Reaction Path and Rate Coefficients Calculations for the Reactions of OH radicals with the Unsaturated Alcohols: 2-methyl-2-propen-1-ol, 3-buten-1-ol and 2-buten-1-ol

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 Computacional Chemists WATOC 2014; 2014

Unsaturated Alcohols are emitted into the atmosphere as primary pollutants from both anthropogenic and biogenic sources. Among them, 2-methyl-2-propen-1-ol (MPO221), 3-buten-1-ol (31BO) and 2-buten-1-ol (21BO) are of particular interest. In this work, the OH addition to MPO221, 31BO and 21BO have been studied at the DFT (Density Functional Theory) level, employing the BHandHLYP functional and the cc-pVDZ and aug-cc-pVDZ basis sets. The thermochemical properties of equilibrium have been determined within the conventional statistical thermodynamics relations and the rate coefficients have been determined on the basis of the microcanonical variational transition state theory (mCVT). The adoption of the mCVT method was proved to be crucial. The rate coefficients obtained for the OH reactions with MPO221, 31BO and 21BO at 298.15 K deviate, respectively, 27%, 10% and 13% from the experimental rate coefficient available in the literature. A non-Arrhenius profile is observed for the rate coefficients. The Arrhenius parameters for each reaction in the temperature range 250-350 K are (in units: 10-11 cm3 molecule-1 s-1 and kcal/mol, for MPO221, 31BO and 21BO, respectively): 0.73 and -1.31, 0.44 and -1.40 and 0.58 and -1.47. All these values show good agreement with the experimental results. Moreover, it is worth comparing the reactivity of the unsaturated alcohols with the alkene analogues. The values of the rate coefficients for the reactions of MPO221, 31BO and 21BO with OH radicals are greater than those for the methyl-propene, 1-butene and 2-butene (Z or E), suggesting that the substitution of the hydrogen atom in an alkene by the ?OH functional group, increases the reactivity toward the hydroxyl radical. The results found in this work can be justified by specific attractive interactions, probably due to hydrogen bonds, that stabilize the stationary points on the unsaturated alcohol + OH potential energy surface compared to those on the alkene + OH surface. The pre-barrier complex is the most affected species in these mechanisms by such interactions. The saddle points generally form five or six-membered rings stabilized also by hydrogen bonds, which are not present in the related alkene + OH saddle points. The electronic interactions that stabilize both pre-barrier complexes and saddle-points are highlighted as the cause of the enhanced reactivity of the unsaturated alcohol with respect to its alkene analogue.