Mechanistic Quantum Chemical Insights into Cl-Atom Initiated Oxidation of Cyc-CF ₂ CF ₂ CF = CH- (HFO-c1325yz) and Subsequent O ₂ /HO ₂ ^• Mediated Degradation of Its Product Radicals and Ecotoxic

This work presents a quantum chemical investigation of the atmospheric oxidation mechanism of 1,3,3,4,4-pentafluorocyclobutene (cyc-CF 2 CF 2 CF=CH–, HFO-1325yz) initiated by Cl atoms. The potential energy surface (PES) of the title reaction was explored at the M11/6-311++G(d,p) and CCSD(T)//M11/6-311++G(d,p) methods. Thermodynamic and energetic evaluations clearly indicate that Cl-atom addition to the C═C bond is substantially more favorable than the competing H-abstraction pathway. Further, rate constant calculations using variational transition state theory indicate that the addition pathways exhibit significantly higher reaction rates than the H-abstraction pathway and unambiguously dominate the overall oxidation mechanism. The overall rate constant at 298.15 K is estimated to be 4.52 × 10 –11 cm 3 molecule –1 s –1, corresponding to an atmospheric lifetime of approximately 25.6 days, suggesting a relatively short persistence and a low global warming potential (GWP) over various time horizons. Secondary degradation processes of the primary product radicals in the presence of O 2 and HO 2 • were also examined to elucidate subsequent oxidation routes. The major stable products identified include CF 2 =CHCl, CF 2 =CFCl, and COF 2, alongside several key radicals such as F(O)CCF 2 (•), H(O)CCF 2 (•), F(O)CCH(Cl)(•), and H(O)CCF(Cl)(•). Ecotoxicity assessments of HFO-c1325yz and its primary oxidation products toward three representative aquatic organisms-fish, daphnia, and green algae- are classified as “Harmful”. On the other hand, the secondary degradation products are largely nontoxic. Further, TD-DFT analysis indicates that HFO-c1325yz and some stable products are photochemically stable under tropospheric conditions, whereas selected oxidation products cyc-CF 2 CF 2 CF(Cl) • CH- (P2) and cyc-CF 2 CF 2 CF=C • - (P3) exhibit absorption in the near-UV/visible region and may undergo photolytic degradation. In general, this work sheds comprehensive insight into the reaction mechanisms, kinetics, and environmental implications of Cl-initiated oxidation of HFO-c1325yz, contributing to a deeper understanding of its atmospheric fate and potential ecological impact.