Structural, FT-IR, FT-Raman and ECD studies on the free base, cationic and hydrobromide species of scopolamine alkaloid

Abstract

The complete vibrational assignments for the alkaloid scopolamine as free base, quaternary scopolammonium cationic and hydrobromide species were reported for first time by using the internal coordinates, the scaled quantum mechanical force field (SQMFF) procedure and the experimental FT-IR, FT-Raman spectra of hydrobromide tri-hydrate. Hybrid B3LYP/6-31G* calculations were employed to study two pairs of R and S enantiomers of free base, cationic and hydrobromide forms in gas and aqueous solution phases. The properties in solution were studied with the self-consistent reaction field (SCRF) method and the integral equation formalism variant polarised continuum model (IEFPCM) model while the solvation energies were computed by using the solvation model. The structural, electronic, topological and vibrational properties were studied only for the most stable structure of each form by using natural bond orbital (NBO) and atoms in molecules (AIM) analyses. Also, the Mulliken and Merz-Kollman (MK) charges were evaluated in both media together with the molecular electrostatic potential (MEP) in order to predict the potential nucleophilic and electrophilic reaction sites for each species. The frontier orbitals and some descriptors were investigated for those three species in order to compare with those reported for heroin, morphine, cocaine and tropane alkaloids. Here, the force fields and the complete assignments of the 123, 126 and 129 vibration normal modes expected for the free, cationic and hydrobromine anhydrate form of scopolamine alkaloid are reported. The predicted infrared, Raman and Electronic Circular Dichroism (ECD) support the presence of cationic S2 form of scopolamine in the solid state, in concordance with the corresponding experimental structure determined by X-ray diffraction. In addition, the force constants are reported for the three species of scopolamine.