Dynamic effects in the E.S.R. spectra of the 5-hydro-6-methyl-6-yl-uracil radical in different solid matrices

Abstract

5-Hydro-6-methyl-6-yl-uracil (HMU) radicals generated from 5,6-dihydro-6-methyluracil or 6-methyluracil by X-irradication have been studied in single crystals, polycrystalline powders and aqueous glasses of both compounds. In all matrices the spectra of the HMU radicals show a strong temperature dependence attributable to the alternating linewidth effect. By simulation of the spectra exchange rates have been determined for the methylene protons at C5. It is shown that structure and conformation of the HMU radical are essentially independent of the precursor molecule and of the matrix. The differences observed in the temperature dependence of the spectra can be explained completely by thermal motions of the radicals, which are differently impeded by the different molecular packings of the respective matrices.