Beta-Thioxoketones: Part 6. Electronic Absorption Spectra of Aromatic beta-Thioxoketones. A Study of Enol-Enethiol Tautomerism

Aromatic β-thioxoketones exist in solution as mixtures of rapidly interconverting Z-enol and Z-enethiol tautomers. The electronic absorption spectra exhibit in general four absorption bands in the u.v.–visible region at ca. 265 (ArC=O, π,π*; ArC=C π,π*), 330 (ArC=S π,π*; O=CC=CS π,π*; C=O n,π*), 415 (OC=CC=S π,π*), and 520 nm (C=S n,π*), respectively. The β-thioxoketones are converted by sodium hydroxide into the corresponding anions. CNDO/B Calculations predict that the negative charge in the β-thioxoketonates is delocalized over the OCCCS system, suggesting simultaneously sickles or W shaped conformations. Two characteristic absorption bands found for the β-thioxoketonates at ca. 275 and 400 nm are assigned to π,π* transitions involving the Ar–C[horiz bar, triple dot above]C[horiz bar, triple dot above]C–Ar′ and S[horiz bar, triple dot above]C[horiz bar, triple dot above]C[horiz bar, triple dot above]C[horiz bar, triple dot above]O chromophores, respectively. The enol–enethiol tautomeric equilibrium has been studied by means of low temperature spectroscopy. At room temperature equilibrium constants (K293) of 3–5 have been found corresponding to a 4 : 1 enol–enethiol concentration ratio. The reaction entropy (ΔSr) has been found to be negative for the enethiol→enol conversion, reflecting the intramolecular O–H···S hydrogen bond to be considerably stronger than the corresponding O···H–S hydrogen bond. Variations in ΔSr and K293 as functions of substitution in the aryl group next to oxygen are discussed.