Rajakumar Balla

A dual-level direct dynamic method is employed to estimate the addition and hydrogen abstraction rate coefficients for the reactions of cis-CHF=CHCHF2, trans-CHF=CHCHF2, CF2=CHCHF2 and CF2=C=CHF with hydroxyl radicals (OH) using variational transition state theory (VTST) with interpolated single-point energies (ISPE) at CCSD(T)/cc-pVTZ//M06-2X/6-31+G(d,p) level of theory. The rate coefficients of cis-CHF=CHCHF2, trans-CHF=CHCHF2, CF2=CHCHF2 and CF2=C=CHF + OH reactions were computed using Canonical Variational Transition state Theory (CVT) with Small Curvature Tunneling (SCT) in the temperature range of 200-400 K. It was found that the contribution of abstraction reactions toward the overall reaction to be insignificant, and hence negligible in comparison to the addition reactions in the studied temperature range of 200-400 K. The total rate coefficient for cis-CHF=CHCHF2, trans-CHF=CHCHF2, CF2=CHCHF2 and CF2=C=CHF + OH reactions were calculated to be 6.83 × 10-13, 3.17 × 10-12, 5.43 × 10-13 and 2.22 × 10-13 cm3 molecule-1 s-1, respectively at 298 K. The atmospheric life times of cis-CHF=CHCHF2, trans-CHF=CHCHF2CHF, CF2=CHCHF2 and CF2=C=CHF are estimated to be 16, 3.4, 21 and 57 days respectively at 277 K. The global warming potentials (GWPs) of title molecules at the time horizons of 20, 100 and 500 years were computed using the rate coefficients and radiative forcing values obtained in this study. Since these title molecules show significantly lower GWPs than many hydrofluorocarbons (HFCs) and chlorofluorocarbons (CFCs), it is conclude that these hydrofluoro-olefines (HFOs) may be possible replacements to HFCs in many industrial applications.

The rate coefficient for the gas-phase reaction of OH radical with α-pinene was measured at 298 K using relative rate methods, with propylene as a reference compound. The ratio of the rate coefficient for the reaction of OH radicals with α-pinene to that of with OH radicals with propylene was measured to be 1.77 ± 0.21. Considering the absolute value of the rate coefficient of the reaction of OH radicals with propylene as (3.01 ± 0.42)×10-11 cm3 molecule-1 s -1, the rate coefficient for the reaction of OH radicals with α-pinene was determined to be (5.33 ± 0.79)×10-11 cm3 molecule-1 s-1. To gain a deeper insight into the reaction mechanism, theoretical calculations were also carried out on this reaction. The rate coefficient of OH radical with α-pinene was calculated using canonical variational transition state theory with small-curvature tunnelling. The kinetics data obtained over the temperature range of 200-400 K were used to derive the Arrhenius expression: k(T) = 3.8×10-28 T5.2 exp[2897/T] cm3 molecule-1 s-1. The OH-driven atmospheric lifetime (τ) and ozone formation potential of α-pinene were calculated and reported in this work. © 2013 Taylor & Francis.

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.

Rate coefficient for the reaction of β-pinene with OH radicals was determined at 298K and 800Torr of N2 using the relative rate technique. Isobutylene was used as a reference compound and the concentrations of the organics were followed by gas chromatographic analysis. The rate coefficient for the reaction of β-pinene with OH radical was measured to be (9.35±2.79)×10-11cm3 molecule-1s-1. Theoretical kinetic calculations were also performed for the title reaction using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT). The kinetics data obtained over the temperature range of 200-400K were used to derive the Arrhenius expression: k(T)=8.24×10-23T3.41 exp[2421/T]cm3molecule-1s-1. The OH-driven atmospheric lifetime (τ) and global warming potential (GWP) for β-pinene were computed and concluded that β-pinene is very short lived (2.5h) in the Earth's atmosphere with a GWP of 1.6×10-2 at 20 years horizon of time and which is negligible. The ozone formation potential of β-pinene was also calculated and reported in this present work. © 2013 Elsevier Ltd.