Hydrogen bonding in the side-on rings of the hydrates with one acetic acid molecule

In chemical industry, hydrogen bonding (H-bonding) in the dilute aqueous solutions of acetic acid (HAc) always causes a low efficiency in the distillation for separating HAc from the HAc/H2O mixture. This research examined the H-bonding in the side-on rings (SO), one of the favorable structures in the HAc hydrates, by analyzing the energies and the infrared frequencies of different hydrates from quantum chemical calculations. It is found that the OO―H···HW H-bond in a hydroxyl SO ring and the OC═O···HW in a carbonyl SO ring are slightly weaker than the HO―H···OW or OC═O···HW in a head-on ring (HO). The H-bonding is obviously strengthened for either the HO or the SO ring containing water molecules from none to one and two. When more water molecules are involved, the HO and SO rings become correlated and show a lower binding energy. These features indicate that the H-bonding in the SO ring is also favorable and the preferable structures are the combination or correlation of the SO and HO rings. In this case, it is very difficult to isolate HAc from these water-surrounded structures. Therefore, it needs much more effort to separate HAc from the dilute HAc aqueous solutions than from the high concentrated solutions, where the main interactions are those between HAc molecules.