ORIGIN OF ROTATION AND INVERSION BARRIERS

This paper discusses the origin of barriers to rotation and inversion at the SCF and Cl levels by use of the theory of atoms in molecules. The barriers are related to the changes in the attractive and repulsive potential energies through the use of the virial theorem. Rotation barriers in C2H6, CH3OH, and CH3NH2 are shown to result from an increase in the attractive potential energy and in spite of a decrease in the repulsive energy. Just the opposite behavior is found for the inversion barriers in NH3, PH3, and H3O+ and for the barrier to bending in H2O. The relative changes in energy found for the rotation barriers are anticipated for conformational changes dominated by an increase in internuclear separations, while those for the inversion barriers are typical of changes accompanied by decreases in these separations. The origins of the barriers are given in terms of the mechanical properties of the atoms. In ethane, the transformation of a S3 to a C3 symmetry axis in attaining the eclipsed geometry induces a quadrupole polarization of the density in the C-C bond, causing it to lengthen. This lengthening leads to a decrease in the magnitude of the attractive interaction of each carbon nucleus for the electronic charge in the basin of the other carbon atom, and this is the origin of the barrier. The changes in the charge distribution that determine these changes in atomic properties are not consistent with a model that relates the barrier in this molecule to Pauli-like exchange repulsions between localized C-H bond orbitals. © 1990, American Chemical Society. All rights reserved.