Understanding the kinetics and atmospheric degradation mechanism of chlorotrifluoroethylene (CF2CFCl) initiated by OH radicals

Abstract

The atmospheric degradation of chlorotrifluoroethylene (CTFE) by OH˙ was investigated using density functional theory (DFT). The potential energy surfaces were also defined in terms of single-point energies derived from the linked cluster CCSD(T) theory. With an energy barrier of −2.62 to −0.99 kcal mol⁻¹ using the M06-2x method, the negative temperature dependence was determined. The OH˙ attack on C_α and C_β atoms (labeled pathways R1 and R2, respectively) shows that reaction R2 is 4.22 and 4.42 kcal mol⁻¹, respectively, more exothermic and exergonic than reaction R1. The main pathway should be the addition of OH˙ to the β-carbon, resulting in ˙CClF–CF₂OH species. At 298 K, the calculated rate constant was 9.87 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹. The TST and RRKM calculations of rate constants and branching ratios were performed at P = 1 bar and in the fall-off pressure regime over the temperature range of 250–400 K. The formation of HF and ˙CClF–CFO species via the 1,2-HF loss process is the most predominant pathway both kinetically and thermodynamically. With increasing temperature and decreasing pressure, the regioselectivity of unimolecular processes of energized adducts [CTFE–OH]˙ gradually decreases. Pressures greater than 10⁻⁴ bar are often adequate for assuring saturation of the estimated unimolecular rates when compared to the RRKM rates (in high-pressure limit). Subsequent reactions involve the addition of O₂ to the [CTFE–OH]˙ adducts at the α-position of the OH group. The [CTFE–OH–O₂]˙ peroxy radical primarily reacts with NO and then directly decomposes into NO₂ and oxy radicals. “Carbonic chloride fluoride”, “carbonyl fluoride”, and “2,2-difluoro-2-hydroxyacetyl fluoride” are predicted to be stable products in an oxidative atmosphere.