Abstract
To shed more light on the mechanisms of UV-induced hydrogen-atom-transfer processes in heterocyclic molecules, phototautomeric thione → thiol reactions were investigated for thione compounds isolated in low-temperature Ar as well as in n-H2 (normal hydrogen) matrices. These studies concerned thione compounds with a five-membered heterocyclic ring and thione compounds with a six-membered heterocyclic ring. The experimental investigation of 2-thioimidazole and 3-thio-1,2,4-triazole (thione compounds with a five-membered heterocyclic ring) revealed that for the compounds isolated in solid n-H2 only trace amounts of thiol photoproducts were photogenerated; even though for the same compounds isolated in the solid Ar matrix, the thione → thiol photoconversion was nearly total. In contrast to that, for 3-thiopyridazine and 2-thioquinoline (thione compounds with a six-membered heterocyclic ring) isolated in solid n-H2, the UV-induced thione → thiol conversion occurred with the yield reaching 25–50% of the yield of the analogous process observed for the same species isolated in solid Ar. The obtained experimental results allow us to conclude that the dissociation–association mechanism nearly exclusively governs the phototransformation in thione heterocycles with high barriers for tautomerization (such as thione compounds with a five-membered ring), whereas the strictly intramolecular hydrogen-atom shift contributes to the mechanism of hydrogen-atom transfer in thione heterocycles with lower barriers (such as thione compounds with a six-membered ring).
Highlights
A distinct class of intramolecular H-atom-transfer processes, leading to the oxo → hydroxy, thione → thiol, or N(i)H → N(j)H change of tautomeric form (Scheme 1), was observed for a number of heterocyclic compounds isolated in lowtemperature argon matrices and exposed to UV light
For the molecules undergoing this type of phototautomerism, the energy gap between the S0 and S1 electronic states of the reactant tautomer is smaller than the corresponding S0−S1 energy gap in the photoproduced tautomer
Phototautomerization processes belonging to this category were observed for the compounds isolated in the matrices of solid Ar as well as in the matrices of solid N2.6−29 It appears that the UV-induced oxo → hydroxy, thione → thiol, or N(i)H → N(j)H hydrogen-atom transfer leading to the change of tautomeric form is a typical pattern of the photochemical behavior of monomeric heterocyclic compounds isolated in low-temperature Ar, Xe, Ne, or N2 matrices
Summary
A distinct class of intramolecular H-atom-transfer processes, leading to the oxo → hydroxy, thione → thiol, or N(i)H → N(j)H change of tautomeric form (Scheme 1), was observed for a number of heterocyclic compounds isolated in lowtemperature argon matrices and exposed to UV light. Phototautomerization processes belonging to this category were observed for the compounds isolated in the matrices of solid Ar (or other noble gases) as well as in the matrices of solid N2.6−29 It appears that the UV-induced oxo → hydroxy, thione → thiol, or N(i)H → N(j)H hydrogen-atom transfer leading to the change of tautomeric form (phototautomerism) is a typical pattern of the photochemical behavior of monomeric heterocyclic compounds isolated in low-temperature Ar, Xe, Ne, or N2 matrices Such phototautomeric reactions may proceed either by a mechanism involving detachment of a hydrogen atom followed by its association to another heteroatom or by a purely intramolecular mechanism involving a shift of a hydrogen atom that occurs during dissipation of the excitation energy, when the molecule is in highly excited vibrational levels of the ground electronic state
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