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

Abstract Radical cations of a number of lactones ( β -butyro-, γ -butyro-, γ -valero-, δ -hexano-, δ -valero- and e -capro-) were radiolytically generated in CF 3 CCl 3 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 (CH 2 CHCH 2 ) 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.