Restricted Internal Rotation of a Single Symmetric Top

Restricted Internal Rotation of a Single Symmetric Top. The potential restricting internal rotation may be expressed as a Fourier cosine and sine sum series, similar to equation (19). In practice, however, the restricting 

The tables of thermodynamic functions have been extended by Li and Pitzer to include internal rotation of symmetric tops having very low moments of inertia. These extended tables were calculated for two coaxial symmetric tops, though they are expected to apply to molecules containing symmetric tops attached to a rigid frame of any symmetry. 

The validity of the simple cosine function in equation (21) as a description of the restricting potential of a symmetric top has been questioned by Blade and Kimball. They examined the applicability of a two-parabola potential for the internal rotation in ethane, 1,1,1-trifluoroethane, and methanol, and concluded that the simple cosine function is not satisfactory. This conclusion for ethane has been criticized in detail by Pitzer, who has shown that the two-parabola potential does not provide a significantly better fit to the calorimetric data than does the simple cosine potential. Experimental heat capacities for ethane over the range 90 to 305 K are 

Application of the tables - to calculate thermodynamic contributions requires knowledge of molecular geometry to determine the appropriate reduced moment of inertia. In cases where the potential barrier height is known from spectroscopic measurements, the tables may be used directly to calculate thermodynamic properties. Examples of molecules for which this procedure has been used include methanol, methanethiol, fluoroethane, 1,1-difluoroethane, 1,1,1-trifluoroethane, pentafluoro-chloroethane, chloroethane, 1,1,1,2-tetrachloroethane, pentachloro- 

Overall rotational contribution Restricted internal rotational 97.61 113.25 12.47 12.47 

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