The molecular structure of 4-tert-butylpyrazoles in the solid state and insolution: an X-ray, NMR and calorimetric study of the buttressing effect of a 4-tert-butyl substituent

The molecular structures of three 4-tert-butylpyrazoles have been determine d at 173 K: 4-tert-butyl-3(5)-isopropylpyrazole 1, 4-tert-butyl-3(5)-neopen tylpyrazole 2 and 4-tert-butyl-3(5)-p-tolylpyrazole 3. H-1, C-13 and N-15 N MR spectroscopies, in solution and in the solid state (CPMAS), have been us ed to complement the structural information. The major tautomers in solutio n correspond to the tautomers present in the crystal: 5-isopropyl 1b, 5-neo pentyl 2b and 3-p-tolyl 3a. All these compounds crystallise as tetramers, f ormed by four identical tautomers, through N-H . . .N hydrogen bonds, the t etramers corresponding to 1b and 2b being folded but that of 3a is the firs t example of a planar tetramer.