No abstract

As the result of biogenic and anthropogenic activities, large quantities of chemical compounds are emitted into the troposphere. Alkanes, in general, and cycloalkanes are an important chemical class of hydrocarbons found in diesel, jet and gasoline, vehicle exhaust emissions, and ambient air in urban areas. In general, the primary atmospheric fate of organic compounds in the gas phase is the reaction with hydroxyl radicals (OH). The oxidation by Cl atoms has gained importance in the study of atmospheric reactions because they may exert some influence in the boundary layer, particularly in marine and coastal environments, and in the Arctic troposphere. The aim of this paper is to study of the atmospheric reactivity of methylcylohexanes with Cl atoms and OH radicals under atmospheric conditions (in air at room temperature and pressure). Relative kinetic techniques have been used to determine the rate coefficients for the reaction of Cl atoms and OH radicals with methylcyclohexane, cis-1,4-dimethylcyclohexane, trans-1,4-dimethylcyclohexane, and 1,3,5-trimethylcyclohexane at 298 ± 2 K and 720 ± 5 Torr of air by Fourier transform infrared) spectroscopy and gas chromatography-mass spectrometry (GC-MS) in two atmospheric simulation chambers. The products formed in the reaction under atmospheric conditions were investigated using a 200-L Teflon bag and employing the technique of solid-phase microextraction coupled to a GC-MS. The rate coefficients obtained for the reaction of Cl atoms with the studied compounds are the following ones (in units of 10(-10) cm(3) molecule(-1) s(-1)): (3.11 ± 0.16), (2.89 ± 0.16), (2.89 ± 0.26), and (2.61 ± 0.42), respectively. For the reactions with OH radicals the determined rate coefficients are (in units of 10(-11) cm(3) molecule(-1) s(-1)): (1.18 ± 0.12), (1.49 ± 0.16), (1.41 ± 0.15), and (1.77 ± 0.23), respectively. The reported error is twice the standard deviation. A detailed mechanism for ring-retaining product channels is proposed to justify the observed reaction products. The global tropospheric lifetimes estimated from the reported OH- and Cl-rate coefficients show that the main removal path for the investigated methylcyclohexanes is the reaction with OH radicals. But in marine environments, after sunrise, Cl reactions become more important in the tropospheric degradation. Thus, the estimated lifetimes range from 16 to 24 h for the reactions of the OH radical (calculated with [OH] = 10(6) atoms cm(-3)) and around 7-8 h in the reactions with Cl atoms in marine environments (calculated with [Cl] = 1.3 × 10(5) atoms cm(-3)). The reaction of Cl atoms and OH radicals and methylcylohexanes can proceed by H abstraction from the different positions.

Relative rate coefficients for the gas-phase reaction of chlorine atoms (Cl) and hydroxyl radicals (OH) with 1,8-cineole were determined by Fourier-transform infrared (FTIR) spectroscopy between 285 and 313 K at atmospheric pressure. The temperature dependence of both reactions shows simple Arrhenius behaviour which can be represented by the following expressions (in units of cm(3) molecule(-1) s(-1)): k(1,8-cineole+OH)=(6.28 ± 6.53) × 10(-8) exp[(-2549.3 ± 155.7)/T] and k(1,8-cineole+Cl)=(1.35 ± 1.07) × 10(-10) exp[(-151.6 ± 237.7)/T]. Major products of the titled reactions were identified by solid-phase microextraction (SPME) coupled to a GC-MS. Additionally, the first step of the reaction was theoretically studied by ab initio calculations and a reaction mechanism is proposed.

The reactions of Cl with a series of fluoroalcohols and deuterated fluoroalcohols, CF(3)CH(2)OH (k(4)), CF(3)CH(OH)CH(3) (k(5)), CF(3)CH(OH)CF(3) (k(6)), CF(3)CH(OD)CF(3) (k(7)) and CF(3)CD(OD)CF(3) (k(8)), are investigated as a function of temperature in the range of 268-378 K by laser photolysis-resonance fluorescence. To our knowledge, only the CF(3)CH(2)OH + Cl reaction has been previously studied from a kinetic point of view. The derived Arrhenius expressions obtained using our kinetic data are: k(4) = (1.79+/-0.17) x 10(-13) exp[(410+/-26)/T], k(5) = (1.20+/-0.11) x 10(-12) exp[(394+/-14)/T], k(6) = (2.32+/-0.18) x 10(-13) exp[-(740+/-12)/T], k(7) = (6.45+/-1.87) x 10(-13) exp[-(1136+/-94)/T] and k(8) = (4.19+/-1.09) x 10(-13) exp[-(1378+/-81)/T] (in units of cm(3) molecule(-1) s(-1) and where errors are +/-sigma). Moreover, a theoretical insight into the mechanisms of these reactions is pursued through ab initio Möller-Plesset second-order perturbation treatment calculations with the 6-311G** basis set. Optimized geometries are obtained for reagents, transition states and molecular complexes appearing along the different reaction pathways. Furthermore, molecular energies are calculated at the quadratic configuration interaction with single, double and triple excitations [QCISD(T)] level to obtain an estimation of the activation energies. Finally, the rate constants are calculated through transition-state theory using Wigner's transmission coefficient in order to include the tunnelling-effect corrections.