Prevision des enthalpies libres de redistribution sur des elements du ivème groupe par le biais d'un schema de decomposition des energies de liaison

Abstract

A method is described which permits the emprical prediction of redistribution free energies on, and between, central groups based on Group IV elements. It is based on a bond-energy decomposition scheme analogous to that of Zahn, using additive second-order parameters assigned to each pair of substituents of the central element. The most useful implications of this treatment are: (1) The possibility of writing linear relationships between redistribution free energies. Thus for example the 27 equilibria linking the 15 molecular species which result from the redistribution of 3 distinct substituents on a tetrafunctional centre can be expressed as a function of 3 redistribution free energies only, for which reliable experimental data are available. The Δ G dev have been calculated for 90 equilibria concerning the exchange of the most usual substituents. When compared with 42 available experimental data, these predictions were in 90% of the cases within experimental error. (2) The Δ G dev corresponding to the exchange of 2 substituents X and Y on a given element are independent of the number of exchangeable sites of the central atom. It therefore suffices to know one single experimental result per couple of substituents and per central element in order to predict the molecular distributions resulting from the exchange of these substituents on any centre that can be derived from this element by the presence of non-exchangeable substituents. A set of 134 experimental data have been examined and have confirmed this prediction. (3) The exchange of 2 substituents between distinct centres is also given by a same quantity, but taken a variable number of times, depending on the relative functionalities of the centres involved. Again, a single experimental vlaue allows the prediction of numerous equilibria. The Δ G dev have been calculated for 85 equilibria of this type, and were found to be in good agreement with the experimental values where known. The analysis of the body of calculated Δ G dev brought to light several general trends concerning the redistribution equilibria on the variously alkylated Group IV elements.