Understanding the Atmospheric Oxidation of HFE-7500 (C3F7CF(OC2H5)CF(CF3)2) Initiated by Cl Atom and NO3 Radical from Theory.

Kinetics and mechanistic pathways for atmospheric oxidation of HFE-7500 ( n-C3F7CF(OCH2CH3)CF(CF3)2) initiated by Cl atom and NO3 radical have been studied using density functional theory. Oxidative degradation pathways facilitated by H-abstraction from the -OCH2 or -CH3 groups in HFE-7500 have been considered. It has been shown that H-abstraction from the α-site (-OCH2) is favored over other reaction pathways. The rate constants were computed employing transition-state theory and canonical variation transition-state theory incorporating small curvature tunnelling correction, over the temperature range of 250-450 K at atmospheric pressure. Calculated rate constants at 298 K and 1 atm compare well with earlier experiments. Temperature dependence of the rate constants and branching ratios for these pathways contributing to overall reaction are described. It has been shown that the rate constants over the studied temperature range was found to fit well to the modified Arrhenius equation (in cm3 molecule-1 s-1) kCl = 1.10 × 10-14  T0.04 exp(-69.87 ± 1.41/T) and kNO3 = 7.66 × 10-26 T3.30 exp(596.40 ± 1.22/T). Standard enthalpies of formation for the reactant (C3F7CF(OCH2CH3)CF(CF3)2) and the products [C3F7CF(OC•HCH3)CF(CF3)2 and C3F7CF(OCH2C•H2)CF(CF3)2] during H-abstraction are derived using the isodesmic approach. Atmospheric implications of the titled molecule are presented.

Kinetics and Mechanistic Investigations of Atmospheric Oxidation of HFO-1345fz by OH Radical: Insights from Theory.

HFO-1345fz (CF3CF2CH═CH2 or 3,3,4,4,4-pentafluoro-1-butene) belongs to a class of hydrofluoro-olefins and represents a new generation of potential foam expansion agents. Its atmospheric impact and environmental acceptability can be estimated from the studies of kinetics and mechanism of its oxidative degradation. The molecular insights accompanying the reaction pathways in terms of the characterization of intermediates or products and radiative properties should prove useful for large-scale industrial applications. Systematic mechanistic gas-phase kinetics investigations on the reactivity of HFO-1345fz with the •OH facilitating a variety of degradation routes have been carried out employing the M06-2x-based density functional theory. Structure and energetics of different reaction pathways such as hydrogen abstraction, •OH addition, isomerization-dissociation, or interaction with atmospheric O2 have been analyzed. The formation of gaseous products from the interaction of HFO-1345fz with •OH in the absence and presence of NOx atmospheric conditions has been reported. Calculated branching ratios have shown that the addition channel dominates such oxidative degradation, whereas the abstraction channel contributes negligibly to the global rate constant and addition of •OH to the terminal carbon is favored over the nonterminal one. The rate constants for all reaction channels were computed by conventional transition state theory (TST) and canonical variation transition state theory (CVT) including small curvature tunneling (SCT) over the temperature range of 200-1000 K at atmospheric pressure. The CVT calculated rate constant for the reaction at 298 K was shown to be 1.17 × 10-12 cm3 molecule-1 s-1, which compares well with the 1.24 × 10-12 cm3 molecule-1 s-1 as obtained from TST and is in excellent agreement with the experiments reported earlier. The atmospheric lifetime, radiative efficiency, and global warming potential (GWP) have also been obtained.

Theoretical investigation on the kinetics and branching ratio of the gas phase reaction of sevoflurane with Cl atom.

The present work deals with the theoretical investigation on the Cl initiated H-atom abstraction reaction of sevoflurane, (CF3)2CHOCH2F. A dual-level procedure has been adopted for studying the kinetics of the reaction. Geometrical optimization and frequency calculation were performed at DFT(BHandHLYP)/6-311G(d,p) while single-point energy calculation was made at CCSD(T)/6-311G(d,p) level of theory. The intrinsic reaction coordinate (IRC) calculation has also been performed to confirm the smooth transition from the reactant to product through the respective transition state. The rate constants were calculated using conventional transition state theory (TST). It has been found that 99 % of the reaction proceeded via the H-atom abstraction from the ­CH2F end of the sevoflurane. The rate constant of the dominant path is found to be 1.13 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹. This is in excellent agreement with the reported experimental rate constant of 1.10 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ obtained by relative rate method using FTIR/Smog chamber and LP/LIF techniques.