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 (CH 3 COOH·(H 2 O) n ( n = 1-10) and CH 3 CO 2 − ⋅(H 2 O) 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 СН 3 СООН–H 2 O solutions: previously found CH 3 COOH·(H 2 O) 2 heterotrimer and CH 3 COOH·(H 2 O) 8 , and three complexes (CH 3 CO 2 − ⋅(H 2 O) 2 , CH 3 CO 2 − ⋅(H 2 O) 6 , and CH 3 CO 2 − ⋅(H 2 O) 16 ) in NaCH 3 CO 2 − –H 2 O 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.