Experimental isolation and spectroscopic characterization of squamocin acetogenin combining FT-IR, FT-Raman and UV-Vis spectra with DFT calculations.

Squamocin, an annonaceous acetogenin has been experimentally isolated and characterized in the solid state using the FT-IR and FT-Raman spectra and in methanol solution by UV-visible spectrum. The main bands observed were assigned combining the IR and Raman spectra with hybrid functional B3LYP/6-31G∗ calculations. Structural, electronic and topological properties were predicted at the same level of theory for the most stable conformer of squamocin in gas phase and methanol solution. A corrected solvation energy value of -147.54 kJ/mol was predicted for squamocin in methanol while the atomic population natural (NPA) charges evidence higher values on O atoms of R2 and R3 rings, as compared with the corresponding to lactone ring. Mapped MEP surfaces suggest that nucleophilic regions are located on the O atoms of three rings and of OH bonds belonging to side chain, in agreement with the higher charges values evidenced on these O atoms while electrophilic regions are predicted on the H atoms of OH groups. High stabilities of squamocin in both media was revealed by AIM studies while only in methanol solution by NBO calculations. The expansion of volume and the higher dipole moment in methanol suggest a clear solvation of squamocin by solvent molecules. Gap values have evidenced that squamocin is most reactive in methanol while that its large aliphatic chain produces an increases the reactivity of this γ-lactone, as compared with ascorbic acid lactone. Reasonable concordances among the predicted UV-visible and IR, Raman spectra with the corresponding experimental ones were found.