Ionic Hydrogen Bond Effects on the Acidities, Basicities, Solvation, Solvent Bridging, and Self-Assembly of Carboxylic Groups

Abstract

Ionic assemblies of acetic acid and water form unlimited hydrogen bond networks. The stabilities of the networks correlate with the intrinsic acidities of the components, leading to strong CH3COO-···HOOCCH3 bonds and weak CH3COO-···H2O bonds. These relations apply from strong bonds in small aggregates to weak bonds in large assemblies, and affect the energies of acid dissociation and self-assembly. Partial solvation of CH3COO- by four H2O molecules facilitates acid dissociation and decreases the CH3COO-−H+ bond dissociation energy by 332 kJ/mol (80 kcal/mol). The stabilites of the hydrogen bond networks increase with CH3COOH content, and aggregation decreases further the acid dissociation energy by forming strong CH3COO-···HOOCCH3 bonds about the ions and by stabilizing the released protons in (CH3COOH)m(H2O)nH+ assemblies. The combination of strong CH3COO-···HOOCCH3 bonds and weak CH3COO-···H2O bonds makes self-assembly with solvent displacement particularly favorable for carboxylic acids, explaining their assembly in bilayers and membranes. Ab initio calculations show that isomeric assemblies with directly bonded and solvent-bridged structures have similar energies. As well, the solvent-bridged species CH3COO-···H2O···HOOCCH3 has similar energy to its cation-bridged isomer CH3COO-···H3O+···-OOCCH3. In this transition state the adjacent anions stabilize the central cation, providing low-energy pathways for proton transfer between carboxylic groups