Low-temperature EPR and quantum chemical study of lactone radical cations and their transformationsстатья
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Аннотация:Radical cations of a number of lactones (beta-butyro-, gamma-butyro-, gamma-valero-, delta-hexano-, delta-valero- and epsilon-capro-) were radiolytically generated in CF3CCl3 matrix and investigated by EPR spectroscopy. The primary radical cation of the 4-membered ring beta-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 carbonylcentred radical cations of the 5-, 6- and 7-membered lactone rings towards intramolecular H-shift from the Cl in alpha-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. (c) 2004 Elsevier Ltd. All rights reserved.