Hydrogen-Atom Tunneling in Monomers of 2,4-Dithiouracil, 1-Methyl-2,4-dithiouracil, and 6-Aza-2,4-dithiouracil Isolated in Low-Temperature Matrices

Hydrogen-atom-tunneling phenomena were observed for monomers of 2,4-dithiouracil, 1-methyl-2,4-dithiouracil, and 6-aza-2,4-dithiouracil isolated in low-temperature argon matrices. In these processes, the photoproduced higher-energy thione-thiol tautomers of the studied 2,4-dithiouracils spontaneously converted to the lowest-energy dithione forms. The time constants for these transformations, which occurred in monomeric 2,4-dithiouracil, 1-methyl-2,4-dithiouracil, and 6-aza-2,4-dithiouracil at a low temperature (3.5 K) and in the dark, were experimentally assessed as 22, 1, and 260 h, respectively. For 2,4-dithiouracil and its analogues, the barriers for hydrogen-atom tunneling were calculated at the MP2 level. Analysis of the obtained data has led to several conclusions concerning the systematic dependence of the rate of the observed H atom-tunneling processes on the structural features of the studied compounds.