A perspective on a century of inert seeds in crystallisation

There are many accounts of inert seeds that grow only slowly and fail to proliferate. In the normal course of unseeded crystallisation this results in a latent period between the first cloud point or crystal precipitation and the main crystallisation event. However, similar behaviour has been also observed in seeded crystallisations. Even a smooth precipitation curve may hide periods of nucleation and periods of proliferation within it. Latent periods have often been observed during crystallisation, but infrequently recognised. There are also many reports of critical seed sizes below which they do not bring about secondary nucleation. Sizes between 1 and 900micrometres have been recorded for different substances under different experimental conditions. It seems likely that the reported failure of small seeds to grow is related to their surface features rather than to their size

A practical guide to the measurement of turbidity curves of cooling crystallisations from solution

Several methods for the determination of the temperatures of crystallisation, crystal proliferation and crystal dissolution in cooling crystallisations are presented and compared. These include curve fitting, statistical and visual determination. It is shown that the quick analyses are as reproducible as the longer, mathematically based methods. The point of crystal proliferation is suggested to be the more useful, more meaningful, more reproducible and more quickly determined piece of information than the universally recommended point of first crystal detection

XPac dissimilarity parameters as quantitative descriptors of isostructurality: the case of fourteen 4,5?-substituted benzenesulfonamido-2-pyridines obtained by substituent interchange involving CF3/I/Br/Cl/F/Me/H

Crystal structures of fourteen 4,5'-substituted benzenesulfonamido-2-pyridines, with tautomeric forms R-1-C6H4-SO2-N=C5NH4-R-2 or R-1-C6H4-SO2-NH-C5NH3-R-2, and with R-1 = CF3, I, Br, Cl, Me, F, H and R-2 = CF3 or I are reported. Comparisons carried out with the program XPac show that all investigated structures display a common 3D arrangement of N-H center dot center dot center dot N bonded centrosymmetric dimers. This isostructural series is exceptional in its completeness and in the diversity of the substituents involved. Accordingly, the XPac dissimilarity index x, a measure of how far two structures deviate from perfect geometrical similarity, varies from 0.9 for the interchange R-1 = Br -> Cl to 9.5 for the simultaneous interchange of R-1 = I -> H and R-2 = CF3 -> I. XPac plots of individual dissimilarity parameters have been used to elucidate details of geometrical similarities and differences between structures. This indicated that the geometry is preserved most closely within bc-layers that are composed of 1D slipped stacks of N-H center dot center dot center dot N bonded dimers, with interlayer van der Waals interactions being dominated by R-1 center dot center dot center dot R-2 contacts. The main mode of geometrical adjustment for the size of R-1 is a parallel shift of neighbouring layers against one another in the direction perpendicular to the layer plane, whilst the relative orientations of molecules within each layer are altered to a much smaller extent, even for those structure pairs representing R-1 substituents that are very different in size. This study shows that the ability of two compounds to crystallise in fundamentally the same crystal structure depends not only on how much their molecules differ in shape but also, critically, on the specific tolerance characteristics of the crystal packing arrangement concerned. As a comparison with the present study, dissimilarity indices are reported for a set of 24 isostructural 4,4'-substituted benzenesulfonamidobenzenes previously subjected to XPac analysis

Eight isostructural 4,4'-disubstituted N-phenylbenzenesulfonamides

The isostructural crystals of 4-cyano-N-(4-methoxyphenyl)benzenesulfonamide, C14H12N2O3S, (I), N-(4-methoxyphenyl)-4-(trifluoromethyl) benzenesulfonamide, C14H12F3NO3S, (II), 4-iodo-N-(4-methoxyphenyl) benzenesulfonamide, C13H12INO3S, (III), 4-bromo-N-(4-methoxyphenyl) benzenesulfonamide, C13H12BrNO3S, (IV), 4-chloro-N-(4-methoxyphenyl) benzenesulfonamide, C13H12ClNO3S, (V), 4-fluoro-N-(4-methoxyphenyl) benzenesulfonamide, C13H12FNO3S, (VI), N-(4-chlorophenyl)-4-methoxybenzenesulfonamide, C13H12ClNO3S, (VII), and 4-cyano-N-phenylbenzenesulfonamide, C13H10N2O2S, (VIII), contain infinite chains composed of N-H ··· O(sulfonyl) hydrogen-bonded molecules. The crystal structures of (I)-(VIII) have been compared using the XPac software and quantitative descriptors of isostructurality were generated [Gelbrich, Threlfall & Hursthouse (2012). Cryst-EngComm, 14, 5454-5464]. Certain isostructural relationships in this series involve molecules with substantially different spatial demands, e. g. (VI) and (VIII) are related by the simultaneous interchange of F -> CN on the benzenesulfonamide ring and OMe -> H on the N-phenyl ring, which indicates that the geometry of the three-dimensional crystal-packing mode of (I)-(VIII) is unusually adaptable to different molecular shapes

The same but different: isostructural polymorphs and the case of 3-chloromandelic acid

The expression "isostructural polymorphs" would appear to be an impossible combination of two mutually incompatible words. "Isostructural" implies a high degree of structural similarity; however, conversely, "polymorph" implies structural distinguishability. The structures of two newly determined polymorphs of 3-chloromandelic acid nevertheless justify the use of this expression, for they differ only in crystal symmetry and hardly at all in molecular position or conformation within the crystalline lattice. We demonstrate that parameters derived by the XPac program can be useful in establishing the limits of isostructurality

A systematic study of the crystallisation products of a series of dicarboxylic acids with imidazole derivatives

Nineteen new crystal structures are reported containing alpha,omega-alkanedicarboxylic acids (HOOC-(CH(2))(n-2)-COOH, n = 0-6), maleic acid and fumaric acid with imidazole, 1-methylimidazole, 2-methylimidazole, 4-methylimidazole and 1,2-dimethylimidazole. These were characterised by single crystal X-ray diffraction at 120 K and their crystal structures are discussed together with five published structures. In all of the characterised acid-base combinations at least one of the acidic hydrogens has been transferred to the base and most have a 1 : 1 stoichiometry. Nearly two thirds of the crystal structures adopt a basic sheet topology containing hydrogen bonded anion chains linked by cations. The sheet topologies have been classified based upon the principles of a scheme proposed by MacDonald et al. (J. C. MacDonald, P. C. Dorrestein and M. M. Pilley, Cryst. Growth Des., 2001, 1, 29-38)