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The pi-pi* and n-pi* valence excited states of dibenzo-p-dioxin (DD) were studied through the complete active space SCF and multiconfigurational second-order perturbation theory employing the cc-pVDZ basis set and full pi-electron active space of 16 electrons in 14 active orbitals. Studies into dependence of the oscillator strengths on the extent of the butterfly-like folding showed that the electronic spectrum is more consistent with a folded equilibrium geometry assumed by DD in solution.

A combination of the multiconfigurational complete active space self-consistent field (CASSCF) and density functional theory (DFT) is employed to study the minimum structures, harmonic vibrational frequencies and electronic spectra of dibenzofuran (DF) and its polychlorinated derivatives. The comparison of the electron transitions in DFs and dioxins reveal various dissimilarities in the origin of the singlet states and a great analogy of the triple states.

A theoretical investigation into unimolecular decomposition paths of primary (POZF) and secondary (SOZF) fluorozonide is carried out by utilizing the multiconfigurational complete active space self-consistent field (CASSCF)/cc-pVTZ level in optimizations of the stationary points and calculations of the harmonic vibrational frequencies.

The reaction C2H5Cl+·OH→C2H5Cl·+H2O (α and β abstraction) has been investigated by ab initio molecular orbital theory with several basis sets and levels of correlation. Optimized geometries and harmonic vibrational frequencies have been calculated for all reactants, transition states, and products at the (U)HF/6-31G(d,p) and (U)MP2/6-31G(d,p) levels of theory. The correlation energy is found to play an important role in determining the barrier heights and reaction enthalpies as well as the geometry and the vibrational frequencies of the transition states. A pseudocyclic transition state is found to be favorable to the β-abstraction reaction since the participation of the chlorine substituent reduces the barrier height by 0.95 kcal/mol, through a relatively large inductive through-space effect. The best results for the barrier heights and reaction enthalpies have been obtained using the second-order Mo\ller–Plesset perturbation theory with spin projection employing the 6-311+G(2d,p) basis set. A satisfactory agreement is found with available experimental values. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A detailed study was conducted to investigate the reaction mechanisms and regioselectivity of ethene, fluoroethene and chloroethene reactions with hydroxyl (OH) radicals. Four stationary points were found along the MEP and geometries were optimized while vibrational frequencies were calculated at the UMP2/6-311+G(2d.p) level of theory. Structures of pre-reaction complexes determined at the reactant side of the MEP are characterized by the weak interaction between the hydrogen atom the OH radical and the pi electron density.

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. G3 calculations give higher reaction barriers than MP2, but also a significantly deeper prereaction complex minimum. The rate constants, calculated with Rice-Ramsperger-Kassel-Marcus (RRKM) theory using G3 energies, are much faster and in much better agreement with the experiment than those calculated with MP2 method, as the deeper well favors the formation of prereaction complex and also increases the final relative populations of adducts. The discrepancies between the experimental and calculated rate constants are attributed to the errors in calculated frequencies as well as to the overestimated G3 reaction barriers and underestimated prereaction complex well depth. It was possible to rectify those errors and to reproduce the experimental reaction rates in the temperature range 230-310 K by treating the relative translation of OH radical and fluorobenzene as a two-dimensional particle-in-the-box approximation and by downshifting the prereaction complex well and reaction barriers by 0.7 kcal mol−1. The isomeric distribution of fluorohydroxycyclohexadienyl radicals is calculated from the reaction rates to be 30.9% ortho, 22.6% meta, 38.4% para, and 8.3% ipso. These results are in agreement with experiment that also shows dominance of ortho and para channels. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]