NMR and IR studies of hydrogen bonded complexes

Nmr and ir spectroscopies were used for studying hydrogen bonding in carboxylic acid-tetrabutyloammonium pentachlorophenolate systems in nitrobenzene. Changes in chemical shift of acid proton and in stretching frequency of the CO group due to formation of hydrogen bonds were determined. These changes have been correlated with the ΔpKa values associated with complex formed. It has been shown that 50% of proton transfer occurs at ΔpKa about 2 for aromatic acids and ΔpKa about 2.5 for aliphatic acids. It is suggested as a general rule that for a given acid-base system this critical ΔpKa value is lower in more polar solvents and for a given solvent, lower when the acid or base or both include π-electrons.