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Electron-nuclear double resonance studies of free radicals trapped in irradiated single crystals of cytosine monohydrate and caffeine hydrochloride dihydrate Lenard, Derek Roy
The technique of electron-nuclear double resonance spectroscopy (ENDOR) has been used to determine the identity and structure of radicals trapped in x-irradiated single crystals of cytosine monohydrate and caffeine hydrochloride dihydrate. The radical studied in cytosine mono-hydrate had previously been observed by several workers using electron paramagnetic resonance (EPR) but its identity was in question. All intramolecular and several intermolecular proton hyperfine couplings were analyzed in detail. The experimentally determined coupling parameters were compared with ones obtained by theoretical means for several possible radical species. The agreement between the observed and calculated parameters fit best for the radical formed by net addition of a hydrogen atom to 0(2) of cytosine, with the hydroxyl proton lying in the nodal plane of the π-system. The appearance of twice the number of certain ENDOR lines than expected from the crystal symmetry was interpreted by postulating that the radical was stabilized in two different conformations at 77°K. The assignment of the hyperfine couplings to specific protons was supported by an ENDOR study of a partially deuterated cytosine monohydrate crystal, grown from heavy water. Three different radical species were identified in caffeine hydrochloride dihydrate x-irradiated at room temperature. The EPR spectrum of this system, which had not previously been reported, was too complex to be analyzed. Furthermore, the EPR spectra of two of the radicals studied by ENDOR could not be distinguished in the overall EPR lineshape. One of these radicals had a lifetime of only a few hours and was tentatively identified as the methyl radical, primarily on the basis of the observed isotropic proton hyerpfine coupling constant of -62.65 MHz. The other radical was indefinitely stable and was identified as that formed by net abstraction of a hydrogen atom from N(9) of the caffeine moiety. This radical is equivalent to that which would be produced by the loss of an electron from a neutral caffeine molecule, the caffeine cation; as such, it is the first reported cation radical in a purine derivative. The 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. Nitrogen (¹⁴N) hyperfine and quadrupole coupling tensors were also obtained from the ENDOR spectra and were attributed to N(7). The observation of ¹⁴N-ENDOR lines, which had not previously been reported in any pyrimidine or purine derivative, 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 was found to agree with those determined previously by EPR and by molecular orbital calculations for the analogous species in other purine derivatives.
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