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

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 CH 3 COO - ···HOOCCH 3 bonds and weak CH 3 COO - ···H 2 O 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 CH 3 COO - by four H 2 O molecules facilitates acid dissociation and decreases the CH 3 COO - −H + bond dissociation energy by 332 kJ/mol (80 kcal/mol). The stabilites of the hydrogen bond networks increase with CH 3 COOH content, and aggregation decreases further the acid dissociation energy by forming strong CH 3 COO - ···HOOCCH 3 bonds about the ions and by stabilizing the released protons in (CH 3 COOH) m (H 2 O) n H + assemblies. The combination of strong CH 3 COO - ···HOOCCH 3 bonds and weak CH 3 COO - ···H 2 O 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 CH 3 COO - ···H 2 O···HOOCCH 3 has similar energy to its cation-bridged isomer CH 3 COO - ···H 3 O + ··· - OOCCH 3 . In this transition state the adjacent anions stabilize the central cation, providing low-energy pathways for proton transfer between carboxylic groups.