Thermodynamic Properties of the Internal Rotation in Methyl Alcohol Vapor from 200 to 500°K

At ordinary and lower temperatures the thermodynamic properties of the methyl alcohol internal rotation are beyond the limits of available tabulations. For any internal rotator, of a given symmetry, with a fixed moment of inertia and potential barrier, two limiting values of each property can be calculated for any chosen temperature. Available tables are restricted to those cases for which the differences between the limiting values can be neglected. When, as with methyl alcohol, the differences are large, the representative values can be found by taking proper account of the symmetry and the dependence of the internal rotator energy levels upon the external rotations. For methyl alcohol, the correct intermediate thermodynamic properties are obtained by using for the partition function and its derivatives the numerical mean of the limiting values of these sums obtained for the general rotator. This simple rule, however, is not necessarily applicable to other compounds. Heat capacities and entropies, calculated from the spectroscopic potential barrier of 932 cal./mole and other molecular constants, are considered in relation to experimental data.