Polymorphism predicted existence

With the growing awareness among chemists of the phenomenon of polymorphism its actual occurrence in any particular system may not be as great a surprise as a generation or two ago. The predicted existence of any particular polymorphic structure for a single compound, the conditions and methods required to obtain it, and the properties it will exhibit are still problems that will challenge researchers for many years to come. ... 

Many molecules are obtained and used in a crystalline form, the nature of which can have e significant impact on their properties and behaviour. Moreover, it is sometimes possible foi a given material to exist in more than one crystalline form, depending upon the conditions under which it was prepared. This is the phenomenon of polymorphism. This can be important because the various polymorphs may themselves have different properties. It is Iberefore of interest to be able to predict the three-dimensional atomic structure(s) that a gi en molecule may adopt, for those cases where it is difficult to obtain experimental data and also where one might wish to prioritise molecules not yet synthesised. 

The isolation of crystalline products having mixed polymorphic compositions (often referred to as concomitant polymorphism) remains a topic of interest, even though the phase rule predicts that a system at equilibrium consisting two components (solvent + solute) and three phases (solution + Form I + Form II) is uni variant. Hence, for crystallizations performed at a fixed pressure (typically atmospheric) the system becomes nonvariant and genuine equilibrium can exist at only one temperature. Therefore, concomitant products must be obtained under nonequilibrium conditions. Flexibility in molecular conformation was attributed to the concomitant polymorphs of a spirobicyclic dione [34] and of 3-acetylcoumarin [35],... 

Octanitrocubane (ONC) is a white solid, somewhat soluble in hexane and readily soluble in polar organic solvents. The density of one of the ONC polymorphs is very high (1.979gcm"3) but is still lower than the calculated value (the latest and most sophisticated calculation predicts a density above 2.1 gem"3 for the most stable polymorph of ONC) which indicates the existence of a crystal form of ONC much more dense than that synthesized. Kamlet-Jacobs equations predicted that ONC is 15-30% better than HMX [109] (a most powerful currently employed military explosive) and 6% better (perhaps also less shock sensitive) than the recently discovered explosive HNIW [121, 253-258] or CL-20 as shown in Table 2.15. It is interesting to note that both HpNC and ONC have decomposition points well above 200 °C and are not detonated by hammer blows. 

Evaluation of the fibre patterns showed that the glucan chains have a stretched form with twofold screw symmetry and a fibre repeat period of 0.835 nm, as predicted by model building and conformational analysis. The differences between the polymers exist therefore in their chain packing. A monoclinic unit cell with a=0.581 nm b=1.00 nm Y=96° was derived for polymorph I and orthorhombic cells for II and III respectively with the base plane axes a=0.502 nm b=0.963 nm f°r II and a=0.457 nm b=0.865 nm for III. These cells contain 4 glucose residues, and on account of spatial considerations the ribbon-like chains in projection on the base plane were supposed to be oriented with the longer axis parallel to the b-axis (i. e. with the pyranose rings near parallel to the bc-plane) in both polymorph II and III, whereas in polymorph I a diagonal position appeared more favourable. 

We were able to develop some simple rules in predicting the packing of molecules based on CA M in one dimensional tapes, but not for predicting the three-dimensional packing of these tapes [98,107,145-148]. Conformational polymorphism due to the diphenyl rings, in addition, increased the complexity of the problem in the CAM system. We have found that crystal structures based on diketopiperazines (DKP) are simpler to predict and rationalize than our previous systems, both from observations of existing structures, and from computational methods [30,109]. 

Polymorphism is important as different polymorphic forms of the same compound may have significantly different chemical/physical properties. In terms of polymorphism, computer simulations may be used to either predict the structure of an unknown polymorph or to simulate the potential of a system to exist in other polymorphic forms. In either case the polymorphic simulation is a three-step process. 

The data collected unambiguously indicate the existence of a large variety of actinide orthophosphates and their analogues (orthovanadates, orthoarsenates, orthosilicates, etc.). They are characterized by iso- and heterovalent isomorphic substitutions of cations and anions, and polymorphic and morphotropic transitions. The data analysis allows to predict many new actinide compounds of the group with different expected stmctures and tailored properties, including stability in different extreme physical fields and chemical media. Certain progress in this research direction has already been achieved. 

Maynard Smith I m curious about the prediction from evolutionary game theory, that a genetic polymorphism can be maintained, with different individuals adopting different strategies. Can you offer any plausible examples of places where a genetic polymorphism or a strategic one exists in the human population maintained by that kind of frequency-dependent selection ... 

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