Vibrational spectra of 3,5-dimethylpyrazole and deuterated derivatives

The infrared (IR) and Raman spectra of 3,5-dimethylpyrazole have been recorded in the vapor, liquid (melt and solution) and solid states. Two deuterated derivatives, C5H7N-ND and C5D7N-NH, were also studied in solid state and in solutions. Instrumental resolution was relatively low, 2.0 cm(-1) in the IR and approximately 2.7 cm(-1) in the Raman spectra. The solids are made of cyclic hydrogen-bonded trimers. These trimers, present also in chloroform and acetone solutions, give rise to characteristic high absorption IR spectra in the 3200-2500 cm(-1) region, related to Fermi resonance involving nu(NH) vibrations. Bands from trimers are not present in water solutions but these solutions show spectral features similar in several ways to those of the trimer, attributable to solvent-bonded complexes. Evidence of H-bonding interactions with the other solvents is also visible in the high-frequency region. The two very intense bands in the Raman spectra of the solids appearing at 115 and 82 cm(-1) in the parent compound are also connected with a trimer formation. To interpret the experimental data, ab initio computations of the harmonic vibrational frequencies and IR and Raman intensities were carried out using the Gaussian 94 program package after full optimization at the RHF/6-31G* level for the three monomeric compounds as well as for three models of the trimer, with C3h, C3 and C1 symmetry. The combined use of experiments and computations allow a firm assignment of most of the observed bands for all the systems. In general, the agreement between theory and experiment is very good, with the exception of the IR and Raman intensities of some transitions. Particularly noticeable is the failure of the theoretical calculation in accounting for the high intensity of the Raman bands of the solid about 115 and 82 cm(-1).