β-Thioxoketones. Part 6. Electronic absorption spectra of aromatic β-thioxoketones. A study of enol–enethiol tautomerism

Abstract

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 (ArCO, π,π*; ArCC π,π*), 330 (ArCS π,π*; OCCCS π,π*; CO n,π*), 415 (OCCCS π,π*), and 520 nm (CS 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–CCC–Ar′ and SCCCO 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 (ΔS_r) 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 ΔS_r and K₂₉₃ as functions of substitution in the aryl group next to oxygen are discussed.