HINDERED ROTATION AND MICROWAVE SPECTROSCOPY

Introduction. One of the important objectives of theoretical chemistry is the prediction of the course of chemical reactions in the absence of experimental data. For very simple molecules, the calculation, by the methods of statistical mechanics, heat capacities, entropies, free energies and equilibrium constants has been quite successful. It is necessary only to know the size and shape of the molecule and its fundamental frequencies of vibration. In slightly more complex molecules, however, one of the fundamental modes of vibration or molecular motion either does not interact with radiation or does so in such a fashion that it is not easily studied by the conventional techniques of infra-red and Raman spectroscopy. This mode of vibration is called torsional vibration, or hindered rotation, and arises when the molecule contains two or more groups which can rotate against each other about a common molecular axis. The calculation of thermodynamics functions in the presence of hindered rotation has been reviewed by Wilson' and Pitzer.2 Experimentally, barrier heights for internal rotation have been determined from the comparison of calculated and measured thermodynamic functions such as the entropy or heat capacity. I n most cases, the values so determined are of somewhat doubtful reliability, because of numerous complicating factors. At best, such data gives information only on the overall density of the lower torsional oscillation levels without giving any specific information about the shape of the potential barrier. The nature of the forces of interaction between internal rotating groups continues to be a b a i n g problem to theoretical chemists. Most recently, Lassetre and Deans have attempted to account for hindered rotation effects in terms of the interaction of bond quadrupole moments. This treatment makes several rather arbitrary assumptions and depends on a correlation with experimental data to verify them. All of these considerations make it extremely worthwhile to use the techniques of microwave spectroscopy in every way possible to cast further light on this puzzling phenomenon. Microwave spectroscopists, who may be quite uninterested in hindered rotation for its own sake, have a special reason for being concerned with the problem. Even such relatively simple molecules as methyl alcohol and acetaldehyde have internal groups of atoms which may rotate independently about a common axis. If the microwave spectrum of such a molecule is being investigated for any reason, it is probable that hindered rotation effects must be accounted for before the desired information can be obtained. The special features of the microwave spectrum of a molecule containing internally rotating groups will depend on two main factors; namely, the height of the potential barrier to the internal rotation and the presence or absence of a component of the dipole moment perpendicular to the symmetry axis within the rotating groups.