UV-induced transformations and spontaneous hydrogen-atom tunneling in 2-selenouracil isolated in low-temperature Ar...

The monomers of 2-selenouracil, in the most stable oxo-selenone tautomeric form, were trapped from the gas phase in low-temperature (12 K) argon matrices and then irradiated with UV light of different wavelengths. When the matrix dopped with 2-selenouracil monomers was irradiated with UV (λ  > 345 nm) light, the initial oxo-selenone tautomer converted into one of the oxo-selenol forms of the compound. In another experiment, matrix-isolated molecules of 2-selenouracil were irradiated with UV light of shorter-wavelength (λ = 305 nm). Upon such excitation, two higher-energy photoproducts (the other oxo-selenol form and the hydroxy-selenol form) were generated. UV-irradiated matrices were subsequently kept in the dark and at low temperature. Under such conditions, one of the photogenerated oxo-selenol forms spontaneously transformed into the most stable oxo-selenone tautomer of the compound. This process must be driven by hydrogen-atom tunneling. The time constant of the observed spontaneous transformation was experimentally estimated to be 39 ± 3 h. The structures of the initial reactant and generated photoproducts were identified by comparing their experimental IR spectra with the spectra theoretically predicted for isomeric forms of 2-selenouracil. The calculations of relative energies of 2-selenouracil tautomers were performed at DFT and MP2 level of theory, whereas the barrier heights for spontaneous selenol → selenone tautomeric transformations were computed at the MP2 level.