Theoretical and matrix-isolation experimental study of the infrared spectra of 5-azauracil and 6-azauracil

The infrared absorption spectra of 5-azauracil and 6-azauracil isolated in low-temperature Ar and N₂ matrices are reported. Within the limits of accuracy of spectroscopic measurements, both compounds appear to exist, in Ar and N₂ matrices, exclusively in dioxo tautomeric forms. An assignment of the observed infrared absorption bands is proposed based on the comparison of experimental frequencies and intensities with those calculated theoretically at ab initio Hartree-Fock level with 6-31G** basis set. Infrared spectra predicted at this level of theory reproduce the experimental spectra sufficiently accurately to enable a reliable assignment. Basis set dependency (6-31G**, 3-21G) of the theoretical result was also accounted for and it turned out that for accurate prediction of frequencies and intensities the use of basis set augmented with polarization functions is of crucial importance.