Polymorphic pair

In the case of DuP747 [24], XRD, DSC, and thermomicroscopic studies determined the polymorphic system to be monotropic. Distinct diffuse reflectance IR, Raman, and solid state 13C NMR spectra existed for each physical form. The complementary nature of IR and Raman gave evidence that the polymorphic pair were roughly equivalent in conformation. It was concluded that the polymorphic character of DuP 747 resulted from different modes of packing. Further crystallographic information is required in order to determine the crystal packing and molecular confirmation of this polymorphic system. 

Using these models we wanted to find answers to several questions. Which of the above models is the best parametrization if we predict not only the liquid but the ice and the vapour phase properties as well What is the performance of these models in general, or to put it differently, what are the inherent limitations of these simple models in terms of estimations of several properties simultaneously As for simulations of ice phases the question is whether such a simple classical model is able to grasp the major characteristics of these systems, i.e., the energy, density and structure variations in terms of the pressure and temperature The question of stability for the proton-ordered, proton-disordered polymorph pairs is a subtle problem. Can one obtain any useful information about these problems by using classical models ... 

Polymorphism can be classified into two types, enantiotropic and monotropic polymorphism. Each type involves two polymorphic forms or a polymorphic pair, whose interconversion is defined by a characteristic transition temperature. The temperature-dependent solid phase transformation is best understood by referring to the respective energy-temperature diagrams at constant pressure, as depicted in Figure 1 (3). 

Figure 2 Phase diagram of a polymorphic pair exhibiting an enantiotropic relationship. (From Ref. 4, Copyright 1 969 American Pharmaceutical Association.)...

Figure 6-33. For this polymorphic pair of sesquiterpenes the match is satisfactory. FEGWAP is Ref. [51] and FEGWAP02 is Ref. [52]...

It is possible to show that the amorphous form is less stable than any crystalline form over all temperatures by considering the following. FromEqs. (1.19) and (1.20), the relative stability of the amorphous or any polymorph compared to the more stable form, as given by 5/u., depends on the temperature. Polymorph pairs can be classified as either monotropic or enantiotropic. A monotropic pair is one in which one form is more stable at all temperatures up to the melting temperature of the less stable form. An enantiotropic pair is one in which there is a crossover or transition temperature Tx such that for temperatures lower than Tx one form is more stable while above Tx the other form is more stable. An implied requirement for enantiotropic pairs is that neither form melts below the crossover temperature, otherwise, the pair would be monotropic. (Once melted, there is no solid structure, so there can be no amorphous or polymorphic properties.)... 

Reproducibility, not accessible for all polymorph pairs - only relates to forms involved in transition Larger sample size, solution-mediated phase transformations... 

Figures 14.2 and 14.3 show the main landscape for polymorphic pairs in organic compounds. 30% of polymorphic crystal stmcture pairs have one centrosymmetric and one non-centrosymmetric partner, and 25% of the cases show one partner with more than one molecule in the asymmetric unit. No correlation appears between crystal density differences, and either centrosymmetricity or difference in the number of molecules in the asymmetric unit. Crystal density differences range from 0 to 10%, and lattice energy differences, as computed in a preliminary way with atom-atom UNI...

Fig. 14.2. Relative lattice energy differences, AEjE, as a function of relative density difference, AD/D. 475 polymorph pairs.

Fig. 143. (a) The packing coefficient difference correlates (solid line) with density difference. The variation in self-occupation factor (circles) does not. (b) The difference in number of molecules in the coordination sphere plotted against density differences complete scatter. 475 polymorph pairs. 

Fig. 14.5. Simulated powder patterns for polymorphic pairs identical (a), different (b). Standard parameters (see Section 5.8, eqnation 5.37 copper wavelength, constant thermal factor of4A2).

Fig. 14.6. (a) A scatterplot of differences in intramolecular energy (MP2/6-3IG calculation on the unoptimized molecular structure extracted from the crystal) versus intermolecular energy differences (calculated by Pixel) for 116 polymorphs pairs. Differences cancel for points below the zero line and add for point above the zero line, (b) Total energy differences (intramolecular + intermolecular) as a function of intermolecular energy differences. Units kJ mol . ... 

Fig. 14.7. Same sample as in Fig. 14.6 relative importance of coulombic and repulsion energy in total intermolecular energy differences (Pixel calculations) between polymorph pairs (kJmol ). Small total energy differences result from cancellation of large partial energy differences.

The first and, we believe, still a unique case of two polymorphic pairs of solid tautomer, that is, polymorphic desmotropes, is related to the nonmethoxy derivative of X, IX (Scheme 13.7) [45]. Crystallization from different alcohols resulted In the isolation of four solid forms of IX, two yellow polymorphs of enol-imino tautomer, and two red polymorphs of keto-amino tautomer. The DFT calculations showed enol tautomers to be far more stable in the gas phase and in the polarizable environment. It was also shown by solution spectroscopically that in protic and polar solvents the fraction of the keto form of IX increases. 

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