RADIOLYSIS MECHANISMS IN CLATHRATES SYSTEMS: EPR SPECTROSCOPY INVESTIGATION OF THE LONG CHAIN CARBOXYLIC ESTERS UREA ADDUCTS

摘要

The mechanism of the gamma radiolysis of long chain aliphatic carboxylic esters from decanoic and hexadecanoic acids, in the state of urea clathrates has been investigated by EPR spectroscopy. The major species identified are the carboxylate anion-radicals, trapped electrons, and the neutral primary alkyl radicals of type RCH_2~· arising from the O-C and C-COOR bond scissions. Also present among the species trapped at 77 K are the secondary alkyl radicals -CH_2CHCH_2-formed by C-H bond rupture in the aliphatic chains. On warming the radical anions are converted into RCH_2~· species and intermediate acyl type radicals RC=O have been observed. At room temperature all the primary and secondary radicals gradually disappear, being partially converted into the α-carboxylate radicals RCH_2CHCOOR. This process is thought to involve the intrachain migration of the free valences by hydrogen abstraction. The comparison with the neat compounds has shown that the clathrate structure has a profound influence on the radiolysis mechanism the major effects observed being the increase of the O-C scission products, the increase of the radical anions yield and the decrease of the C-H bond ruptures in the aliphatic chains.