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 relati on of the intermolecular interactions with the acid concentration is discus sed. In the debate concerning the structure of pure liquid acetic acid at r oom temperature, we favour the assignment of the major species to the centr osymmetric 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. O n increasing the acid concentration from dilute solutions, the hydrated mon omer is progressively replaced by more or less hydrated linear dimer and th en by the cyclic dimer. At a molar fraction of 0.5, the three kinds of spec ies 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 publi shed ab initio calculation for monomer, cyclic and linear dimers. A narrow well resolved satellite band is observed for the hydrated monomer some 45 c m(-1) above the CC bond stretching vibration (nu CC), not only for CH3COOD and CD3COOD but also for C2H5COOD and CH2ClCOOD, specifically in dilute hea vy water solutions. It is not easily assigned to overtones or combination b ands simultaneously for the three molecules. Both a blue-shift and a narrow ing of the nu CC band are usually observed by ionisation of a carboxylic ac id 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.