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 center dot(H2O) (n) (n = 1-10) and CH3CO2 (-)ai...(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 CH3COOH-H2O solutions: previously found CH3COOH center dot(H2O)(2) heterotrimer and CH3COOH center dot(H2O)(8), and three complexes (CH3CO2-center dot(H2O)(2), CH3CO2-center dot(H2O)(6), and CH3CO2-center dot(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.