Calculation of Energy Levels for Internal Torsion and Over-All Rotation. I. CH3BF2 Type Molecules

Methods are described for calculating the energy levels for the over-all rotation and internal torsion of molecules consisting of a rigid symmetrical top attached to a rigid asymmetrical framework in such a way that the symmetry axis of the top coincides with a principal axis of the molecule. Probable examples are nitromethane and CH3BF2. Matrix perturbation methods are employed to obtain finite rotational secular equations valid in each of the cases: low barrier, high barrier, low asymmetry. These secular equations are modifications of the ordinary Wang equation for the rigid asymmetric rotor and can usually be solved by the continued fraction method. The symmetry groups applicable to this problem are also discussed.