Composition and Structure of Complexes Formed in Aqueous Solutions of Trifluoroacetic Acid According to IR Spectroscopy Data

The composition and structure of complexes that formed in aqueous solutions of trifluoroacetic acid were studied by frustrated multiple total internal reflection IR spectroscopy (FMTIR). Two types of complexes with a molecular structure formed: trimers CF3COOH · (H2O)2 and cyclic tetramers (CF3COOH)2 · (H2O)2, in which the molecules of the components are arranged in pairs. In the range of acid concentrations from 100% to [H2O]/[CF3COOH] = 1: 1, only these tetramers formed, and all added water was bound into these hydrates. In more dilute solutions (up to [H2O]/[CF3COOH] = 2: 1), CF3COOH · (H2O)2 complexes formed along with tetramers; at a double excess of H2O, the components of the solution were completely bound into these trimers. In dilute solutions (from 0 to 3.6 M CF3COOH), the acid is completely dissociated into H5O 2 + and CF3COO– ions hydrated with water molecules. In the range of medium concentrations (from 3.6 M to [H2O]/[CF3COOH] = 2: 1), the solutions contain both these ions and CF3COOH · (H2O)2 dihydrates. For this range of compositions of the CF3COOH−H2O system, the concentrations of H5O 2 + ions and CF3COOH · (H2O)2 dihydrates were calculated.