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. Scaledforce constants are also reported. One of two optimized structures corresponds to the experimental reported. Thediminishing of scaled force constants in water is justified because both acceptors of H bonds, N atoms of quinuclidineand quinoline rings, and the O atom of O-CH3 group are hydrated in solution. Topological propertiesconfirm the high stability observed for quinine by using NBO calculations and reveal higher densities of threefused 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 thismedium. The UV spectra of quinine in ethanol show electronic and vibronic signatures when the concentration ofthe solution is 1 mg/mL and reveal that the positions peaks change with the concentration due to the effects ofinteractions.