Matrix-isolation FTIR studies and theoretical calculations of hydrogen-bonded complexes of imidazole. A comparison between experimental results and different calculation methods

The hydrogen bond interaction between water and imidazole was investigated with the matrix-isolation FTIR spectroscopy coupled to ab initio calculations performed with the RHF and MP2 methods and the parametrized DFT method with the B3LYP hybrid functional. The 6-31G** and 6-31++G** basis sets were used in the calculations. Evaluation of the accuracy of the three methods and the two basis sets was made for noncomplexed imidazole. All three of the methods gave geometries for imidazole in good agreement with the experimental structure. Also, all three levels of theory with both basis sets gave similarly accurate vibrational frequency predictions for monomeric imidazole with a best mean deviation for the DFT/B3LYP/ 6-31++G** method. The assignment of the matrix spectra of the two isomeric H-bond complex species, N-H⋯OH₂ and N⋯H-OH, was performed by comparison with the theoretically predicted IR frequencies and intensities and was further assisted by asymmetrical deuteration experiments. The MP2 and DFT methods employed with the basis set augmented with diffuse functions gave good predictions of the frequency shifts for the vibrational modes directly influenced by the H-bond interaction. For the other vibrational modes, the RHF method performed almost as equally well as the MP2 and DFT methods and we can conclude that this method can provide qualitative and quantitively reliable data on hydrogen-bonded systems.