FT-IR and FT-Raman spectra of 6-chlorouracil: Molecular structure, tautomerism and solid state simulation. A comparison between 5-chlorouracil and 6-chlorouracil

•All the tautomers of 6-chlorouracil are more stable than those in uracil molecule.•The Raman and IR spectra of 6-ClU in the solid state were simulated and interpreted.•The solid state simulation with scaled values was carried out by a tetramer form.•Halogenation produces a smaller effect for 5-XU than for 6-XU derivatives.•The low energy gap HOMO–LUMO facilitates the intra molecular charge transfer.

A Raman and IR study of the biomolecule 6-chlorouracil was carried out in the solid state. The unit cell found in the crystal was simulated as a tetramer form by density functional calculations. Specific scale factors and scaling equations deduced from uracil molecule were employed in the predicted wavenumbers of 6-chlorouracil. The scaled wavenumbers were used in the reassignment of the IR and Raman experimental bands. Good reproduction of the experimental wavenumbers is obtained and the % error is very small in the majority of cases. A comparison between the molecular structure and charge distribution of 6-chlorouracil and 5-chlorouracil molecules was presented. The effect of the hydration with the PCM model in the molecular structure and charges was discussed. The optimum tautomers of 6-chlorouracil were optimized and analyzed. Six of them were related to those of uracil molecule. The effect of the halogen substitution in the sixth position of the pyrimidine ring in the stability of the different tautomers was evaluated. HOMO and LUMO orbital energy analysis were carried out.

Graphical abstractHalogenation effect in the energy of 5- and 6-uracil derivatives.Figure optionsDownload full-size imageDownload as PowerPoint slide