Vibrational spectra of 3,5-dimethylpyrazole and deuterated derivatives

The infrared (IR) and Raman spectra of 3,5-dimethylpyrazole have been recor ded in the vapor, liquid (melt and solution) and solid states. Two deuterat ed 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 th e IR and approximate to 2.7 cm(-1) in the Raman spectra, The solids are mad e of cyclic hydrogen-bonded trimers. These trimers, present also in chlorof orm and acetone solutions, give rise to characteristic high absorption IR s pectra in the 3200-2500 cm(-1) region, related to Fermi resonance involving nu(NH) vibrations. Bands from trimers are not present in water solutions b ut these solutions show spectral features similar in several ways to those of the trimer, attributable to solvent-bonded complexes. Evidence of H-bond ing interactions with the other solvents is also visible in the high-freque ncy region. The two very intense bands in the Raman spectra of the solids a ppearing at 115 and 82 cm(-1) in the parent compound are also connected wit h a trimer formation. To interpret the experimental data, ab initio computa tions of the harmonic vibrational frequencies and IR and Raman intensities were carried out using the Gaussian 94 program package after full optimizat ion at the RHF/6-31G* level for the three monomeric compounds as well as fo r three models of the trimer, with C-3h, C-3 and C-1 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 theo ry and experiment: is very good, with the exception of the IR and Raman int ensities 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). (C) 2000 Elsevier Science B.V. All rights reserved.