Atmospheric oxidation of halogenated aromatics: comparative analysis of reaction mechanisms and reaction kinetics.

Atmospheric transport is the major route for global distribution of semi-volatile compounds such as halogenated aromatics as well as their major exposure route for humans. Their major atmospheric removal process is oxidation by hydroxyl radicals. There is very little information on the reaction mechanism or reaction-path dynamics of atmospheric degradation of halogenated benzenes.

Reactions of 2-Propanol Radical with Halogenated Organics in Aqueous Solution: Theoretical Evidence for Proton-Coupled Electron Transfer and Competing Mechanisms.

The reactions of α-hydroxyalkyl radicals in aqueous medium are of interest because they exhibit a rich variety of fundamentally important competing mechanisms, such as proton-coupled electron transfer (PCET), hydrogen atom transfer, free radical substitutions, abstractions and additions, etc. We present a theoretical study of the mechanism and kinetics of the aqueous reactions of α-hydroxyisopropyl (2-propanol) radical with four halogenated organic substrates: iodoacetate (IAc), iodoacetamide (IAm), 5-bromouracil (5-BrU), and carbon tetrachloride (CCl). The reactions are studied using density functional theory (DFT) (M06-2X), and the solvent is modeled as a polarizable continuum, either without the explicit solvent molecules or with one added water molecule.

Atmospheric oxidation of hexachlorobenzene: New global source of pentachlorophenol.

Hexachlorobenzene is highly persistent, bioaccumulative, toxic and globally distributed, a model persistent organic pollutant. The major atmospheric removal process for hexachlorobenzene is its oxidation by hydroxyl radicals. Unfortunately, there is no information on the reaction mechanism of this important atmospheric process and the respective degradation rates were measured in a narrow temperature range not of environmental relevance.

Mechanisms and reaction-path dynamics of hydroxyl radical reactions with aromatic hydrocarbons: the case of chlorobenzene.

All geometries and energies significant for the first step of tropospheric degradation of chlorobenzene were characterized using the MP2/6-31+G(d,p) and G3 methods. A pre-reaction complex for the addition of OH radical to chlorobenzene was found and the associated transition state was determined for the first time. The reaction path for the association of OH radical and chlorobenzene into the pre-reaction complex was extrapolated from the selected low frequency normal mode of pre-reaction complex.

Theoretical study on the mechanism and kinetics of addition of hydroxyl radicals to fluorobenzene.

Geometries, frequencies, reaction barriers, and reaction rates were calculated for the addition of OH radical to fluorobenzene using Möller-Plesset second-order perturbation (MP2) and G3 methods. Four stationary points were found along each reaction path: reactants, prereaction complex, transition state, and product. A potential for association of OH radical and fluorobenzene into prereaction complex was calculated, and the associated transition state was determined for the first time.

CASSCF/CASPT2 and TD-DFT study of valence and Rydberg electronic transitions in fluorene, carbazole, dibenzofuran, and dibenzothiophene.

A combination of multireference CASSCF/CASPT2 and time-dependent DFT (TD-B3P86) theoretical treatments was employed to test their predictions against recently proposed assignments of the vacuum-UV spectra of fluorene and its three heteroanalogues-dibenzofuran, carbazole, and dibenzothiophene-up to the ionization threshold. For the low-lying transitions, the theoretically based assignments are generally not problematic because of the well-resolved bands, although, even in this region, the two methods yield some opposing predictions. Further on toward the vacuum region, the assignments prove increasingly challenging because of predicted crowding of transitions, many of which exhibit significant intensity.

Bioavailability of xenobiotics in the soil environment.

It is often presumed that all chemicals in soil are available to microorganisms, plant roots, and soil fauna via dermal exposure. Subsequent bioaccumulation through the food chain may then result in exposure to higher organisms. Using the presumption of total availability, national governments reduce environmental threshold levels of regulated chemicals by increasing guideline safety margins.

Evaluation of artificial intelligence based models for chemical biodegradability prediction.

This study presents a review of biodegradability modeling efforts including a detailed assessment of two models developed using an artificial intelligence based methodology. Validation results for these models using an independent, quality reviewed database, demonstrate that the models perform well when compared to another commonly used biodegradability model, against the same data. The ability of models induced by an artificial intelligence methodology to accommodate complex interactions in detailed systems, and the demonstrated reliability of the approach evaluated by this study, indicate that the methodology may have application in broadening the scope of biodegradability models.

Theoretical study of structure, vibrational frequencies, and electronic spectra of dibenzofuran and its polychlorinated derivatives.

Minimum structures and harmonic vibrational frequencies of dibenzofuran (DF), 2,3,7,8-tetrachlorodibenzofuran (TCDF), and octachlorodibenzofuran (OCDF) were calculated using the multiconfigurational complete active space self-consistent field (CASSCF) and density functional theory (DFT) methods. The electronic transitions in these compounds were studied via the single-state multireference second-order perturbation theory (CASPT2) based on the CASSCF(14,13) references, as well as the time-dependent DFT (TD-B3P86) employing the cc-pVDZ (CASSCF/CASPT2) and 6-31G(d,p) (TD-B3P86) basis sets. The B3P86 geometry and harmonic vibrational frequencies of ground state DF agree very well with the experimental data, and the CASSCF/CASPT2 excitation energies and oscillator strengths are accurate enough to provide a reliable assignment of the absorption bands in the 200-300 nm region.

Ozonolysis of fluoroethene: theoretical study of unimolecular decomposition paths of primary and secondary fluorozonide.

A theoretical investigation into unimolecular decomposition paths of primary (POZF) and secondary (SOZF) fluorozonide was carried out by utilizing the multiconfigurational CASSCF/cc-pVTZ level in optimizations of the stationary points and calculations of the harmonic vibrational frequencies. The dynamical electron correlation was accounted for via the multireference CASPT2/cc-pVTZ treatment based on the zeroth-order CASSCF/cc-pVTZ reference. The CASPT2 was substituted with the CCSD(T)/6-311G(2d,2p) correction whenever the former resulted in negative activation barriers.

Dibenzo-p-dioxin. An ab initio CASSCF/CASPT2 study of the pi-pi* and n-pi* valence excited states.

The pi-pi* and n-pi* valence excited states of dibenzo-p-dioxin (DD) were studied via the complete active space SCF and multiconfigurational second-order perturbation theory employing the cc-pVDZ basis set and the full pi-electron active spaces of 16 electrons in 14 active orbitals. The geometry and harmonic vibrational wavenumbers of the ground state correlate well with the experimental and other theoretical data. In particular, significant improvements over previously reported theoretical results are observed for the excitation energies.

Theoretical study of structure, vibrational frequencies, and electronic spectra of polychlorinated dibenzo-p-dioxins.

The structure, vibrational frequencies, and excited states of 2,3,7,8-tetrachloro-, 1,4,6,9-tetrachloro-, and octachlorodibenzo-p-dioxin (2,3,7,8-TCDD, 1,4,6,9-TCDD, and OCDD) were studied via complete active space SCF followed by the multireference second-order perturbative approach (CASSCF/CASPT2), as well as the time-dependent density functional theory (TD-B3LYP). The cc-pVDZ basis set and the full pi-electron active spaces of 16 electrons in 14 active orbitals were employed. Whereas 2,3,7,8-TCDD assumes a planar D(2)(h)() minimum, 1,4,6,9-TCDD and OCDD are slightly folded exhibiting the C(2)(v)() symmetry.