Chapter 21 Architecture of Hydrates and Local Structure of Acetic Acid Aqueous Solution: Ab Initio Calculations and Car–Parrinello Molecular Dynamics (CPMD) Simulations on Hydrogen-Bonding Rings, Network, and Intra-Hydrate Protonation in Multi-Hydrates of Acetic Acid Monomer

Abstract

Abstract The protonation and deprotonation phenomena and molecular association of solute molecule with water via intermolecular hydrogen bonding forming various hydration compounds are very common in aqueous solution and in biological cell in nature. In the aqueous solution, more complicated type of hydrogen bond, hydrogen-bonding rings, various kinds of hydration compounds (hydrates), and even hydrogen-bonding network can be expected. The nature of hydrogen bonding, the bonds networking, the rule in architecture of larger hydration compounds, deprotonation of acetic acid in solution, stability of the hydrated proton, and the local structure of its aqueous solution are the most fundamental problems to understanding solute molecule living style in aqueous solution. Hydrogen-bonding rings and network in the multi-hydrates of acetic acid monomer have been investigated by ab initio calculations, and ab initio molecular dynamics (CPMD) simulations on acetic acid monomer–water system were also performed to explore the local structure of acetic acid aqueous solution. More than hundreds of multi-hydrates have been selected as candidates during our calculations. The structural optimizations and energy calculations have been performed at the MP2/6-31+g (d, p) and MP4/6-31+g (d, p) levels which are adequate for our large hydrates calculations with reliable results and reasonable cost as we stated in the Section 2. The most stable structure of the smallest hydration compound of acetic acid monomer, i.e., acetic acid water dimer, has a four-membered head-on ring with the smallest dipole moment. To verify the existence of it, the infrared spectra experiment data were collected in the dilute CCl 4 /HAc and CCl 4 /H 2 O ratio condition. The hydroxyl (O–H) stretching vibrations in molecules of water, acetic acid, and the dimer are distinguished, for the dissolved species are isolated from each other by surrounded solvent molecules CCl 4 . The calculated and measured vibration frequencies are almost lain in line with 0.872 scaling. The four-, five-, and six-membered head-on rings are the most favorable in the small multi-hydrates with a stable planar structure and the side-on ring with a weak hydrogen-bonding interaction of C − H ⋯ O w − H . The six-membered ring is most important in the large multi-hydrates and in the local structure of dilute solution also verified by our CPMD simulations. A larger ring of or more than seven-membered could no longer maintain the planar structure. Larger multi-hydrates of acetic acid monomer could be constructed from these basic building blocks. The strength order for the five kinds of bonds is in the descending order as C – O – H ⋯ O w – H > O w – H ⋯ O w – H > C = O ⋯ H – O w > O w – H ⋯ O a – H >> C – H ⋯ O w – H . The three hydroxyl bonds O–H in the hydrated proton could be divided into two types: one of the hydroxyl bonds is hydrogen bonded to the deprotonated oxygen of the acetic acid, other two to waters in the hydrate. The larger the R O – H becomes, the smaller the R O ⋯ H and the stronger the associated hydrogen bond are. The hydrated proton is stable and liberated from the deprotonated acetic acid in the large multi-hydrate with more complicit head-on ring. In small hydrates, the protonating hydrogen is shared partly by the acetic acid.