A Screening Tool for the Environmental Impact of New Fluids

F. Louis, C. Gonzalez, V. L. Orkin, M. J. Kurylo, and R. E. Huie

Objective: To develop a screening tool, based on quantum mechanics, for the prediction of the environmental impact of new classes of halogenated compounds.

Problem: Experimental studies from this Division have demonstrated that the reactivity of the hydroxyl radical toward halogenated organic compounds is not adequately correlated by simple structure-activity relationships. This was particularly evident when an ether linkage was introduced, where even the order of reactivity could not be predicted. This implied that it would be necessary to measure rate constants for a large number of members of any new class of reactants in order to predict the environmental impact of these possible new solvents, refrigerants, or fire suppressants. It was clear that a new approach was needed.

Approach: The basic idea underlying this project is to establish a level of theory which will predict the reactivity of the hydroxyl radical with a series of simple molecules, at the lowest possible degree of computational difficulty. This level of theory is then applied to more complex molecules and, ultimately, it is applied to the new class of interest. Then it is validated by a limited number of experimental determinations. The reality of the approach is much more complex and multidimensional.

Results and Future Plans: In the initial study, several levels of theory were explored for the reaction of OH with CH₂Br₂. This study included the treatment of tunneling in three different manners. This molecule was chosen both because of the importance of bromine as a fire suppressant and because the relatively large electron system of the molecule makes this reaction a serious test of the various levels of theory. Building on the results of this study, the reactions of OH with the other halogen-substituted methanes, up to bromine, were investigated. From these studies, we chose a level of theory and have been investigating the reactions of OH with several fluoroethanes and the ethers derived from them. These pairs were chosen to represent the extremes of behavior observed experimentally: an increase in reactivity upon addition of the ether linkage; a reduction in reactivity; and a small change in reactivity upon addition of the ether linkage. Theory has been able to reproduce the observed trends, with predictions in absolute reactivity within a factor of three. For bis-(difluoromethyl) ether, we have carried out a more exhaustive theoretical analysis of the reaction surface in order to understand better these reactions in general. At the present, we are further refining this approach, with a particular emphasis on better tunneling corrections and the use of pseudo-potentials. Calculations are being extended up to ethers with several carbons and containing fluorine and one or more bromine atoms. In order to verify these calculations, a sample of 2-bromo-1,1-difluoroethyl methyl ether is being synthesized for us, which we will use for an experimental determination of the rate constant.

Publications:

Orkin, V.L., Villenave, E., Huie, R.E., and Kurylo, M.J., "Atmospheric Lifetimes and Global Warming Potentials of Hydrofluoroethers: Reactivity Toward OH, UV Spectra, and IR Absorption Cross Sections," J. Phys. Chem. A (in press).

Louis, F.,Gonzalez, C., Huie, R.E., and Kurylo, M.J., "An Ab Initio Study of the Reaction of Halomethanes with the Hydroxyl Radical. Part 1: CH₂Br₂," J. Phys Chem. A (in press).