Theoretical calculations of rate coefficients for the OH addition reactions to C2-C4 alkenes

VICTOR S. LOPES, THAÍS S. BARBOSA, JAVIER A. BARRERA, SILVINA PEIRONE, SILVIA I. LANE, GRACIELA ARBILLA, GLAUCO F. BAUERFELDT

Congreso; 10th. Congress of the World Association of Theoretical and Computational Chemists (WATOC) 2014; 2014

Among the most intriguing reactions of atmospheric interest are the OH additions to unsaturated compounds. From the Atmospheric Chemistry point of view, the OH radical is the most important daytime oxidant for the chemical removal of pollutants, from either anthropogenic or biogenic sources. The unsaturated compounds are included in this group and, especially the alkene + OH reactions have received great attention since the 1970s. Rate constants were shown to follow negative temperature dependence, with a non-Arrhenius behavior. Despite all the experimental observations and knowledge, considerable effort is still devoted to the theoretical description of these reactions. In this work, we try to answer some open issues: the dependence and sensitivity of the rate constants with the level of theory and the origin of electronic effects that govern the differences in reactivity of the unsaturated compounds.Theoretical calculations were performed for several alkenes (ethene, propene, methyl-propene, 1-butene, E-2-butene and Z-2-butene) at the DFT (Density Functional Theory) level, adopting the BHandHLYP functional and the cc-pVDZ, cc-pVTZ, aug-cc-pVDZ and aug-cc-pVTZ basis sets. Calculations at ab initio levels (MP2, CCSD(T) and QCISD) were also performed for comparison. Rate Constants were evaluated from both the canonical (CVTST) and microcanonical (RRKM) variational transition state methods.The alkene + OH reactions were best described at the BHandHLYP/aug-cc-pVDZ level. Pre-barrier complexes (-PC) are stabilized (with respect to isolated reactants) by 2.5 ? 3.6 kcal/mol. Saddle points (TS(c) and TS(t), for the OH addition to the central or terminal carbon atoms, respectively) also lie below the reactants energies, although with zero-point vibrational energy (zpe) corrections energy differences become slightly positive. Barrier heights are, however, less sensitive to the chain size or structure. For the calculation of the rate constants, the adoption of the RRKM method was proved to be crucial (Figure 1).Figure 1: Theoretical (RRKM/BHandHLYP/aug-cc-pVDZ, solid lines) and Experimental (circles) Rate Coefficients. Dashed Lines are the RRKM results, including the corrections for the internal rotation.The good agreement of our results with the experimental available data represents not only a satisfactory quantitative prediction of rate coefficients but a fundamental contribution to the understanding at the microscopic molecular level of the observed experimental results.