Full FTIR, FT-Raman assignments of natural quinine alkaloid combined with DFT and SQMFF calculations

Abstract

Here, experimental vibrational spectra of natural quinine alkaloid have been combined with hybrid B3LYP/6-311++G** and SQMFF calculations to perform full assignments of the expected 138 vibration modes. Scaled force constants are also reported. One of two optimized structures corresponds to the experimental reported. The diminishing of scaled force constants in water is justified because both acceptors of H bonds, N atoms of quinuclidine and quinoline rings, and the O atom of O-CH₃ group are hydrated in solution. Topological properties confirm the high stability observed for quinine by using NBO calculations and reveal higher densities of three fused quinuclidine rings than the quinoline one. The high reactivity observed for quinine in aqueous solution (3.631 eV) is supported by the high ω value and by its solvation energy (−75.98 kJ/mol) observed in this medium. The UV spectra of quinine in ethanol show electronic and vibronic signatures when the concentration of the solution is 1 mg/mL and reveal that the positions peaks change with the concentration due to the effects of interactions.