Proton transfer in rubazoic acid derivatives in solution and in the solid state. An NMR study

Solution 1H and 13C, and solid-state 13C CPMAS NMR data are reported for the rubazoic acid derivatives 4-(5-hydroxy-1,3-dimethylpyrazol-4-ylmethylidene)-1,3-dimethyl-2-pyrazolin-5-one 1, 4-(5-hydroxy-1,3-dimethylpyrazol-4-ylimino)-1,3-dimethyl-2-pyrazolin-5-one 2, 4-(5-hydroxy-1-phenyl-3-aminopyrazol-4-ylmethylidene)-1-phenyl-3-amino-2-pyrazolin-5-one 3, and 4-(5-hydroxy-1-p-sulfophenylene-3-methylpyrazol-4-ylmethylidene)-1-p-sulfophenylene-3-methyl-pyrazolin-5-one 4. Solution 15N NMR chemical shifts have also been measured, lsotopic 2H shifts for the OH signal in CDCI3 for the first two compounds were found to be +0.28 ± 0.03 and +0.40 ± 0.03 ppm respectively. AM1-calculated molecular geometries are also reported. The overall results support the idea of a fast proton transfer equilibrium between two enolic tautomers both in solution and in the solid state. The degeneracy of the tautomers is removed in crystals.