Objectives. The paper is devoted to the crystal structure characterization of 5-methyl-2-phenyl4H-pyrazol-3-one (compound I) and 2-(4-chlorophenyl)-5-methyl-4H-pyrazol-3-one (compound II).Methods. Single-crystal X-ray diffraction studies and theoretical calculations: Density functional theory and quantum theory of atoms in molecules.Results. In the solid state, the crystal structure of compound I is characterized by the alternation of OH and NH tautomers connected via O–H---O and N–H---N hydrogen bonds. For compound II, the existence of chains built from the NH monomers via hydrogen bonding can be explained by the peculiarities of cooperative effects. In the framework of quantum theory of atoms in molecules, the following topological characteristics are calculated for all dimers: electron density, Laplacian of electron density, density of kinetic, potential, and total energy in the critical point of the intermolecular hydrogen bond. It is concluded that the hydrogen bond in dimers 1–4, 7 (compound I), and 8–11 (compound II) can be assigned to the intermediate (between covalent and dispersion types) interaction owing to hydrogen bond formation with the participation of electronegative oxygen- (and/or nitrogen-) atoms, whereas H-bond in dimers 5 and 6 (compound I) can be attributed to the dispersion one (no hydrogen bond formation or weak H-bond formation), and it represents the weak interaction, being in agreement with length for intermolecular hydrogen bond in dimers. The electron density and total energy density values demonstrate that the strongest intermolecular H-bonds take place in dimers 1 (OH---O), 4 (OH---O), 7 (OH---N), 8 (OH---O), 9 (NH---N), and 11 (OH---N). The results obtained for compounds I and II are compared with data for antipyrine (1,2-dihydro-1,5-dimethyl-2-phenyl-3H-pyrazol-3-one; compound III)Conclusions. An important role of intermolecular hydrogen bonding in the crystal packing, molecule association and self-organization via dimer- or more extended species formation has been demonstrated.
Excited-state intramolecular proton transfer in a bioactive flavonoid provides fluorescence observables for recognizing its engagement with target proteins
