Symmetry of hydrogen bonds, infrared continuous absorption and proton transfer

Abstract

In liquid systems in which BH⁺⋯ B hydrogen bonds with a symmetrical double minimum potential well are present, i.r. continuous absorption is observed. This shows that the protons have a continuous energy level distribution, caused by various interactions resulting from the extremely high polarizability of such hydrogen bonds. Mixtures of carboxylic acids and nitrogen bases were investigated to see how the occurrence of this continuum depends on the symmetry when the two acceptors B are not identical. The AH ⋯ B hydrogen bond between acid and base always produces i.r. bands and not continuous absorption. The occurrence of continuous energy level distribution of the protons thus depends to a large extent on the symmetry in the systems investigated in this work. As well as these bands, however, the continuum is observed with some of the above systems, since BH⁺⋯ B bonds form between the base molecules. In the AH ⋯ B bonds proton transfer to the base is observed in anhydrous systems when the acid is more acidic than the base by ΔpK_a≈ 4. This proton transfer is favoured by excess acid and by the presence of water molecules. In aqueous solutions of mixtures of carboxylic acids and N bases, proton transfer occurs in the AH ⋯ B bond at ΔpK_a≈ 2. According to these results the hydrogen bonds between the acidic and basic amino-acid residues could well be largely symmetrical. The possible significance of this result for biological systems is discussed.