Combined experimental and quantum chemical studies on spectroscopic (FT-IR, FT-Raman, UV–Vis, and NMR) and structural characteristics of quinoline-5-carboxaldehyde

Abstract Comparative experimental and theoretical studies have been performed on the structure and spectral (FT-IR, FT-Raman, UV–Vis and NMR) features of quinoline-5-carboxaldehyde. Quantum chemical calculations have been carried out at Hartree-Fock and density functional B3LYP levels with the triple-zeta 6-311++G** basis set. Two stable conformers of quinoline-5-carboxaldehyde arising from the orientation of the carboxaldehyde moiety have been located at the room temperature. The energetic separation of these conformers is as small as 2.5 kcal/mol with a low transition barrier (around 9 kcal/mol). Therefore, these conformers are expected to coexist at the room temperature. Several molecular characteristics of quinoline-5-carboxaldehyde obtained through B3LYP and time-dependent B3LYP calculations, such as conformational stability, key geometry parameters, vibrational frequencies, IR and Raman intensities, UV–Vis vertical excitation energies and the corresponding oscillator strengths have been analyzed. The 1 H and 13 C NMR chemical shifts of quinoline-5-carboxaldehyde were also investigated.