An Electrostatic Theory of the Potential Barriers Hindering Rotation around Single Bonds

Abstract

A number of simplifying hypotheses concerning the distribution of molecular electrons are discussed. On the basis of these hypotheses, the contribution of electrostatic forces to the interaction energy between two non-adjacent covalent bonds in a molecule has been estimated by expansion of the electrostatic potential in inverse powers of the distance. The expansion is carried as far as the quadrupole term. A discussion is given of the magnitudes to be expected for quadrupole moments of covalent bonding distributions. It is found that the electrostatic interactions, when extended to the quadrupole approximation, are sufficient to account for potential barriers hindering internal rotation in ethane, methylamine, methyl alcohol and dimethylacetylene. Using reasonable assumptions concerning the distribution of unshared electron pairs, the unsymmetrical configuration of hydrogen peroxide is found to be stable, in agreement with experiment. Extension of the treatment to the more complex molecules propane, isobutane, and neopentane indicates that the interaction between non-adjacent groups is not negligible. A relation between bond quadrupole moments and interatomic distance is given, and possible refinements to the treatment are discussed.