The Structure of Pyrazoles in the Solid State: A Combined CPMAS, NMR, and Crystallographic Study

The structures of six N-unsubstituted pyrazoles, three already known and three newly synthesized, have
been studied by a combination of X-ray crystallography, multinuclear NMR (solution and solid state),
and density functional theory (DFT) calculations. In those cases where crystal structure and CPMAS
NMR were available, the agreement was almost perfect, allowing a prediction of the tautomer (with
certitude) and the tetrameric structure (with high probability) in the case of 5-isopropyl-3-phenyl-1H-pyrazole without knowing the X-ray structure. In the case of the 5-(2-benzylphenyl)-3-trifluoromethyl-1H-pyrazole above represented, the DFT calculations at the B3LYP/6-31G** level justify the great stability
of this tautomer by the presence of an intramolecular N−H···π interaction, present in solution.