Vibrational spectra and structure of 5,6-diamino uracil and 5,6-dihydro-5-methyl uracil by density functional theory calculations.

The molecular vibrations of 5,6-diamino uracil and 5,6-dihydro-5-methyl uracil were investigated in polycrystalline sample, at room temperature, by FT-IR and FT-Raman spectroscopies. The spectra were interpreted with the aid of normal coordinate analysis following a full structure optimization and force field calculations based on the density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** methods and basis set combinations. The DFT force field transformed to natural internal coordinates was corrected by a well-established set of scale factors that were found to be transferable to the title compounds. The infrared and Raman spectra were also predicted from the calculated intensities. Comparison of the simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes.