The crystal structure of the title compound was determined by X-ray analysis at 200K. Three independent molecules form a trimer joined by strong and linear N- H. . . N hydrogen bonds. There is another centrosymmetrically related trimer in the unit cell. Both tautomers (1H and 2H) are present in each trimer. Disorder of the NHprotons involved in the NH . . N hydrogen bonds has been observed. Solid-state 13C CPMAS NMR was used to establish the dynamic nature of the NH-proton disorder, the title compound being the first example of proton transfer in a tautomeric mixture of pyrazoles with an equilibrium constant other than 1.

The authors thank the European Community for financial support, Project No. CHRX-CT94-0582. 0.H. gratefully acknowledges a grant from the same project. We are indebted to Dr. Marfisi (Bruker Es- pabola) for the "C CPMAS NMR spectrum.

Peer reviewed