Some exemplary studies of conformational polymorphism

One of the common features of the growing number of studies on conformational polymorphs is the utilization of a number of analytical and computational techniques to characterize the systems (e.g. Bauer et al. 2001). The emphasis differs from study to study, as demonstrated below, but these multidisciplinary approaches are to be encouraged and hopefully expanded. Another common feature of these investigations is the nature of the molecules investigated—relatively small molecules, with a limited number of conformational parameters. This allows a more direct comparison of conformational differences and the energies associated with those differences. As our understanding of the phenomenon increases and the computational capabilities to deal with larger systems improve, we can expect that more and more complex systems will be studied. 

The energetic relationships among the polymorphs were established by combining a number of techniques, providing a prototypical example of how this can be done. The pure melting endotherms of four forms (R, Y, OP and ON) could be determined by DSC (at 10 °C min Fig. 5.10). Each one exhibits homogeneous melting, without 

The data obtained from the DSC measurements could be used to determine free energy and entropy differences from expressions developed earlier by Yu (1995)  

Similar data obtained for other pairs of polymorphic modifications are presented in Fig. 5.13. The thermodynamic data and enantiotropic or monotropic relationship between modifications are given in Table 5.2, summarizing the relative stability of the phases from room temperature to the melting points (save the difficult to obtain ORP form). 

The authors have made some interesting comments regarding these data in particular and the relative importance of energy and entropy contributions to the relative 

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