FTIR spectroscopy and molecular level insight of diluted aqueous solutions of acetic acid

Aqueous solutions of acetic acid (AA) have been intensively explored for decades with a particular attention addressed to the hydrogen bond network generated by COOH group at different concentrations. In majority of studies conducted so far the envelope originated from νCO is decomposed into two bands assigned to differently hydrated monomers: the one presumably to AA···H₂O, and another one to AA···(H₂O)₂. In order to examine if species other than the mentioned monomers produce this spectral signature, we performed computational and FTIR spectroscopic study of AA in aqueous solutions. Dilute solutions of deuterated acetic acid (CD₃COOD) in D₂O and in C₂Cl₄ as a reference were prepared (c₀ = 0.001, 0.01 and 0.1 mol dm^-3) as well as of deuterated sodium acetate (CD₃COONa) in D₂O. CD₃COOD in 0.1 mol dm^-3 solution in D₂O displays a feature that separated in two signals with maxima at 1706 cm^-1 and 1687 cm^-1. A combined DFT and molecular dynamics study performed in this work showed the assignation of those spectral bands to be a more complex problem than previously thought, with syn-anti isomerism and hydration contributing to the experimentally observed broad νCO envelope.