Revealing new pathways for the reaction of Criegee intermediate CH2OO with SO2.

Criegee intermediates play an important role in the tropospheric oxidation models through their reactions with atmospheric trace chemicals. We develop a global full-dimensional potential energy surface for the CH2OO + SO2 system and reveal how the reaction happens step by step by quasi-classical trajectory simulations. A new pathway forming the main products (CH2O + SO3) and a new product channel (CO2 + H2 + SO2) are predicted in our simulations. The new pathway appears at collision energies greater than 10 kcal/mol whose behavior demonstrates a typical barrier-controlled reaction. This threshold is also consistent with the ab initio transition state barrier height. For the minor products, a loose complex OCH2O ∙ ∙ ∙ SO2 is formed first, and then in most cases it soon turns into HCOOH + SO2, in a few cases it decomposes into CO2 + H2 + SO2 which is a new product channel, and rarely it remains as ∙OCH2O ∙ + SO2.

First-principles mode-specific reaction dynamics.

Controlling the outcome of chemical reactions by exciting specific vibrational and/or rotational modes of the reactants is one of the major goals of modern reaction dynamics studies. In the present Perspective, we focus on first-principles vibrational and rotational mode-specific dynamics computations on reactions of neutral and anionic systems beyond six atoms such as X + C2H6 [X = F, Cl, OH], HX + C2H5 [X = Br, I], OH- + CH3I, and F- + CH3CH2Cl. The dynamics simulations utilize high-level ab initio analytical potential energy surfaces and the quasi-classical trajectory method. Besides initial state specificity and the validity of the Polanyi rules, mode-specific vibrational-state assignment for polyatomic product species using normal-mode analysis and Gaussian binning is also discussed and compared with experiment.

Full-dimensional automated potential energy surface development and detailed dynamics for the CH2OO + NH3 reaction.

With the help of the ROBOSURFER program package, a global full-dimensional potential energy surface (PES) for the reaction of the Criegee intermediate, CH2OO, with the NH3 molecule is developed iteratively using different ab initio methods and the monomial symmetrization fitting approach. The final permutationally-invariant analytical PES is constructed based on 23447 geometries and the corresponding ManyHF-based CCSD(T)-F12b/cc-pVTZ-F12 energies. The accuracy of the PES is confirmed by the excellent agreement of its stationary-point properties and one-dimensional potential energy curves compared with the corresponding ab initio data. The reaction probabilities and integral cross sections are calculated for the ground-state and several vibrationally excited-state reactions by quasi-classical trajectory simulations. Remarkable is that the maximum impact parameter b where reactivity vanishes is almost independent of collision energy ranging from 1 to 40 kcal mol-1, and the reaction probability increases with increasing collision energy for this negative-barrier reaction. At the same time, a slight mode-specificity effect is observed. In addition, the deuterium effect is investigated and the sudden vector projection is discussed.

Competition between the H-abstraction and the X-abstraction pathways in the HX (X = Br, I) + C2H5 reactions.

The recently-developed high-level full-dimensional spin-orbit-corrected potential energy surfaces based on ManyHF-UCCSD(T)-F12a/cc-pVDZ-F12 + SOcorr(MRCI-F12+Q(5,3)/cc-pVDZ-F12) (cc-pVDZ-PP-F12 for the Br and I atoms) energy points for the reactions of HX (X = Br, I) with C2H5 are improved by adding three to four thousand new geometries with higher energies at the same ab initio level to cover a higher-energy range. Quasi-classical trajectory simulations in the 30-80 kcal mol-1 collision energy range on the new surfaces are performed and show that as collision energy increases, the reaction probability of the submerged-barrier H-abstraction reaction pathway decreases a bit but the reactivity of the X-abstraction reaction, which has an apparent barrier, increases significantly, which leads to the co-domination of the two reaction pathways at high collision energies. The excitation in HX vibrational mode helps both reaction pathways, but more for X-abstraction. The mode-specific excitations in C2H5 inhibit the H-abstraction, especially for CH2 wagging mode, but almost no effect is found for X-abstraction. The deuterium effect is similar for both pathways. The sudden vector projection model can only predict the HX-stretching vibrational enhancements in X-abstraction. Forward/backward scattering is favored for H/X-abstraction, indicating the dominance of the direct stripping/rebound mechanism. The decrease of reactivity for the H-abstraction reaction pathway partly comes from the fact that the H-abstraction is much pickier about the initial attack angle. The reactivity of both reaction pathways increases when side-on CH3CH2 attack happens. The major part of the initial translational energy is preserved as translational energy in the products in H-abstraction, while for X-abstraction a large amount of it is transferred into the internal energy of C2H5X.

Vibrational mode-specific quasi-classical trajectory studies for the two-channel HI + C2H5 reaction.

We report a detailed dynamics study on the mode-specificity of the HI + C2H5 two-channel reaction (H-abstraction and I-abstraction), through performing quasi-classical trajectory computations on a recently developed high-level ab initio full-dimensional spin-orbit-corrected potential energy surface, by exciting four different vibrational modes of reactants at five collision energies. The effect of the normal-mode excitations on the reactivity, the mechanism, and the post-reaction energy flow is investigated. Both reaction pathways are intensely promoted when the HI-stretching mode is excited while the excitations imposed on C2H5 somewhat surprisingly inhibit the dominant H-abstraction reaction pathway. The enhancement effect of the excitation in the HI vibrational mode is found to be much more effective than increasing the translational energy, similar to the HBr + C2H5 reaction. Not like the Br-abstraction pathway, however, the I-abstraction reaction pathway could be comparable to the dominant H-abstraction reaction pathway. The dominance of the direct stripping mechanism is indicated in H-abstraction while the direct rebound mechanism is observed in I-abstraction. The H-abstraction is much pickier about the initial attack angle distributions for HI than I-abstraction is, which leads to a decrease in reactivity in the H-abstraction reaction pathway. The dominance of side-on CH3CH2 attack in I-abstraction is more obvious than in the case of H-abstraction. In the case of the H-abstraction reaction pathway, the major part of the initial translational energy ends up in translational recoil, while for I-abstraction most energy excites the product C2H5I.

Theoretical vibrational mode-specific dynamics studies for the HBr + C2H5 reaction.

A quasi-classical trajectory (QCT) study is performed for the HBr + C2H5 multi-channel reaction using a recently-developed high-level ab initio full-dimensional spin-orbit-corrected potential energy surface (PES) by exciting five different vibrational modes of reactants at five collision energies. The effect of the normal-mode excitations on the reactivity, the mechanism, and the post-reaction energy flow is followed. A significant decrease of the reactivity caused by the longer initial distances of the reactants for the vHBr = 1 reaction at low collision energy (Ecoll) is observed due to the intramolecular vibrational-energy redistribution and the classical nature of the QCT method. All of the three reaction pathways (H-abstraction, Br-abstraction, and H-exchange) are intensely promoted when the HBr-stretching mode is excited. No clear promotion is observed when excitation is imposed to C2H5 except that asymmetric CH-stretching helps the H-exchange process. The enhancement effect of the excitation in the HBr vibrational mode is found to be much more effective than increasing the translational energy, in contrast to the HBr + CH3 reaction. The forward scattering mechanism can be clearly promoted by the excitation of the HBr-stretching mode, or by the high collision energy, indicating the dominance of the direct stripping mechanism in these cases. At low collision energy with no excitation or excitation of any vibrational mode of C2H5, the forward scattering feature is less obvious. At Ecoll = 1 kcal mol-1, when HBr-stretching is excited, the product clearly gains more relative translational energy. However, it is interesting to see that when the excitation is in C2H5, the effect is the opposite, i.e., the product gains less relative translational energy compared to the ground-state reaction.

Automated full-dimensional potential energy surface development and quasi-classical dynamics for the HI(X1Σ+) + C2H5 → I(2P3/2) + C2H6 reaction.

A full-dimensional spin-orbit-corrected analytical coupled-cluster-quality potential energy surface (PES) is developed for the HI(X1Σ+) + C2H5 → I(2P3/2) + C2H6 reaction using the ROBOSURFER program package, and a quasi-classical trajectory (QCT) study on the new PES is reported. The stationary-point relative energies obtained on the PES reproduce well the benchmark values. Our simulations show that in the 0.5-40 kcal mol-1 collision energy (Ecoll) range, the b = 0 reaction probability, where b denotes the impact parameter, increases first and then stays steady with increasing Ecoll, reaching around 10% when Ecoll = 5 kcal mol-1. No significant Ecoll dependence is observed in the range of 5-40 kcal mol-1. The reaction probabilities decrease monotonically with increasing b, and the maximum b where the reactivity vanishes becomes smaller and smaller as Ecoll increases. Scattering angle distributions show a forward scattering preference, indicating the dominance of the direct stripping mechanism, which is more obvious than in the case of HBr + C2H5 → Br + C2H6. The reaction clearly favors H-side attack over side-on HI and the least-preferred I-side approach, and favors side-on CH3CH2 attack marginally over CH2-side and the least-preferred CH3-side approach at high Ecoll. At low Ecoll, however, the dominant effect of H-side attack becomes weaker, while the side-on CH3CH2 attack becomes comparable with CH2-side and the former is a little less favored when Ecoll = 0.5 kcal mol-1. It turns out that the initial translational energy is converted mostly into product recoil, whereas the reaction energy excites the C2H6 vibration. The vibrational and rotational distributions of the C2H6 product slightly blue-shift as Ecoll increases, and none of the reactive trajectories violates the zero-point energy (ZPE) constraint. The energy transfer in the HI + C2H5 → I + C2H6 reaction is very similar to the case in the HBr + C2H5 → Br + C2H6 system that we investigated recently.

Full-dimensional potential energy surface development and dynamics for the HBr + C2H5 → Br(2P3/2) + C2H6 reaction.

We report a full-dimensional spin-orbit-corrected analytical potential energy surface (PES) for the HBr + C2H5 → Br + C2H6 reaction and a quasi-classical dynamics study on the new PES. For the PES development, the ROBOSURFER program package is applied and the ManyHF-based UCCSD(T)-F12a/cc-pVDZ-F12(-PP) energy points are fitted using the permutationally-invariant monomial symmetrization approach. The spin-orbit coupling at the level of MRCI-F12+Q(5,3)/cc-pVDZ-F12(-PP) is taken into account, since it has a significant effect in the exit channel of this reaction. Our simulations show that in the 1-40 kcal mol-1 collision energy (Ecoll) range the b = 0 reaction probability increases first and then decreases with increasing Ecoll, reaching around 15% at the medium Ecoll. No significant Ecoll dependence is observed in the range of 5-20 kcal mol-1. The reaction probabilities decrease monotonically with increasing b and the maximum b where reactivity vanishes is smaller and smaller as Ecoll increases. Unlike in the case of HBr + CH3, the integral cross-section decays sharply as Ecoll changes from 5 to 1 kcal mol-1. Scattering angle distributions usually show forward scattering preference, indicating the dominance of the direct stripping mechanism. The reaction clearly favors H-side attack over side-on HBr and the least-preferred Br-side approach, and favors side-on CH3CH2 attack over the CH2-side and the least-preferred CH3-side approach. The initial translational energy turns out to convert mostly into product recoil, whereas the reaction energy excites the C2H6 vibration. The vibrational and rotational distributions of the C2H6 product slightly blue-shift as Ecoll increases, and very few reactive trajectories violate zero-point energy.

Surprisingly long lifetime of methacrolein oxide, an isoprene derived Criegee intermediate, under humid conditions.

Ozonolysis of isoprene, the most abundant alkene, produces three distinct Criegee intermediates (CIs): CH2OO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). The oxidation of SO2 by CIs is a potential source of H2SO4, an important precursor of aerosols. Here we investigated the UV-visible spectroscopy and reaction kinetics of thermalized MACRO. An extremely fast reaction of anti-MACRO with SO2 has been found, kSO2 = (1.5 ± 0.4) × 10-10 cm3 s-1 (±1σ, σ is the standard deviation of the data) at 298 K (150 - 500 Torr), which is ca. 4 times the value for syn-MVKO. However, the reaction of anti-MACRO with water vapor has been observed to be quite slow with an effective rate coefficient of (9 ± 5) × 10-17 cm3 s-1 (±1σ) at 298 K (300 to 500 Torr), which is smaller than current literature values by 1 or 2 orders of magnitude. Our results indicate that anti-MACRO has an atmospheric lifetime (best estimate ca. 18 ms at 298 K and RH = 70%) much longer than previously thought (ca. 0.3 or 3 ms), resulting in a much higher steady-state concentration. Owing to larger reaction rate coefficient, the impact of anti-MACRO on the oxidation of atmospheric SO2 would be substantial, even more than that of syn-MVKO.

Effects of water vapor on the reaction of CH2OO with NH3.

The reaction of the simplest Criegee intermediate, CH2OO, with ammonia and water vapor has been investigated at 278-308 K and under 100-760 Torr by monitoring the strong UV absorption of CH2OO. We found that the observed decay rate of CH2OO becomes much larger when ammonia and water vapor are both present; the combinational effect of ammonia and water vapor is significantly greater than the sum of their individual contributions, revealing a strong synergic effect. The kinetic data are consistent with a termolecular process of CH2OO + NH3 + H2O reaction, of which the reaction rate coefficient was determined to be kNH3+H2O = (8.2 ± 1.2) × 10-31 cm6 s-1 at 298 K with a negative activation energy, Ea = -8.0 ± 0.8 kcal mol-1 [kNH3+H2O(T) = 1.04 × 10-36 exp(4047/T)]. Quantum chemistry calculation (at the QCISD(T)/aug-cc-pVTZ//B3LYP/6-311+G(2d,2p) level) found a low-energy reaction pathway, on which water accepts a hydrogen atom (or proton) from ammonia and releases another hydrogen atom to the terminal oxygen of CH2OO. The predicted products are H2NCH2OOH and a new H2O molecule, indicating water catalysis. This reaction is very fast and probably barrierless, which poses a theoretical challenge to modeling the related kinetics.

Hydrogen-Bonding Mediated Reactions of Criegee Intermediates in the Gas Phase: Competition between Bimolecular and Termolecular Reactions and the Catalytic Role of Water.

Criegee intermediates have substantial Zwitterionic character and interact strongly with hydrogen-bonding molecules like H2O, NH3, CH3OH, etc. Some of the observed reactions between Criegee intermediates and hydrogen-bonding molecules exhibit third-order kinetics. The experimental data indicate that these termolecular reactions involve one Criegee intermediate and two hydrogen-bonding molecules; quantum chemistry calculation shows that one of the hydrogen-bonding molecules acts as a catalytic bridge, which receives a hydrogen atom and donates another one. In this Feature Article, we will discuss the roles of the hydrogen-bonding molecules and the trend of the reactivity for the title reactions. To better predict the competition between a catalyzed reaction (a termolecular process) and its bare reaction (a bimolecular process), we analyzed the free energy landscape of the competing reaction paths under pseudo-first-order conditions. The results indicate that the entropy reduction in the translational degrees of freedom is the main cause to hinder a catalyzed termolecular process under typical experimental concentrations at near ambient temperatures. For such a termolecular process to be significant, its energy gain (barrier lowering) by adding the catalytic molecule has to be large enough to compensate the corresponding entropy cost. One great advantage of this analysis is that the translational entropy only depends on simple parameters like temperature, reactant masses, and concentrations and thus can be easily estimated.

Temperature and isotope effects in the reaction of CH3CHOO with methanol.

Carbonyl oxides, also known as Criegee intermediates, are generated from ozonolysis of unsaturated hydrocarbons in the atmosphere. Alcohols are often used as a scavenger of the Criegee intermediates in laboratory studies. In this work, the reaction kinetics of CH3CHOO with methanol vapor was investigated at various temperatures, pressures, and isotopic substitutions using time-resolved UV absorption spectroscopy. The observed rate coefficients of the reaction of anti-CH3CHOO with methanol show a linear dependence on [CH3OH]. The bimolecular rate coefficient was determined to be k1Ha = (4.8 ± 0.5) × 10-12 cm3 s-1 at 298 K and 250 Torr with a negative activation energy Ea = -2.8 ± 0.3 kcal mol-1 for T = 288-315 K [k(T) = A exp(-Ea/RT)]. For the reaction of syn-CH3CHOO with methanol vapor, the observed rate coefficients show a quadratic dependence on [CH3OH], indicating that two methanol molecules participate in the reaction. The termolecular rate coefficient was determined to be k2Hs = (8.0 ± 1.0) × 10-32 cm6 s-1 at 298 K and 250 Torr with a strong negative temperature dependence (Ea = -13.2 ± 0.3 kcal mol-1) at 273-323 K. No significant pressure effect was observed at 250-760 Torr. A kinetic isotope effect, k2Hs/k2Ds = 2.5, was observed by changing CH3OH to CH3OD. Quantum chemistry and transition state theory calculations suggest that the observed isotope effect is mainly attributed to the changes of the vibrational zero-point energies and partition functions while tunneling plays a very minor role. The reaction of syn-CH3CHOO with one CH3OH molecule was not observed in the studied concentration range.

Temperature-Dependent Rate Coefficient for the Reaction of CH3SH with the Simplest Criegee Intermediate.

The kinetics of the reaction of the simplest Criegee intermediate CH2OO with CH3SH was measured with transient IR absorption spectroscopy in a temperature-controlled flow reaction cell, and the bimolecular rate coefficients were measured from 278 to 349 K and at total pressure from 10 to 300 Torr. The measured bimolecular rate coefficient at 298 K and 300 Torr is (1.01 ± 0.17) × 10-12 cm3 s-1. The results exhibit a weak negative temperature dependence: the activation energy Ea ( k = Ae- Ea/ RT) is -1.83 ± 0.05 kcal mol-1, measured at 30 and 100 Torr. Quantum chemistry calculations of the reaction rate coefficient at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level (1.6 × 10-12 cm3 s-1 at 298 K; Ea = - 2.80 kcal mol-1) are in reasonable agreement with the experimental results. The experimental and theoretical results of the reaction of CH2OO with CH3SH are compared to the reactions of CH2OO with methanol and hydrogen sulfide, and the trends in reactivity are discussed. The results of the present work indicate that this reaction has a negligible influence to atmospheric CH2OO or CH3SH.

Synergy of Water and Ammonia Hydrogen Bonding in a Gas-Phase Reaction.

We report a very significant cooperative effect of water-ammonia hydrogen bonding in their reactions with a Criegee intermediate, syn-CH3CHOO. Under near ambient conditions, we found that the reaction of syn-CH3CHOO with NH3 becomes much faster (by up to 138 times) at high humidity. Intriguingly, merely adding NH3 (or H2O) alone has almost no effect on the rate of syn-CH3CHOO decay. Quantum chemistry calculation shows that the main reaction involves a hydrogen-atom (or proton) relay in a hydrogen-bonded ring structure; on the product side, a C-N bond is formed and H2O is regenerated as a catalyst. This result demonstrates a new category of reaction in which having two types of hydrogen-bond players (NH3 and H2O) is much more effective than having only one.

Criegee Intermediate Reaction with Alcohol Is Enhanced by a Single Water Molecule.

The role of water in gas-phase reactions has gained considerable interest. Here we report a direct kinetic measurement of the reaction of syn-CH3CHOO (a Criegee intermediate or carbonyl oxide) with methanol at various relative humidity (RH = 0-80%) under near-ambient conditions (298 K, 250-755 Torr). The data indicate that a single water molecule expedites the reaction by up to a factor of three. The rate coefficient of the corresponding reaction, syn-CH3CHOO + CH3OH + H2O → products, has been determined to be (1.95 ± 0.11) × 10-32 cm6 s-1 at 298 K, with no observable pressure dependence for 250-755 Torr. Quantum chemistry calculation shows that the dominating pathway involves a hydrogen-bonded ring structure, in which methanol is donating a hydrogen atom to water, water is donating a hydrogen atom to the terminal oxygen atom of the Criegee intermediate, and, on the product side, H2O is reformed and acts as a catalyst.

Kinetics of the reaction of the simplest Criegee intermediate with ammonia: a combination of experiment and theory.

The kinetics of the reaction of the simplest Criegee intermediate (CH2OO) with ammonia has been measured under pseudo-first-order conditions with two different experimental methods. We investigated the rate coefficients at 283, 298, 308, and 318 K at a pressure of 50 Torr using an OH laser-induced fluorescence (LIF) method. Weak temperature dependence of the rate coefficient was observed, which is consistent with the theoretical activation energy of -0.53 kcal mol-1 predicted by quantum chemistry calculation at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level. At 298 K, the rate coefficient at 50 Torr from the OH LIF experiment was (5.64 ± 0.56) × 10-14 cm3 molecule-1 s-1 while at 100 Torr we obtained a slightly larger value of (8.1 ± 1.0) × 10-14 cm3 molecule-1 s-1 using the UV transient absorption method. These experimental values are within the theoretical error bars of the present as well as previous theoretical results. Our experimental results confirmed the previous conclusion that ammonia is negligible in the consumption of CH2OO in the atmosphere. We also note that CH2OO may compete with OH in the oxidation of ammonia under certain circumstances, such as at night-time, high altitude and winter time.

Effect of unsaturated substituents in the reaction of Criegee intermediates with water vapor.

Criegee intermediates (CIs), formed in the reactions of unsaturated hydrocarbons with ozone, are very reactive carbonyl oxides and have recently been suggested as important oxidants in the atmosphere. In this work, we studied the substituent effect on the water monomer and dimer reaction with CIs which include up to three carbon atoms at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level. Our calculation showed that for saturated CIs with a hydrogen atom on the same side as the terminal oxygen atom, the reaction with water vapor would likely dominate the removal processes of these CIs in the atmosphere. On the other hand, for unsaturated CIs, the reactivity toward water vapor decreases compared to the saturated species allowing them to survive in humid atmospheric environments. We also evaluated the kinetic isotope effect in the reaction between CI and water vapor by performing calculations with deuterated water. We found that tunneling is not important and the kinetic isotope effect mainly comes from the difference in the zero point energy between water and deuterated water.

How big is the substituent dependence of the solar photolysis rate of Criegee intermediates?

Criegee intermediates (CIs) can actively oxidize trace gases in the troposphere, and it is important to quantify their solar photolysis rates. However, experimental measurement has been challenging, and there are differences even in the UV spectra of the simplest CH2OO. In this study, we calculated the absolute UV cross sections for C1 to C3 CIs with multireference quantum chemistry and quantum dynamics methods. Our results gave peak positions, cross sections and spectral widths reproducing the experimental results for CH2OO and (CH3)2COO. For vinyl-CIs, the peak position is greatly redshifted compared to CH2OO, and the cross section is three times larger. This knowledge should help in the future detection of CIs with vinyl groups. Lastly, we showed that for C1 to C3 CIs the solar photolysis rate only varies between 0.08 and 1.03 s-1. This small substituent dependence is very different from other CI decay pathways, such as thermal decomposition and reaction with water vapor, which varied by three orders of magnitude. These rates are too slow to compete with other atmospheric decay pathways such as CI thermal decomposition or CI reaction with water vapor.

How does substitution affect the unimolecular reaction rates of Criegee intermediates?

To gain an understanding of the substitution effect on the unimolecular reaction rate coefficients for Criegee intermediates (CIs), we performed ab initio calculations for CH2OO, CH3CHOO, (CH3)2COO, CH3CH2CHOO, CH2CHCHOO and CHCCHOO. The energies of the CIs, products and transition states were calculated with QCISD(T)/CBS//B3LYP/6-311+G(2d,2p), while the rate coefficients were calculated with anharmonic vibrational correction by using second order vibrational perturbation theory. It was found that for single bonded substitutions, the hydrogen transfer reaction dominates for the syn-conformers, while the OO bending reaction dominates for the anti-conformers. However once a double bond or a triple bond is added, the OO bending reaction dominates for both syn and anti-conformers. The rate coefficients for OO bending reaction show a significant increase when adding a methyl group or ethyl group. On the other hand, the addition of unsaturated vinyl and acetylene groups usually results in a slower thermal decomposition compared to the substitution with saturated carbon groups. Interestingly, for syn_Syn-CH2CHCHOO, a special five member ring closure reaction forming dioxole was calculated to have an extremely fast rate coefficient of 9312 s-1 at room temperature.