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 NO_x 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.