Quantum-mechanical and Car–Parrinello molecular dynamics simulations of infrared spectra of crystalline imidazole

Abstract

Theoretical simulation of the bandshape and fine structure of the N–H(D) stretching band is presented for imidazole and its deuterated derivative taking into account adiabatic coupling between the high-frequency N–H(D) stretching and the low-frequency N⋯N stretching vibrations, anharmonicity of the potentials for the low-frequency vibrations in the ground and excited state of the N–H(D) stretching mode, Fermi resonance between the N–H(D) stretching and the first overtone of the N–H(D) bending vibrations, and electric anharmonicity. The vibrational potential functions describing N–H and N⋯N stretching modes have been obtained from ab initio calculations. The effect of deuteration has been successfully reproduced by our model calculations. Infrared, far-infrared, Raman and low-frequency Raman spectra of the polycrystalline imidazole have been recorded. The geometry and experimental frequencies are compared with the results of harmonic MP2/6-311++G** and anharmonic B3LYP/6-311++G** calculations.Car–Parrinello molecular dynamics was used to calculate geometry, power and infrared spectra of crystalline imidazole. The results were compared with the results of ab initio MP2/6-311++G** static calculations previously performed for the imidazole dimer. The reconstruction of the νN–H bandshape obtained by Car–Parrinello molecular dynamics method was compared with the results of quantum mechanical model of vibrational couplings in hydrogen-bonded dimer and with the experimental data.