POLARIZED PI-FRONTIER MOLECULAR-ORBITALS - A METHOD FOR PREDICTING DIASTEREOFACIAL SELECTIVITIES

A method of breaking the symmetry of the pi-orbitals of nonsymmetrical molecules with respect to their nodal planes is described. The new (polarized) orbitals have different coefficients associated with each face of each pi-center. These coefficients can be used for predictive purposes in a manner analogous to simple frontier orbital theory. In order to break the symmetry of the atomic p-functions, each new p-orbital is constructed from a normal p-function and two additional Gaussian s-type functions, one superimposed over each lobe of the p-function. This new method (polarized pi-frontier orbitals, PPFMO) is illustrated using the placement of different functional groups (H, CH3, OH, CN) on a carbon adjacent to pi-systems containing from one to four carbon pi-centers (methyl, ethylene, allyl, butadiene). The effects of the various substituents adjacent to carbon pi-systems cannot be categorized completely in terms of electron-withdrawing and electron-donating effects. The effects are greatest on the smaller pi-systems and on the nearest carbon centers. The substituent effects on the polarization of the FMO's are shown to be quite different from those on their energies. FMO theory is adapted to include both effects. A comparison of PPFMO theory with earlier proposals made by Anh and Cieplak is discussed in terms of the predicted effects on FMO's.