CIDNP DETECTED FLASH PHOTOLYSIS OF cis–syn 1,3 DIMETHYLTHYMINE DIMER

Abstract— Theanthraquinone–2-sulfonate photo-sensitized splitting of cis-syn 1,3-dimethylthymine dimer gives rise to large CIDNP effects in the reaction product 1,3-dimethylthymine. The polarization originates from a radical ion pair formed by electron transfer from the dimer to the triplet state sensitizer. In a deoxygenated solution the sign of the polarization of theC–6 proton is reversed compared to the predicted one on account of the CIDNP sign rules. In an aerated solution the correct sign is observed. This can be accounted for by assuming reduction of the lifetime of the radical pair in the presence of oxygen. The time-resolved photo-CIDNP technique was used to study the time dependence of the 1,3-dimethylthymine signal. To account for this time-dependence a cation radical disproportionation reaction is proposed.