Tautomerism of 1-(2′,4′-dinitrophenyl)-3-methyl-2-pyrazolin-5-one:: theoretical calculations, solid and solution NMR studies and X-ray crystallography

To gain a better understanding of the tautomerism of 1-(2',4'-dinitrophenyl)-3-methyl-2-pyrazolin-5-one, 2, different studies were performed. In order to simulate the gas phase, several MO calculations at the semiempirical (AM1 and PM3) and ab initio (HF/6-31G* and B3LYP/6-31G*) levels were carried out on the different tautomers of this compound and on those of the corresponding l-phenyl derivative 4. The (1)H and (13)C NMR spectra were recorded in solution for compound 2. Finally, to investigate the solid state, 13C CPMAS NMR studies and the crystal structure analysis of this pyrazolinone and that of its isomer [1-(2',4'-dinitrophenyl)-3-hydroxy-5-methylpyrazol 3, whose chemical structure was incorrectly reported in the literature] were performed. In solution, the most abundant tautomer for both pyrazolinones, 2 and 4, depends on the solvent used. For compound 2 it was found that the CH tautomer was the most stable in the gas and solid states as opposed to its l-phenyl analogue, which appears as the CH form in the gas phase and as NH and OH tautomers in the crystal.