Semiempirical quantum chemical calculations of molecular π- complexes

A method is suggested to calculate the geometrical and thermodynamical characteristics of organic π- complexes. The interaction energy is considered as a sum of two terms: the specific quantum chemical interaction of the π- electron systems depending essentially on the chemical nature of reactants, and the nonspecific atom-atom (van-der-Waals) interaction depending only on the kind of atoms belonging to the fragments of a complex. An attractive quantum chemical interaction is described in terms of the PPP-method; the van-der-Waals interaction is expressed in terms of the empirical exp-6-potential. The geometries of complexes are found by the complete energy minimization with respect to six parameters characterizing the mutual orientation of the complex fragments. Energies, entropies and equilibrium constants of several tetracyanoethylene π-complexes are calculated by this method. The results agree satisfactorily with experimental data.