Gas-Phase Reaction Kinetics of Pyruvic Acid with OH Radicals: The Role of Tunneling, Complex Formation, and Conformational Structure

The gas-phase reaction of pyruvic acid (PA) with the OH radical is studied theoretically using accurate quantum chemistry and transition state theory. Two chemically distinct H-atom abstraction reactions and two distinct OH addition reactions have been identified. The rate coefficients for these four processes were calculated. Quantum tunneling was included in each rate using the small curvature tunneling method. The influence of the conformational structure of PA was found to be particularly intriguing. While the trans-cis structure was found to dominantly react by H-atom abstraction from the methyl site, the trans-trans conformer was found to react mostly through H-atom abstraction from the acid site. A general formalism was developed to model the kinetics of the reactions that involve multiple conformers, interconverting prereactive complexes, and multiple transition states. Comparison of the results obtained with available experimental rate observations reveals agreement with the trans-trans conformer of PA but disagreement with the results obtained for a full statistical mixture of reagents. The role of these reactions in the atmospheric processing of PA is discussed.