Atmospheric chemistry models are used to predict the environmental impact of organic pollutants. To model the atmospheric fate of an organic pollutant such as the alternative fuel dimethyl ether, CH3OCH3 (DME), or dimethoxymethane, (CH3-O)2-CH2 (DMM), it is necessary to develop a chemical reaction mechanism that describes the degradation. A multi-disciplinary approach is used to develop the mechanism, which includes organic/atmospheric chemistry, theoretical computational chemistry, and sensitivity analysis.

Mechanism formulation entails using fundamental principles of atmospheric chemistry to propose credible reaction pathways for the degradation of DME and DMM (including both photochemical reactions and reactions with ambient species) and to estimate reaction rate constants. Reaction rate constants may be estimated using Structure Activity Relationships (SAR). Unfortunately, for many compounds (e.g., polyethers, halogenated compounds, nitrogen compounds, etc.) the SAR method can fail. Furthermore, it typically does not provide temperature dependent information. Experimental studies1 can provide information about reaction rate constants and about the formation of degradation products. However, these studies can be expensive, of questionable accuracy, and inconclusive when used to determine partitioning of multiple pathways.

The work presented here focuses on the use of computational chemistry and a novel dynamics approach2 to predict reaction rate constants. In this study, we illustrate how state-of-the-art theoretical computational chemistry techniques can be used to (a) map out the energetics of the proposed reaction mechanism (i.e., determine the activation energies), and (b) calculate the rate constants (kOH) for the important reactions in the mechanism. This methodology can also be applied to any of the other organic atmospheric pollutants.

1(a) Kwok, E.; Atkinson, R.; Atmospheric Environment 1995, 29 (14), 1685-1695; (b) Atkinson, R.; J. Phys. Chem. Ref. Data 1989, Monograph No. 1, 201-203. 2Runge, K. et al., J. Chem. Phys. 2000, 114, 5141.