Low-temperature EPR study of radical cations of 2,5- and 2,3-dihydrofuran and their transformations in freon matrices

Abstract

Using low-temperature EPR spectroscopy, transformations of radical cations of 2,3- and 2,5-dihydrofuran (DHF) radiolytically generated in freon matrices (CFCl₂CF₂Cl and CF₃CCl₃) were investigated. In addition, quantum chemical calculations were performed to obtain information on the electronic structure of the transient species observed. The radical cation 2,3-DHF⁺˙ (a(H)/mT: 4.3(2H), 1.4(2H), 1.5(1H), 0.5(1H)) formed in frozen 2,3-DHF solutions is stable at 77 K. At higher temperatures it transforms into dihydrofuryl radical DHF˙ (a(H)/mT: 3.6(2H), 1.3(2H), 0.2(1H)). The oxygen-centred radical cation 2,5-DHF⁺˙ (a(H)/mT: 8.4(4H), 0.5(2H)) is rather unstable at 77 K and transforms via a first-order process (half life 2.3 or 4 min, according to the matrix type), which is apparently an intramolecular H-shift, into a transient distonic radical cation 2,4-DHF⁺˙ (a(H)/mT: 3.9(2H), 4.0(1H), 3.6(1H), 2.3(1H), 0.02(1H)). The latter yields at higher temperatures again the DHF˙ radical. Transformations of the DHF radical cations into the dihydrofuryl radical proceed most likely by ion–molecule reactions, when due to the softening of the matrix diffusion becomes possible. At high DHF concentrations the DHF˙ radical is formed in the case of 2,5-DHF already during irradiation at 77 K and in the case of 2,3-DHF after warming to 95 K. This is explained by the presence of preformed DHF dimer agglomerates favouring deprotonation of the DHF cation (the attached DHF molecule acts as the proton acceptor). The most stable species, the dihydrofuryl radical, finally disappears by a simple bimolecular recombination reaction.