Theoretical Study of Hydrogen Bond Formation in Four Rare Sugars: Gulose, Allose, Altrose, and Talose

Rare sugars are monosaccharides with tremendous potential for applications in pharmaceutical, cosmetics, nutraceutical, and flavors industries. The four rare sugars, including; gulose, allose, altrose and talose are stereoisomers that are different in the hydroxyl group orientation (axial or equatorial) on the C2-4 atoms. The DFT, AIM, and, NBO calculations were used to probe the probability of formation of internal H-bonds in four rare sugars. The AIM analysis identified that altrose and talose can form three predominantly intramolecular H-bonds, whereas gulose and allose revealed one and two H-bonds, respectively and these normal intramolecular H-bonds are mostly closed-shell interactions. The theoretical calculated O-H stretching FT-IR vibrational frequencies confirmed that the intramolecular H-bonds shifted toward low frequencies in comparison to the free hydroxyl group, which caused the red-shift. Also, the lowest IR frequency in each sugar was related to the structure with the highest stabilization energy and the most strongest intramolecular H-bonds.

Vibrational Studies of Species Derived from Potent S(+) and R(-) Ecstasy Stimulant by Using Ab-initio Calculations and the SQM Approach

B3LYP/6-311++G** calculations and the scaled quantum mechanical force field (SQMFF) approach have been used to study the structures and vibrational spectra of three species derived from potent S(+) and R(-) ecstasy stimulant. The complete vibrational assignments of free base, cationic, and hydrochloride species of both enantiomeric forms of ecstasy have been reported by using the normal internal coordinates and the experimental available attenuated total reflectance ATR-IR and FT-Raman spectra. SQM calculations predicted that the three species could be present in the IR spectrum of hydrochloride species because the IR bands of medium intensity at 2794 cm-1 is assigned to the stretching C4-H15 and symmetric CH3 modes of the free base while the strong IR band at 1508 cm-1 is assigned easily to NH2 stretching mode of hydrochloride species and NH2 deformation modes of cationic species. The calculations reveal the same energy values for both enantiomers, indicating that both could exist simultaneously in the two media with similar corrected solvation energies in solution probably because the R(-) form is quickly converted to the S(+) one. Three types of charges studied in both media evidence higher effect on the N atoms belonging to N-CH3 groups of three species of S(+) form of ecstasy in both media than on the O atoms of R1 ring. The high gap value predicted for the hydrochloride species of S(+) form in solution supports the low reactivity of this species, in agreement to its higher stability evidenced in this medium by AIM and NBO calculations. The predicted Ultraviolet-visible and Electronic Circular Dichroism ecstasy (ECD) support the presence of both enantiomeric forms in solution while excellent concordance evidence the comparisons between the predicted 1H- and 13C-NMR chemical shifts for the three species of S(+) form of ecstasy with the corresponding experimental ones.