Formation of 3,4-dihydrofuran radical cation through intramolecular H-shift: quantum chemical calculations and low-temperature EPR study

Abstract

Intramolecular transformations of dihydrofuran (DHF) radical cations radiolytically generated in Freon matrix were investigated using low temperature EPR spectroscopy. Radical cation 2,4-DHF + has been identified earlier as a product of intramolecular rearrangement of unstable primary oxygen-centred radical cation of 2,5-DHF. 2,4-DHF + is stable at 77 K but its further intramolecular rearrangements through 2→3 and 3→4 H-shifts can be induced by illumination with visible light. Both transformations proceed simultaneously, with more-or-less same yields of the two isomers of the DHF radical cation, namely 2,3-DHF + and 3,4-DHF + . The latter is a newly identified species, stable at 77–145 K, characterised by hfs splitting constants a(2 H)=1.59 mT and a(4 H)=2.82 mT and is the only of the four isomers of DHF radical cation showing a strong optical absorption in visible. The interpretation of the experimental results is strongly supported by DFT quantum chemical calculations.