Theoretical study on formation mechanism of acetic acid associating configurations and their distributions under saturated conditions.

Context: The vapor-liquid equilibrium (VLE) properties of acetic acid systems generally behave strong non-ideality due to the associating interaction among acetic acid molecules. Theoretical study of the associating mechanism will provide guidance for the VLE property prediction, which is crucial for the designing on the separation process of the acetic acid systems. In this work, the association conformers and their distribution on acetic acid molecules in saturated gas and liquid phase were firstly studied. The proportions of the acetic acid monomer and multimers were obtained, which will contribute to the foundation for the vapor-liquid equilibrium simulations. The association mechanism on acetic acid molecules was then investigated by comparing among the structures and non-bonded interaction energies of different dimers. The structure of the cyclic dimer containing two OC-HO hydrogen bonds, may be found probably when acetic acid molecules approached. Electronic properties of different acetic acid dimers showed that the electrons around carbonyl oxygen atoms were deflected by the attraction of hydrogen atoms in the other molecule, which polarized the acetic acid molecules when the hydrogen bonds between acetic acid molecules were formed, providing theoretical basis for the polarized acetic acid molecular model.

Methods: In this work, the molecular dynamics (MD) simulations and DFT calculations were conducted through the software GROMACS and Gaussian 09, respectively. For the MD simulations, the OPLS-AA force field was used as the atomic force field, with the cubic simulation cells constructed by Packmol program. For the DFT calculations, the M06-2X functional was employed for the optimization of the associating structures with the 6-311G** basis sets. Hydrogen bonding energies of dimers were corrected for the basis set superposition error (BSSE) and the deformation energies of monomers. Furthermore, the energy decomposition analysis was conducted at DFT/M06-2X/def-tzp level by the ADF software, and the wave function analysis was conducted by the Multiwfn software including the atom in molecule (AIM) topology analysis, the electronic potential analysis, and the electron density difference analysis.