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, NsH‚‚‚OH 2 and N‚‚‚HsOH, 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.

Ab initio calculations performed in this work found positive electron affinities for all three possible doubly H-bonded complexes of the uracil molecule with a single water molecule. In all cases the excess electron is bound by the the dipole field of the complex. No conventional stable "valence" anionic states were found with the theoretical procedure used in this work (SCF + second-order perturbation theory corrections for the electron correlation effects). The attachment of the excess electron lowers the relative energy differences between the three complexes, making their coexistence more probable. Structural changes in the uracilwater complex upon attachment of an electron were also found. The anion's equilibrium geometry had noticeably shortened hydrogen-bond lengths and a shifted orientation of the water molecule with respect to the uracil molecule compared to the neutral system.

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