ISING-MODELS OF POLYPROTIC ACIDS AND BASES

Ising models, which have received much attention in statistical mechan ics, are used to predict acidity constants (pK values) of polyprotic a cids or bases. The present model circumvents the calculation of micros copic pK values, which is necessary in the classical approach, and lea ds directly to all experimentally observable macroscopic pK values. Th e Ising model allows a transparent parametrization of the problem wher e the basic parameters are the microscopic pK values of the acid-base groups (given all other groups in the molecule are deprotonated) and i nteraction energies between pairs and triplets of protonated sites. Th e classical description is recovered if only pair interactions are con sidered while the model can be systematically improved by including tr iplet (and higher order) interactions. As an illustration, the model i s applied to linear aliphatic amines and polyphosphoric acids.