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A new potential energy surface (PES) for the atmospheric formation of sulfuric acid from OH+SO2 is investigated using density functional theory and high-level ab initio molecular orbital theory. A pathway focused on the new PES assumes the reaction to take place between the radical complex SO3·HO2 and H2O. The unusual stability of SO3·HO2 is the principal basis of the new pathway, which has the same final outcome as the current reaction mechanism in the literature but it avoids the production and complete release of SO3. The entire reaction pathway is composed of three consecutive elementary steps:(1) HOSO2+O2→SO3·HO2, (2) SO3·HO2+H2O→SO3·H2O·HO2, (3) SO3·H2O·HO2→H2SO4+HO2. All three steps have small energy barriers, under 10 kcal/mol, and are exothermic, and the new pathway is therefore favorable both kinetically and thermodynamically. As a key step of the reactions, step (3), HO2 serves as a bridge molecule for low-barrier hydrogen transfer in the hydrolysis of SO3. Two significant atmospheric implications are expected from the present study. First, SO3 is not released from the oxidation of SO2 by OH radical in the atmosphere. Second, the conversion of SO2 into sulfuric acid is weakly dependent on the humidity of air.

The effects on the hydrolysis of NO2 in the presence of methylamine and dimethylamine molecules were investigated by theoretical calculations of a series of the molecular clusters 2NO2-mH2O–CH3NH2 (m = 1–3) and 2NO2-mH2O-(CH3)2NH (m = 1, 2). With methylamine included in the clusters, the energy barrier is reduced by 3.2 kcal/mol from that with ammonia, and the corresponding products may form without an energy barrier. The results show that amines have larger effects than ammonia in promoting the hydrolysis of NO2 on thermodynamics. The additional water molecules can stabilize the transition states and the product complexes, and we infer that adding more water molecules in the reactions mainly act as solvent and promoting to form the methylamine nitrate (CH3NH3+NO3–). In addition, the interactions of CH3NH2 and (CH3)2NH on the hydration of HNO3 are also more effective than NH3, and the NH4NO3, CH3NH3NO3, and (CH3)2NH2NO3 complexes tend to form the larger aerosols with the increasing of water molecules. The...

The hydrolysis reaction of sulfur dioxide (SO2) to form sulfurous acid involving additional sulfurous acid (H2SO3) was investigated using high-level computational methods. With H2SO3, the reaction takes place via a double proton transfer process with a cage-like structure, which is different from the planar ring structure involved in a corresponding process with an additional water molecule (served as a catalyst). Our results show that H2SO3 is a better catalyst than water, as the barrier height for the H2SO3-catalyzed reaction is only 5.5 kcal/mol, compared to over 25.0 and 15.0 kcal/mol for the reaction without a catalyst and the H2O-catalyzed reaction, respectively. In addition, the sulfurous acid dimer from the H2SO3-catalyzed reaction is more stable than hydrated H2SO3 from the H2O-catalyzed reaction. Considering the existence of sulfurous acid in the aqueous phase and acidic aerosols, as well as the importance of SO2 and H2O in the atmosphere, our results will have potentially significant implications on the homogeneous and heterogeneous nucleation processes.

The deposition and hydrolysis reaction of SO2 + H2O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO2-(H2SO4)n-(H2O)m (m = 1,2; n = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO2 as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO2 + H2O + H2SO4 - H2O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H2SO4-(H2O)2 forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO2 + (H2SO4)-H2OSO2 + (H2SO4)2-H2OSO2 + H2SO4-H2O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO2). Consequently, sulfuric acid promotes the hydrolysis of SO2 both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO2. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO2, leading to the formation and growth of new particles.

Effects of ammonia and water molecules on the hydrolysis of sulfur dioxide are investigated by theoretical calculations of two series of the molecular clusters SO2-(H2O)n (n = 1–5) and SO2-(H2O)n-NH3 (n = 1–3). The reaction in pure water clusters is thermodynamically unfavorable. The additional water in the clusters reduces the energy barrier for the reaction, and the effect of each water decreases with the increasing number of water molecules in the clusters. There is a considerable energy barrier for reaction in SO2-(H2O)5, 5.69 kcal/mol. With ammonia included in the cluster, SO2-(H2O)n-NH3, the energy barrier is dramatically reduced, to 1.89 kcal/mol with n = 3, and the corresponding product of hydrated ammonium bisulfate NH4HSO3-(H2O)2 is also stabilized thermodynamically. The present study shows that ammonia has larger kinetic and thermodynamic effects than water in promoting the hydrolysis reaction of SO2 in small clusters favorable in the atmosphere.

A systematic ab initio study was carried out to investigate the π–π intermolecular interaction in the pyridine dimer. Potential energy curves were calculated for eight major configurations at second-order Moller–Plesset approximation (MP2) using various large basis sets augmented with bond functions. The basis set superposition error (BSSE) was considered by the counterpoise method. The study shows that bond functions can effectively give accurate description of π–π interactions. The intermolecular energies at the MP2 potential minima of the eight configurations were recalculated using the coupled-cluster method including single, double, and perturbative triple configurations, CCSD(T), with the aug-cc-pVDZ basis set augmented with the bond functions {6s6p4d2f}. Two most stable geometries were found to be antiparallel-displaced, with the CCSD(T) interaction energies of −3.05 and −2.95 kcal/mol, respectively. The CCSD(T) interaction energies of other configurations were found to be −1.05, −2.30, −1.42, −2.29, −1.79, and −1.83 kcal/mol, respectively. Dispersion is largely responsible for attraction in the pyridine dimer and, as a result, electron correlation beyond MP2 is necessary as in the case of heteroaromatic molecules.

Recent discoveries involving the roles of nitric oxide in humans have stimulated intense interest in transition metal nitrosyl complexes. A series of dinitrosyl iron complexes with the formula [(DPPX)Fe(NO)2], {DPPX = 1,2-bis(diphenylphosphino)benzene (1), 1,3-bis(diphenylphosphino)propane (2), and cis-1,2-bis(diphenylphosphino)ethylene (3)} has been prepared and characterized through a combination of FT-IR, NMR, UV–Vis, X-ray crystallography, and electrochemical techniques. Infrared spectroscopy showed NO shifts to the region of 1723 and 1674 cm−1 for complexes 1 and 3, and 1708 and 1660 cm−1 for 2, indicating that ligand 2 acts as a stronger σ-donor. The X-ray crystallographic data showed that 1 and 3 possess the rare repulso conformation while 2 has the attracto conformation. CV studies on compounds 1, 2 and 3 display two quasi-reversible oxidations with the E°1/2 values at 0.101 and 0.186 V, 0.121 and 0.184 V, and 0.019 and 0.342 V, respectively. The larger ΔE value for compound 2 compared with that of 1 and 3 is attributed to the lack of π-bonds between the two phosphorus atoms. Theoretical calculations using density functional theory were carried out on the synthesized compounds and model compounds and the results are consistent with the experimental data. The calculated HOMO–LUMO gaps for compounds 1, 2 and 3 are 3.736, 4.060, and 3.669 eV, respectively, which supports the stronger back-donation for compound 2 than that of compounds 1 and 3.

Thermodynamically stable small clusters of oxalic acid (CO2H)2, ammonia (NH3), and water (H2O) are studied through quantum chemical calculations. The (CO2H)2-NH3 core system with up to three waters of hydration was examined by B3LYP density functional theory and MP2 molecular orbital theory with the aug-cc-pVDZ basis set. The (CO2H)2-NH3 core complexes are observed to hydrogen bond strongly and should be found in appreciably significant concentrations in the atmosphere. Subsequent hydration of the (CO2H)2-NH3 core, however, is found to be somewhat prohibitive under ambient conditions. Relative populations of the examined clusters are predicted and the binding patterns detailed. Atmospheric implications related to new particle formations are discussed.

The reaction mechanism of nitric oxide with four hydroxycyclohexadienyl peroxyl radicals from the oxidation of benzene in the troposphere has been investigated using the UB3LYP/6-311++G(d,p) density functional theory (DFT) method. The title reaction was found to involve five steps and produce the following intermediates and products: cis -peroxynitrite, trans -peroxynitrite, hydroxycyclohexadienyl nitrate, 2-hydroxy cyclohexadienyl oxyl radical, 3-hydroxy-1,2-epoxy-5-cyclohexene radical (and NO 2 ), hexa-2,4-diene-1,6-dial ( trans -HONO), hydroxyl-cyclohexadienone (HNO 2 ). The results confirm that aromatic compounds contribute to tropospheric ozone production, the rain acidity, and secondary organic aerosol (SOA) pollution via the title reaction with a sufficiently high concentration of NO x .

The decomposition of CH3OH on clean and oxygen-predosed V(100) surfaces was studied on the basis of periodic density functional calculations and microkinetic modeling. The results indicate that the O−H bond scission of CH3OH is thermodynamically and kinetically favorable on clean V(100) while the C−H and C−O bond scissions are unlikely to occur at low temperature, and as a result, CH3O is the major intermediate in the decomposition process. The C−O bond scission of CH3O to form CH3 is much easier than the C−H bond scission to form HCHO. Hydrogenation of CH3 by the surface hydrogen from dissociating CH3OH and CH3O is responsible for the desorption of CH4 at low and high temperatures, respectively. HCHO further undergoes decomposition or/and coupling to form CO or/and C2H4. When oxygen is preadsorbed on the surface at low coverage, the O−H bond scission of CH3OH is virtually not affected, while the cleavages of the C−O and C−H bonds from CH3O are inhibited in different degrees, leading to the decrease in the ratio of CH4 produced at the low temperature relative to that at the high temperature. All products are delayed in temperature. The results are in good agreement with experimental observations.

The importance of aerosols to humankind is well-known, playing an integral role in determining Earth's climate and influencing human health. Despite this fact, much remains unknown about the initial events of nucleation. In this work, the molecular properties of common organic atmospheric pollutant oxalic acid and its gas phase interactions with water have been thoroughly examined. Local minima single-point energies for the monomer conformations were calculated at the B3LYP and MP2 level of theory with both 6-311++G(d,p) and aug-cc-pVDZ basis sets and are compared with previous works. Optimized geometries, relative energies, and free energy changes for the stable clusters of oxalic acid conformers with up to six waters were then obtained from B3LYP calculations with 6-31+G(d) and 6-311++G(d,p) basis sets. Initially, cooperative binding is predicted to be the most important factor in nucleation, but as the clusters grow, dipole cancellations are found to play a pivotal role. The clusters of oxalic acid hydrated purely with water tend to produce extremely stable and neutral core systems. Free energies of formation and atmospheric implications are discussed.

The reaction mechanism and kinetics for the addition of hydroxyl radical (OH) to phenol have been investigated using the hybrid density functional (B3LYP) method with the 6-311++G(2dp, 2df) basis set and the complete basis set (CBS) method using APNO basis sets, respectively. The equilibrium geometries, energies, and thermodynamics properties of all the stationary points along the addition reaction pathway are calculated. The rate constants and the branching ratios of each channel are evaluated using classical transition state theory (TST) in the temperature range of 210 to 360 K, to simulate temperatures in all parts of the troposphere. The ortho addition pathway is dominant and accounts for 99.8%–96.7% of the overall adduct products from 210 to 360 K. The calculated rate constants are in good agreement with existing experimental values. The addition reaction is irreversible.

Reaction mechanisms for the OH-initiated oxidation of o-cresol in the troposphere were investigated using B3LYP hybrid density functional method with the 6-311++G(2df,2pd) basis set. Single-point energy calculations with the same basis set were performed using the coupled cluster method with single, double, and perturbative triple configurations, CCSD(T). The equilibrium geometries, energies, and thermodynamics properties of all the stationary points along the addition reaction pathway and hydrogen abstraction reaction pathway were calculated. The rate constants and the branching ratios of each pathway were evaluated using classical transition state theory (TST) in the temperature range of 200–360 K, to simulated temperatures in all parts of the troposphere. The ortho (at position ortho- to the hydroxyl group) addition pathway was found dominant, accounting for 95.3–75.0% of the overall products from 200 to 360 K. The calculated rate constants are in good agreement with available experimental values. The addition reaction and hydrogen abstraction reaction are both thermodynamically irreversible.

The homogeneous nucleation of three gases, NO2, H2O, and NH3, to form NH4NO3 aerosols and HONO gas is proposed to be a new significant source of atmospheric HONO. Four reaction pathways are examined by density functional theory calculations of various complexes of reactants and products involved. NH3 is shown to play a critical role in lowering the Gibbs free energy barrier and stabilizing the HNO3 product by forming particulate ammonium nitrate. In conclusion, the system of gaseous NO2, H2O, and NH3 is expected to undergo a spontaneous homogeneous nucleation process, which is likely an important source of nighttime atmospheric HONO concentrations.

The interaction of alkyl sulfate surfactant with water molecules was studied by using DFT. It was revealed for the first time that alkyl sulfate surfactant formed stable hydrate with six water molecules when it was saturation adsorption at the air-water interface. The influence of alkyl chain length on the hydration area and the structure of hydrophilic group of complexes CH3 (H2O)6 (m = 0–8) was investigated. The calculated results showed that: (1) the hydration area of the hydrophilic group was 53.6 A2, which was in good agreement with the experimental results; (2) change in the length of alkyl chain had almost no effect on the structure and hydration area of the hydrophilic group.

Density functional theory calculations of polychlorinated dibenzo-p-dioxins (PCDDs) were carried out to obtain the electronic descriptors, polarizabilities, and traceless quadrupole moments of 76 PCDD congeners. No simple relationships were found for the electronic descriptors with the relevant aryl hydrocarbon receptor (AhR) binding affinities of PCDDs, which suggests that they alone may not be sufficient to explain the variation in toxicity. However, quantitative structure-activity relationships (QSARs) were developed with the polarizabilities and traceless quadrupole moments, explaining about 74% and 59% of variation in AhR binding affinities of PCDDs, respectively. To explain the nature of toxic interaction, a mathematical model based on the ligand-receptor binding and solute-solvent interaction was presented, and then a multiple regression analysis of all the above parameters was performed to evaluate the contributions of the parameters to the bonding affinities. Other data for PCDFs found in the literature were also included in the regression analysis to minimize data over-fitting. The results suggest that dispersion and electrostatic interactions are equally important for the interaction of PCDD/Fs with the AhR.

Ionic dissociation of chlorosulfonic acid (HSO3Cl) in the molecular clusters HSO3Cl-(H2O) n (n = 1–4) and HSO3Cl-NH3-(H2O) n (n = 0–3) was investigated by density functional theory and ab initio molecular orbital theory. The equilibrium structures, binding energies, and thermodynamic properties, such as relative enthalpy and relative Gibbs free energy, and were calculated using the hybrid density functional (B3LYP) method and the second order Moller-Plesset approximation (MP2) method with the 6-311++G** basis set. Chlorosulfonic acid was found to require a minimum of three water molecules for ionization to occur and at least one water molecule to protonate ammonia. The corresponding clusters with fewer water molecules were found to be strongly hydrogen-bonded. The related properties and acid strength of chlorosulfonic acid were discussed and compared to the acid strengths of perchloric acid and sulfuric acid in the context of clusters with ammonia and water. The relative stabilities of these clusters were also investigated.

Density functional theory calculations at the B3LYP/6-31+G(d) and B3LYP/aug-cc-pVDZ levels were performed to obtain the equilibrium structures, thermodynamic properties, and electron affinities (EA) of 14 polybrominated diphenyl ether (PBDE) congeners in the gas phase. All congeners except for those of symmetric BDE are found to have two or more conformational isomers. The optimized geometries of the most stable conformational isomers are in agreement with recently published X-ray crystallographic data. The thermodynamic properties of the congeners with a given number of bromine substitutions are strongly dependent on the substitution pattern, whereas the EA values also depend on the number of bromine substitutions. The vertical electron affinities (EAVer) calculated for the selected BDE congeners at the B3LYP/aug-cc-pVDZ level are all positive except for di-BDEs, and are correlated with the initial reductive debromination rate constants obtained recently [Keum, Y.-S., Li, Q.X., 2005. Reductive debromination of polybrominated diphenyl ethers by zerovalent iron. Environ. Sci. Technol. 39, 2280]. All adiabatic electron affinities (EAAda) are positive, and suggest that the BDE congeners act as electron acceptors when reacting with receptors in living cells. The calculated EAAda values differ considerably from those of EAVer because of the large geometrical relaxation from the neutral to the anionic BDE congeners, highlighted by the lengthening of a C–Br bond. The elongated C–Br bond, which occurs at the α position, is directly involved in the debromination of n-bromodiphenyl to (n − 1)-bromodiphenyl ethers in the reductive debromination experiments.

Density functional theory calculations have been preformed on a series of hydrogen-bonded complexes of substituted aliphatic and aromatic carboxylic acids with ammonia. Molecular properties, particularly those related to hydrogen bonding, have been carefully examined for their interdependence as well as dependence on the acidity of the acid. The bond length and stretching frequency of the hydroxyl group and the hydrogen-bond length and energy of the complex are shown to be highly correlated with each other and are linearly correlated with available literature pKa values of the carboxylic acids. The linear correlations resulting from the fit to the available pKa values can be used to predict the pKa values of similar carboxylic acids. The pKa values so predicted using the different molecular properties are highly consistent and in good agreement with the literature values. This study suggests that calculated molecular properties of hydrogen-bonded complexes allow effective and systematic prediction of pKa values for a large range of organic acids using the established linear correlations. This approach is unique in its capability to determine the acidity of a particular functional group or the local acidity within a large molecular system such as a protein.

This article presents a theoretical reaction mechanism for the oxidation of thiourea by hydrogen peroxide using density functional theory and ab initio methods. This work also focuses on the analysis of the potential energy surface (PES), thermodynamic and kinetic properties for the designed oxidation mechanism of thiourea using BH&HLYP and B3LYP methods. Both DFT calculations lead to similar reaction paths. The results of the thermodynamic analysis for the designed oxidation mechanism of thiourea are reasonable and feasible. Based on the research on the activity energies, activity enthalpies and activity Gibbs free energies obtained from the determined reaction pathways, we conclude that the oxidation of thiourea by hydrogen peroxide in vacuum was well completed. Because the active relative energies, the relative enthalpies and the relative Gibbs free energies of Reaction 4 predicted by Path II are higher than those predicted by Path I. Kinetics calculations suggest that the designed Path I is more favorable than Path II. The basis sets and calculation methods have strong effects on the thermodynamics and kinetics calculations.

Ionization dissociation of methanesulfonic acid (MSA) in the clusters MSA–NH3–(H2O)n, n = 0–4, is investigated by quantum chemical calculations. The MSA–NH3 unit without water or with one water molecule is hydrogen-bonded. In the presence of two or more water molecules, it converts to the NH 4 + · CH 3 SO 3 - ion pair as a result of direct and spontaneous proton transfer from MSA to NH3. All of the clusters are shown thermodynamically stable in the gas phase with respect to separate monomers. Harmonic frequencies of the clusters confirm the transition from hydrogen-bonded MSA–NH3 to the ion pair as the water molecules are introduced.

The structure, stability, and properties of the hydrated clusters of calcium hydroxide, Ca ( OH )2( H2O )n, n = 1–6, were investigated using density functional and ab initio quantum chemical methods. The results show that six water molecules are needed to result in the complete dissociation of Ca ( OH )2. The stable and ionic conformer of Ca ( OH )2( H2O )6has C3symmetry. Its surprising stability and high IR activity render hydrated clusters of Ca ( OH )2potentially significant in the nucleation of noctilucent clouds in the mesosphere. Trends in the interaction energies (ΔEe) of the complexes show that water molecules in the first shell of Ca2+are highly stable, further alluding to the role of hydrated Ca ( OH )2in aerosol formation.

Ionic dissociation of methanesulfonic acid (MSA) in the aqueous environment is studied by density functional theory and ab initio calculations of the water clusters MSA–(H2O)n, n = 1–5. Molecular structures and stabilities are discussed based on the calculated stable conformers and relative energies. The clusters with up to three water molecules (n = 1–3) are dominated by strong hydrogen bond interactions. With a minimum of n = 4, MSA is dissociated as a result of proton transfer from MSA to H2O. All of the clusters are shown thermodynamically stable in the gas phase with respect to separate monomers.

Density functional theory calculations were performed to study the stereo-controlling step of the direct aldol reaction between acetone and 4-nitrobenzaldehyde catalyzed by (S,S)-proline dipeptide. Four transition state structures have been determined using B3LYP functional with the 6-31G* basis set, corresponding to the anti and syn arrangements of the methylene moiety with respect to the carbonyl group in enamine intermediate, and to the si and re attacks to the aldehyde carbonyl carbon, respectively. Solvent effects of DMSO on the stereo-controlling step were investigated with Onsager model. The energy results of the transition states reveal the origin of poor enantioselectivity for the reaction. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Reaction mechanisms and kinetics of the hydrogen abstraction of acetone by chlorine radical are studied by theoretical calculations. Density functional theory and ab initio methods are used to explore the reaction pathways. The hydrogen abstraction is found to be highly favored over the addition pathway, and the latter is insignificant in atmospheric conditions. Classical transition state theory with Eckart tunneling corrections is employed to obtain rate constants of the reaction from 210 K to 360 K. The reaction is characterized by a small barrier, and rate constants approximated by the expression k ( T ) = 6.44 × 10 −12 e (−672/ T ) cm 3 molecule −1 s −1 , which is in reasonable agreement with experiment.

Ionic dissociations of the halosulfonic acids HSO3X, X = F, Cl, Br in an aqueous environment are studied by density functional theory and ab initio method calculations of the water clusters HSO3X–(H2O)n, n = 1–4. With a minimum of n = 3, HSO3X is dissociated as a result of proton transfer from HSO3X to H2O. Two competing stable configurations are reported: one with the halogen pointing away from water and the other with the halogen participating in hydrogen bonding with water. In the former, the S–X bond is stable with the successive addition of water molecules while in the latter the bond length increases gradually.

Ionization or ionic dissociation of fluorosulfonic acid (HSO3F) in the clusters HSO3F–(H2O)n (n = 1–3) and HSO3F–NH3–(H2O)n (n = 0, 1) is investigated by density functional theory and ab initio molecular orbital theory. The equilibrium structures, binding energies, and harmonic frequencies are calculated using the hybrid density functional (B3LYP) method with the 6-311++G** basis set and the second-order Moller–Plesset approximation (MP2) method with the 6-311++G** basis set. Harmonic frequencies are obtained from the B3LYP/6-311++G** calculations. Fluorosulfonic acid is found to require a minimum of three water molecules for ionization to occur and at least one water molecule to protonate ammonia. The corresponding clusters with fewer water molecules are found to be strongly hydrogen-bonded. The acid strength and related properties of fluorosulfonic acid are discussed and compared to those of perchloric acid and sulfuric acid in the context of clusters with ammonia and water.

Interactions and proton transfer reactions of halosulfonic acids (HSO3X, X = F, Cl, Br) with ammonia (NH3) in the hydrated clusters, HSO3X–NH3–(H2O)n (n = 0,1,2,3), are investigated by density functional theory and ab initio molecular orbital theory. The HSO3X–NH3 systems alone are hydrogen-bonded despite of the high acidity of HSO3X, while one water molecule is enough to assist the transfer of a proton from the acid to ammonia. Large hydration energies and favorable free energies are found for each of the first few water molecules, indicating high stabilities of the small hydrated clusters in the gas phase.

Effects of ammonia and water molecules on the hydrolysis of sulfur dioxide are investigated by theoretical calculations of two series of the molecular clusters SO2-(H2O)n (n = 1-5) and SO2-(H2O)n-NH3 (n = 1-3). The reaction in pure water clusters is thermodynamically unfavorable. The additional water in the clusters reduces the energy barrier for the reaction, and the effect of each water decreases with the increasing number of water molecules in the clusters. There is a considerable energy barrier for reaction in SO2-(H2O)5, 5.69 kcal/mol. With ammonia included in the cluster, SO2-(H2O)n-NH3, the energy barrier is dramatically reduced, to 1.89 kcal/mol with n = 3, and the corresponding product of hydrated ammonium bisulfate NH4HSO3-(H2O)2 is also stabilized thermodynamically. The present study shows that ammonia has larger kinetic and thermodynamic effects than water in promoting the hydrolysis reaction of SO2 in small clusters favorable in the atmosphere.

The reaction mechanisms and kinetics for DMS + O 3 ⇒ DMSO + O 2 in water vapor are studied using density functional theory. A series of reaction pathways are determined with molecular clusters containing the reacting species and up to three water molecules. The results show that the energy barrier, defined as the energy difference between the reactant complex and the transition state, decreases progressively as each water molecule is added to the reacting system. A decreasing energy barrier is attributed to favorable electrostatic interactions between the reacting species and water at the transition state and at the more polar product. Rate constants for the second-order reactions, involving different combinations of hydrated reactants up to three water molecules, are calculated using transition state theory with Eckart tunneling corrections. Effective rate constants for DMS + O 3 ⇒ DMSO + O 2 are obtained using the calculated second-order rate constants and the concentrations of hydrated reactants present in saturated water vapor. The results show that the rate of reaction for DMS + O 3 ⇒ DMSO + O 2 increases dramatically in the presence of water vapor, by up to seven orders of magnitude for reactions involving three water molecules. The study implies that the gas-phase reaction of DMS with ozone is significant in the troposphere and can greatly influence the global climate.

The structures of thiophene–hydrogen halide (HX) complexes are examined at the second-order Moller-Plesset (MP2) level to understand hydrogen bonding and intermolecular interactions of a heteroaromatic ring as the hydrogen bond acceptor. In contrast with the previous results on similar furan complexes, only the face-on geometry type is observed for the thiophene complexes. Variations in complex geometry can be accounted for by the differences in the electrostatic potential on the aromatic ring. A decomposition of interaction energy reveals that thiophene–hydrogen halide is bounded largely by orbital and electrostatic energy contributions.