Dimerization of Carboxylic Acids and Salts:  An IR Study in Perfluoropolyether Media

It was found that perfluoropolyethers (PFPE) with carboxylic acid end groups, PFPE-COOH, and their sodium salts, PFPE-COONa, afford an anhydrous fluid medium in which the direct pair-wise interaction of these end groups may be examined. An IR study (of the carbonyl band) of these materials revealed not only the dimerization of acid units, but also the formation of salt dimers, (salt)2, and mixed dimers, (acid)(salt). The carbonyl spectra of PFPE-acid/salt mixtures exhibit temperature-dependent broadening, shift, and, in some cases, coalescence of the bands which are attributed to the pseudocoalescence phenomenon stemming from the presence of relatively low energy cation exchange passages among the three possible dimers. Examination of the corresponding CH3COOH/CH3COONa system by ab initio calculations (6-31G) yielded the heat of dimerization of ∼20, ∼50, and ∼35 kcal/mol for the acid−acid, the salt−salt, and the acid−salt dimers, respectively. The calculated dimer structures are shown and discussed. The calculations also yielded the carbonyl stretching frequencies in good accord, and in support of the analyses/postulates made for the PFPE-acid/salt system.