Polycyclic molecules steroid ring system

One impediment to straightforward comparison is the lack of solid-phase enthalpies of formation for most low molecular weight molecules. Examination of the few examples for which there are both solid- and liquid-phase values shows, however, that the liquid- and solid-phase enthalpies of reaction do not vary too much, at least within the uncertainties of the steroid combustion measurements. We are additionally hampered by not having available enthalpies of formation for those small molecules which most resemble the steroid substructure. The prototype small molecules required for the analysis are primarily those of substituted five- or six-membered carbocyclic rings. Although the functional groups on the available prototypes and the steroids are identical, stereochemical aspects (such as axial vs. equatorial substituent positions) and their consequent interactions are different due to the differences in conformational flexibility of the monocyclic and polycyclic systems. The prototype molecules and their liquid-phase enthalpies of formation are listed in Table 3. 

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