Infrared and Raman spectroscopic study of carboxylic acids in heavy water

Infrared and Raman spectra of CH3COOD and CD3COOD were obtained below 1900 cm−1, from dilute solutions in heavy water up to neat liquids. The relation of the intermolecular interactions with the acid concentration is discussed. In the debate concerning the structure of pure liquid acetic acid at room temperature, we favour the assignment of the major species to the centrosymmetric dimer. This is based not only on the literature data for crystal, liquid and vapour acetic acid, but also on the infrared and Raman spectra of liquid acetic and propanoic acids and their 0.1 M solutions in water. On increasing the acid concentration from dilute solutions, the hydrated monomer is progressively replaced by more or less hydrated linear dimer and then by the cyclic dimer. At a molar fraction of 0.5, the three kinds of species coexist. At still higher concentrations, longer oligomers appear while the cyclic dimer dominates the other species. A comprehensive assignment of the infrared and Raman spectra is given on the basis of a previously published ab initio calculation for monomer, cyclic and linear dimers. A narrow well resolved satellite band is observed for the hydrated monomer some 45 cm−1 above the CC bond stretching vibration (νCC), not only for CH3COOD and CD3COOD but also for C2H5COOD and CH2ClCOOD, specifically in dilute heavy water solutions. It is not easily assigned to overtones or combination bands simultaneously for the three molecules. Both a blue-shift and a narrowing of the νCC band are usually observed by ionisation of a carboxylic acid into a carboxylate ion in water. The hypothesis of a contact ion pair {R–COO−, D3O+}, whose protonated equivalent species would not exist in normal water, is thus discussed.