Rajakumar Balla

The rate coefficients for the reactions of hydrogen (H) atom with propylene and isobutylene were studied at M06-2X/6-31+G(d,p) and MPWB1K/6-31+G(d,p) level of theories between 200 and 2500 K. The possible mechanism for the reactions of propylene and isobutylene with H atom were examined. The rate coefficients for each reaction channels were calculated over a wide range of temperature using Conventional Transition State Theory (CTST). The quantum mechanical tunneling effect was computed using parabolic model and were included in the rate coefficient calculations. The Arrhenius expressions for the reactions, propylene + H and isobutylene + H were estimated to be kpropylene+H = (9.68 ± 0.17) × 10−18 T2.16exp[−(131 ± 10)/T] cm3 molecule−1 s−1 and kisobutylene+H = (1.40 ± 0.22) × 10−16 T1.89exp[−(215 ± 12)/T] cm3 molecule−1 s−1, respectively. Theoretically calculated rate coefficients are found to be in good agreement with the available experimentally measured rate coefficients.

Cl-initiated photo-oxidation reaction of methyl propionate was investigated experimentally using relative rate method. Gas chromatography/mass spectrometry (GC-MS) and GC/infrared spectroscopy (GC-IR) were used as analytical tools to follow the concentrations of reactants and products during reaction. The gas-phase kinetics of methyl propionate with Cl atoms was measured over the temperature range of 263–363 K at 760 Torr in N2 atmosphere using C2H6 and C2H4 as reference compounds. The temperature-dependent rate coefficient for the reaction of methyl propionate with Cl atom was obtained as k(T) = [(3.25 ± 1.23) × 10−16] T2 exp [− (33 ± 4) / T] cm3 molecule−1 s−1. Theoretical calculations were also performed at CCSD(T)/cc-pVDZ//B3LYP/6-31G(d,p) level of theory, and the rate coefficients for H abstraction reactions were evaluated using canonical variational transition state theory (CVT/SCT) with interpolated single point energy (ISPE) method over the temperature range of 200–400 K. The rate coefficients over the studied temperature range yielded the Arrhenius expression k(T) = (7.22 × 10−16) T1.5 exp (466 / T) cm3 molecule−1 s−1. The reaction mechanism based on product analysis, thermochemistry, branching ratios, atmospheric implications, degradation pathways, and cumulative lifetime of methyl propionate is also presented in this manuscript.

The temperature-dependent rate coefficients were calculated for the reactions of Cl atoms with propene (R1), 1-butene (R2), 1-pentene (R3), and 1-hexene (R4) over the temperature range of 200–400 K. Canonical variational transition state theory (CVT) with small curvature tunneling (SCT) and conventional transition state theory (CTST) in combination with MP2/6-31G(d,p), MP2/6-31G+(d,p), and MP2/6–311 + G(d,p) level of theories were used to calculate the kinetic parameters. The obtained rate coefficients at 298 K for the reactions of Cl atoms with propene, 1-butene, 1-pentene, and 1-hexene are 1.36 × 10−10 cm3 molecule−1 s−1, 1.53 × 10−10 cm3 molecule−1 s−1, 4.61 × 10−10 cm3 molecule−1 s−1, and 4.76 × 10−10 cm3 molecule−1 s−1, respectively. In all these reactions, strong negative temperature dependence was observed over the studied temperature range. Cl atom addition across the double bond is the most dominant pathway. The contribution of abstraction channels towards their global rate coefficients was observed to be increasing from propene to 1-hexane. Atmospheric implications such as effective lifetimes and thermodynamic parameters of the test molecules were investigated in the present study.

The rate coefficients of hydroxyl radical (OH) reaction with limonene were computed using canonical variational transition state theory with small-curvature tunnelling between 275 and 400 K. The geometries and frequencies of all the stationary points are calculated using hybrid density functional theory methods M06-2X and MPWB1K with 6-31+G(d,p), 6-311++G(d,p), and 6-311+G(2df,2p) basis sets. Both addition and abstraction channels of the title reaction were explored. The rate coefficients obtained over the temperature range of 275-400 K were used to derive the Arrhenius expressions: k(T) = 4.06×10-34 T7.07 exp[4515/T] and k(T) = 7.37×10-25 T3.9 exp[3169/T] cm3 molecule-1 s-1 at M06-2X/6-311+G(2df,2p) and MPWB1K/6-311+G(2df,2p) levels of theory, respectively. Kinetic study indicated that addition reactions are major contributors to the total reaction in the studied temperature range. The atmospheric lifetime (τ) of limonene due to its reactions with various tropospheric oxidants was calculated and concluded that limonene is lost in the atmosphere within a few hours after it is released. The ozone production potential of limonene was computed to be (14-18) ppm, which indicated that degradation of limonene would lead to a significant amount of ozone production in the troposphere.

The reactivity of phenyl radical towards propionaldehyde and butyraldehyde was investigated by the CVT/SCT/ISPE method using the energetics computed at the CCSD(T)//B3LYP/cc-pVTZ level of theory over the temperature range of 200–2000 K. The rate coefficients (k × 1014 cm3 molecule−1 sec−1) for two aldehydes were computed to be 7.7 and 15.2 at 298 K. The branching ratios explained the contribution of H-abstraction reaction channels to the global reactivity. The degradative mechanism explained the formation of acids, aldehydes, and peroxy nitrates. The reactivity of these aldehydes towards phenyl radicals was also compared with their reactivity towards other atmospheric oxidants.

The rate coefficients of hydroxyl radical (OH) reaction with CF3CHFCH2F (HFC-245eb) were computed using G3MP2 and G3B3 theories between 200 and 400 K. Nine different transition states were identified for the title reaction and confirmed by intrinsic reaction coordinate (IRC) calculations. The contributions of all the individual hydrogen atoms in different rotamers for the title reaction were computed and compared with the results obtained by experimental methods and structure activity relationships (SAR). The rate coefficients for the title reaction were computed to be k = (1.57 ± 0.14) × 10-13 exp[-(690 ± 26)/T] cm3 molecule-1 s-1at G3MP2, and k = (0.97 ± 0.13) × 10-13 exp[-(513 ± 39)/T] cm3 molecule-1 s-1 at G3B3 theories. Theoretically calculated rate coefficients are found to be in good agreement with the experimental results. The tropospheric lifetime of CF3CHFCH2F because of its reaction with OH radicals are computed to be 2.52 and 2.14 years at G3MP2 and G3B3 level of theories, respectively. © 2010 Elsevier B.V. All rights reserved.