CHAPTER 19: Theoretical Studies on Mechanism and Kinetics of Atmospheric Chemical Reactions

A chemical reaction is defined as the conversion of one minimum energy compound into another minimum energy compound through possible intermediates and transition states. Theoretical studies on atmospheric chemical reactions are very important to learn about the degradation of pollutants in the atmosphere and their implications. The complexity of these reactions requires both experimental and theoretical approaches to characterize these reactions. Finding the stationary points on the potential energy surface of a reaction complex is the major task in the theoretical modeling of chemical reactions. Theoretical rate constants can be calculated based on transition state theory and results obtained from electronic structure calculations. In this chapter, the key concepts behind the identification and characterization of stationary points and reaction pathways are discussed. Basic ideas behind the transition state theory to calculate the rate constants are summarized. The application of electronic structure methods and transition state theory to study chemical reactions is explored by studying the atmospheric reaction between 2,3-dimethylphenol and OH radical. The secondary reactions resulting from the principal oxidation step play a vital role in determining the lifetime of the pollutant. The reaction between m-cresol, which is formed as a degradation product of 2,3-dimethylphenol and O₂, is studied in detail. The atmospheric implications of these reactions are also discussed.