Experimental and DFT studies on the structure, infrared and Raman spectral properties of dinitropyrazoles

Experimental and theoretical studies on the structure and vibrational properties of the possible isomers of dinitropyrazoles have been investigated. The infrared and Raman spectra of 1,3-dinitropyrazole, 1,4-dinitropyrazole, 3,4-dinitropyrazole and 3,5-dinitropyrazole were recorded in the solid state. To interpret the experimental infrared and Raman spectra, the vibrational frequency calculations were carried out at the HF/6-311++G(df,pd), B3PW91/6-311++G(df,pd), B3LYP/6-311++G(df,pd) and B3LYP/aug-cc-pVDZ levels of theory. The combined use of experiments and computations allowed a firm assignment of the majority of vibrational bands of dinitropyrazoles. The calculated stretching frequencies were found to be in good agreement with the experimental values. However, the structural parameters and fundamental vibrational frequencies calculated at the B3LYP/aug-cc-pVDZ level are superior compared with those values that are obtained from the HF/311++G(df,pd), B3PW91/311++G(df,pd) and B3LYP/311++G(df,pd) levels. The discrepancies in the structural and vibrational properties are restricted to the relative positions of NO2 groups that present the greatest deviation from the planarity.