Elucidation of the hydration pattern of trifluoroacetic acid in dilute solutions: FTIR and computational study

The atmospheric partitioning of trifluoroacetic acid (TFA) in aerosol is a complex function of the size of suspended water droplets and their pH value. The unraveling of the affinity of TFA towards basic but not acidic conditions may be accomplished by providing an insight into the hydration pattern of undissociated TFA. Owing to rather scarce details on very dilute aqueous solutions of trifluoroacetic acid (TFA), we examined CF₃COOD and CF₃COONa solutions in D₂O in the concentration range 0.001-0.1 mol dm^-3 using transmission FTIR spectroscopy and computational methods. Besides detecting the signals originated from undissociated species in both CF₃COOD (1787 cm^-1 and 1766 cm^-1 at c₀ = 0.1 mol dm^-3) and CF₃COONa (1807 cm^-1 at c₀ = 0.1 mol dm^-3) D₂O solutions, through computational techniques we identified different TFA hydrates that contribute to the complexity of the spectral appearance. The combination of experimental and computational data suggested the concentration dependence of the predominant hydrogen bonding pattern of TFA. The results obtained in this work should help in understanding the partitioning of TFA into micron-size water droplets in the atmosphere in molecular and structural terms, i.e. the eventual stability of a hydrated complex for a particular TFA conformer.