An Interdisciplinary Approach to Chemical Mechanism Development for Atmospheric Degradation of Organic Pollutants

Atmospheric chemistry models are used to predict the environmental impact of organic pollutants. To more accurately model the atmospheric fate of an organic pollutant such as the potential alternative diesel fuel dimethoxymethane, CH₃OCH₂OCH₃ (DMM), or trimethoxymethane, (CH₃O)₃CH (TMM), it is necessary to develop chemical reaction mechanisms that describe the degradation. Aspects of the methodology used to develop such chemical reaction mechanisms include theoretical computational chemistry and sensitivity analysis.

Mechanism formulation entails predicting all conceivable transformation products and estimating the reaction rate constants. Fundamental principles of atmospheric chemistry are used to propose possible reaction pathways for the degradation of DMM and TMM. Unfortunately, the prediction of rate constants can be troublesome since Structure Activity Relationships (SARs) have not been parameterized for many of the proposed reactions. Furthermore, the SAR expressions can fail for ether compounds such as these. In order to overcome this limitation, state-of-the-art theoretical computational chemistry techniques are used to (a) map out and understand the thermodynamics (Delta G, Delta H) of the proposed reaction mechanism, and (b) predict the reaction rate constants for the important reactions in the mechanism. Other researchers have performed experimental chamber studies with TMM¹ and DMM² to obtain reaction rate constants as well as information about the formation of various degradation products. The work presented here focuses on (1) the sensitivity analysis that was used to identify the reactions in the mechanism to which the model output is most sensitive (2) the computational chemistry used to understand the thermodynamics of the mechanistic pathways, and (3) the application of theoretical dynamics models to predict reaction rate constants. The results of the thermodynamic calculations and the sensitivity analysis can be used to focus the more expensive experimental and theoretical efforts on specific reactions in the putative reaction mechanism.

¹ J. Platz, J. Sehested, O.J. Nielsen, and T.J. Wallington, J. Phys. Chem. A 1999, 103, 2632-2640.

² T.J. Wallington, M.D. Hurley, J.C. Ball, A.M. Straccia, J. Platz, L.K. Christensen, J. Sehested, and O.J. Nielsen, J. Phys. Chem. A 1997, 101, 5302-5308.