Ab initio studies on the reactivity of the CF_3OCH_2O radical: Thermal decomposition vs. reaction with O_2

摘要

Hydrofluoroethers are being considered as potential candidates for third generation refrigerants. The present investigation involves the ab initio quantum mechanical study of the decomposition mechanism of CF _3OCH_2O radical formed from a hydrofluoroether, CF _3OCH_3 (HFE-143a) in the atmosphere. The geometries of the reactant, products and transition states involved in the decomposition pathways are optimized and characterized at the DFT (B3LYP) level of theory using 6-311G(d,p) basis set. Energy calculations have been performed at the G2(MP2) and G2M(CC,MP2) level of theory. Two prominent decomposition channels, C-O bond scission and reaction with atmospheric O_2 have been considered for detailed investigation. Studies performed at the G2(MP2) level reveals that the decomposition channel involving C-O bond scission occurs with a barrier height of 23.8 kcal mol~(-1) whereas the oxidative pathway occurring with O_2 proceeds with an energy barrier of 7.2 kcal mol~(-1). On the other hand the corresponding values at G2M(CC,MP2) are 24.5 and 5.9 kcal mol~(-1) respectively. Using canonical transition state theory (CTST) rate constants for the two pathways considered are calculated at 298 K and 1 atm pressure and found to be 5.9∈×∈10~(-6) s~(-1) and 2.3∈×∈10~(-5) s~(-1) respectively. The present study concludes that reaction with O_2 is the dominant path for the consumption of CF_3OCH_2O in the atmosphere. Transition states are searched and characterized on the potential energy surfaces involved in both of the reaction channels. The existence of transition state on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation.