Rate coefficient for the reaction of OH radicals with (z)-3-hexene: an experimental-theorectical study

THAÍS S. BARBOSA, JAVIER A. BARRERA, SILVINA PEIRONE, JUAN P. ARANGUREN ABRATE, SILVIA I. LANE, GRACIELA ARBILLA, GLAUCO F. BAUERFELDT

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

Volatile organic compounds (VOCs) are emitted into the troposphere by both biogenic andanthropogenic sources, playing an important role in atmospheric chemistry. Unsaturatedcompounds from anthropogenic source, such as hexenes, are used in the production ofpolymers. In this work, the rate coefficient at 298 K was experimentally obtained for theaddition reaction of OH to cis-3-hexene and, in parallel, a theoretical model for the reaction isinvestigated at the Density Functional Theory (DFT) level.The kinetics of the reaction was investigated using the relative rate method. The experimentalset-up at INFIQC (Argentine) consisted of an 200 L Teflon bag located in a wooden box withthe internal walls covered with aluminum foil, and operated at atmospheric pressure and 298K. 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 chamberfor about 2 h. Photolysis reactions of the (Z)-3-hexene or the reference compounds were notobserved. Relative rate coefficients were determined by comparing the rate coefficients forthe 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 ratecoefficient 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 beenperformed, at the DFT level with BHandHLYP functional with the cc-pVDZ and aug-cc-pVDZbasis sets, in order to investigate the reaction mechanism, to clarify the experimentalobservations 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-barriercomplexes (for the up and down addition to conformer 1 and to the OH addition to conformer2) stabilized by 4.72-6.11 kcal/mol in relation to the isolated reactants. The OH additionreactions to cis-3-hexene yield four distinct hydroxyl radicals, passing through four saddlepoints, all located above the corresponding -PCs, but below the isolated reactants, stabilizedby 1.96-3.32 kcal/mol. This mechanism is in agreement with others, previously investigatedin our laboratory. Rate Constants are finally evaluated from both the canonical (CVTST) andmicrocanonical (RRKM) variational transition state methods, the latter found in goodagreement with the experimental data.Our results suggest that the theoretical mechanism, based on DFT calculations, is satisfactoryfor understanding the experimental results.