Composition and structure of hydrates of CH3COOH molecules and CH3CO2− anions in aqueous solutions

The optimal configurations, energy parameters, and normal vibrational frequencies of hydrates of the acetic acid molecule and anion (CH3COOH·(H2O)n (n = 1-10) and CH3CO2−⋅(H2O)n (n = 1-8, 16)) are calculated by density functional theory (B3LYP/6-31++G(d,p)). The comparison of the calculation results with the known experimental data (Raman, NMR, and so on) gives arguments in favor of the existence of two stable heteroassociates (HAs) in СН3СООН–H2O solutions: previously found CH3COOH·(H2O)2 heterotrimer and CH3COOH·(H2O)8, and three complexes (CH3CO2−⋅(H2O)2, CH3CO2−⋅(H2O)6, and CH3CO2−⋅(H2O)16) in NaCH3CO2−–H2O solutions. Each of them is most stable in a series of isomers, contains unstrained H bonds, and is characterized by the maximum molecular packing density among HAs with similar n values. The structure of the subsequent complex formed in solution uniquely follows the structure of the preceding complex and is based on it.