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

Abstract

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 abinitio (HF/6-31G* and B3LYP/6-31G*) levels were carried out on the different tautomers of this compound and on those of the corresponding 1-phenyl derivative 4. The ¹H and ¹³C 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-methylpyrazole, 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 1-phenyl analogue, which appears as the CH form in the gas phase and as NH and OH tautomers in the crystal.