The Nature of Bond Orbitals and the Origin of Potential Barriers to Internal Rotation in Molecules

Twenty years ago Kemp and Pitzer(1) discovered that the relative rotation of the two methyl groups about the carbon-carbon bond in ethane is not completely free but is restricted by a potential barrier about 3.0 kcal/mole high, with three maxima and three minima in a complete rotation, corresponding to the trigonal symmetry of the methyl groups. It was soon found that rotation about single bonds is restricted in many molecules, and many experimental values for the height of the potential barrier have been obtained by the analysis of thermodynamic quantities (entropy, heat capacity) and especially by the methods of microwave spectroscopy.