Topological aspects of chemical reactivity: valency changes in the course of chemical reactions
The Jug and Gopinathan approach to the use of valency changes for the characterization of chemical reactions is generalized by incorporating it into the framework of the recently proposed topological description of chemical reactivity in terms of the overlap determinant method. The conclusions of the simple topological analysis agree with the results of the direct quantum chemical treatment, indicating thus that the overlap determinant method depicts correctly the principal features of the electron density reorganization occuring during the chemical reactions. One of the fundamental concepts of organic chemistry is the concept of valency, on the basis of which an immense number of empirical findings accumulated by the classical structural theory could be classified and systemized. For a deeper justification of this theory, frequent attempts have been made over the past decade to incorporate the valency concept into the wider scope of the theoretically founded quantum chemical description of molecular structure. The first to attempt at such generalization were probably Armstrong and coworkers1 who defined the valency within the CNDO approximation in terms of the so-called Wiberg index2 • Although this .original definition was later extended to more sophisticated semi empirical and ab initio techniques in both orthogonal and nonorthogonal basis sets 3 - 10, the basic idea of all such generalizations remains virtually unchanged. The most significant consequence arising from this quantum chemical generalization concerns the fact that the valency no longer needs to attain, as in the classical theory, only integer values. Nevertheless, the calculated valencies, at least in the case of usual molecules, approach the classical integer values rather closely. The advantage of the quantum chemical generalization thus appears above all for various special structures such as the hypervalent clusters CLi6 , OLi4 , OLi6 , etc., where the classical description completely fails, whereas the new definition allows us to obtain a sufficiently iIlustrative picture of bonding. Similar complications can also occur for various nonclassical structures which may appear as intermediates or transition states in chemical reactions. In view of the successful application of the valency concept to the description of hypervalent structures, Gopinathan and JugU - 13 have recently proposed its use also to the characterization of the above nonclassical species. Stimulated by the interesting results of these studies we suggest in this paper a modification of the original approach by incorporating the valency concept into the framework of the recently formulated description of chemical reactivity in terms of the generalized overlap determinant method.