SELF-ASSOCIATION OF CARBOXYLIC-ACIDS AS STUDIED BY H-1-NMR SPECTROSCOPY

The self-association of the family of aliphatic carboxylic acids has been studied by analyzing the concentration dependence of carboxyl proton chemical shifts at various fixed temperatures. Specifically the increase of chemical shift of carboxyl protons with the acid dilution in inert solvent to 0.1 mole fraction has been found to be essentially dependent on the structure and volume of the radical of the carboxylic acid. The chemical shifts of cyclic dimers and open associates, and the thermodynamic parameters of the self-association have been calculated. The increase of the cyclic dimer chemical shifts and their temperature coefficient, and the decrease of enthalpy change in the range of acetic, methylacetic, ethylacetic, dimethylacetic, and trimethylacetic acid have been obtained. The unexpected interrelation of these parameters has been quantitatively interpreted assuming the interaction between the cyclic dimers, polymerization and the existence of cis- and transmonomers of acid. © 1979.