The Role of Lattice Matching Techniques in the Characterization of Polymorphic Forms
An inspection of the recent literature reveals that polymorphism is a frequently encountered phenomenon. The recognition of polymorphic forms plays a vital role in the materials sciences because such structures are characterized by different crystal packing and accordingly have different physical properties. In the pharmaceutical industry, recognition of polymorphic forms can be critical for, in certain cases, a polymorphic form of a drug may be an ineffective therapeutic agent due to its unfavorable physical properties. A check of the recent literature has revealed that in some cases new polymorphic forms are not recognized. In other instances, a supposedly new polymeric form is actually the result of an incorrect structure determination. Fortunately, lattice-matching techniques, which have proved invaluable in the identification and characterization of crystal structures, represent a powerful tool for analyzing polymorphic forms. These lattice-matching methods are based on either of two strategies: (a) the reduced cell strategy–the matching of reduced cells of the respective lattices or (b) the matrix strategy–the determination of a matrix or matrices relating the two lattices coupled with an analysis of the matrix elements. Herein, these techniques are applied to three typical cases–(a) the identification of a new polymorphic form, (b) the demonstration that a substance may not be a new polymorphic form due to missed symmetry, and (c) the evaluation of pseudo polymorphism because of a missed lattice. To identify new polymorphic forms and to prevent errors, it is recommended that these lattice matching techniques become an integral part of the editorial review process of crystallography journals.
Structure of triphenylphosphine oxide hemihydrate.
C18H15OP.1/2H2O, Mr = 287.30, orthorhombic, Fdd2, a = 19.794 (18), b = 32.540 (12), c = 9.459 (6) A, V = 6092.5 (22) A3, Z = 16, Dx = 1.253 g cm-3, lambda(Mo K alpha) = 0.71073 A, mu = 1.76 cm-1, F(000) = 2416, T = 294 K, R = 0.037 for 1677 reflections with I greater than 3 sigma(I). The water O atoms sit on a crystallographic twofold axis and are linked by hydrogen bonds to two of the oxide molecules with O...OW distances of 2.856 (3) A. Considerable conformational flexibility is conferred on the molecule by the long P--C bonds (around 1.8 A each).
A spectroscopic and crystallographic study of polymorphism in an aza-steroid.
The crystal structures of two enantiotropic polymorphs of the aza-steroid finasteride (N-(1-1-di-methylethyl)-3-oxo-4-aza-5 alpha-and rost-1-ene-17 beta-carboxam ide ) have been determined. The solid-state nuclear magnetic resonance spectra, infrared spectra, and physical property data of these two polymorphs are discussed in relation to both their solid-state structures and hydrogen-bonding networks.