ON THE ORIGIN OF POTENTIAL BARRIERS TO INTERNAL ROTATION IN MOLECULES

Abstract

It is well known that in molecules such as ethane (CHa-CCH3) one part of the molecule can rotate relative to the other part about the single bond (here CC) joining the two halves. This phenomenon has considerable importance; for instance , it permits the coiling and uncoiling of protein molecules and other polymers. It has also been known1 for some time that this internal rotation is not free but is hindered by a potential energy barrier of a magnitude beyond theoretical expectations. Various hypotheses have been put forward concerning the origin of these forces, but no satisfactory conclusion has been reached. Recent developments in microwave spectroscopy and the related theory2'-10 have provided powerful new tools for the study of this phenomenon. For a certain class of molecules it is now possible to obtain values of the potential barrier with an accuracy of 5 per cent or better, and with much greater certainty than was associated with older methods. Further, the structure and interatomic distances can be obtained and the equilibrium orientation of the rotating groups, as well as other detailed information mentioned below. Current theories of the forces between atoms suggest a number of types of interaction which could possibly account for the observed barriers. All these forces are of course fundamentally electrostatic interactions among the electrons and nuclei involved. Eyring""12 and collaborators early made a quite detailed attempt to find the origin of the barriers by means of the quantum-mechanical approximations then available, but without success. Recently Mason and Kreevoy'3 and also van Dranen14 have made new estimates of the importance of the van der Waals repulsion between the attached groups, a repulsion which appears between separate gas molecules at close distances, due to overlap of the charge clouds and quantum-mechanical exchange. At somewhat longer distances than occur in most examples of hindered rotation, this repulsion should be replaced by a weak attraction due to inductive and dispersion effects. The separate atoms and chemical bonds in a molecule will surely interact, due to the direct electrostatic force between the charge distributions, even if there is no important contribution from overlap, exchange, dispersion, or induction. If the electron distribution were known, this term could be calculated with purely classical 816 PRoc. N. A S.