GEOMETRIES OF MOLECULAR COMPLEXES - AN EXTENDED HUCKEL APPROACH

The extended Hückel scheme of Hoffmann is approximated to allow relatively easy energy cal-calculations as a function of relative intermolecular geometry for a variety of planar charge transfer and charge resonance molecular complex pairs. The Hoffmann scheme is briefly discussed and the pertinent features of the method are illustrated with particular reference to weakly interacting molecular pairs. A dominant experimental structural feature is noted for all the complexes studied, in which a π-bond of one molecule is centered over and parallel to two edges of a hexagonal ring of the second molecule; it is basically a bonds-over-bonds arrangement. The calculated structures show good agreement with those observed for systems containing TCNE-like fragments and fair agreement for those in which this dominant structural feature occurs through the projection of two six-membered rings. In general no absolute binding is predicted. However, the agreement of relative minima between calculated and observed geometries coupled with the ease of calculation indicate the simple method may be useful in predicting relative geometries and further elucidating the more complicated structural features which must be involved. © 1969 Springer-Verlag.