SIMULATION OF MC-SCF RESULTS ON COVALENT ORGANIC MULTIBOND REACTIONS - MOLECULAR MECHANICS WITH VALENCE BOND (MM-VB)

A scheme, MM-VB, for modelling (i.e., simulating MC-SCF results) covalent multi-bond reactivity problems (pericyclic and radical reactions) using a combination of molecular mechanics (MM) and valence bond (VB) theory is presented. The efficiency of the model in locating equilibrium geometries, transition structures, reaction intermediates, excited-state structures, and structures associated with conical intersections of ground and excited states is documented by comparison with ab initio MC-SCF results for structures belonging to four different potential energy surfaces for hydrocarbon reactions. The results have been obtained using the same pool of parameters (71 parameters) for the valence bond parametrization in all computations. The method performs very well for the semiquantitative prediction of structural parameters, and it is always possible to deduce the correct shape (topology) of the global potential energy surface. The application of the method to larger molecules is illustrated with an example where the conformational change in the ring system of ergosterol during electrocyclic ring opening is discussed. From a conceptual point of view, it has been demonstrated that quantum mechanics of the complicated process of the reorganization of the spin recoupling that occurs in a transition state or the driving force for the change in CC bond lengths that occurs in delocalized pi-systems can both be described by the solution of a simple VB problem which allows for the resonance of the possible VB structures involving the active bonds alone. The inactive framework of the molecule can be described by MM. The central parameters in such a VB calculation are the exchange integrals (K(ab)) and Coulomb integrals (Q(ab)). The functional form of these Coulomb and exchange parameters ran be transferred from two active-center model molecules (in the present case ethane and ethylene) to a multi-active-center system via a delocalization algorithm.