Reactions of O2+ with OH-isoprene adduct isomers: Exothermicity, product ions, and rate constants

Quantitative structure-property prediction of ion-molecule rate constants for proton transfer reaction between H3O+ and volatile organic compound

A quantitative structure-property relationship (QSPR) study based on multiple linear regression (MLR) and artificial neural network (ANN) techniques was carried out to investigate the ion-molecules rate constants for proton transfer reaction between hydronuim ion (H(3)O(+)) and some important volatile organic compounds (VOCs). A collection of 50 VOCs was chosen as data set that was randomly divided into three groups, training, internal and external test sets consist of 40, 5 and 5 molecules, respectively. A total of five independent variables selected by stepwise multilinear regression are electronic, geometric, topological type descriptors. The ANN model was developed by using the five descriptors appearing in the MLR model as inputs. Among developed models, the best QSPR model was the ANN model that produced a reasonable level of mean square error MSE(train) = 0.021, MSE(external) = 0.186, MSE(internal) = 0.110. The rate constants calculated by this model are in very good agreement with experimental values. The result of this study reveals the applicability of QSPR approaches in prediction of ion-molecules rate constants for proton transfer reaction of VOCs from their molecular structural descriptors.

Thermodynamics of the hydroxyl radical addition to isoprene

Oxidation of isoprene by the hydroxyl radical leads to tropospheric ozone formation. Consequently, a more complete understanding of this reaction could lead to better models of regional air quality, a better understanding of aerosol formation, and a better understanding of reaction kinetics and dynamics. The most common first step in the oxidation of isoprene is the formation of an adduct, with the hydroxyl radical adding to one of four unsaturated carbon atoms in isoprene. In this paper, we discuss how the initial conformations of isoprene, s-trans and s-gauche, influences the pathways to adduct formation. We explore the formation of pre-reactive complexes at low and high temperatures, which are often invoked to explain the negative temperature dependence of this reaction's kinetics. We show that at higher temperatures the free energy surface indicates that a pre-reactive complex is unlikely, while at low temperatures the complex exists on two reaction pathways. The theoretical results show that at low temperatures all eight pathways possess negative reaction barriers, and reaction energies that range from -36.7 to -23.0 kcal x mol(-1). At temperatures in the lower atmosphere, all eight pathways possess positive reaction barriers that range from 3.8 to 6.0 kcal x mol(-1) and reaction energies that range from -28.8 to -14.4 kcal x mol(-1).

Quantification of Hydroxycarbonyls from OH−Isoprene Reactions

Hydroxycarbonyls arising from OH-initiated reactions of isoprene have been quantified by the technique of a flow reactor coupled to proton-transfer reaction mass spectrometry (PTR-MS) detection. The yields of C5- and C4-hydroxycarbonyls are (19.3 +/- 6.1)% and (3.3 +/- 1.6)%, respectively, measured at a flow tube pressure of about 100 Torr and at a temperature of 298 +/- 2 K. A yield of (8.4 +/- 2.4)% is obtained for the unsaturated carbonyl C5H8O, confirming that internal OH addition represents the minor channel in the initial OH-isoprene reaction. The results show that those carbonyl compounds account for the most previously unquantified carbon, enabling the isoprene carbon closure. The study also reveals novel aspects of the delta-hydroxyalkoxy radical degradation mechanism, which is essential for modeling tropospheric O3 formation. In addition, this work demonstrates the application of PTR-MS for quantification of products of hydrocarbon reactions, which should have profound impacts on elucidation of the chemistry of atmospheric anthropogenic and biogenic hydrocarbons.