Investigating atmospheric implications for the reaction of ●OH with CH2=C(CH3)-CX3, X=(H,F): Perspective on future refrigerant

Abstract

The Montreal protocol aimed at phasing out refrigerants with high global warming potentials (GWPs), high ozone depletion potential, and then expanding research in developing refrigerants that are more environmentally friendly. Therefore, the investigation of the atmospheric implications on the hydrofluoroolefins 2-methylpropene (MP) and 2-trifluoromethylpropene (tFMP) were done using computational means. These compounds have major applications as refrigerant. The thermochemistry of the process of removal was studied using the PW6B95/Def2-TZVP. This has resulted in the exothermic and spontaneous reactions of MP, tFMP with ^●OH radicals. The enthalpy and free energy of reactions as well as the bond dissociations show that the methyl group, -CH₃, easily releases the H-atom than the other sites of hydrogen abstraction. The kinetics of the said reactions were performed through the MPWB1K/Def2-QZVPP//Def2-TZVP method. The branching ratios of the reaction channels were studied and show that the H-abstraction from the =CH₂ contributes more to rate constant than the other sites containing H-atom. The latter was calculated over a temperature range of 200 – 400 K and was accurately fitted. The rate coefficient at 298 K and 1 atm yielded 9.28 × 10^-14 and 2.16 × 10^-13 cm³molecule^-1s^-1. These allowed to assess the atmospheric lifetimes of MP and tFMP, which gave 124.7 days and 53.6 days, respectively. Using the B3LYP/6-31 G (3df,p) level of theory, the radiative forcing efficiencies (REs) were estimated through the IR absorption cross section. The potential energy distribution (PED) of vibrational modes shows that RE is mainly obtained by the torsional vibration H-C-C-C for MP and the stretching of C-F bonds for tFMP. The global warming potentials (GWPs) of MP and tFMP, obtained using their atmospheric lifetimes and REs, yielded low values over the time horizon times 20, 100, and 500 years. The functional group -CF₃ showed lowering effect of GWP. The estimated photochemical ozone creation potential (POCP) are also low for both compounds indicating their very low contribution to ozone production in the troposphere. Therefore, MP and tFMP are both environmentally friendly compounds.