FTIR and FT-Raman spectra, assignments, ab initio HF and DFT analysis of xanthine

The molecular vibrations of xanthine were investigated in polycrystalline sample, at room temperature by Fourier transform infrared (FTIR) and FT-Raman spectroscopies. The spectra of the molecule have been recorded in the regions 4000–50 cm−1 and 3500–100 cm−1, respectively. Theoretical information on the optimized geometry, harmonic vibrational frequencies, infrared and Raman intensities were obtained by means of ab initio Hartree–Fock (HF) and density functional theory (DFT) gradient calculations with complete relaxation in the potential energy surface using 6-311++G(d,p) basis set. The vibrational frequencies which were determined experimentally from the spectral data are compared with those obtained theoretically from ab initio and DFT calculations. A close agreement was achieved between the observed and calculated frequencies by refinement of the scale factors. The infrared and Raman spectra were also predicted from the calculated intensities. Thermodynamic properties like entropy, heat capacity, zero point energy have been calculated for the molecule. Unambiguous vibrational assignment of all the fundamentals was made using the potential energy distribution (PED).

Graphical abstractThe molecular vibrations of xanthine were investigated in polycrystalline sample, at room temperature by Fourier transform infrared (FTIR) and FT-Raman spectroscopies. Theoretical information on the optimized geometry, harmonic vibrational frequencies, infrared and Raman intensities were obtained by means of ab initio Hartree–Fock (HF) and density functional theory (DFT) gradient calculations with complete relaxation in the potential energy surface using 6-311++G(d,p) basis set. The vibrational frequencies which were determined experimentally from the spectral data are compared with those obtained theoretically from ab initio and DFT calculations. A close agreement was achieved between the observed and calculated frequencies by refinement of the scale factors. Unambiguous vibrational assignment of all the fundamentals was made using the potential energy distribution (PED).Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The properties of xanthine and its derivatives are of interest due to their biological and pharmaceutical importance. ► The optimized geometrical parameters and vibrational frequencies of the fundamental modes of xanthine have been obtained from ab initio and DFT calculations. ► The theoretical results were compared with the experimental vibrations. The computed geometrical parameters are in good agreement with the observed X-ray diffraction data ► The PED calculation regarding the normal modes of vibration provides a strong support for the frequency assignment. ► The assignments proposed at higher level of theory with higher basis set with only reasonable deviations from the experimental values seem to be correct.