Infrared and Raman Spectroscopy Studies of Light and Heavy Trifluoroacetic Acids

Abstract

An infrared and Raman spectroscopic study of light and heavy trifluoroacetic acids shows that their vapors at room temperature are composed of a mixture of monomeric and associated molecules. The principal bands corresponding to vibrations of the COOH and COOD groups have been identified for both the monomeric and the associated molecules by means of a comparative study of the two acids (a) in the vapor state at different pressures, (b) in CCl₄ solutions at various concentrations, and (c) in the liquid state. The monomeric carbonyl frequencies, influenced by the presence of the perfluoromethyl group, are the highest yet reported in the literature; this is true not only for CF₃COOH (1826 cm⁻¹, vapor), CF₃COOD (1823 cm⁻¹, vapor), but also for the ethyl ester, CF₃COOC₂H₅ (1802 cm⁻¹, vapor), the sodium salt, CF₃COONa (1687 cm⁻¹, solid), and the anhydride (CF₃COO)₂O (1884 cm⁻¹ and 1818 cm⁻¹, vapor). On the basis of a comparative study of the spectra of all these compounds, as well as of other perfluoromethyl compound spectra reported in the literature, the C–F stretching vibrations for the two acids have been assigned to the strong infrared absorptions at 1244 cm⁻¹ and 1185 cm⁻¹. The corresponding Raman bands are very weak.