Oxidation Mechanism and Kinetics of cyc-CF ₂ CF ₂ CF═CH– (HFO-c1325yz) Initiated by ^• OH Radicals and Fate of Its Product Radicals in the Presence of O ₂ and HO ₂ ^•

The current work presented the investigation of the atmospheric repercussions of reactions of 1,3,3,4,4-pentafluorocyclobutene (cyc-CF2CF2CF═CH–, HFO-c1325yz) with the •OH radical using quantum chemical methods. To understand the reaction mechanism, we first explored the potential energy surface of the title reaction using M11/6-311++G(d,p) and CCSD(T)//M11/6-311++G(d,p) methods. The estimated relative energy and thermodynamic parameters for both methods reveal that the •OH addition reactions to the C═C bond of cyc-CF2CF2CF═CH– are more favorable than the H- and F-abstraction reactions by the •OH radical. Additionally, kinetic analysis demonstrates that these two •OH addition pathways are faster than the H- and F-abstraction reaction pathways. The calculated overall rate coefficient (3.68 × 10–14 cm3 molecule–1s–1) using variational transition state theory (VTST) at 298.15 K agrees with the experimental result. Additionally, we evaluated the atmospheric lifetime, global warming potential (GWP), and photochemical ozone creation potential (POCP) of this compound. Furthermore, the degradation pathways of the product radicals are analyzed by using the same quantum chemical methods in the presence of O2 and HO2•. The key products formed in the degradation pathways include stable compounds like CF2═CHOH, CF2═CFOH, F(O)CCH(OH)CF2COF, HOF, and COF2, along with radicals such as F(O)CCF2(·), H(O)CCF2(·), F(O)CCH(OH)(·), and H(O)CCF(OH)(·). Moreover, to understand the photolytic degradation mechanism, a time-dependent density functional theory (TD-DFT) calculation by the M11/6-311++G(d,p) method was performed for cyc-CF2CF2CF═CH– as well as for all intermediates and products. The TD-DFT calculations suggest that most products are susceptible to photodissociation, except F(O)CCH(OH)CF2COF and COF2, which exhibit photolytic stability under sunlight.