1-Phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) is a beta-diketone and
exists either as a yellow enol-form or as a colorless keto-form. The
yellow PMBP exists as a mixture of two enol conformers. The enol form
is known to form strong intramolecular hydrogen bonds where the shorte
ning (O---O) and lengthening (O---H) distances are associated with a s
trong increase in the delocalization of the electrons in the O=C-C=C-O
H pi-conjugated system. On the other hand, the characteristic structur
e in the keto form is the formation of a short intermolecular hydrogen
bond [2.673 (3) angstrom] between an imino nitrogen atom and a carbon
yl oxygen atom [N-H---O=C]. In the IR spectra of the PMBPs, the keto-f
orm exhibits a broad strong band centered at 2550 cm-1, which is attri
buted to the NH stretching vibration of the intermolecular hydrogen bo
nds (N-H---O). The enol form also gives a strong band, at around 3100
cm-1, which is attributed to the OH vibrations of the intramolecular h
ydrogen bonds (OH---O). The DSC curves of the keto and enol tautomers
show endothermic peaks at about 92-degrees-C and 121-degrees-C respect
ively, due to a phase change on melting. The obtained enthalpies were
found to be 27 and 24 kJ/mol for the keto and the enol forms, respecti
vely. The higher enthalpy and melting point of the keto form are due t
o the strong intermolecular hydrogen bonds.