Temperature-dependent long-range proton hyperfine coupling in pyrimidine H-addition radicals trapped in single crystals

Abstract

H-addition radicals at position C6 of 1-CH₃-uracil · HBr and 1-CH₃-cytosine · HBr are produced by X-irradiation of single crystals at 300 K. The coupling constants of the methylene β-protons are equivalent at 300 K. Upon cooling they become progressively non-equivalent while a long-range δ coupling appears simultaneously. This additional hyperfine splitting, with an isotropic coupling equal to 4·5–6·5 G, arises from one proton of the methyl group. INDO calculations support the view that the appearance of the long-range proton coupling is correlated with the non-equivalence of the methylene protons. Energy considerations favour a slight asymmetric disposition of the methylene protons relative to the ring, instead of a nonplanar ring, as the cause of the non-equivalence. Symmetry arguments indicate that the non-equivalence of methylene proton coupling is due to close-packing requirements.