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

HFO-1345fz (CFCFCH═CH 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 O have been analyzed. The formation of gaseous products from the interaction of HFO-1345fz with OH in the absence and presence of NO 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 cm molecule s, which compares well with the 1.24 × 10 cm molecule s 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.