STRUCTURE OF MOLECULAR COMPLEXES FORMED IN AQUEOUS SOLUTIONS OF TRIFLUOROACETIC ACID
E. G. Tarakanova, G. V. Yukhnevich
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
Keywords: solution of CF3COOH—H2O, trifluoroacetic acid, structure of trifluoroacetic acid hydrates, IR spectroscopy, hydrogen bond
Abstract
Optimized structures and vibration frequencies of H-bonded complexes formed by one or two molecules of CF3COOH (or CF3COO– ion) with water molecules and the monomers and dimers of formic, acetic, trifluoroacetic, and tribromoacetic acids were calculated using density functional theory (B3LYP/6-31++G(d,p)). The results were compared with available experimental data on the vibrational spectra and equilibrium composition of the CF3COOH—H2O system and the acids listed above. The structure of the hydrated forms of CF3COOH molecules and CF3COO– anions which are present in aqueous solutions of trifluoroacetic acid was determined. In these solutions at concentrations 100 % > [CF3COOH] > 35 %, (CF3COOH)2×(H2O)2 cyclic tetramers form in which the acid and water molecules are arranged pairwise side-by-side, and starting at 75 % CF3COOH, CF3COO–×(H2O)2 cyclic dihydrates of the anion are formed. The average strength of the hydrogen bonds in these heteroassociates is ~9–10 kcal/mol, and the properties and mutual position of their constituent molecules almost completely determine their vibrational spectra in solution. This approach provides a good reproduction of the IR and Raman spectra of molecular complexes, that have a cyclic structure and an average H–bond energy not less than 7 kcal/mole, in solution.
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