Structural, spectral, and theoretical investigations of 5-methyl-1-phenyl-1H-pyrazole-4-carboxylic acid

The present research work has focused on combined experimental and theoretical studies of one of the biologically important pyrazole-4-carboxylic acid derivatives, viz. 5-methyl-1-phenyl-1H-pyrazole-4-carboxylic acid (C11H10N2O2). The starting material 5-methyl-1-phenyl-1H-4-pyrazolecarboxylate (1) was obtained by the cyclocondensation of ethyl acetoacetate, N,N-dimethylformamide dimethyl acetal (DMF-DMA), and phenylhydrazine, which upon basic hydrolysis yielded the corresponding acid (2). The target compound (2) was characterized by 1H and 13C NMR (solution in DMSO), Fourier transform infrared (FT-IR) spectroscopy, thermo gravimetric analysis, and by single-crystal X-ray diffraction technique. The single crystals of compound (2) were obtained at room temperature by slow evaporation of ethanol as solvent and crystallized in the space group P21/n of monoclinic system. The experimental FT-IR and 1H and 13C NMR chemical shifts have been compared to those calculated by means of density functional theory (DFT) at the B3LYP/TZ2P level of theory. The continuum-like screening model was used for geometry optimization of a single molecule and for subsequent calculations of NMR shielding constants in solution (DMSO). Finally, the HOMO–LUMO energy levels were also constructed to study the electronic transition within the molecule by time-dependent TD-DFT method.