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

Abstract

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.