Unimolecular proton-transfer photoreactions in 2,4-dithiouracil and 6-aza-2,4-dithiouracil: A matrix isolation study

Unimolecular proton-transfer photoreactions were studied for 2,4-dithiouracil and 6-aza-2,4-dithiouracil isolated in low-temperature Ar matrices. The IR spectra have revealed that before UV irradiation the matrix-isolated dithiouracils adopted exclusively the dithione tautomeric forms. UV (λ > 320 nm) irradiation of the matrices led to transfer of protons from N(1)-H or N(3)-H groups to the sulfur atoms of the C(2) = S and C(4) = S thiocarbonyl moieties. The ultimate products of these photoreactions were dithiol tautomeric forms of the studied dithiouracils. For 6-aza-2,4-dithiouracil, photogeneration of a second species was clearly observed. This photoproduct was identified as the thiol–thione tautomer of 6-aza-2,4-dithiouracil. During UV irradiation, the population of this form first grew but then decreased, whereas the population of the dithiol tautomer was increasing monotonically. For 2,4-dithiouracil, the phototautomeric transformations are strongly dominated by the generation of the dithiol product. The second product, the thiol–thione form of 2,4-dithiouracil, was photogenerated only in trace amounts. The products of the investigated photoreactions were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-311++G(2d,p) level.