Some aspects of the structure and interaction potential of hydrogen bonded complexes

The different aspects which are considered here are the influence of the medium on the formation of hydrogen bonds, the cyclization of aggregates, the polarity of the bonds and the ionization processes, and the influence of the first hydrogen bond on the formation of a second one. The stability of the complexes is influenced by the non-specific and specific interactions between the complex and the separated constituents, respectively, and the molecules of the surrounding medium. When the solvent molecules exhibit specific sites which can compete with those of the donor or of the acceptor, the complexation is reduced. This is also the case, when the solvent molecules are highly polarizable. On the other hand, when polar species are present in the medium, interaction parameters between these species and the polarizable complexes must be taken into account. This is particularly the case for the interaction between the complex triethylamine-acetic acid and the dimer of this acid.

Cyclization can only occur to an appreciable extent for low aggregates because the cyclization of higher aggregates is completely disfavoured by the loss of entropy which is not compensated for by the energy of the additional bond. Thus whilst dimers of the higher carboxylic acids are mostly in the cyclic form, and whilst the lower aggregates (trimers, tetramers …) of the alcohols and of the phenols show an appreciable cyclization, the higher aggregates of the latter substances predominantly assume non-cyclic forms. Sometimes, as a necessary prerequisite, cyclization requires a specific conformation of the monomer. This possibly explains why the cyclization is less effective in formic acid where the “trans” isomer is more important, and is almost completely absent in propionamide, where the cis-form is practically nonexistent.

In many cases there exist two energy levels of potential energy for the proton participating in a hydrogen bond. The equilibrium constant K_i between the two protomers depends (for a given family of complexes and in a given medium) on the difference between the pK_BH⁺ of the base and the pK_HA of the acid. Below a certain range of ΔpK the normal bond entirely predominates, whereas the reverse is true above this range. The predominancy of a given form can be shown by various methods such as NMR spectrometry, measurements of dipole moments, solubility, etc. Solvation of the complex by molecules of the medium can influence the energy of the two protomers and thus K_i.

When a molecule has an H-bond as proton donor, the tendency of other sites of this molecule to act as proton donor decreases, whereas their tendency to act as electron-donor sites is enhanced. The reverse is true for molecules acting as electron donors. These effects are particularly strong if the hydrogen bond formed involves a (partial) proton transfer.