Solid-state thiotropolone: an extremely rapid intramolecular proton transfer.

Through variable-temperature solution-state NMR and molten- and solid-state CP/MAS 13 C NMR spectra, thiotropolone is found to exist as two rapidly equilibrated tautomeric structures, thione and enethiol, even in the solid state far below the melting point. The crystal structure shows an almost perpendicular packing, suggesting that the intramolecular hydrogen bond is dominant.