Carboxylic acid—N-base systems are studied by i.r. spectroscopy. The position of the proton transfer equilibrium in OH ⋯ N ⇌ O^–⋯ H⁺N bonds can be determined by considering the i.r. bands. Thus, in the case of pure 1 : 1 mixtures, 50 % proton transfer is observed in these bonds for ΔpK_a= 2.3, i.e., if the pK_a of the acid is 2.3 units smaller than that of the protonated base. It is shown that a double minimum energy surface is present in these bonds. An i.r. continuum indicates that these bonds are easily polarizable. Thus, B₁H ⋯ B₂⇌ B^–₁⋯ H⁺B₂ bonds may be easily polarizable, too, when B₁≠ B₂. This polarizability is greatest for bonds with 50 % proton transfer, i.e., when the weights of the proton boundary structures OH ⋯ N and O^–⋯ H⁺N are equal. With increasing degree of asymmetry of such bonds, the polarizability does not decrease appreciably within a very considerable ΔpK_a region. A major decrease is found only when the ΔpK_a becomes < 1.0 or > 3.5. Even when the proton is largely located at one side of the bond, as indicated by the bands, the polarizability is weaker but can still be observed and only decreases slowly with increasing asymmetry, as indicated by the continuum. The continua show band-like structures in the region 3200–1800 cm^–1 which is also explained.

The value ΔpK_a= 2.3 for 50 % proton transfer is only valid when systems are considered with which the N-bases have additional NH groups. When no additional NH groups are present, 50 % proton transfer is obtained at ΔpK_a= 4. Hence, changes of the structure of the molecules which alter their interaction characteristics with the environment, change the symmetry of the OH ⋯ N ⇌ O^–⋯ H⁺N bonds, too.

Acid—N-base 1 : 1 mixtures in which water molecules are present are also studied. Water shifts the proton transfer equilibrium, i.e., it increases the weight of the proton boundary structure O^–⋯ H⁺N. In the presence of four water molecules per acid base pair, the ΔpK_a is reduced from 2.3 to 0.9 (N-bases with additional NH groups) or from 4 to 2 (N-bases without additional NH groups). The reasons for this environmental influence on the energy surfaces are discussed. Finally, the dissociation of the acid—N-base bonds, which occurs with increasing water content in the systems is considered.