The search for proton mobility in solid pyrazoles: molecular and crystal structure of 3(5)-phenyl-4-bromo-5(3)-methylpyrazole

The crystal structure of 3(5)-phenyl-4-bromo-5(3)-methylpyrazole, 3, has be en determined by X-ray crystallography. Both tautomers are present simultan eously in the crystals forming cyclic tetramers. The proton disorder, deter mined by the crystallographic methods, has been established to be dynamic u sing solid state C-13 CPMAS NMR spectroscopy. This is a new example of an a symmetrically 3,5-disubstituted pyrazole that presents solid state intermol ecular proton transfer. The arrangement of the tautomers in the cyclic tetr amer is different to the case of 3(5)-phenyl-5(3)-methylpyrazole, 2, previo usly described. (C) 1999 Elsevier Science B.V. All rights reserved.