welz et al.5 (H2COO produced from a CH2I + O2 reaction) and rates measured in experiments where the sCI is generated by ozonolysis is the possible reaction of sCI with ozone. To our knowledge, there are no previous studies on the reactions of Criegee intermediates with ozone. Previously, Kroll et al. considered the possibility that ozone might react with sCI, but noted that in their experiment ozone concentrations were too low to react significantly with the sCI even if the rate was fast.15 Despite both sCI and ozone being relatively reactive molecules, it might seem reasonable to discount such a reaction for two reasons: First, if the sCI + O3 reaction proceeds analogously to other studied bimolecular sCI reactions (such as those with SO2, NO2 or H2O), it requires the formation of an intermediate species with four or five oxygen atoms (and one carbon) in a ring, which, based on chemical intuition, would seem like a unlikely or unstable molecule. Second, many of the potential thermodynamically favorable sCI + O3 reaction channels involve the formation of ground-state (triplet) molecular oxygen (3O2), and could thus be expected to be spin-forbidden and hence slow. The stationary points identified for the reaction of H2COO with O3 are shown in Figure 1, and the corresponding reaction

Figure 2. Relative energies (ΔE) and Gibbs free energies (ΔG, 298K) for the reaction of H2COO with O3 with a loss of two O2 to form formaldehyde. The UHF-UCCSD(T)/aug-cc-pVTZ//UB3LYP/augcc-pVTZ energies are corrected for zero point vibrational energy with the B3LYP/aug-cc-pVTZ harmonic frequencies. The rate of this reaction is determined by the barrier to SP1. However, the existence of SP1 and hence the barrier height, could not be unambiguously determined.

(ΔG) is shown in Figure 2, and the zero point vibrational energies (ZPVE) are given in the Supporting Information. In the following discussion, values correspond to UHF-UCCSD(T)/aug-cc-pVTZ//UB3LYP/aug-cc-pVTZ energy unless otherwise noted. Formation of the interesting singlet cyclic adduct (RAO5) with five oxygen atoms in the same ring was found to be exothermic by almost 50 kcal/mol (ΔG = 32 kcal/mol). The partial biradical nature of the two compounds reacting, H2COO and O3, lead elegantly to the formation of two bonds, O−O and C−O, and the formation of the cyclic adduct with a structure much like that of the chair form of cyclohexane. Despite extensive searches, we were unable to conclusively determine whether a saddle point (SP1, Figure 2) leading to RAO5 exists, see later discussion and Supporting Information. Initially, we thought that the reaction of H2COO with O3 would proceed analogously to that with inert molecules such as CO2, and searched for saddle points regenerating O3 together with formic acid (HCOOH).16,17 However, we were unable to find such a saddle point, as all the saddle points structures we found corresponded to O2 rather than O3 dissociation. This seemed problematic, as the ground state of O2 is a triplet, and a singlet reactant cannot dissociate to a triplet and a singlet product without the inherently slow intersystem crossing from

Figure 1. Mechanism for the H2COO + O3 reaction. Triplet structures are indicated by a square box around them.

energies are given in Table 1 (and Table S1). The energy diagram for both electronic energy (ΔE) and Gibbs free energy Table 1. Energies (kcal/mol) of the H2COO + O3 Reaction geometry

UB3LYP/aVTZ

UHF-UCCSD(T)/VDZ

compound

B3LYPa

F12b

RCCSD(T)d

UCCSD(T)c

UCCSD(T)/VDZe

UCCSD(T)/aVTZf

H2COO+O3 RAO5 SP2 RIO3 + 3O2 SP3 + 3O2 H2CO + 2 3O2

0.0 −44.1 −36.2 −59.2 −57.6 −96.2

0.0 −46.0 −38.5 −45.6 −34.6 −86.8

0.0 −47.8 −40.2 −48.0 −43.5 −88.0

0.0 −47.9 −33.8 −48.1 −43.7 −88.2

0.0 −44.4 −35.4 −55.8 −48.8 −100.7

0.0 −47.3 −32.8 −48.3 −40.9 −87.7

a

UB3LYP/aug-cc-pVTZ optimized geometry. bRHF-UCCSD(T)-F12a/VDZ-F12 single point energy calculated at the UB3LYP/aug-cc-pVTZ optimized geometry. cUHF-UCCSD(T)/aug-cc-pVTZ single point energy calculated at the UB3LYP/aug-cc-pVTZ optimized geometry. dRHFRCCSD(T)/aug-cc-pVTZ single point energy calculated at the UB3LYP/aug-cc-pVTZ optimized geometry. eUHF-UCCSD(T)/cc-pVDZ optimized geometry. fUHF-CCSD(T)/aug-cc-pVTZ single point at the UHF-UCCSD(T)/cc-pVDZ optimized geometry. 2526

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stable, compared to the RAO5 for the H2COO + O3 reaction, but still energetically favorable by about 25 kcal/mol compared to the reactants (see Supporting Information). Based on estimates of the possible SP1 for this reaction, it is expected to be slower than the H2COO reaction as expected for a larger sCI. A fast sCI + O3 reaction may have significant implications for interpretation of previous studies of sCI chemistry. Because many previous studies of sCI have been performed in high O3 environments, this chemistry may have been a significant interference. For example, it may influence the estimated yield of OH radicals from decomposition of the sCI. To the extent that this chemistry is competitive with H2O reactions in laboratory systems, the products of the sCI would lead to more formaldehyde and less formic acid and hydroxymethylhydroperoxide. To assess the atmospheric importance of the sCI+O3 reaction, we compare the reaction of H2COO with H2O, SO2, and O3. Anti-CH3CHOO + H2O reaction was recently measured to have a rate constant of 1 × 10−14 molecule−1 cm3 s−1.14 The H2COO + H2O is predicted theoretically to be 50 times slower.6 The atmospheric H2O concentration is very dependent on temperature. If we assume a rate constant for the H2COO + H2O reaction of 2 × 10−16 molecule−1 cm3 s−1 and 50% RH, then the first order loss rate of sCI to H2O would range from about 1 s−1 at 225 K to 1000 s−1 at 298 K. The rate of the H2COO + SO2 reaction is 3.9 × 10−11 molecule−1 cm3 s−1.5 For typical atmospheric concentrations of SO2, 0.01−10 ppb,22 sCI will react with SO2 at a rate in the range of 0.01−10 s−1. If the proposed reaction of H2COO + O3 has a rate as slow as our suggested lower limit, it will be unimportant in the atmosphere. If the rate constant is much faster, i.e., 10−13 molecule−1 cm3 s−1, sCI would be lost to O3 at 0.1 s−1 for typical concentrations of 30 ppb ozone. Thus the sCI + H2O reaction is likely the main bimolecular “sink reaction” for sCI as has been suggested.14 Only at very high altitude, where H2O is low, could the reaction of O3 with sCI play any important role in the atmosphere. In summary, we have found, theoretically, that the reaction of the one carbon Criegee intermediate (H2COO) with ozone (O3) leads to formation of formaldehyde and two 3O2 molecules. Key to this reactivity of sCI with O3 is the strongly exothermic formation of a remarkable intermediate consisting of five oxygen and one carbon atoms in a six-membered ring structure.

a singlet to the triplet potential energy surface (via spin−orbit coupling). This apparent issue was resolved when we found that the ground state of the OCH2OO species formed (RIO3) in the dissociation of RAO5 is a triplet rather than a singlet, and the dissociation of a singlet into two triplets is spin-allowed. The energy difference between the triplet and singlet ground state of RIO3 is 0.3 kcal/mol at the UB3LYP/aVTZ level. The saddle point (SP2) leading to 3O2 and RIO3 is 14 kcal/mol above the RAO5 intermediate, while the RIO3 + 3O2 products are essentially at the same energy as RAO5 (see Table 1 and Figure 2). RIO3 can then further decompose via a low-lying saddle point (SP3) into formaldehyde (H2CO, singlet) and another 3 O2. The saddle point SP3 lies only 4 kcal/mol above RIO3, while the 3O2 + H2CO products are 40 kcal/mol below it. The level of spin contamination and multireference character are indicated by the ⟨S2⟩ and T1 diagnostic values which are given in Table S2 and S3. It is worth noting that, while SP2 is significantly spin contaminated prior to spin annihilation (not surprising as it corresponds to a singlet splitting into two triplets), all of the triplet structures have B3LYP ⟨S2⟩ values very close to the ideal value of 2. T1 diagnostic values for several of the structures are relatively high, indicating that the coupled-cluster energies should be considered qualitative rather than quantitative. We have estimated the reaction rates for each barrier using the Multiwell program suite assuming a pressure of 1 atm and temperature of 298K.18 The net result is that the rate of the sCI + O3 reaction is controlled solely by the formation of the RAO5 adduct. The SP2 and SP3 barriers are low enough that no stabilization of the RAO5 will occur at 1 atm. Despite extensive searches (constrained optimizations and scans) at the UB3LYP/6-31G(d) level, we were unable to find a transition state connecting the free reactants to the RAO5 adduct. However, with the spin-restricted RB3LYP/aug-ccpVTZ method, we located a first order saddle point with a C (H2COO) to nearest O (ozone), RC···O, distance of 2.0 Å Figure S3). However, this was not a stationary point at the UB3LYP/aug-cc-pVTZ level. Geometry optimizations at higher levels methods (CASSCF, CASPT2, CCSD(T)) did not unambiguously resolve this issue (see Supporting Information). We also included CCSDT single point energy calculations (where the triple excitations are treated nonperturbatively) as these have been shown recently to improve CCSD(T) energies at intermediate distances corresponding to bond dissociations.19 The saddle points found with the multireference methods (CASSCF, CASPT2) do suggest the existence of SP1. The range of results in Tables S5 and S6 suggest that the barrier to SP1, if this exists, is relatively small in the range of 0.5−4 kcal/mol for the electronic energy with a Gibbs free energy in the range from 12 to 15.5 kcal/mol. (see Supporting Information). Based on this 4 kcal/mol upper limit of the barrier to SP1, we find that the rate of the H2COO + O3 reaction is faster than 10−18 cm3 molecule−1 s−1, with the rate being highly dependent on more accurate calculations for the potential SP1 followed by more accurate rate calculation. We suggest that a more precise determination of the size of the barrier height would require large scale multi reference calculations, which should be followed by a more precise reaction rate calculation using variational TST methods.20,21 To investigate the reactivity of ozone with a larger sCI, we performed calculations on the (CH3)2COO sCI and found that the equivalent RAO5 adduct also exists for this sCI. It is less

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THEORETICAL METHODS All scans and initial optimizations were done using the B3LYP hybrid density functional with the standard 6-31G(d), 631+G(d) and/or 6-31+G(d,p) double-ζ basis sets. The optimized structures were then further refined using B3LYP with the aug-cc-pVTZ triple-ζ basis set (aVTZ). All UB3LYP calculations correspond to spin-unrestricted calculations, where the spin symmetry of the initial guess has been explicitly broken using the Gaussian 09 keyword Guess=(Mix,Always) to ensure that the UB3LYP solution is found.4 The UB3LYP/aVTZ optimized structures were also optimized with the UHFUCCSD(T)/cc-pVDZ method. Harmonic vibrational frequencies were calculated for all B3LYP/aVTZ optimized structures to confirm that each structure is either a minimum or a firstorder saddle point (SP, one imaginary frequency). For saddle points SP2 and SP3, intrinsic reaction coordinate (IRC) calculations were performed at the UB3LYP/aVTZ level to 2527

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Notes

confirm that the presented SP indeed connected to the desired reactants and products (Figure S1 and S2). For H2COO, O3, and RAO5, anharmonic vibrational frequencies were calculated with the UB3LYP/aVTZ method to facilitate possible detection of RAO5. Single-point energies were computed at the UB3LYP/aVTZ geometries using three different coupled-cluster methods: UHF-UCCSD(T)/aug-cc-pVTZ, RHF-RCCSD(T)/aug-ccpVTZ, and RHF-UCCSD(T)-F12a/VDZ-F12. The CCSD(T)-F12/VDZ-F12 method has been shown to be similar to the accurate and well-recognized CCSD(T)/aug-cc-pVQZ level of theory.23,24 In addition UHF-UCCSD(T)/aug-cc-pVTZ single point energies were computed at the UCCSD(T)/cc-pVDZ geometries. Zero point vibrational energies (ZPVE) were calculated with the UB3LYP/aVTZ harmonic frequencies. Spin contamination values ⟨S2⟩ and T1 diagnostics have been computed for all structures and are given in the Supporting Information. The CCSD(T)-F12 calculations were carried out using the MOLPRO2012 program suite, while the B3LYP and CCSD(T) calculations were carried out using Gaussian 09.25 The Cartesian coordinates of all studied structures, optimized at the UB3LYP/aug-cc-pVTZ level, as well as computed absolute energies, enthalpies, and free energies are given in the Supporting Information. Visualizations of molecular structures and orbitals were carried out using GaussView 5.0.26 Default energy and geometry convergence criteria were used, except for the steplength and gradient in MOLPRO calculations, which were chosen to be 10−4 a.u. Thermal contributions to enthalpies and free energies were computed using the standard rigid rotor and harmonic oscillator approximations. For the possible SP1 (and the reactants CI and O3), additional calculations were performed. Single point calculations at the full T coupled cluster level, RHF-RCCSDT/ccpVDZ, were performed with CFOUR using default convergence criteria.27 Harmonic and anharmonic vibrational frequencies were calculated for the RB3LYP/aVTZ optimized saddle point SP1. Recently, it has been shown that the CCSDT method performs better (by several kcal/mol for F2 and H2O2) than the CCSD(T) method at intermediate dissociation distances, whereas at the equilibrium distance and at long distances the two methods give very similar energies.19 Multireference methods, complete active space self-consistent field (CASSCF), and second-order perturbation theory (CASPT2) with the 6-31+G(d) basis set were performed with MOLPRO and default convergence criteria.28 The active space was chosen based on CISD natural orbital occupation numbers and are shown in the Supporting Information.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Rasmus Faber and Sten Rettrup for helpful discussions. We thank The Danish Council for Independent Research - Natural Sciences, NASA (NNX08AD29G), and the NSF (ATM-0934408) for funding and the Danish Center for Scientific Computing (DCSC) and CSC Finland for computer resources.

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(1) Jiang, L.; Xu, Y. S.; Ding, A. Z. Reaction of Stabilized Criegee Intermediates from Ozonolysis of Limonene with Sulfur Dioxide Ab Initio and DFT Study. J. Phys. Chem. A 2010, 114, 12452−12461. (2) Kurten, T.; Lane, J. R.; Jorgensen, S.; Kjaergaard, H. G. A Computational Study of the Oxidation of SO2 to SO3 by Gas-Phase Organic Oxidants. J. Phys. Chem. A 2011, 115, 8669−8681. (3) Mauldin, R. L., III; Berndt, T.; Sipila, M.; Paasonen, P.; Petaja, T.; Kim, S.; Kurten, T.; Stratmann, F.; Kerminen, V. M.; Kulmala, M. A New Atmospherically Relevant Oxidant of Sulphur Dioxide. Nature 2012, 488, 193−196. (4) Vereecken, L.; Harder, H.; Novelli, A. The Reaction of Criegee Intermediates with NO, RO2, and SO2, and Their Fate in the Atmosphere. Phys. Chem. Chem. Phys. 2012, 14, 14682−14695. (5) Welz, O.; Savee, J. D.; Osborn, D. L.; Vasu, S. S.; Percival, C. J.; Shallcross, D. E.; Taatjes, C. A. Direct Kinetic Measurements of Criegee Intermediate (CH2OO) Formed by Reaction of CH2I with O2. Science 2012, 335, 204−207. (6) Anglada, J. M.; Gonzalez, J.; Torrent-Sucarrat, M. Effects of the Substituents on the Reactivity of Carbonyl Oxides. A Theoretical Study on the Reaction of Substituted Carbonyl Oxides with Water. Phys. Chem. Chem. Phys. 2011, 13, 13034−13045. (7) Criegee, R. Mechanism of Ozonolysis. Angew. Chem., Int. Ed. Engl. 1975, 14, 745−752. (8) Donahue, N. M.; Drozd, G. T.; Epstein, S. A.; Presto, A. A.; Kroll, J. H. Adventures in Ozoneland: Down the Rabbit-Hole. Phys. Chem. Chem. Phys. 2011, 13, 10848−10857. (9) Johnson, D.; Marston, G. The Gas-Phase Ozonolysis of Unsaturated Volatile Organic Compounds in the Troposphere. Chem. Soc. Rev. 2008, 37, 699−716. (10) Johnson, D.; Lewin, A. G.; Marston, G. The Effect of CriegeeIntermediate Scavengers on the OH Yield from the Reaction of Ozone with 2-Methylbut-2-ene. J. Phys. Chem. A 2001, 105, 2933−2935. (11) Berndt, T.; Jokinen, T.; Mauldin, R. L.; Petaja, T.; Herrmann, H.; Junninen, H.; Paasonen, P.; Worsnop, D. R.; Sipila, M. Gas-Phase Ozonolysis of Selected Olefins: The Yield of Stabilized Criegee Intermediate and the Reactivity toward SO2. J. Phys. Chem. Lett. 2012, 3, 2892−2896. (12) Hatakeyama, S.; Akimoto, H. Reactions of Criegee Intermediates in the Gas-Phase. Res. Chem. Intermed. 1994, 20, 503−524. (13) Taatjes, C. A.; Welz, O.; Eskola, A. J.; Savee, J. D.; Osborn, D. L.; Lee, E. P. F.; Dyke, J. M.; Mok, D. W. K.; Shallcross, D. E.; Percival, C. J. Direct Measurement of Criegee Intermediate (CH2OO) Reactions with Acetone, Acetaldehyde, and Hexafluoroacetone. Phys. Chem. Chem. Phys. 2012, 14, 10391−10400. (14) Taatjes, C. A.; Welz, O.; Eskola, A. J.; Savee, J. D.; Scheer, A. M.; Shallcross, D. E.; Rotavera, B.; Lee, E. P. F.; Dyke, J. M.; Mok, D. K. W.; et al. Direct Measurements of Conformer-Dependent Reactivity of the Criegee Intermediate CH3CHOO. Science 2013, 340, 177−180. (15) Kroll, J. H.; Clarke, J. S.; Donahue, N. M.; Anderson, J. G.; Demerjian, K. L. Mechanism of HOx Formation in the Gas-Phase Ozone-Alkene Reaction. 1. Direct, Pressure-Dependent Measurements of Prompt OH Yields. J. Phys. Chem. A 2001, 105, 1554−1560. (16) Aplincourt, P.; Ruiz-Lopez, M. F. Theoretical Study of Formic Acid Anhydride Formation from Carbonyl Oxide in the Atmosphere. J. Phys. Chem. A 2000, 104, 380−388.

ASSOCIATED CONTENT

S Supporting Information *

Absolute energies and structures; relative energies; IRC curves; reaction kinetics; energies and structure of SP1; harmonic and anharmonic vibrational frequencies; MOs included in CAS; relative energies of the (CH3)2COO + O3 reaction. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 45-35320322. Tel.: 4535320334. 2528

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