Impact of Post-CCSD(T) Corrections on Reaction Energetics and Rate Constants of the OH• + HCl Reaction

Kinetic Computation of Cyclization Reactions of Large Keto-Hydroperoxide Radicals in Low Temperature Combustion

The cyclization reactions of keto-hydroperoxide (KHP) radicals leading to the formation of keto cyclic ethers and OH radicals play an important role in low temperature combustion for hydrocarbon fuels or oxygenated hydrocarbon fuels. However, due to the lack of kinetic data of cyclization reactions of KHP radicals, researchers often derive high-pressure-limit rate constants of cyclization reactions of KHP radicals from analogous cyclization reactions of hydroperoxyl alkyl radicals during construction of the combustion mechanism. This study aims to systematically investigate the kinetics of cyclization reactions of KHP radicals involving short-to-large-sized radicals. The studied reactions are divided into 7 reaction classes, according to the size of the cyclic transition state, the conjugative effect (whether KHP radicals are resonance-stabilized or not), and the position of the carbonyl group (whether the carbonyl group is inside or outside of the reaction center). The isodesmic reaction method, in conjunction with transition state theory, is utilized for each reaction class to compute the energy barriers and high-pressure-limit rate constants at the DFT level. The study revealed that energy barriers calculated at the DFT level with correction by the isodesmic reaction method are close to the results from the benchmark CCSD(T) method. To develop more accurate rate rules, these reaction classes are further divided into subclasses based on the relative site of the OOH group with the carbonyl group, the type of carbon atoms where the OOH group is located, and the type of carbon atoms where the radical site is located. For each subclass, high-pressure-limit rate rules are derived by averaging the rate constants of reactions in the subclass, and it is found that the maximum absolute deviation of the energy barrier and the ratio of the largest rate constant to the smallest rate constant among reactions in each subclass are within chemical accuracy limits, indicating acceptable use of the developed rate rules. A comparison of the rate constants for cyclization reactions of KHP radicals with the values of analogous cyclization reactions of hydroperoxyalkyl radicals as provided in reported mechanisms is made. Additionally, a comparison is drawn between our developed rate rules for subclasses of the cyclization reactions of KHP radicals and the rate rules for analogous subclasses of cyclization reactions of hydroperoxyl alkyl radicals. These comparisons demonstrate significant differences and highlight the necessity for improved rate rules for cyclization reactions of KHP radicals to enhance the automatically generated combustion mechanisms for hydrocarbon and oxygenated hydrocarbon fuels.

Ammonolysis of Glyoxal at the Air-Water Nanodroplet Interface

The reactions of glyoxal (CHO)2 ) with amines in cloud processes contribute to the formation of brown carbon and oligomer particles in the atmosphere. However, their molecular mechanisms remain unknown. Herein, we investigate the ammonolysis mechanisms of glyoxal with amines at the air-water nanodroplet interface. We identified three and two distinct pathways for the ammonolysis of glyoxal with dimethylamine and methylamine by using metadynamics simulations at the air-water nanodroplet interface, respectively. Notably, the stepwise pathways mediated by the water dimer for the reactions of glyoxal with dimethylamine and methylamine display the lowest free energy barriers of 3.6 and 4.9 kcal ⋅ mol-1 , respectively. These results showed that the air-water nanodroplet ammonolysis reactions of glyoxal with dimethylamine and methylamine were more feasible and occurred at faster rates than the corresponding gas phase ammonolysis, the OH+(CHO)2 reaction, and the aqueous phase reaction of glyoxal, leading to the dominant removal of glyoxal. Our results provide new and important insight into the reactions between carbonyl compounds and amines, which are crucial in forming nitrogen-containing aerosol particles.

Mechanistic insight into the N2O + O(1D,3P) reaction: role of post-CCSD(T) corrections and non-adiabatic effects

In the present work, we have studied the N2O + O(1D,3P) reaction using high level quantum chemical calculations along with non-adiabatic kinetics. For quantum chemical calculations, we used the post-CCSD(T) method, which includes corrections from full triple excitations and partial quadratic excitations at the coupled-cluster level. For both the paths (N2 + O2 and 2NO), we have computed the rate constants over a wide range of temperatures (100-500 K for singlet paths and 700-4000 K for triplet paths). To assess the accuracy of our computations, we have compared our results with various experimentally measured quantities (absolute rate constant, branching fraction, and crossover temperature) and found a good match with all of them. We recommend the Arrhenius expressions for singlet paths, which turn out to be 4.46 × 10-11 exp(0.022/RT) cm3 molecule-1 s-1 and 7.12 × 10-11 exp(0.024/RT) cm3 molecule-1 s-1 for N2 + O2 and NO paths, respectively. For triplet paths, our recommended Arrhenius expressions are 5.15 × 10-12 exp(-15.35/RT) cm3 molecule-1 s-1 and 1.59 × 10-10 exp(-27.76/RT) cm3 molecule-1 s-1 for N2 + O2 and NO paths, respectively.

Effect of (H2O)n (n = 1 and 2) on HOCl + Cl reaction

In the present work, we investigate the effect of water molecules (H₂O and (H₂O)₂) on HOCl + Cl˙ → ClO˙ + HCl (R1), and HOCl + Cl˙ → OH˙ + Cl₂ (R2) reactions using quantum chemical and kinetics calculations. The present investigation suggests that a water molecule decreases the energy barrier of both reactions significantly, compared to uncatalyzed reaction. However, the effective rate constants for the water catalyzed path for both channels (R1 and R2) were found to be lower than the bimolecular rate constant of the uncatalyzed path. Further, it was found that the R2 reaction will dominate over the R1 reaction, with or without catalyst. Interestingly, the uncatalyzed title reaction was found to be two times faster than the HOCl + OH˙ reaction, but in the presence of water, HOCl + OH˙ becomes the dominant reaction compared to the HOCl + Cl˙ reaction in the atmosphere. In addition, the concentration of bimolecular complexes formed in the presence of a catalyst are found to be higher than the precursor molecule of the uncatalyzed reaction, which suggests that in the presence of catalyst, the HOCl + Cl˙ reaction would favor the catalyzed path rather than the uncatalyzed path.

Accurate determination of reaction energetics and kinetics of the HO2˙ + O3 → OH˙ + 2O2 reaction

In the present work, we have studied the HO₂˙ + O₃ → HO˙ + 2O₂ reaction using chemical kinetics and quantum chemical calculations. We have employed the post-CCSD(T) method to estimate the barrier height and reaction energy for the title reaction. In the post-CCSD(T) method, we have included zero point energy corrections, contributions from full triple excitations and partial quadratic excitations at the coupled-cluster level, and core corrections. We have also computed the reaction rate in the temperature range of 197-450 K and found good agreement with all the available experimental results. In addition, we have also fitted the computed rate constants with the Arrhenius expression and obtained an activation energy of 1.0 ± 0.1 kcal mol^-1, almost identical to the value recommended by IUPAC and JPL.

Oxidation of HOSO˙ by O2 (3Σg-): a key reaction deciding the fate of HOSO˙ in the atmosphere

In the present work, we have studied the oxidation of HOSO˙ by O₂ (³Σ_g^-) employing quantum chemical and kinetic calculations. The present work reveals that HOSO˙ + O₂ (³Σ_g^-) is a barrierless reaction which proceeds through a stable hydrogen-bonded complex. The estimated atmospheric lifetime of HOSO˙ in the presence of O₂ (³Σ_g^-) is found to be several orders of magnitude less compared to the other oxidation paths of HOSO˙, suggesting that the oxidation of HOSO˙ by O₂ (³Σ_g^-) might be the most dominant oxidation path of HOSO˙ in the atmosphere.

Nitrous acid (HONO) as a sink of the simplest Criegee intermediate in the atmosphere

In the present work, we have investigated the reaction of nitrous acid with the simplest Criegee intermediate using chemical kinetics and quantum chemical calculations. It was found that reactions can occur through four different paths. Among them, one path involves hydrogen atom transfer and leads to the formation of hydroperoxymethyl nitrite, while two paths involve cycloaddition leading to the formation of ozonide and formic acid and the remaining path involves oxygen atom transfer leading to the formation of HNO₃ as a final product. Although there are various oxidation reactions of HONO present in the literature, which produce nitrogen dioxide as a final product, the possibility of in situ generation of HNO₃ and formic acid from HONO is reported for the first time. Nevertheless, the hydrogen atom transfer path, which leads to the formation of hydroperoxymethyl nitrite as a final product, was found to be the fastest, and hence dominating the Criegee reaction with HONO. By comparing the title reaction with other dominant Criegee reactions, it was found that although it will be more effective than Criegee oxidation by Cl˙ or ClO˙ and can compete with Criegee oxidation by OH˙ under special circumstances, it is negligible compared to the reaction of the Criegee intermediate with a water molecule.

OH + HCl Reaction on the Surface of Ice: An Ab Initio Molecular Dynamics Study

We have investigated the OH + HCl reaction on the surface of ice using Born-Oppenheimer molecular dynamics (BOMD) simulation. The present work revealed that the OH + HCl reaction becomes ∼1 order of magnitude faster on the ice surface compared to the bare reaction. The BOMD simulation also indicates that the Cl radical formed on the ice surface through the title reaction can form two hydrogen bonds at a time with the water molecules present on the ice surface; hence, the Cl radical cannot escape from the ice surface easily.

Role of post-CCSD(T) corrections in predicting the energetics and kinetics of the OH • +O 3 reaction

The present work investigates the OH • +O 3 reaction by means of chemical kinetics and quantum chemical calculations. To predict the reaction barrier height and reaction energy, we have...

Effect of microsolvation on the mode specificity of the OH˙(H2O) + HCl reaction

The present study investigates the mode specificity in the microsolvated OH˙(H₂O) + HCl reaction using on-the-fly direct dynamics simulation. To the best of our knowledge, this is the first study which aims to gain insights into the effect of microsolvation on the mode selectivity. Our investigation reveals that, similar to the gas phase OH˙ + HCl reaction, the microsolvated reaction is also predominantly affected by the vibrational excitation of the HCl mode, whereas the OH vibrational mode behaves as a spectator. Interestingly, in contrast to the behavior of the bare reaction, the integral cross section at the ground state of the microsolvated reaction decreases with an increase in translational energy. However, for the vibrational excited states, the reactivity of the microsolvated reaction is found to be higher than that of the bare reaction within the selected range of translational energies.

Oxidation of HOSO˙ by Cl˙: a new source of SO2 in the atmosphere?

In the present work, we have studied the formation of SO₂ in the atmosphere from the oxidation of HOSO˙ by Cl˙ at the CCSD(T)/aug-cc-pV(+d)TZ//MP2/aug-cc-pV(+d)TZ level of theory. The present work reveals that the title reaction is a barrierless reaction that proceeds through a stable intermediate sulfurochloridous acid having a stabilization energy of ∼-56.5 kcal mol^-1. The rate constant values within the temperature range of 213-400 K indicate that the rate of HOSO˙ + Cl˙ = SO₂ + HCl reaction does not change much with the change in temperature. Besides, the reaction was also found to be insensitive towards pressure change. Interestingly, the relative rate of HOSO˙ + Cl˙ reaction with respect to HOSO˙ + OH˙ reaction indicates that HOSO˙ + Cl˙ is always much slower than HOSO˙ + OH˙ reaction, within the temperature range of 213-400 K.

Effect of water on the oxidation of CO by a Criegee intermediate

The present work employs the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level of theory to investigate the effect of a water monomer and dimer on the oxidation of carbon-monoxide by a Criegee intermediate (CH2OO). The present work suggests that in the presence of a water monomer the energy barrier of the title reaction reduced to ∼3.4 kcal mol-1 from the corresponding uncatalyzed barrier (∼12.4 kcal mol-1), whereas, in the presence of a water dimer it became as low as ∼-3.2 kcal mol-1. It has also been found that, in the presence of catalysts, additional channels become available from which the title reaction can proceed. The estimated values of rate constants suggest that within the temperature range of 210-320 K, the effective bimolecular rate constant for the water monomer catalyzed channel is 10 to 100 times lower than the bimolecular rate constant of the uncatalyzed channel, whereas in the case of the water dimer it is ∼5-10 times higher than that of the uncatalyzed channel.

Effect of ammonia and formic acid on the CH3O˙ + O2 reaction: a quantum chemical investigation

In the present work, the catalytic effect of ammonia and formic acid on the CH3O˙ + O2 reaction has been investigated employing the MN15L density functional. The investigations suggest that, in the presence of ammonia, the reaction can proceed through two different pathways, namely a single hydrogen atom transfer and a double hydrogen atom transfer path, but due to the high energy barrier associated with the double hydrogen atom transfer channel, it prefers the single hydrogen atom transfer channel. On the other hand, in the case of formic acid, only the single hydrogen atom transfer path is found to be feasible. Interestingly, it has been found that, in the presence of ammonia and formic acid, the reaction becomes a barrierless reaction. The calculated rate constant values at various temperatures indicate an anti-Arrhenius behavior for both the ammonia and formic acid catalyzed channels.

CO2 as an auto-catalyst for the oxidation of CO by a Criegee intermediate (CH2OO)

The present work investigates the effect of CO₂ on the CH₂OO + CO reaction, employing the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level of theory.

Probing the Partial Activation of Water by Open-Shell Interactions, Cl(H2O)1-4

The partial chemical activation of water by reactive radicals was examined computationally for small clusters of chlorine and water, Cl^•(H₂O)_n=1-4. Using an automated isomer-search procedure, dozens of unique, stable structures were computed. Among the resulting structural classes were intact, hydrated-chlorine isomers, as well as hydrogen-abstracted (HCl)(OH)(H₂O)_n-1 configurations. The latter showed increased stability as the degree of hydration increased, until n = 4, where a new class of structures was discovered with a chloride ion bound to an oxidized water network. The electronic structure of these three structural classes was investigated, and spectral signatures of this hydration-based evolution were connected to these electronic properties. An ancillary outcome of this detailed computational analysis, including coupled-cluster benchmarks, was the calibration of cost-effective quantum chemistry methods for future studies of these radical-water complexes.

Influence of water on the CH3O˙ + O2 → CH2O + HO2˙ reaction

Electronic structure calculations employing density functional theory have been used to study the effect of a single water molecule on the CH3O˙ + O2 → CH2O + HO2˙ reaction. The investigation suggests that in the presence of water the reaction barrier reduces from 3.01 kcal mol-1 to -1.86 kcal mol-1. Consequently, when we consider the bimolecular rate constants for the water catalyzed channel, they were found to be 104 to 105 times higher than that of the uncatalyzed reaction. Interestingly, the Arrhenius plot indicates a negative temperature dependency of the catalyzed channel (anti-Arrhenius behavior); as a result of this the domination of the catalyzed channel over the bare reaction increases with the lowering of the temperature. But the effective bimolecular rate constant values for the catalyzed channel were found to be approximately four orders of magnitude lower than that of the uncatalyzed one, which implies that the contribution of the catalyzed channels to the overall rate of the reaction is very small.

Effects of water, ammonia and formic acid on HO2 + Cl reactions under atmospheric conditions: competition between a stepwise route and one elementary step

Quantum chemical calculations at M06-2X and CCSD(T) levels of theory have been performed to investigate the effects of H₂O, NH₃, and HCOOH on the HO₂ + Cl → HCl + O₂ reaction. The results show that catalyzed reactions with three catalysts could proceed through two different mechanisms, namely a stepwise route and one elementary step, where the former reaction is more favorable than the latter. Meanwhile, for the stepwise route, a single hydrogen atom transfer pathway in the presence of all catalysts has more advantages than the respective double hydrogen atom transfer pathway. Then, the relative impacts of catalysts under tropospheric conditions were investigated by considering the temperature dependence of the rate constants and the altitude dependence of catalyst concentrations. The calculated results show that at 0 km altitude, the HO₂ + Cl → HCl + O₂ reaction with catalysts, such as H₂O, NH₃, or HCOOH, cannot compete with the reaction without a catalyst, as the effective rate constant with a catalyst is smaller by 2–6 orders of magnitude than the naked reaction within the temperature range 280–320 K. The calculated results also show that at altitudes of 5, 10 and 15 km, the effective rate constant of the HCOOH-catalyzed reaction increases obviously with an increase in altitude. At 15 km altitude, its value is up to 9.63 × 10⁻¹¹ cm³ per molecule per s, which is close to the corresponding value of the reaction without a catalyst, showing that the contribution of HCOOH to the HO₂ + Cl → HCl + O₂ reaction cannot be neglected at high altitudes. The new findings in this investigation are not only of great necessity and importance for elucidating the gas-phase reaction of HO₂ with Cl in the presence of acidic, neutral and basic catalysts, but are also of great interest for understanding the importance of other types of hydrogen abstraction in the atmosphere.

The effects of acidic (FA), neutral (WM) and basic (AM) catalysts on the energetic and kinetic aspects of the HO₂ + Cl reaction have been studied. At 298 K, the catalytic order of FA, WM and AM is WM > FA > AM.

Revisiting the reaction energetics of the CH3O˙ + O2 (3Σ−) reaction: the crucial role of post-CCSD(T) corrections

The CH₃O˙ + O₂ reaction has been studied by means of high level ab initio calculations to predict the reaction energy and barrier height with chemical accuracy.