Predictions of Environmental Properties of Halogenated Organics by a Computational Chemistry Screening Tool
The abstraction of hydrogen atoms by hydroxyl radicals is the determining factor in the tropospheric lifetimes of most saturated organic compounds, including halogenated species containing one or more C-H bonds. The kinetics of thesereactions has attracted considerable attention from experimentalists and theoreticians. Ab initio studies on the kinetics of these reactions have been limited, however, and no theoretical study has been published for predicting the kinetic parameters of the reactions involving bromine-containing halomethanes. This is undoubtedly due to the significant computational expense involved in the treatment of large electronic systems containing bromine atoms. This results in a particularly acute problem for the assessment of the environmental suitability of candidate replacement halons. Thus, we have set about developing a computational tool that will allow the prediction of the reactivity of potential replacements towards the hydroxyl radical.The plan has been to find the minimum level of ab initio molecular orbital theory, suitable for predicting the reactivity of a set of well-known halocarbons, and apply this level of theory uniformly to the prediction of the reactivity of new species. This was done on a series of halomethanes. For these compounds, the aim was to describe the reaction enthalpy, the evolution of the rate constant as function of the temperature, and to gain insight into the reactivity trends along the series of reactions:OH + CHXYZ H2O + CXYZ (X, Y, Z = H, F, Cl or Br)The values of Arrhenius parameters and their evolution along the series of reactions estimated by these calculations will be discussed and compared to the experimental values. Excellent agreement is observed between the calculated rate constants at room temperature and their experimental counterparts.The next step in this process has been to extend these studies to a series of bromine-containing halomethanes for which experimental data are not available: CH2FBr, CHFBr2, CHFClBr, CHCl2Br, and CHClBr2. This has been done both by the use of ab initio calculations employing transition-state theory and by utilizing the computed heats of reaction. The estimated uncertainties associated with these predictive tools will be discussed along with the extension of the techniques to largermolecules.