Origin of Rotational Barriers. I. Many-Electron Molecular Orbital Wavefunctions for Ethane, Methyl Alcohol, and Hydrogen Peroxide

An investigation into the origin of rotation barriers has been carried out with the aid of LC(Hartree—Fock) AO MO SCF calculations on CH3CH3, CH3OH, and H2O2. The general shape and magnitudes of the barriers are reasonably well represented by the wavefunctions and one may conclude that the origin of rotational barriers is to be found within the framework of the Hartree—Fock approximation. It is also found that study of several molecules, particularly ones of low symmetry, is required for a detailed understanding of barriers. The electronic energy has been decomposed into one- and two-electron parts. The relative phase and relative amplitude of these two components provide the best invariants available for analyzing the barrier mechanism. On the other hand the nuclear—nuclear repulsion is shown not to be an appropriate invariant. A physical picture of the barrier mechanism constructed from the electronic-energy decomposition depicts the barrier origin as a balance between the interactions of lone pairs, bonds, and the protons.