Water Catalysis of the Reaction of Methanol with OH Radical in the Atmosphere is Negligible

Computational study of the post-transition state dynamics for the OH + CH3OH reaction probed by photodetachment of the CH3O-(H2O) anion

Dissociative photodetachment dynamics simulations were conducted to study the CH3O-(H2O) → CH3O + H2O + e- reaction using classical molecular dynamics (MD) and ring-polymer molecular dynamics (RPMD) techniques on two newly formulated neutral potential energy surfaces (PES1 and PES2) by different research groups. While the dissociation dynamics exhibited similarities between classical MD and RPMD, there were noticeable differences in the fluctuation of probability densities for the internal modes due to nuclear quantum effects. Upon comparison of our findings with experimental data concerning the electron binding energy distribution and photofragment relative energy, it suggests that the potential energy landscapes of PES2 are reasonably precise. The time evolution of occupied vibrational states of the H2O photofragment is presented in this study.

Intramolecular Catalytic Hydrogen Atom Transfer (CHAT)

Intramolecular catalysis (IntraCat) is the acceleration of a process at one site of a molecule catalyzed by a functional group in the same molecule; an external agent such as a solvent typically facilitates it. Here, we report a general first-principles-based IntraCat mechanism, which strictly occurs within a single molecule with no coreagent being involved─we call it intramolecular catalytic transfer of hydrogen atoms (CHAT). A reactive part of a molecule (chat catalyst moiety or chat agent, represented by -OOH, -COOH, -SH, -CH2OH, -HPO4, or another bifunctional H-donor/acceptor group) catalyzes an interconversion process, such as keto-enol or amino-imino tautomerization, and cyclization in the same molecule, while being regenerated in the process. It can thus be regarded as an intramolecular version of the intermolecular H atom transfer processes mediated by an external molecular catalyst, e.g., dihydrogen, water, or a carboxylic acid. Earlier, we proposed a general mechanistic systematization of intermolecular processes, illustrated in the simplest case of the H2-mediated reactions classified as dihydrogen catalysis [Asatryan, R.; et al. Catal. Rev.: Sci. Eng., 2014, 56, 403-475]. Following this systematization, the CHAT catalysis belongs to the category of relay transfer of H atoms, albeit in an intramolecular manner. A broader class of intramolecular processes includes all types of H-transfer reactions stimulated by an H-migration, which we call self-catalyzed H atom transfer (SC-HAT). The CHAT mechanism comprises a subset of SC-HAT in which the catalytic moiety is regenerated (i.e., acts as a true catalyst and not a reagent). We provide several characteristic examples of CHAT mechanism based on detailed analysis of the corresponding potential energy surfaces. All such cases showed a dramatically reduced activation barrier relative to the corresponding uncatalyzed H-transfer reactions. For example, we show that CHAT can facilitate long-range H-migration in larger molecules and can occur multiple times in one molecule with multiple interconverting groups. It also facilitates amino-imino tautomerization of unsaturated GABA-analogues and peptides, as well as intramolecular cyclization processes to form heterocycles, e.g., oxygenated rings. CHAT pathways may also explain the pH-dependent increase of mutarotation rate of glucose-6-phosphate demonstrated in pioneering experiments that introduced the classical IntraCat concept. In addition, we identify a ground electronic state CHAT pathway as an alternative to the UV-promoted long-range molecular crane keto-enol conversion with a remarkably low activation energy. To initially assess the possible impact of the new keto-enol conversion pathway on combustion of n-alkanes, we present a detailed kinetic analysis of isomerization and decomposition of pentane-2,4-ketohydroperoxide (2,4-KHP). The results are compared with key alternative reactions, including direct dissociation and Korcek channels (for which a new alkyl group migration channel is also identified), revealing the competitiveness of the CHAT pathway across a range of conditions. Taken together, this work provides insight into a general class of reaction pathways that has not previously being systematically considered and that may occur in a broad range of contexts from combustion to atmospheric chemistry to biochemistry.

Kinetics of Sulfur Trioxide Reaction with Water Vapor to Form Atmospheric Sulfuric Acid

Although experimental methods can be used to obtain the quantitative kinetics of atmospheric reactions, experimental data are often limited to a narrow temperature range. The reaction of SO3 with water vapor is important for elucidating the formation of sulfuric acid in the atmosphere; however, the kinetics is uncertain at low temperatures. Here, we calculate rate constants for reactions of sulfur trioxide with two water molecules. We consider two mechanisms: the SO3···H2O + H2O reaction and the SO3 + (H2O)2 reaction. We find that beyond-CCSD(T) contributions to the barrier heights are very large, and multidimensional tunneling, unusually large anharmonicity of high-frequency modes, and torsional anharmonicity are important for obtaining quantitative kinetics. We find that at lower temperatures, the formation of the termolecular precursor complexes, which is often neglected, is rate-limiting compared to passage through the tight transition states. Our calculations show that the SO3···H2O + H2O mechanism is more important than the SO3 + (H2O)2 mechanism at 5-50 km altitudes. We find that the rate ratio between SO3···H2O + H2O and SO3 + (H2O)2 is greater than 20 at altitudes between 10 and 35 km, where the concentration of SO3 is very high.

Quantitative Kinetics of HO2 Reactions with Aldehydes in the Atmosphere: High-Order Dynamic Correlation, Anharmonicity, and Falloff Effects Are All Important

Kinetics provides the fundamental parameters for elucidating sources and sinks of key atmospheric species and for atmospheric modeling more generally. Obtaining quantitative kinetics in the laboratory for the full range of atmospheric temperatures and pressures is quite difficult. Here, we use computational chemistry to obtain quantitative rate constants for the reactions of HO₂ with HCHO, CH₃CHO, and CF₃CHO. First, we calculate the high-pressure-limit rate constants by using a dual-level strategy that combines conventional transition state theory using a high level of electronic structure wave function theory with canonical variational transition state theory including small-curvature tunneling using density functional theory. The wave-function level is beyond-CCSD(T) for HCHO and CCSD(T)-F12a (Level-A) for XCHO (X = CH₃, CF₃), and the density functional (Level-B) is specifically validated for these reactions. Then, we calculate the pressure-dependent rate constants by using system-specific quantum RRK theory (SS-QRRK) and also by an energy-grained master equation. The two treatments of the pressure dependence agree well. We find that the Level-A//Level-B method gives good agreement with CCSDTQ(P)/CBS. We also find that anharmonicity is an important factor that increases the rate constants of all three reactions. We find that the HO₂ + HCHO reaction has a significant dependence on pressure, but the HO₂ + CF₃CHO reaction is almost independent of pressure. Our findings show that the HO₂ + HCHO reaction makes important contribution to the sink for HCHO, and the HO₂ + CF₃CHO reaction is the dominant sink for CF₃CHO in the atmosphere.

Catalytic Effect of CO2 and H2O Molecules on •CH3 + 3O2 Reaction

The methyl (•CH3) + 3O2 radical is an important reaction in both atmospheric and combustion processes. We investigated potential energy surfaces for the effect of CO2 and H2O molecules on a •CH3+ O2 system. The mechanism for three reaction systems, i.e., for •CH3 + 3O2, •CH3 + 3O2 (+CO2) and •CH3 + 3O2 (+H2O), were explored using ab initio/DFT methods [CCSD(T)//M062X/6-311++G(3df,3pd)] in combination with a Rice−Ramsperger−Kassel−Marcus (RRKM)/master-equation (ME) simulation between a temperature range of 500 to 1500 K and a pressure range of 0.0001 to 10 atm. When a CO2 and H2O molecule is introduced in a •CH3 + 3O2 reaction, the reactive complexes, intermediates, transition states and post complexes become thermodynamically more favorable. The calculated rate constant for the •CH3 + 3O2 (3 × 10−15 cm3 molecule−1 s−1 at 1000 K) is in good agreement with the previously reported experimentally measured values (~1 × 10−15 cm3 molecule−1 s−1 at 1000 K). The rate constant for the effect of CO2 (3 × 10−16 cm3 molecule−1 s−1 at 1000 K) and H2O (2 × 10−17 cm3 molecule−1 s−1 at 1000 K) is at least one–two-order magnitude smaller than the free reaction (3 × 10−15 cm3 molecule−1 s−1 at 1000 K). The effect of CO2 and H2O on •CH3 + 3O2 shows non-RRKM behavior, however, the effect on •CH3 + 3O2 shows RRKM behavior. Our results also demonstrate that a single CO2 and H2O molecule has the potential to accelerate a gas-phase reaction at temperature higher than >1300 K and slow the reaction at a lower temperature. The result is unique and observed for the first time.

Effect of Water and Formic Acid on ·OH + CH4 Reaction: An Ab Initio/DFT Study

In this work, we used ab initio/DFT method coupled with statistical rate theory to answer the question of whether or not formic acid (HCOOH) and water molecules can catalyze the most important atmospheric and combustion prototype reaction, i.e., ·OH (OH radical) + CH4. The potential energy surface for ·OH + CH4 and ·OH + CH4 (+X) (X = HCOOH, H2O) reactions were calculated using the combination of hybrid-density functional theory and coupled-cluster theory with Pople basis set [(CCSD(T)/ 6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd)]. The results of this study show that the catalytic effect of HCOOH (FA) and water molecules on the ·OH + CH4 reaction has a major impact when the concentration of FA and H2O is not included. In this situation the rate constants for the CH4 + HO···HCOOH (3 × 10−9 cm3 molecule−1 s−1) reaction is ~105 times and for CH4 + H2O···HO reaction (3 × 10−14 cm3 molecule−1 s−1 at 300 K) is ~20 times higher than ·OH + CH4 (~6 × 10−15 cm3 molecule−1 s−1). However, the total effective rate constants, which include the concentration of both species in the kinetic calculation has no effect under atmospheric condition. As a result, the total effective reaction rate constants are smaller. The rate constants when taking the account of the FA and water for CH4 + HO···HCOOH (4.1 × 10−22 cm3 molecule−1 s−1) is at least seven orders magnitude and for the CH4 + H2O···HO (7.6 × 10−17 cm3 molecule−1 s−1) is two orders magnitude smaller than ·OH + CH4 reaction. These results are also consistent with previous experimental and theoretical studies on similar reaction systems. This study helps to understand how FA and water molecules change the reaction kinetic under atmospheric conditions for ·OH + CH4 reaction.

Effect of a single water molecule on ˙CH2OH + 3O2 reaction under atmospheric and combustion conditions

The hydroxymethyl (˙CH₂OH) radical is an important intermediate species in both atmosphere and combustion reaction systems. The rate coefficients for ˙CH₂OH + ³O₂ and (˙CH₂OH + ³O₂ (+H₂O)) reactions were calculated using the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulation and canonical variational transition state theory (CVT) between the temperature range of 200 to 1500 K based on the potential energy surface constructed using CCSD(T)//ωB97XD/6-311++G(3df,3pd). The results show that ˙CH₂OH + ³O₂ leads to the formation of CH₂O and HO₂ at temperatures below 800 K, and goes back to reactants at high temperature (>1000 K). When a water molecule is added to the reaction, the formation of CH₂O and HO₂ is favored at all temperatures. The calculated rate coefficient for the ˙CH₂OH + ³O₂ (2.8 × 10^-11 cm³ molecule^-1 s^-1 at 298 K) is in good agreement with the previous experimental values (∼1 × 10^-11 cm³ molecule^-1 s^-1 at 298 K). The rate coefficients for the water-assisted reaction (2.4 × 10^-16 cm³ molecule^-1 s^-1 at 1000 K) is at least 3-4 orders of magnitude smaller than the water-free reaction (6.2 × 10^-12 cm³ molecule^-1 s^-1 at 1000 K). This result is consistent with the similar types of reaction system. Our calculations also predict that the effect of a single water molecule favors the formation of CH₂O in the combustion condition. However, the water-free reaction favors the formation of CH₂O in the atmospheric condition. The current study helps to understand how a single water molecule changes the reaction mechanism and chemical kinetic behaviour under atmospheric and combustion conditions.

The impact of water vapor on the OH reactivity toward CH3CHO at ultra-low temperatures (21.7-135.0 K): Experiments and theory

The role of water vapor (H2O) and its hydrogen-bonded complexes in the gas-phase reactivity of organic compounds with hydroxyl (OH) radicals has been the subject of many recent studies. Contradictory effects have been reported at temperatures between 200 and 400 K. For the OH + acetaldehyde reaction, a slight catalytic effect of H2O was previously reported at temperatures between 60 and 118 K. In this work, we used Laval nozzle expansions to reinvestigate the impact of H2O on the OH-reactivity with acetaldehyde between 21.7 and 135.0 K. The results of this comprehensive study demonstrate that water, instead, slows down the reaction by factors of ∼3 (21.7 K) and ∼2 (36.2-89.5 K), and almost no effect of added H2O was observed at 135.0 K.

Atmospheric oxidation of fluoroalcohols initiated by ˙OH radicals in the presence of water and mineral dusts: mechanism, kinetics, and risk assessment

The transport and formation of fluorinated compounds are greatly significant due to their possible environmental risks. In this work, the ˙OH-mediated degradation of CF3CF2CF2CH2OH and CF3CHFCF2CH2OH in the presence of O2/NO/NO2 was studied by using density functional theory and the direct kinetic method. The formation mechanisms of perfluorocarboxylic/hydroperfluorocarboxylic acids (PFCAs/H-PFCAs), which were produced from the reactions of α-hydroxyperoxy radicals with NO/NO2 and the ensuing oxidation of α-hydroxyalkoxy radicals, were clarified and discussed. The roles of water and silica particles in the rate constants and ˙OH reaction mechanism with fluoroalcohols were investigated theoretically. The results showed that water and silica particles do not alter the reaction mechanism but obviously change the kinetic properties. Water could retard fluoroalcohol degradation by decreasing the rate constants by 3-5 orders of magnitude. However, the heterogeneous ˙OH-rate coefficients on the silica particle surfaces, including H4SiO4, H6Si2O7, and H12Si6O18, are larger than that of the naked reaction by 1.20-24.50 times. This finding suggested that these heterogeneous reactions may be responsible for the atmospheric loss of fluoroalcohols and the burden of PFCAs. In addition, fluoroalcohols could be exothermically trapped by H12Si6O18, H6Si2O7, and H4SiO4, in which the chemisorption on H12Si6O18 is stronger than that on H6Si2O7 or H4SiO4. The global warming potentials and radiative forcing of CF3CF2CF2CH2OH/CF3CHFCF2CH2OH were calculated to assess their contributions to the greenhouse effect. The toxicities of individual species were also estimated via the ECOSAR program and experimental measurements. This work enhances the understanding of the environmental formation of PFCAs and the transformation of fluoroalcohols.

Effect of ammonia and water molecule on OH + CH3OH reaction under tropospheric condition

The rate coefficients for OH + CH3OH and OH + CH3OH (+ X) (X = NH3, H2O) reactions were calculated using microcanonical, and canonical variational transition state theory (CVT) between 200 and 400 K based on potential energy surface constructed using CCSD(T)//M06-2X/6-311++G(3df,3pd). The results show that OH + CH3OH is dominated by the hydrogen atoms abstraction from CH3 position in both free and ammonia/water catalyzed ones. This result is in consistent with previous experimental and theoretical studies. The calculated rate coefficient for the OH + CH3OH (8.8 × 10-13 cm3 molecule-1 s-1), for OH + CH3OH (+ NH3) [1.9 × 10-21 cm3 molecule-1 s-1] and for OH + CH3OH (+ H2O) [8.1 × 10-16 cm3 molecule-1 s-1] at 300 K. The rate coefficient is at least 8 order magnitude [for OH + CH3OH(+ NH3) reaction] and 3 orders magnitude [OH + CH3OH (+ H2O)] are smaller than free OH + CH3OH reaction. Our calculations predict that the catalytic effect of single ammonia and water molecule on OH + CH3OH reaction has no effect under tropospheric conditions because the dominated ammonia and water-assisted reaction depends on ammonia and water concentration, respectively. As a result, the total effective reaction rate coefficients are smaller. The current study provides a comprehensive example of how basic and neutral catalysts effect the most important atmospheric prototype alcohol reactions.

Chemical reactivity from the vibrational ground-state level. The role of the tunneling path in the tautomerization of urea and derivatives

Recent developments of low-temperature techniques are providing valuable knowledge about chemical processes that manifest in the quantum regimen. The tunneling effect from the vibrational ground-state is the main mechanism of these reactions, which usually involves the motion or transfer of hydrogen atoms. Theoretical methods can enrich the information supplied by these experimental methods through an insightful analysis of the tunneling process. In this context, canonical variational transition state theory with multidimensional tunneling corrections (CVT/MT) can handle this type of reaction, and it has been applied to several systems within the small-curvature approximation for tunneling (SCT). This method is of proven reliability for polyatomic reactions occurring at room temperature and above, but no tests have been performed to check its performance when only the lowest energy level is populated. In this work, we compare SCT against the least-action tunneling (LAT) method to study the tautomerization and cis-trans interconversion reactions in the enol forms of urea, thiourea, and selenourea. To the best of our knowledge, this is the first time that the LAT method is applied to a polyatomic reaction occurring in the deep-tunneling region. The theoretical results indicate that the reaction mechanisms are controlled by tunneling. The SCT and LAT tautomerization reaction times are in good agreement with the experimental values; however, LAT seems superior to SCT for reactions (tautomerizations) that involve moderate reaction path curvature, whereas the opposite is true for reactions with small curvature (interconversions). These results led us to introduce and recommend the microcanonically optimized tunneling path that selects the tunneling probability as the maximum between the SCT and LAT tunneling probabilities.

Reactions of Criegee Intermediates are Enhanced by Hydrogen-Atom Relay Through Molecular Design

We report a type of highly efficient double hydrogen atom transfer (DHAT) reaction. The reactivities of 3-aminopropanol and 2-aminoethanol towards Criegee intermediates (syn- and anti-CH₃ CHOO) were found to be much higher than those of n-propanol and propylamine. Quantum chemistry calculation has confirmed that the main mechanism of these very rapid reactions is DHAT, in which the nucleophilic attack of the NH₂ group is catalyzed by the OH group which acts as a bridge of HAT. Typical gas-phase DHAT reactions are termolecular reactions involving two hydrogen bonding molecules; these reactions are typically slow due to the substantial entropy reduction of bringing three molecules together. Putting the reactive and catalytic groups in one molecule circumvents the problem of entropy reduction and allows us to observe the DHAT reactions even at low reactant concentrations. This idea can be applied to improve theoretical predictions for atmospherically relevant DHAT reactions.