Electron Spin Resonance Study of X‐Irradiated Single Crystals of Di‐n‐butyl Oxalate–Urea and Di‐n‐butyl Malonate–Urea Inclusion Compounds

Urea inclusion compounds have been used in numerous x-irradiation studies of aliphatic esters.1-4 Single crystals are easily prepared for ESR experi ments, and the hexagonal urea matrix provides a com mon environment for the various guest molecules. To date all published work on saturated and unsaturated ester-urea inclusion compounds indicates that the x-ray produced stable free radicals occur near the carboxyl end of the fatty acid residue. In saturated ester mole cules, the observed radicals simply correspond to re moval of one hydrogen atom from the carbon atom adjacent to the carboxyl group.1'2 Thus, the radicals observed in saturated esters are essentially the same as the well-known radicals5 derived from the carboxylic acids. The purpose of this note is to point out that in aliphatic esters the dominant radicals are not always produced adjacent to the carboxyl groups. As examples, we discuss below the free radicals observed in urea inclusion crystals of di-M-butyl oxalate and di-«-butyl