Abstract
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.
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