Radiation Damage in Solid 5-halouracils: Free Radicals in Single Crystals of 5-fluorouracil

Summary

X-irradiation at 300 K of single crystals of 5-fluorouracil results in the formation of two different radical species observable by e.s.r. and ENDOR-spectroscopy One is an α-fluoro radical RĊF(CH₂)R' formed by saturation of the 5,6-double bond of the pyrimidine ring. The principal values of its α-fluorine interaction are 170, −9 and −18 G; the isotropic part of the methylene β-proton couplings are 46·3 and 28 G, respectively; the g-tensor has principal values of 2·0029, 2·0066, and 2·0052. The other radical species is formed by enolization of the C₄ carbonyl function. The resulting spin-density distribution gives rise to three observable interactions, an α-proton (−5·2, −14·9, −11·2 G) at C₆, an OH-proton from the C_4⁻ OH group (−1·7, −5·0, −4·1 G) and a residual α-fluorine interaction (0,0, 11·2 G). Irradiation at 77 K yields an e.s.r.-pattern which is tentatively assigned to the molecular anion radical. These findings are related to the radiation chemistry of solid 5-halouracils.