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

Abstract

The molecular structures of three 4-tert-butylpyrazoles have been determined at 173 K: 4-tert-butyl-3(5)-isopropylpyrazole 1, 4-tert-butyl-3(5)-neopentylpyrazole 2 and 4-tert-butyl-3(5)-p-tolylpyrazole 3. ¹H, ¹³C and ¹⁵N NMR spectroscopies, in solution and in the solid state (CPMAS), have been used to complement the structural information. The major tautomers in solution correspond to the tautomers present in the crystal: 5-isopropyl 1b, 5-neopentyl 2b and 3-p-tolyl 3a. All these compounds crystallise as tetramers, formed by four identical tautomers, through N–H···N hydrogen bonds, the tetramers corresponding to 1b and 2b being folded but that of 3a is the first example of a planar tetramer.