A theoretical and experimental NMR study of the tautomerism of two phenylene-bis-C-substituted pyrazoles

Abstract

Two bis-pyrazolylbenzenes, one meta- and the other para-substituted, have been studied by multinuclear magnetic resonance both in solution and in the solid state, including ¹³C CP-MAS NMR variable temperature experiments. The tautomerism in DMSO-d₆ solution has been studied and the most stable tautomers (the 3,3 and the 3,5) identified. In the solid state the meta derivative exists as the 3,3-tautomer while the tautomer of the para derivative is the 3,5 one. This latter tautomer exists in a dynamic equilibrium with the 5,3 one, representing a new and original example of proton transfer in the solid state (SSPT).