Rotation of an Asymmetric Top restricted by a Complex Potential Barrier

Internal Rotation of an Asymmetric Top Restricted by a Complex Potential Barrier. Simple general procedures for calculating thermodynamic contributions from internal rotation of asymmetric tops restricted by a potential more complex than a simple cosine function have not been developed. Nevertheless, several calculations using appropriate approximations have been reported, and are discussed below. 

For light rotors it may be possible to observe sufficient torsional transitions for the internal rotation partition function to be determined by direction summation (p. 271). If only a few low-lying torsional levels are observed, they may be fitted to a suitable potential from which higher torsional energy levels can be computed. Thermodynamic functions of hydrogen peroxide calculated by use of this procedure give a value of 5 (298.15 K) in excellent agreement with the calorimetric value.  

For heavy asymmetric rotors the torsional levels may be sufficiently low for thermodynamic properties to be estimated by using the classical approximation. Gwinn and Pitzer have expressed the classical approximation to the complete internal partition function as 

Equation (28) has been used to calculate thermodynamic functions for 1,2-dichloroethane, 1,1,2-trichloroethane, and but-l-ene. For each molecule a potential function was selected to give satisfactory agreement between calculated and calorimetric values of entropy and heat capacity. 

A breakdown of part of the calculation for 1,2-dichloroethane is given in Table 9. The potential selected to give best fit to calorimetric and dipole moment data is 

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