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

Abstract

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.

Tropospheric Degradation of Perfluorinated Aromatics: A Case of Hexafluorobenzene

“…These degrees of freedom were treated as internal rotors and barrier of the internal rotation was calculated by scanning the rotational potential of the OH group. Furthermore, as demonstrated in our previous studies, [10,12] the relative translation of OH radical with respect to hexafluorobenzene in the prereaction complex was treated as a two-dimensional particle-in-the-box and the box size was set at 2.8 Å which is the diameter of benzene ring. Energy transfer, used in the Gillespie stochastic modeling, between prereaction complex and bath gas was modeled with the exponential down collision model at pressure of 1 atmosphere.…”

“…Energy transfer parameters are unknown for this system and a value of 175 cm -1 was assumed for exponential down model by analogy with monohalogenated benzenes. [10,12] Energy density was integrated by double array integration scheme with 7 cm -1 energy grain. The maximum energy from the coarser part of the double array was fixed at 85000 cm -1 .…”

“…Contrary to this, the number of studies on aromatic compounds is quite limited. The detailed investigations on reaction mechanisms and reaction kinetics are available only for benzene, [7,8] toluene, [9] fluorobenzene, [10] chlorobenzene, [11,12] phenol [13] and naphthalene. [14] The major reason for a small number of studies on aromatic compounds is their significant size and complex reaction mechanism with OH radicals, thus sophisticated theoretical methods must be used in order to obtain meaningful results.…”

“…[15][16][17][18] We have successfully developed a reliable methodology for the reaction rate constants and tropospheric lifetime of fluorinated alkanes [19] and halogenated alkenes. [18,20,21] In the last several years, we are extending those research efforts into the tropospheric degradation reactions of halogenated aromatic hydrocarbons and were successful in modeling gas-phase reactions of OH radicals with fluorobenzene [10] and chlorobenzene. [12] This study on hexafluorobenzene is the next step in our endeavor to produce methodology for calculating the tropospheric degradation rates of persistent organic pollutants (POPs), [22,23] normally perhalogenated or polyhalogenated biphenyls, diphenylethers, dibenzo-p-dioxins or -furans, and analogous chemicals (DDT, chlordane, heptachlor).…”

“…In our previous studies on monosubstituted halobenzenes, [10,12] it was proven that theoretical methods MP2 and G3 can give relevant information on the reaction mechanism of OH radical addition to haloaromatic systems as well as on the corresponding thermodynamic data. Those results coupled with the RRKM (Rice-RamspergerKassel-Marcus) kinetic theory have also successfully reproduced the experimental reaction rates.…”