Rate Coefficient for the Reaction of OH radical with (z)-3-hexene ? an Experimental-Theoretical Study

THAÍS DA SILVA BARBOSA; JAVIER A. BARRERA; SILVINA A. PEIRONE; JUAN P. ARANGUREN ABRATE; SILVIA I. LANE; GRACIELA ARBILLA; GLAUCO FAVILLA BAUERFELDT

Santiago

Congreso; 10th Congress of the World Association of Theoretical and Computational Chemists; 2014

Volatile organic compounds (VOCs) are emitted into the troposphere by both biogenic and anthropogenic sources, playing an important role in atmospheric chemistry. Unsaturated compounds from anthropogenic source, such as hexenes, are used in the production of polymers. In this work, the rate coefficient at 298 K was experimentally obtained for the addition reaction of OH to cis-3-hexene and, in parallel, a theoretical model for the reaction is investigated at the Density Functional Theory (DFT) level. The kinetics of the reaction was investigated using the relative rate method. The experimental set-up at INFIQC (Argentine) consisted of an 200 L Teflon bag located in a wooden box with the internal walls covered with aluminum foil, and operated at atmospheric pressure and 298 K. OH radicals were produced by the photolysis of H2O2 at 254 nm. The mixtures of (Z)-3-hexene and reference compounds with H2O2 were stable in the dark when left in the chamber for about 2 h. Photolysis reactions of the (Z)-3-hexene or the reference compounds were not observed. Relative rate coefficients were determined by comparing the rate coefficients for the reaction of OH radicals with the target compound to that with three reference compounds (cyclohexene, 2-buten-1-ol and allyl ether). The mean value for the second-order rate coefficient was (0.6±0.1)x10-10 cm3 molecule-1 s-1 . Theoretical calculations for the addition reaction of OH to cis-3-hexene have also been performed, at the DFT level with BHandHLYP functional with the cc-pVDZ and aug-cc-pVDZ basis sets, in order to investigate the reaction mechanism, to clarify the experimental observations and to model the reaction kinetics and dynamics. Two conformers of cis-3-hexene were located and considered in the mechanism, leading to three distinct pre-barrier complexes (for the up and down addition to conformer 1 and to the OH addition to conformer 2) stabilized by 4.72-6.11 kcal/mol in relation to the isolated reactants. The OH addition reactions to cis-3-hexene yield four distinct hydroxyl radicals, passing through four saddle points, all located above the corresponding -PCs, but below the isolated reactants, stabilized by 1.96-3.32 kcal/mol. This mechanism is in agreement with others, previously investigated in our laboratory. Rate Constants are finally evaluated from both the canonical (CVTST) and microcanonical (RRKM) variational transition state methods, the latter found in good agreement with the experimental data. Our results suggest that the theoretical mechanism, based on DFT calculations, is satisfactory for understanding the experimental results.