Free Internal Rotation of Several Symmetric Tops

Free Internal Rotation of Several Symmetric Tops. Extension of statistical thermodynamic calculations from internal rotation of one top to several tops is straightforward. The rotational partition function may be expressed by 

The first approximation for the reduced moment of the mth top is given by equation (18). The next approximation is given by 

Thermodynamic contributions from the internal rotation of several symmetric tops may be readily calculated by appropriate summation of terms in Table 4. Few reliable calculations, however, have been reported. Thermodynamic properties of propane and several methyl-substituted benzenes have been reported, for example, but subsequent more accurate work has shown the necessity for considering that the internal rotation may be restricted. Although the subsequent calculations for m-xylene and p-xylene used 6-fold internal rotation barriers of 2.1 to 3.1 kJ mol , more recent statistical calculations for toluene employing the presence of free rotation suggest that internal rotation in the two xylenes may be effectively unrestricted. 

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