Intramolecular hydrogen-atom tunneling in matrix-isolated heterocyclic compounds: 2-thiouracil and its analogues.

Hydrogen-atom tunneling leading to spontaneous tautomeric conversion in monomeric heterocyclic molecules without intramolecular hydrogen bonds has been experimentally detected for the first time. For monomers of 2-thiouracil, 6-aza-2-thiouracil and 1-methyl-2-thiouracil isolated in low-temperature matrices, higher-energy thiol forms were generated upon UV ( = 305 nm) excitation of the most stable thione tautomers. When the matrices were subsequently kept in the dark and at low temperature, hydrogen-atom tunneling occurred, leading to the thiol → thione conversion. During this process, the photoproduced thiol form spontaneously converted into the lowest energy thione tautomer.