Gas Phase Reaction of Nitric Acid with Hydroxyl Radical without and with Water. A Theoretical Investigation

Unveiling the Influence of Water Molecules for NF3 Removal by the Reaction of NF3 with OH: A DFT Study

The removal of nitrogen trifluoride (NF3) is of significant importance in atmospheric chemistry, as NF3 is an important anthropogenic greenhouse gas. However, the radical species OH and O(1D) in atmospheric conditions are nonreactive towards NF3. It is necessary to explore possible ways to remove NF3 in atmosphere. Therefore, the participation of water molecules in the reaction of NF3 with OH was discussed, as water is abundant in the atmosphere and can form very stable complexes due to its ability to act as both a hydrogen bond donor and acceptor. Systemic DFT calculations carried out at the CBS-QB3 and ωB97XD/aug-cc-pVTZ level of theory suggest that water molecules could affect the NF3 + OH reaction as well. The energy barrier of the SN2 mechanism was decreased by 8.52 kcal/mol and 10.58 kcal/mol with the assistance of H2O and (H2O)2, respectively. Moreover, the presence of (H2O)2 not only reduced the energy barrier of the reaction, but also changed the product channels, i.e., formation of NF2O + (H2O)2-HF instead of NF2OH + (H2O)2-F. Therefore, the removal of NF3 by reaction with OH is possible in the presence of water molecules. The results presented in this study should provide useful information on the atmospheric chemistry of NF3.

Triplet State Radical Chemistry: Significance of the Reaction of 3SO2 with HCOOH and HNO3

The triplet excited states of sulfur dioxide can be accessed in the UV region and have a lifetime large enough that they can react with atmospheric trace gases. In this work, we report high level ab initio calculations for the reaction of the a3B1 and b3A2 excited states of SO2 with weak and strong acidic species such as HCOOH and HNO3, aimed to extend the chemistry reported in previous studies with nonacidic H atoms (water and alkanes). The reactions investigated in this work are very versatile and follow different kinds of mechanisms, namely, proton-coupled electron transfer (pcet) and conventional hydrogen atom transfer (hat) mechanisms. The study provides new insights into a general and very important class of excited-state-promoted reactions, opening up interesting chemical perspectives for technological applications of photoinduced H-transfer reactions. It also reveals that atmospheric triplet chemistry is more significant than previously thought.

The influence of (H2O)1-2 in the HOBr + HO2 gas-phase reaction

The HOBr + HO₂ reaction in the absence of water has three different channels for the abstraction of H to generate the corresponding products. The dominant channel is the generation of BrO + H₂O₂. The introduction of water molecules influences this dominant reaction via the way the reactants interact with the water molecules. The addition of water molecules decreases the energy barrier and increases the rate coefficient of the reaction. Interestingly, water works as a catalyst and we obtain BrO + H₂O₂, like in the reaction without water, or the water works as a reactant and we obtain products other than BrO + H₂O₂. The rate coefficients of the HOBr + HO₂ reaction in the presence of water are calculated to be faster than the reaction in the absence of water. However, other pathways in the presence of water are slower than the reaction in the absence of water. The water-assisted effective rate coefficients for the HOBr + HO₂ reaction are also larger than those for the reaction in the absence of water. The influence of a water dimer is not as important when compared with one water molecule. In summary, a single water molecule has a positive catalytic influence in enhancing the HOBr + HO₂ reaction.

Impact of a single water molecule on the atmospheric oxidation of thiophene by hydroperoxyl radical

Water as an important assistant can alter the reactivity of atmospheric species. This project is designed to investigate the impact of a single water molecule on the atmospheric reactions of aromatic compounds that have not been attended to comprehensively. In the first part, the atmospheric oxidation mechanisms of thiophene initiated by hydroperoxyl radical through a multiwell-multichannel potential energy surface were studied to have useful information about the chemistry of the considered reaction. It was verified that for the thiophene plus HO₂ reaction, the addition mechanism is dominant the same as other aromatic compounds. Due to the importance of the subject and the presence of water molecules in the atmosphere with a high concentration that we know as relative humidity, and also the lack of insight into the influence of water on the reactions of aromatic compounds with active atmospheric species, herein, the effect of a single water molecule on the addition pathways of the title reaction is evaluated. In another word, this research explores how water can change the occurrence of reactions of aromatic compounds in the atmosphere. For this, the presence of one water molecule is simulated by higher-level calculations (BD(T) method) through the main interactions with the stationary points of the most probable pathways. The results show that the mechanism of the reaction with water is more complicated than the bare reaction due to the formation of the ring-like structures. Also, water molecule decreases the relative energies of all addition pathways. Moreover, atoms in molecule theory (AIM) along with the kinetic study by the transition state (TST) and the Rice–Ramsperger–Kassel–Marcus (RRKM) theories demonstrate that the overall interactions of a path determine how the rate of that path changes. In this regard, our results establish that the interactions of water with HO₂ (thiophene) in the initial complex 1WHA (1WTA or 1WTB) are stronger (weaker) than the sum of its interactions in transition states. Also, for the water-assisted pathways, the ratio of the partition function of the transition state to the partition functions of the reactants is similar to the respective bare reaction. Therefore, the reaction rates of the bare pathways are more than the water-assisted paths that include the 1WHA complex and are less than the paths that involve the 1WTA and 1WTB complexes.

Reactivity of Undissociated Molecular Nitric Acid at the Air-Water Interface

Recent experiments and theoretical calculations have shown that HNO₃ may exist in molecular form in aqueous environments, where in principle one would expect this strong acid to be completely dissociated. Much effort has been devoted to understanding this fact, which has huge environmental relevance since nitric acid is a component of acid rain and also contributes to renoxification processes in the atmosphere. Although the importance of heterogeneous processes such as oxidation and photolysis have been evidenced by experiments, most theoretical studies on hydrated molecular HNO₃ have focused on the acid dissociation mechanism. In the present work, we carry out calculations at various levels of theory to obtain insight into the properties of molecular nitric acid at the surface of liquid water (the air-water interface). Through multi-nanosecond combined quantum-classical molecular dynamics simulations, we analyze the interface affinity of nitric acid and provide an order of magnitude for its lifetime with regard to acid dissociation, which is close to the value deduced using thermodynamic data in the literature (∼0.3 ns). Moreover, we study the electronic absorption spectrum and calculate the rate constant for the photolytic process HNO₃ + hν → NO₂ + OH, leading to 2 × 10^-6 s^-1, about twice the value in the gas phase. Finally, we describe the reaction HNO₃ + OH → NO₃ + H₂O using a cluster model containing 21 water molecules with the help of high-level ab initio calculations. A large number of reaction paths are explored, and our study leads to the conclusion that the most favorable mechanism involves the formation of a pre-reactive complex (HNO₃)(OH) from which product are obtained through a coupled proton-electron transfer mechanism that has a free-energy barrier of 6.65 kcal·mol^-1. Kinetic calculations predict a rate constant increase by ∼4 orders of magnitude relative to the gas phase, and we conclude that at the air-water interface, a lower limit for the rate constant is k = 1.2 × 10^-9 cm³·molecule^-1·s^-1. The atmospheric significance of all these results is discussed.

Photoinduced Oxidation Reactions at the Air-Water Interface

Chemistry on water is a fascinating area of research. The surface of water and the interfaces between water and air or hydrophobic media represent asymmetric environments with unique properties that lead to unexpected solvation effects on chemical and photochemical processes. Indeed, the features of interfacial reactions differ, often drastically, from those of bulk-phase reactions. In this Perspective, we focus on photoinduced oxidation reactions, which have attracted enormous interest in recent years because of their implications in many areas of chemistry, including atmospheric and environmental chemistry, biology, electrochemistry, and solar energy conversion. We have chosen a few representative examples of photoinduced oxidation reactions to focus on in this Perspective. Although most of these examples are taken from the field of atmospheric chemistry, they were selected because of their broad relevance to other areas. First, we outline a series of processes whose photochemistry generates hydroxyl radicals. These OH precursors include reactive oxygen species, reactive nitrogen species, and sulfur dioxide. Second, we discuss processes involving the photooxidation of organic species, either directly or via photosensitization. The photochemistry of pyruvic acid and fatty acid, two examples that demonstrate the complexity and versatility of this kind of chemistry, is described. Finally, we discuss the physicochemical factors that can be invoked to explain the kinetics and thermodynamics of photoinduced oxidation reactions at aqueous interfaces and analyze a number of challenges that need to be addressed in future studies.

Pressure-Dependent Rate Constant Caused by Tunneling Effects: OH + HNO3 as an Example

Tunneling effects on chemical reactions are well-known and have been unambiguously demonstrated by processes that involve the motion of hydrogen atoms at low temperature. However, the process by which tunneling effects cause a falloff curve (i.e., how reaction rate constants depend on pressure) has apparently not been previously documented. This work points out that falloff curves can indeed be caused by tunneling and explains the effect in simple terms. This is an interesting feature of quantum tunneling, which can appear in low temperature chemistry (such as in atmospheric or interstellar environments). In this Letter, we use high-level coupled-cluster calculations in combination with master-equation methods on the well-studied reaction of OH with HNO₃, which plays an important role in the upper troposphere and lower stratosphere. Our results in combination with available experimental data clearly demonstrate that the tunneling correction depends on not just temperature, but also pressure.

CH3NO as a potential intermediate for early atmospheric HCN: a quantum chemical insight

Hydrogen cyanide (HCN) has played a central role in the production of several biological molecules under prebiotic conditions on primitive Earth. Previously, K. J. Zahnle (J. Geophys. Res.: Atmos., 1986, 91, 2819) and Tian et al. (Earth Planet. Sci. Lett., 2011, 308, 417) emphasized that HCN production in the early Earth's CH₄-rich atmosphere could have been possible through the reaction between active nitrogen atoms (N) and methane photolysis products. Here, we have proposed alternative pathways for the formation of early atmospheric HCN via the decomposition of CH₃NO as an intermediate. In the early Earth's O₂-free atmosphere, CH₃˙ could preferentially attach to NO, which was generated via early atmospheric volcanism or lightning and photochemical processes. We have quantum chemically explored both unimolecular and bimolecular decomposition pathways of CH₃NO via the assistance of another CH₃NO molecule and via H₂O, NH₃, HCl, HCOOH, HNO₃ and H₂SO₄ catalysis. Both energetic and kinetic analyses reveal that H₂SO₄ is more efficient in this regard than other atmospheric species. Overall, it has been suggested that the proposed bimolecular decomposition pathways might have been alternative pathways for the formation of HCN under certain conditions on prebiotic Earth, while the unimolecular decomposition of CH₃NO could lead to the formation of HCN in the high temperature volcanic environment on early Earth.

Atmospheric chemistry of the self-reaction of HO2 radicals: stepwise mechanism versus one-step process in the presence of (H2O)n (n = 1-3) clusters

The effects of water on radical-radical reactions are of great importance for the elucidation of the atmospheric oxidation process of free radicals. In the present work, the HO2 + HO2 reactions with (H2O)n (n = 1-3) have been investigated using quantum chemical methods and canonical variational transition state theory with small curvature tunneling. We have explored both one-step and stepwise mechanisms, in particular the stepwise mechanism initiated by ring enlargement. The calculated results have revealed that the stepwise mechanism is the dominant one in the HO2 + HO2 reaction that is catalyzed by one water molecule. This is because its pseudo-first-order rate constant (kRWM1') is 3 orders of magnitude larger than that of the corresponding one-step mechanism. Additionally, the value of kRWM1' at 298 K has been found to be 4.3 times larger than that of the rate constant of the HO2 + HO2 reaction (kR1) without catalysts, which is in good agreement with the experimental findings. The calculated results also showed that the stepwise mechanism is still dominant in the (H2O)2 catalyzed reaction due to its higher pseudo-first-order rate constant, which is 3 orders of magnitude larger than that of the corresponding one-step mechanism. On the other hand, the one-step process is much faster than the stepwise mechanism by a factor of 105-106 in the (H2O)3 catalyzed reaction. However, the pseudo-first-order rate constants for the (H2O)2 and (H2O)3-catalyzed reactions are lower than that of the H2O-catalyzed reaction by 3-4 orders of magnitude, which indicates that the water monomer is the most efficient one among all the catalysts of (H2O)n (n = 1-3). The present results have provided a definitive example that water and water clusters have important influences on atmospheric reactions.

Role of (H2O) n (n = 1-2) in the Gas-Phase Reaction of Ethanol with Hydroxyl Radical: Mechanism, Kinetics, and Products

The effect of water on the hydrogen abstraction mechanism and product branching ratio of CH₃CH₂OH + ^•OH reaction has been investigated at the CCSD(T)/aug-cc-pVTZ//BH&HLYP/aug-cc-pVTZ level of theory, coupled with the reaction kinetics calculations, implying the harmonic transition-state theory. Depending on the hydrogen sites in CH₃CH₂OH, the bared reaction proceeds through three elementary paths, producing CH₂CH₂OH, CH₃CH₂O, and CH₃CHOH and releasing a water molecule. Thermodynamic and kinetic results indicate that the formation of CH₃CHOH is favored over the temperature range of 216.7-425.0 K. With the inclusion of water, the reaction becomes quite complex, yielding five paths initiated by three channels. The products do not change compared with the bared reaction, but the preference for forming CH₃CHOH drops by up to 2%. In the absence of water, the room temperature rate coefficients for the formation of CH₂CH₂OH, CH₃CH₂O, and CH₃CHOH are computed to be 5.2 × 10^-13, 8.6 × 10^-14, and 9.0 × 10^-11 cm³ molecule^-1 s^-1, respectively. The effective rate coefficients of corresponding monohydrated and dihydrated reactions are 3-5 and 6-8 orders of magnitude lower than those of the unhydrated reaction, indicating that water has a decelerating effect on the studied reaction. Overall, the characterized effects of water on the thermodynamics, kinetics, and products of the CH₃CH₂OH + ^•OH reaction will facilitate the understanding of the fate of ethanol and secondary pollutants derived from it.

Functionalized hollow MnFe2O4 nanospheres: design, applications and mechanism for efficient adsorption of heavy metal ions

A l-cysteine functionalized magnetic hollow MnFe₂O₄ nanosphere material has been synthesised, with high magnetism, large interior cavities, and high porosity and surface activity. It has high adsorption efficiency and regenerated adsorption capacity for the removal of Cr⁶⁺ and Pb²⁺ in contaminated water.

Triplet state promoted reaction of SO2 with H2O by competition between proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) processes

The excited triplet electronic state of SO₂ (a³B₁) reacts with water through a proton coupled electron transfer (pcet) mechanism rather than via a conventional hydrogen atom transfer (hat) process.

Catalytic effect of (H2O)n (n = 1–3) clusters on the HO2 + SO2 → HOSO + 3O2 reaction under tropospheric conditions

The HO₂ + SO₂ → HOSO + ³O₂ reaction without and with (H₂O)n (n = 1–3) have been investigated using CCSD(T)/CBS//M06-2X/aug-cc-pVTZ methods, and canonical variational transition state theory with small curvature tunneling.

Theoretical investigation of the chiral transition of serine and the roles of water, hydroxyl radical and hydroxide ion

The serine chiral transition and the roles of water, hydroxyl radical and hydroxide ion as catalysts were theoretically investigated.

Effect of a single water molecule on the HO2 + ClO reaction

The catalytic effect of a single water molecule on the HO2 + ClO reaction has been investigated at the CCSD(T)/aug-cc-pVTZ//B3LYP-D3/aug-cc-pVDZ level of theory. Four H-abstraction paths and two kinds of products, among which the paths for HOCl + O2 formation are dominant, have been found for the HO2 + ClO reaction without water. The rate constant of the most favorable path for the reaction without water is computed to be 4.53 × 10-12 cm3 molecule-1 s-1 at room temperature, in good agreement with the experiment. In the presence of a water molecule, although the reaction becomes more complex, the dominant products do not change. Four main channels, starting from HO2H2O + ClO, H2OHO2 + ClO, ClOH2O + HO2 and H2OClO + HO2, are investigated. The most favorable paths, reactions between H2OHO2 and ClO, and between ClOH2O and HO2, are 7-10 and 6-9 orders of magnitude slower than the reaction in the absence of water, respectively. It is concluded that the presence of a single water molecule does not play an important role in enhancing the HO2 + ClO reaction under tropospheric conditions.

Reactivity of hydropersulfides toward the hydroxyl radical unraveled: disulfide bond cleavage, hydrogen atom transfer, and proton-coupled electron transfer

Hydropersulfides (RSSH) are highly reactive as nucleophiles and hydrogen atom transfer reagents. These chemical properties are believed to be key for them to act as antioxidants in cells. The reaction involving the radical species and the disulfide bond (S-S) in RSSH, a known redox-active group, however, has been scarcely studied, resulting in an incomplete understanding of the chemical nature of RSSH. We have performed a high-level theoretical investigation on the reactions of the hydroxyl radical (˙OH) toward a set of RSSH (R = -H, -CH₃, -NH₂, -C(O)OH, -CN, and -NO₂). The results show that S-S cleavage and H-atom abstraction are the two competing channels. The electron inductive effect of R induces selective ˙OH substitution at one sulfur atom upon S-S cleavage, forming RSOH and ˙SH for the electron donating groups (EDGs), whereas producing HSOH and ˙SR for the electron withdrawing groups (EWGs). The H-Atom abstraction by ˙OH follows a classical hydrogen atom transfer (hat) mechanism, producing RSS˙ and H₂O. Surprisingly, a proton-coupled electron transfer (pcet) process also occurs for R being an EDG. Although for RSSH having EWGs hat is the leading channel, S-S cleavage can be competitive or even dominant for the EDGs. The overall reactivity of RSSH toward ˙OH attack is greatly enhanced with the presence of an EDG, with CH₃SSH being the most reactive species found in this study (overall rate constant: 4.55 × 10¹² M^-1 s^-1). Our results highlight the complexity in RSSH reaction chemistry, the extent of which is closely modulated by the inductive effect of the substituents in the case of the oxidation by hydroxyl radicals.

Water catalysis of the reaction between hydroxyl radicals and linear saturated alcohols (ethanol and n-propanol) at 294 K

Water accelerates the title reaction by lowering the energy barrier and increasing the dipole moments of the reactants.

Impact of the acidic group on the hydrolysis of 2-dinitromethylene-5,5-dinitropyrimidine-4,6-dione

Unique and incredible catalysis of the titled hydrolysis using HSO₄⁻ is proposed and verified in the gas and solvent phases.

Reaction kinetics of OH + HNO3 under conditions relevant to the upper troposphere/lower stratosphere

Key upper atmosphere reaction of HNO₃ + OH studied over extended pressure and temperature range using new alternative detection method.

The atmospheric oxidation of CH3OOH by the OH radical: the effect of water vapor

The relative humidity can enhance the atmospheric oxidation of CH₃OOH by OH into CH₃O₂ + H₂O up to 19% whereas the formation of H₂CO + OH + H₂O is enhanced up to 5% only under the same conditions.

The catalytic effect of water, water dimers and water trimers on H2S +3O2formation by the HO2+ HS reaction under tropospheric conditions

Catalyst X (X = H₂O, (H₂O)₂and (H₂O)₃) is incorporated into the channel of H₂S +³O₂formation and the catalytic effect of water, water dimers and water trimers is mainly taken from the contribution of a single water vapor molecule.

Influence of water on the homogeneous gas-phase formation mechanism of polyhalogenated dioxins/furans from chlorinated/brominated phenols as precursors

Water is of great chemical importance due to its ability to form hydrogen bond. Polyhalogenated dibenzo-p-dioxin/benzofurans (PHDD/Fs) are notorious due to their persistence, bioaccumulation and extremely high toxicity. Water is ubiquitous, and a deep knowledge of its influence on the formation mechanism of PHDD/Fs is necessary. This work investigated the influence of water on the homogeneous gas-phase formation of PHDD/Fs from halogenated phenols (HPs) as precursors by using quantum chemical calculations with the aid of the MPWB1K theoretical approach in connection with the 6-31+G(d,p) and 6-311+G(3df,2p) basis sets. The schematic energy profile in the presence of water was constructed and compared with the situation without water. This study reveals for the first time that the introduction of water promotes the formation of halogenated phenoxy radicals (HPRs) from the H abstraction reactions of HPs with atomic H and OH radicals by lowering the reaction energy barriers and opening new low-energy pathways. Another intriguing finding of this work is that the inclusion of a water molecule produces a catalytic effect on the H-shift step involved in the formation of PHDFs and thus their formation potential is enhanced.

New insights in atmospheric acid-catalyzed gas phase hydrolysis of formaldehyde: a theoretical study

A two-step mechanism of the gas phase hydrolysis of formaldehyde catalyzed by nitric acid.

A theoretical study on the catalytic role of water in methanol steam reforming on PdZn(111)

The catalytic role of water in the methanol steam reforming process on the PdZn(111) surface is explored theoretically.

Atmospheric formation of the NO3 radical from gas-phase reaction of HNO3 acid with the NH2 radical: proton-coupled electron-transfer versus hydrogen atom transfer mechanisms

The amidogen radical abstracts the hydrogen from nitric acid through a proton coupled electron transfer mechanism rather than by an hydrogen atom transfer process.

The gas-phase reaction of methane sulfonic acid with the hydroxyl radical without and with water vapor

The gas phase reaction between methane sulfonic acid (CH3SO3H; MSA) and the hydroxyl radical (HO), without and with a water molecule, was investigated with DFT-B3LYP and CCSD(T)-F12 methods. For the bare reaction we have found two reaction mechanisms, involving proton coupled electron transfer and hydrogen atom transfer processes that produce CH3SO3 and H2O. We also found a third reaction mechanism involving the double proton transfer process, where the products and reactants are identical. The computed rate constant for the oxidation process is 8.3 × 10(-15) cm(3) s(-1) molecule(-1). CH3SO3H forms two very stable complexes with water with computed binding energies of about 10 kcal mol(-1). The presence of a single water molecule makes the reaction between CH3SO3H and HO much more complex, introducing a new reaction that consists in the interchange of H2O between HO and CH3SO3H. Our kinetic calculations show that 99.5% of the reaction involves this interchange of the water molecule and, consequently, water vapor does not play any role in the oxidation reaction of methane sulfonic acid by the hydroxyl radical.