Restricted Internal Rotation of Several Symmetric Tops

Restricted Internal Rotation of Several Symmetric Tops. The tables of thermodynamic functions for an internal rotation of a single symmetric top may be used for several symmetric tops [with moments of inertia calculated from equation (20)] provided both potential energy and kinetic energy cross-terms between the tops can be neglected. Both assumptions have been generally made in calculations for molecules with several tops. Where there are reliable calorimetric data at one or more temperatures, the tables have been used to calculate appropriate potential barriers. Using this procedure thermodynamic contributions have been calculated for propane, 2-methylpropane, 2,2-dimethylpropane, cis-but-2-ene, rm a -but-2-ene, isobutene, o-xylene, m-xylene, p-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, dimethyl sulphide,2-chloro-2-methylpropane, and dimethyl-amine. In several cases thermodynamic contributions have been calculated using potential barriers estimated from those of related molecules. Examples of this procedure are found in calculations for 2-fluoro-2-methylpropane, 2-chloropropane, 2-bromopropane, 2-iodopro-pane, 2,2-dichloropropane, 2-bromo-2-methylpropane, 2-iodo-2-methylpropane, 1,3,5-trimethylbenzene, 1,2,3,5-tetramethylbenzene, 1,2,4,5-tetramethylbenzene, 1,2,3,4-tetramethylbenzene, pentamethyl-benzene, and hexamethylbenzene.  

Only a few cases have been reported, where spectroscopically determined barriers have been used in conjunction with the tables to calculate thermodynamic properties. These include trimethylamine, dimethyl ether, i and acetone.  

By developing the treatment described by Eidinoff and Aston for the classical partition function for internal rotation as 

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