Experimental and theoretical studies on the structure and vibrational properties of nitropyrazoles

• Pyrazoles as models of simple aromatic systems.
• Observed vibrational frequencies with the calculated at the B3LYP/aug-cc-pVDZ was superior to B3LYP/311++G∗∗ basis set.
• Calculated geometries of pyrazoles are in excellent agreement with the average experimental geometries.
• Differences in the bond distances and angles are confined to the twist of the nitro group.

We report a theoretical and experimental study on the structure and vibrational properties of pyrazole and its mononitropyrazoles. The infrared (IR) and Raman spectra of pyrazole, N-nitropyrazole, 3-nitropyrazole and 4-nitropyrazole have been recorded in the solid state. To interpret the experimental data, ab initio computations of the vibrational frequencies were carried out using the Gaussian 03 program following the full optimizations at the HF/6-311++G(d,p), B3P86/6-311++G(d,p), B3LYP/6-311++G(d,p) and B3LYP/aug-cc-pVDZ levels. The combined use of experiments and computations allowed a firm assignment of the majority of observed bands for all compounds. The calculated stretching frequencies have been found to be in good agreement with the experimental frequencies. However, the fundamental vibrational frequencies of nitropyrazoles calculated at the B3LYP/aug-cc-pVDZ level are superior compared with those values that are obtained at the HF/311++G(d,p), B3P86/311++G(d,p) and B3LYP/311++G(d,p) levels of theory. The differences in the bond distances and bond angles are confined to the twist of the nitro group that present the greatest deviation from planarity in molecules.