Revealing new pathways for the reaction of Criegee intermediate CHOO with SO.

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 CHOO + SO system and reveal how the reaction happens step by step by quasi-classical trajectory simulations. A new pathway forming the main products (CHO + SO) and a new product channel (CO + H + SO) are predicted in our simulations.

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 + CH [X = F, Cl, OH], HX + CH [X = Br, I], OH + CHI, and F + CHCHCl. The dynamics simulations utilize high-level analytical potential energy surfaces and the quasi-classical trajectory method.

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

With the help of the ROBOSURFER program package, a global full-dimensional potential energy surface (PES) for the reaction of the Criegee intermediate, CHOO, with the NH molecule is developed iteratively using different 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 data.

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

The recently-developed high-level full-dimensional spin-orbit-corrected potential energy surfaces based on ManyHF-UCCSD(T)-F12a/cc-pVDZ-F12 + SO(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 CH are improved by adding three to four thousand new geometries with higher energies at the same level to cover a higher-energy range. Quasi-classical trajectory simulations in the 30-80 kcal mol 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.

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

We report a detailed dynamics study on the mode-specificity of the HI + CH two-channel reaction (H-abstraction and I-abstraction), through performing quasi-classical trajectory computations on a recently developed high-level 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 CH somewhat surprisingly inhibit the dominant H-abstraction reaction pathway.

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

A quasi-classical trajectory (QCT) study is performed for the HBr + CH multi-channel reaction using a recently-developed high-level 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 = 1 reaction at low collision energy () is observed due to the intramolecular vibrational-energy redistribution and the classical nature of the QCT method.

Automated full-dimensional potential energy surface development and quasi-classical dynamics for the HI(XΣ) + CH → I(P) + CH reaction.

A full-dimensional spin-orbit-corrected analytical coupled-cluster-quality potential energy surface (PES) is developed for the HI(XΣ) + CH → I(P) + CH 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.

Full-dimensional potential energy surface development and dynamics for the HBr + CH → Br(P) + CH reaction.

We report a full-dimensional spin-orbit-corrected analytical potential energy surface (PES) for the HBr + CH → Br + CH 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.

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): CHOO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). The oxidation of SO by CIs is a potential source of HSO, an important precursor of aerosols. Here we investigated the UV-visible spectroscopy and reaction kinetics of thermalized MACRO.

Effects of water vapor on the reaction of CHOO with NH.

The reaction of the simplest Criegee intermediate, CHOO, with ammonia and water vapor has been investigated at 278-308 K and under 100-760 Torr by monitoring the strong UV absorption of CHOO. We found that the observed decay rate of CHOO 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 CHOO + NH + HO reaction, of which the reaction rate coefficient was determined to be k = (8.

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 HO, NH, CHOH, 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.

Temperature and isotope effects in the reaction of CHCHOO 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 CHCHOO with methanol vapor was investigated at various temperatures, pressures, and isotopic substitutions using time-resolved UV absorption spectroscopy.

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

The kinetics of the reaction of the simplest Criegee intermediate CHOO with CHSH 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.

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-CHCHOO. Under near ambient conditions, we found that the reaction of syn-CHCHOO with NH becomes much faster (by up to 138 times) at high humidity. Intriguingly, merely adding NH (or HO) alone has almost no effect on the rate of syn-CHCHOO decay.

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-CHCHOO (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.

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.

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.

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.

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 CHOO, CHCHOO, (CH)COO, CHCHCHOO, CHCHCHOO 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.