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 (A M1 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 corresp onding l-phenyl derivative 4. The H-1 and C-13 NMR spectra were recorded in solution for compound 2. Finally, to investigate the solid state, 13C CPMA S NMR studies and the crystal structure analysis of this pyrazolinone and t hat of its isomer [1-(2',4'-dinitrophenyl)-3-hydroxy-5-methylpyrazol 3, who se chemical structure was incorrectly reported in the literature] were perf ormed. In solution, the most abundant tautomer for both pyrazolinones, 2 an d 4, depends on the solvent used. For compound 2 it was found that the CH t automer 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 a nd OH tautomers in the crystal.