Photochemical transformations of 4,6-dihydroxypyrimidine and 2-methyl-4,6-dihydroxypyrimidine isolated in low-temperature Ar, Ne and H2 matrices

Effect of a Solid-Hydrogen Environment on UV-Induced Hydrogen-Atom Transfer in Matrix-Isolated Heterocyclic Thione Compounds

“…This significant (25–50%) efficiency of the phototautomerization processes, observed for 3-thiopyridazine and 2-thioquinoline isolated in solid n-H 2, is quite surprising in comparison to the results obtained for other heterocycles (4-oxopyrimidine, 6-hydroxy-4-oxopyrimidine, 7-azaindole, and 3-thio-1,2,4-triazole) ,− isolated in n-H 2 matrices. For these latter compounds, UV excitation led to no (or nearly no) phototautomeric conversion.…”

“…The compounds with a six-membered heterocyclic ring and the CO group directly attached to it (4-oxopyrimidine and 6-hydroxy-4-oxopyrimidine) do readily phototransform from the oxo to the hydroxy form. However, this photoinduced oxo → hydroxy conversion occurred only for the species isolated in Ar matrices; in a solid n-H 2 environment, the oxo → hydroxy phototransformation did not occur at all. , This type of photochemical behavior must be related to the presence of oxygen atom in the CO group (which is the target for the transferring hydrogen atom) because in thione compounds (3-thiopyridazine and 2-thioquinoline), having an analogous six-membered heterocyclic ring but the CS group as a target of hydrogen shift, the thione → thiol phototransformation occurred in both Ar and n-H 2 low-temperature environments.…”

“…The investigations on UV-induced hydrogen-atom-transfer processes, carried out so far for such compounds as 7-azaindole, 4-oxopyrimidine, and 6-hydroxy-4-oxopyrimidine ,, isolated in Ar and n-H 2 low-temperature matrices, might have led to the conclusion that the replacement of solid argon by a solid-hydrogen environment inevitably precludes phototautomeric transformations. The current study, performed for a number of thione compounds with the H–N–CS moiety in the structure, demonstrated that this is not always the case.…”

“…The strength of the van der Waals interactions between a molecule and its solid Ar environment is nearly the same as the strength of the interactions between the same molecule and its solid-hydrogen environment. This is demonstrated by almost identical spectral positions (indicating similar matrix shifts) of IR bands observed in the spectra of molecules isolated in Ar and in normal hydrogen (n-H 2 ) matrices. , Assuming that the UV-induced hydrogen-atom transfer (occurring in an Ar matrix) is a purely intramolecular process, it would be only natural to expect that the analogous UV-induced hydrogen-atom transfer should also occur for the same molecules but isolated in normal hydrogen (n-H 2 ) matrices.…”

“…The results of the recent studies on phototransformations, occurring for 7-azaindole, 4-oxopyrimidine, and 6-hydroxy-4-oxopyrimidine monomers isolated in Ar and n-H 2 matrices and exposed to UV light, contradict this supposition. For the monomers of 4-oxopyrimidine and 6-hydroxy-4-oxopyrimidine isolated in Ar matrices and excited with UV light, oxo tautomers were converted into hydroxy tautomers by hydrogen-atom transfer from the N–H group to the vicinal oxygen atom. , These phototautomerizations were accompanied by minor photoisomerization processes leading to the formation of the nonplanar Dewar isomers and the open-ring ketenes. Contrary to what was expected, UV irradiation of the same molecules but isolated in n-H 2 matrices resulted solely in the formation of the Dewar isomers and the open-ring ketenes; the oxo → hydroxy phototautomeric conversion was not detected.…”