Acid-base interaction from the viewpoint of molecular clustering - Effects of solvent, pKa and size of alkyl group

Acid-base interactions were investigated by mass spectrometry of clusters isolated from solutions containing a carboxylic acid (propionic, butyric or hexanoic acid), an aromatic base (pyridine or pyrazine) and a solvent (water, acetonitrile or propionitrile). In water, self-aggregation of the carboxylic acid molecules due to hydrophobic interaction to form carboxylic acid clusters, such as (butyric acid)(n), was prominent, and acid-base interaction proceeded between the carboxylic acid clusters and the basic molecules. In acetonitrile, the acid-base interaction was sensitive to the relative strength of the base. When pyridine (relatively strong base) was used as a base, clustering proceeded through the formation of a polarized acid-base complex, (acid)(delta-)(base)(delta+). However, when pyrazine (relatively weak base) was used as a base, self-aggregation of the acid molecules became favorable. In propionitrile, such clusters were not observed because each molecule was separated by individual solvation. This solvent effect is related to the solvation structure, which determines the balance between the self-aggregation of the acid molecules and the formation of the polarized acid-base complex. It has also been demonstrated that the balance of the above intermolecular interactions is also dependent on the size of the alkyl group of the carboxylic acid. The results of the mass spectrometry partially show the microscopic view for the effect of solvent, pK(a) and the size of the alkyl group on the acid-base interaction in solution.