Low-temperature EPR and quantum chemical study of lactone radical cations and their transformations

Radical cations of a number of lactones (β-butyro-, γ-butyro-, γ-valero-, δ-hexano-, δ-valero- and ε-capro-) were radiolytically generated in CF3CCl3 matrix and investigated by EPR spectroscopy. The primary radical cation of the 4-membered ring β-butyrolactone is unstable even at 77 K and undergoes spontaneous ring opening and fragmentation, leading to the deprotonated neutral (CH2CHCH2)radical. The stability of the primary carbonyl-centred radical cations of the 5-, 6- and 7-membered lactone rings towards intramolecular H-shift from the C1 in α-position to carbonyl oxygen depends primarily on the ring size, which determines the activation energy of the transformation and distance L(H-O) of the carbonyl oxygen to the nearest H-atom on the ring. The larger the ring, the smaller the L(H-O) and also activation energy of the H-shift, making the transformation of the primary radical cation more feasible. The quantum chemical calculations facilitated the interpretation of the EPR spectra of the secondary radical cations.