The phase diagram of cyclopentane has been studied by powder neutron diffraction, providing diffraction patterns for phases I, II, and III, over a range of temperatures and pressures. The putative phase IV was not observed. The structure of the ordered phase III has been solved by single-crystal diffraction. Computational modeling reveals that there are many equienergetic ordered structures for cyclopentane within a small energy range. Molecular dynamics simulations reproduce the structures and diffraction patterns for phases I and III and also show an intermediate disordered phase, which is used to interpret phase II.
A predicted orthorhombic crystal structure of carbamazepine is isostructural with an experimentally determined dihydrocarbamazepine crystal structure, providing a rationale for synthesising a novel 1 : 1 solid solution that has carbamazepine in a rare catemeric configuration
A combined computational and experimental polymorph search was undertaken to establish the crystal forms of 7-fluoroisatin, a simple molecule with no reported crystal structures, to evaluate the value of crystal structure prediction studies as an aid to solid form discovery. Three polymorphs were found in a manual crystallisation screen, as well as two solvates. Form I (P2 1 /c, Z 0 ¼ 1), found from the majority of solvent evaporation experiments, corresponded to the most stable form in the computational search of Z 0 ¼ 1 structures. Form III (P2 1 /a, Z 0 ¼ 2) is probably a metastable form, which was only found concomitantly with form I, and has the same dimeric R 2 2 (8) hydrogen bonding motif as form I and the majority of the computed low energy structures. However, the most thermodynamically stable polymorph, form II (P1, Z 0 ¼ 2), has an expanded four molecule R 4 4 (18) hydrogen bonding motif, which could not have been found within the routine computational study. The computed relative energies of the three forms are not in accord with experimental results. Thus, the experimental finding of three crystalline polymorphs of 7-fluoroisatin illustrates the many challenges for computational screening to be a tool for the experimental crystal engineer, in contrast to the results for an analogous investigation of 5-fluoroisatin.
Key indicatorsSingle-crystal X-ray study T = 150 K Mean (C-C) = 0.003 Å R factor = 0.041 wR factor = 0.101 Data-to-parameter ratio = 12.6For details of how these key indicators were automatically derived from the article, see
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