Influence of Proton Transfer on the Formation of Hydrogen-Bonded Complexes of Higher Stoichiometry and on their Dissociation into Free Ions.

Proton transfer in a given H-bond ⊂A—H—B—(H)→⊂A₋— H⁺—B—(H) considerably enhances the strength of the electron donor sites of the first partner and that of proton donor sites possibly present in the second one. This leads to the formation of complexes of higher stoichiometry of the type B— H⁺—(A⁻—H—A--H—A--H--)or A⁻---(H—B⁺—H----B—H—B—H----) where the self-association bonds are much strengthened. This is due to the high stability of the homoconjugated anions or cations in the corresponding ion pairs. In polar solvents like acetonitrile, the ion pairs may dissociate into free ions. The variety of the entities that can be formed necessitates a diversification of the quantitative concepts connected with the proton transfer process. Besides the average value 〈x₁〉 of the fractions of the various complexes in the ionized form, other quantities are defined that can also be used in the case of partial dissociation: (1) the percentage of ionized base molecules and (2) the fraction of donor molecules AH ionized after direct interaction with B. A further characteristic used in this generalized treatment is the average number 〈n〉 of proton donor molecules perturbed by one base molecule. Examples of determinations of these various parameters from calorimetric, infrared, or NMR data from the literature are presented and new quantitative correlations established. [ABSTRACT FROM AUTHOR]