The orange crystalline form of 4-cinnamoyl-1,3-dimethylpyrazol-5-one (3) is shown to be the OH tautomer (3e) in which the OH proton is involved in a three-centre hydrogen bond to the intramolecular carbonyl oxygen and to the OH oxygen of an adjacent molecule related by a centre of inversion. Crystals of (3e) are monoclinic, P21/n, a 7.530(3), b 7.960(3), c 20.705(8) Ǻ, β 100.16(3)°, Z = 4; the structure was refined to a conventional R(I > 2σ(I)) = 0.051. The yellow form of (3) exists as a hydrate in which the NH tautomer (3b) is hydrogen-bonded to the oxygen of a water molecule which in turn is hydrogen-bonded to the carbonyl oxygens of two adjacent molecules. Crystals of (3b) are monoclinic, P21/n, a 8.295(3), b 12.813(4), c 12.888(4) Ǻ, β 98.55(3)°, Z = 4; the structure was refined to a conventional R(I > 2σ(I)) = 0.054. The structure of (3) in solution is also discussed.
On the basis of 1H n.m.r. n.O.e. Measurements and an X-ray crystal structure determination, it is shown that a large series of pharmacologically active pyridazine derivatives should be represented as aromatic pyridazine tautomers [e.g. (1b)-(3b)], rather than the previously reported arylidene-4,5-dihydropyridazines [e.g. (1a)-(3a)]. Crystals of (7b) are monoclinic, P21/c, a 13.312(3), b 7.269(1), c 11.753(2) Ǻ, β 101.38(3)°, Z = 4; the structure was refined to a conventional R[I > 2σ(I)] 0.037.
Contrary to a recent report, 3-hydroxy-5-phenyl-1,2,4-thiadiazole is shown to exist as the OH tautomer (2a). At 130 K crystals of (2a) are monoclinic, P21/n, a 5.453(3), b 4.956(2), c 27.28(2) Ǻ, β 94.12(5)°, Z = 4; the structure was refined to a conventional R[I > 2σ(I)] 0.051. 5-Benzoylamino-1,2,4-thiadiazol-3-one is shown to exist as the N(2)H tautomer (3b) but with a contribution to the structure from a bicyclic or zwitterioni resonance contributor containing an S-O bond. At 130 K crystals of (3b) are monoclinic, Pn , a 3.771(1), b 15.465(4), c 8.105(1) Ǻ, β 98.72(2)°, Z = 2; the structure was refined to a conventional R[I > 2σ(I)] 0.038.
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