Hydrogen Bonding in Hydrates with one Acetic Acid Molecule

Hydrogen bonding (H-bond) interaction significantly influences the separation of acetic acid (HAc) from the HAc/H2O mixtures, especially the dilute solution, in distillation processes. It has been examined from the HAc mono-, di-, tri-, and tetrahydrates by analyzing the structures, binding energies, and infrared vibrational frequencies from quantum chemical calculations. For the first coordinate shell the 6-membered head-on ring is surely the most favorable structure because it has (1) the most favorable H-bonding parameters. (2) almost the largest binding energy per H-bond, (3) the biggest wavenumber shifts, and (4) the highest ring distribution (the AIMD) simulations). Moreover, the comparison of the calculations with the experiments the X-ray scattering data and IR frequencies) suggests that the possible structures in dilute aqueous solution arc those involving two or more coordinate shells. The H-bonding in these water-surrounded HAc hydrates are the origin of the low-efficiency problem of isolating HAc Iron) the dilute HAc/H2O mixtures. It is apparently a tougher work to break the H-bonds among HAc and the surrounded H2O molecules with respect to the case of more concentrated solutions, where the dominant structures are HAc or H2O aggregates.