Derivatives of 1-phenyl-3-methylpyrazol-2-in-5-thione and their oxygen analogues in the crystalline phase and their tautomeric transformations in solutions and in the gas phase

1-Phenyl-3-methylpyrazol-2-in-5-thione, crystallised from methanol, was sho wn to exist in the tautomeric NH-form, stabilised by intermolecular N-H . . .S hydrogen bonds. In solutions, however, the molecule is found predominan tly as the SH-tautomer, accompanied (in low-polar solvents) by a small amou nt of the CH-tautomer. 1-Phenyl-3-methyl-4-benzoylpyrazol-2-in-5-thione occurs in the crystal as w ell as in solution in the SH-tautomeric form, stabilised by an intramolecul ar SH . . .O bridge. In dimethylsulfoxide solution indications were found f or an additional SH-tautomer in a conformation lacking the intramolecular H -bridge. The structure of 1-phenyl-3-methylpyrazol-2-in-5-one was redetermined by X- ray single crystal diffraction at 120 degreesK in order to obtain more accu rate geometry and hydrogen bonding parameters. (C) 2001 Elsevier Science B. V. All rights reserved.