Determination of structural and vibrational properties of 6-quinolinecarboxaldehyde using FT-IR, FT-Raman and Dispersive-Raman experimental techniques and theoretical HF and DFT (B3LYP) methods

The FT-IR (4000–50 cm−1), FT-Raman (4000–50 cm−1) and Dispersive-Raman (3500–50 cm−1) spectra of solid sample of 6-quinolinecarboxaldehyde (6QC) have been recorded. The structure, vibrational frequencies, IR intensities, Raman activities and thermodynamic properties of the two possible aldehyde rotamers of 6QC have been calculated at the Hartree–Fock (HF) and density functional B3LYP levels employing 6-311++G(d,p) basis set. The complete assignments were performed on the basis of the potential energy distribution (PED) of the all vibrational modes. Since HF and B3LYP mode definitions of this molecule are quite similar to each other, we only give in Table 3 PED of Rot1 calculated at B3LYP level for the sake of simplicity. Potential energy surface has been scanned over the C3–C2–C1O16 torsion angle. When the O atom of the aldehyde is farther away than the nitrogen atom of the quinoline, 6QC has the lowest possible energy, and thus is in its ground state. The scaled theoretical frequencies of the lowest energy rotamer agree also slightly better than those of the higher energy rotamer with the experimental frequencies. The thermodynamic characteristics of the ground state of 6QC have been theoretically investigated at 298.15 K temperature.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The conformational analysis of 6-quinolinecarboxaldehyde was performed. ► The HF and B3LYP results were compared with the experimental data. ► Frequencies and optimized geometries of two conformers of 6QC were calculated. ► Assignments were performed on the basis of PED of the all vibrational modes. ► Thermodynamic parameters were computed.