Endor studies of free radicals trapped in X-ray irradiated single crystals of caffeine hydrochloride dihydrate

Abstract

ENDOR spectroscopy has been used to identify two different radical species in caffeine hydrochloride dihydrate X-irradiated at room temperature. One radical was identified as that formed by the net abstraction of a hydrogen atom from N(9). This radical is equivalent to that which would be produced by the loss of an electron from a neutral caffeine molecule, the caffeine cation. The other radical, which dominated the EPR spectrum, was identified as that resulting from net addition of a hydrogen atom to C(8) of the protonated caffeine molecule. The identification of this radical was based on the analysis of four different proton hyperfine couplings. Both the nitrogen ( 14N) hyperfine and quadrupole coupling tensors for a ring nitrogen atom were also obtained from the ENDOR spectra, and are attributed to the N(7) nucleus. The observation of 14N ENDOR lines, provided a second, independent estimate of the unpaired spin density centred on N(7). An indirect second-order effect, giving rise to a non-crossing phenomenon, was observed between the methylene protons, which were also found to be non-equivalent. The structure of this radical agrees with those determined previously by EPR and by INDO molecular orbital calculations for the analogous species in other purine derivatives.