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

Abstract

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 H₅O₂⁺ 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 H₅O₂⁺ ions and CF₃COOH · (H₂O)₂ dihydrates were calculated.