ESR study of photodecomposition mechanism of a long-lived radical perfluoro-2,4-dimethly-3, ESR spectrum of trifluoromethyl radical formed during solid-phase photodecomposition at 77 K in glassy matrix

摘要

Formation under some perfluorocompounds radiolisys of long-lived radicals (LR) incapable to mutual recombination in liquid was first discovered by our group. This work is concerned the [(CF{sub}3){sub}2 CF]{sub}2 C·C{sub}2F{sub}5 (LRl) formed during radiolysis or fluorination of hexafluoropropylene trimer (HFPT) by addition of F atom to double bond of HFPT. During the heating or UV irradiation of LRl its decomposition takes place to give a molecule of perfluoroolefine and ·CF3 radical initiating the polymerization of monomers but subjected to fast recombination in liquid. ESR study of mechanism of photolytic decomposition of LR1 and the analysis of spectrum of ·CF3 radical formed during solid-phase photodecomposition of LRl at 77 K has been performed. A{sub}‖ = 25.15 mT, A{sub}⊥ = 9.1 mT, and g{sub}‖ = 1.9996, g{sub}⊥ = 2.0056 parameters were found by using computer-simulated ESR spectra. The further destiny of ·CF3 radicals formed under the decomposition depends on the phase state of the matrix. About 90% of them recombine in liquid at 300 K. The rest attaches to the HFPT molecule giving another long-lived [(CF{sub}3){sub}2 CF]{sub}3 C ·radical. On photolysis at 77 K, a part of ·CF3 radicals is stabilized in the glassy HFPT matrix. The ·CF3 radicals formed during γ-radiolysis of HFPT at 77 K are not stabilized. Quantum-chemical calculations showed that the mechanism of LR1 photodecomposition is different from the mechanism of thermal decomposition. That is the consequence of the fact that decay of photoexcited radical is predetermined by a radical structure and results in whereas the reaction of ·CF3 detachment from CF-CF{sub}3 group being as twice as less endothermic than that from CF{sub}2-CF{sub}3 group (48-56 kJ/mole instead 98-104 kJ/mole) is to be dominant under thermolisys. The LR1 photodecomposition red cut-off has experimentally shown to be 320nm. In accordance with this the simulations manifest that LRl electronic excitation of 4eV is sufficient for its decomposition, and, contrary to thermolisys, ·CF3 radical detachment proceeds from CF{sub}2-CF{sub}3 group.