Theoretical study of the oxidation reactions of sulfurous acid/sulfite with ozone to produce sulfuric acid/sulfate with atmospheric implications.

Herein, theoretical studies were performed on the atmospheric oxidation of sulfurous acid (HSO) and sulfite ions (HSO ) by ozone (O) to produce sulfuric acid and hydrosulfate ions. The most favorable path for the HSO + O reaction has been found to be initiated from concerted H-abstraction and oxygen addition, with an overall energy barrier of 18.3 kcal mol.

Hydrolysis of Sulfur Dioxide in Small Clusters of Sulfuric Acid: Mechanistic and Kinetic Study.

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.

Mechanism of the gaseous hydrolysis reaction of SO₂: Effects of NH₃ versus H₂O.

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.

Theoretical study of the hydrogen abstraction of substituted phenols by nitrogen dioxide as a source of HONO.

The mild yet promiscuous reactions of nitrogen dioxide (NO2) and phenolic derivatives to produce nitrous acid (HONO) have been explored with density functional theory calculations. The reaction is found to occur via four distinct pathways with both proton coupled electron transfer (PCET) and hydrogen atom transfer (HAT) mechanisms available. While the parent reaction with phenol may not be significant in the gas phase, electron donating groups in the ortho and para positions facilitate the reduction of nitrogen dioxide by electronically stabilizing the product phenoxy radical.

A combined experimental and theoretical study of dinitrosyl iron complexes containing chelating bis(diphenyl)phosphinoX (X = benzene, propane and ethylene): X-ray crystal structures and properties influenced by the presence or absence of π-bonds in chelating ligands.

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)], {DPPX = 1,2-bis(diphenylphosphino)benzene (), 1,3-bis(diphenylphosphino)propane (), and cis-1,2-bis(diphenylphosphino)ethylene ()} 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 for complexes and , and 1708 and 1660 cm for , indicating that ligand acts as a stronger σ-donor.

Theoretical study on stable small clusters of oxalic acid with ammonia and water.

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.

Ab initio calculations of the Ar-ethane intermolecular potential energy surface using bond function basis sets.

The intermolecular potential energy surface (PES) of argon with ethane has been studied by ab initio calculations at the levels of second-order Møller-Plesset perturbation (MP2) theory and coupled-cluster theory with single, double, and noniterative triple configurations (CCSD(T)) using a series of augmented correlation-consistent basis sets. Two sets of bond functions, bf1 (3s3p2d) and bf2 (6s6p4d2f), have been added to the basis sets to show a dramatic and systematic improvement in the convergence of the entire PES. The PES of Ar-ethane is characterized by a global minimum at a near T-shaped configuration with a well depth of 0.

Theoretical study on the structure and stabilities of molecular clusters of oxalic acid with water.

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.

Titanium-mediated rearrangement of cyclopropenylmethyl acetates to (E)-halodienes.

TiCl(4) and TiBr(4) rapidly transform cyclopropenylmethyl acetates to (E)-halodienes via ring-opening to allyl-vinyl cations. DFT calculations suggest that the regioselectivity of the halogenation of this cationic intermediate by [TiX(4)OAc](-) is under thermodynamic control, while the stereoselectivity is governed by kinetics.

Interpreting vibrational sum-frequency spectra of sulfur dioxide at the air/water interface: a comprehensive molecular dynamics study.

We investigated the solvation and spectroscopic properties of SO(2) at the air/water interface using molecular simulation. Molecular interactions from both Kohn-Sham (KS) density functional theory (DFT) and classical polarizable models were used to understand the properties of SO(2):(H(2)O)(x) complexes in the vicinity of the air/water interface. The KS-DFT was included to allow comparisons with vibrational sum-frequency spectroscopy through the identification of surface SO(2):(H(2)O)(x) complexes.

Theoretical study of the quantitative structure-activity relationships for the toxicity of dibenzo-p-dioxins.

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.

Correlations and predictions of carboxylic acid pKa values using intermolecular structure and properties of hydrogen-bonded complexes.

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.

Structures, reductive dechlorination, and electron affinities of selected polychlorinated dibenzo-p-dioxins: density functional theory study.

Density functional theory calculations were performed to obtain the structures, vertical electron affinities, and adiabatic affinities of 15 polychlorinated dibenzo-p-dioxins (PCDDs), including several extremely toxic congeners. A three-parameter hybrid density functional, B3LYP, was utilized with two different basis sets, 6-311G(d,p) and 6-311+G(2d,2p). The optimized structures of all PCDDs under consideration were planar, while all corresponding anions attained nonplanar geometries.

Theoretical study on the chemical properties of polybrominated diphenyl ethers.

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.

Physical origin for the nonlinear sorption of very hydrophobic organic chemicals in a membrane-like polymer film.

Bioconcentration factor (BCF) is often assumed to be linearly associated with the octanol-water partition coefficient K(ow) for hydrophobic organic chemicals (HOCs). However, a large amount of data has suggested that the correlation between the logBCF and logK(ow) is curvilinear for HOCs. Similar curvilinear relationship has also been noticed for sorption of HOCs into poly(dimethyl)siloxane (PDMS), a polymer with cross-linked interior structures.

Efficient bond function basis set for pi-pi interaction energies.

Systematic study has been carried out to investigate the accuracy of mid-bond functions in describing pi-pi interactions in benzene dimer. Potential energy curves are calculated for the sandwich, T-shaped, and parallel-displaced configurations of benzene dimer by adding bond functions in MP2 (second-order Møller-Plesset perturbation theory) calculations with a wide range of split-valence and augmented, correlation-consistent basis sizes. At MP2 level, the largest basis set used with a bond function (denoted aug-cc-pVDZf-6s6p4d2f) differs by only approximately 0.

Molecular structures and properties of the complete series of bromophenols: Density functional theory calculations.

The complete series of 19 bromophenols have been studied by density functional theory (DFT) calculations at the B3LYP/6-311G++(d,p) level. The molecular structures and properties of bromophenols are strongly influenced by intramolecular hydrogen bonding of ortho-bromine, steric and inductive effects of substituted bromine, and other intramolecular electrostatic interactions. Systematic trends in several structural parameters and molecular properties of bromophenols have been found with the increasing number of bromine substitutions, including increase in O-H bond length, decrease in C-O bond length, red shift in O-H stretching frequency, and blue shift in O-H torsional frequency.

Theoretical study of hydrogen-bonded complexes of chlorophenols with water or ammonia: Correlations and predictions of pKa values.

A simple practical method for predicting the acidity constants (as pKa values) of chlorophenols is proposed based on density functional theory calculations of a series of hydrogen-bonded complexes of phenol and 19 different congeners of chlorophenol, with a single probe molecule, either water or ammonia. Relevant structural parameters and molecular properties of these complexes, primarily involving the acidic hydroxyl group, are examined and plotted against the known pKa values of 14 chlorophenols and phenol. Strong linear correlations are found for these compounds.

Reaction mechanisms and kinetics for the oxidations of dimethyl sulfide, dimethyl disulfide, and methyl mercaptan by the nitrate radical.

This study examines the initial oxidation routes of the three major reduced sulfur compounds (CH(3)SH, CH(3)SCH(3), and CH(3)SSCH(3)) by the nitrate radical using density functional and ab initio methods. Stationary points along each reaction pathway are examined using different levels of theory and basis sets to ensure the convergence of the results. Kinetics calculations follow on the determined reaction pathways to obtain the rate constants.

Correlations and predictions of pKa values of fluorophenols and bromophenols using hydrogen-bonded complexes with ammonia.

Density functional theory calculations have been preformed on a series of the hydrogen-bonded fluorophenol-ammonia and bromophenol-ammonia complexes. Intermolecular and intramolecular properties, particularly those related to hydrogen bonding, have been carefully analyzed. Several properties, such as the bond length and stretching frequency of the hydroxyl group, the hydrogen bond length and binding energy, are shown to be highly correlated with each other and are linearly correlated with known experimental pKa values of the halogenated phenols.

Asymmetrical linear structures including three-electron hemibonds or other interactions in the (ABA)-type triatomic cations: Ne3+, (He-Ne-He)+, (Ar-Ne-Ar)+, (Ar-O-Ar)+, (He-O-He)+, and (Ar-He-Ar)+.

By the counterpoise geometry optimization at the level of CCSD(T)aug-cc-pVDZ, the asymmetrical linear structures with all the real frequencies were obtained for the triatomic cations of (ABA)+ type: Ne3+, (He-Ne-He)+, (Ar-Ne-Ar)+, (Ar-He-Ar)+, (He-O-He)+, and (Ar-O-Ar)+. The validity of this optimization method is confirmed by comparing with the method of the potential-energy surface for the calculations of Ne3+ and (He-Ne-He)+. Using the molecular-orbital theory, it is found that the interaction within the triatomic cations is dominated by the contribution from the first two atoms while the contribution from the third atom is small.

Potential energy surfaces for HenNe+ ions: ab initio and diatomics-in-molecule results.

The potential energy surface of He2Ne+ has been reinvestigated using a combination of ab initio and diatomics-in-molecule (DIM) calculations. In contrast to the reports of two recent studies the ion is found to have an asymmetric linear He-Ne-He structure, with no barrier to formation from the separated atoms on the ground-state surface. The He-Ne+ bond lengths at the potential minimum are 1.

Concanavalin A in a dimeric crystal form: revisiting structural accuracy and molecular flexibility.

A structure of native concanavalin A (ConA), a hardy perennial of structural biology, has been determined in a dimeric crystal form at a resolution of 1.56 A (space group C222(1); unit-cell parameters a = 118.70, b = 101.