Light-induced displacement of hydrogen in pyrimidine H-addition radicals

Abstract
After x irradiation of single crystals of 1‐CH3‐uracil, 1‐CH3‐uracil⋅HBr, cytosine⋅H2O, and deoxycytidine⋅HCl at room temperature C5‐addition radicals, which result from H addition at carbon 5 of the unsaturated C5–C6 bond of the pyrimidine ring, are formed. It is shown that, upon irradiation with light of wavelength λ≳400 nm, these radicals transform into C6‐addition radicals where the added hydrogen atom is localized on carbon 6. INDO calculations show that, whereas C5‐addition radicals are neutral, the C6‐addition radicals must be positively charged in order to reproduce the experimental couplings.