Conclusion
Theoretical analysis was performed to explore the influence of H-bond on acidity of Deoxy-hexose sugars including CH3-pentose and aldo-pentose sugars. The results of the calculation reveal that acidity in the Deoxy-hexose sugars significantly depend on their OH groups orientation (equatorial and axial position) and the nature and strength of H-bonds in their conjugate bases. The axial OH groups in aldo-pentose were found to be more acidic than the equatorial OH groups in CH3-pentose sugars. Compared to aldo-pentose sugars, methy-pentose sugars are less acidic due to existent of CH3 group on C5. It was found that these sugars are more acidic than normal alcohols such as ethanol and propanol because their conjugate bases are stabilized through intramolecular H-bonds. Thus, the nature of these H-bond interactions was analyzed by means of the natural bonding orbital (NBO) and quantum theory atoms in molecules (QTAIM) approaches. Analysis of the electron density in methy-pentose reveals the existence O-H…O and C-H..O intramolecular hydrogen bonds in methyl-pentose sugars. Also, the results indicated the presence of two types of intramolecular (normal and bifurcated) H-bonds in the conjugate bases of Deoxy-hexoses sugars. The conjugate bases of D-ribose, L-lyxose and L-arabinose show bifurcated intramolecular H-bonds; in contrast, those of other sugars indicate only normal H-bonds. It was shown that intramolecular H-bonds are electrostatic (closed-shell) interactions in the conjugate bases of Deoxy-hexoes sugars due to at BCPs of H-bond. Although, depending on the signs, some sugars are partially covalent-partially electrostatic and some are van der Waals interaction in nature.