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| - Proton-Transfer Processes in Thiourea: UV Induced Thione → Thiol Reaction and GroundState Thiol → Thione Tunneling
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| - Thiourea monomers isolated in a low-temperature argon matrix adopt exclusively the thione tautomeric form.Upon UV (λ > 300 nm) irradiation a photoreaction converting the initial isomer of the compound into itsthiol tautomer occurred. Two structures of the photoproduct (with syn and anti orientation of the iminoCN−H group with respect to the C−S bond) were identified in the matrix. Subsequently, a back reactiontransforming the thiol form into the initial thione tautomer was observed for the matrix kept at 10 K and indarkness. The molecules with the anti conformation underwent this process, whereas those in the syn formremained unchanged. The only possible mechanism of the ground-state thiol → thione transformation at lowtemperature is proton-tunneling through the very high energy barrier of 9030 cm-1 (108 kJ mol-1) (calculatedat the MP2/6-31++G(d,p) level). The experimentally obtained time constant of this process was 52 h. Thestructure of the photoproduced species was identified by comparing the experimental IR spectra with thetheoretically calculated spectra obtained at the DFT(B3LYP)/6-31++G(d,p) level. UV-induced thione →thiol reaction of thiourea as well as the ground-state thiol → thione proton tunneling in the absence of lightare reported for the first time.
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