Isomerism and polymorphism

Moulton, B., Zaworotko, M. J., From molecules to crystal engineering Supramolecular isomerism and polymorphism in network solids. Chem. Rev. 2001,101, 1629-1658. 

B. Moulton, M.J. Zaworotko, From Molecules to Crystal Engineering Supramolecular Isomerism and Polymorphism in Network Solids , Chem. Rev., 101, 1629 (2001)... 

Biradha, K. and Zaworotko, M. (1998). Supramolecular isomerism and polymorphism in dianion salts of pyromellitic acid. Cryst. Eng., 1, 67-78. [81]... 

Silver fulminate was recognized by Gay-Lussac16 to have the same composition as silver cyanate, thus providing the first example of isomerism. Two polymorphs of silver fulminate are known and their structures have been determined.17 Silver fulminate is regarded as a poor detonator due to its dangerous sensitivity this may depend on the presence or absence of the second form.17... 

It has been argued above that molecular crystal polymorphism can be seen as a form of crystal isomerism, and that the change in crystal structure associated with polymorph interconversion, i.e. a solid-to-solid phase transition between ordered... 

The work of Sanders et al. (1984), Jackson and Catlow (1988) and Tomlinson et al. (1990) showed that with Born model potentials it is possible accurately to reproduce the crystal structures of silicates including zeolites. A typical example is shown in Plate I which illustrates the calculated and experimental structures of a purely siliceous (i.e. pure Si02 polymorph) zeolite, silicalite. (A closely related, isostructural material, ZSM-5, which contains a low concentration of Al is an effective isomerization and hydrocarbon synthesis catalyst.) The agreement between theory and experiment is evidently good more discussion follows in Chapter 9. 

Organometaiiic Crystal Polymorphism, Crystal Isomerization, and Phase Transitions 579... 

Fosinopril sodium, an angiotensin-converting enzyme inhibitor, has been found to exist in two anhydrous polymorphic forms. No single crystal of sufficiently high crystallographic quality was obtained for the polymorphic phases and polymorphism was characterized by solid state and P NMR. The analysis of chemical shifts in C spectra suggested that polymorphism originates from conformational differences in the acetal side chains and/or cis-trans isomerization about the peptide C-N bond. The difference in chemical shift between P resonances in the two spectra is 2.2 ppm and demonstrates the existence of various envirorunents of phosphorus nuclei in each polymorph [43]. 

All of these subdivisions of supramolecular isomerism can be related to previously used terminology thus. Class I can be considered as polymorphism Class IT as structural isomerism, and Class 111 as pseudo-polymorphism. 

Furthermore, it is often possible to extract from the structural analysis of solid solvates a significant information on solvation patterns and their relation to induced structural polymorphism. An interesting illustration has been provided by crystal structure determinations of solvated 2,4-dichloro-5-carboxy-benzsulfonimide (5)35). This compound contains a large number of polar functions and potential donors and acceptors of hydrogen bonds and appears to have only a few conformational degrees of freedom associated with soft modes of torsional isomerism. It co-crystallizes with a variety of solvents in different structural forms. The observed modes of crystallization and molecular conformation of the host compound were found to be primarily dependent on the nature of the solvent environment. Thus, from protic media such as water and wet acetic acid layered structures were formed which resemble intercalation type compounds. 

Analogous to the DuP 747 study, complete crystallographic information was not possible on the fosinopril sodium polymorphic system [25], Two known polymorphs (A and B) were studied via a multidisciplinary approach (XRD, IR, NMR, and thermal analysis). Complementary spectral data from IR and solid state 13C NMR revealed that the environment of the acetal sidechain of fosinopril sodium differed in the two forms. In addition, possible cis-trans isomerization about the CgN peptide bond may exist. These conformational differences are postulated as the origin of the observed polymorphism in fosinopril sodium in the absence of the crystallographic data for form B (single crystals not available). 

The most recent converts are in the health care industry. Pharmaceutical and biological applications have become myriad since the early 1980s. The first widespread application was for the identification/ qualification of incoming raw materials. Since then, applications have appeared for moisture (bound and free), blend uniformity of powders, tablet and capsule assays, counterfeiting, polymorphism, degree of crystallinity, hardness (of tablets), dissolution prediction, isomerism, as... 

In 2007, in a very exhaustive paper, Paradies and coworkers carried out a comprehensive structural characterization of the colorless and yellow forms of Af-hydroxyphthalimide (NHPI) by means of single-crystal X-ray diffraction, FTIR and Raman spectroscopies and scanning electron microscopy. In the yellow form, the Af-hydroxyl group is significantly out of the plane (1.19°), but the Af-hydroxyl group in the colorless form is only 0.06° out of the plane. The irreversible conversion of the colorless crystalhne form to the yellow crystalhne form is more like a dynamic isomerism than a polymorphic transformation. 

The [C4Qlm]+ cations in the two polymorphs were found predominantly to differ with respect to conformation The structural results showed that the polymorphism is due to a rotational isomerism of the butyl group of fhe [C4Qlm]+ cation around C7-C8, as defined in Figure 12.1. In the monoclinic polymorph, the butyl chain is in anti (or trans) conformation around C7-C8, and in the orthorhombic polymorph it is gauche around C7-C8. The conformational difference reveals itself in fhe rofafion of the butyl chain... 

POLYMORPHISM. 1, A phenomenon in which a substance exhibits different forms. Dimorphic substances appear in two solid forms, whereas trimorphic exist in three, as sulfur., carbon, tin, silver iodide, and calcium carbonate. Polymorphism is usually restricted to the solid state, Polymorphs yield identical solutions and vapors (if vaporizable). The relation between them has been termed physical isomerism. See Allotropes under Chemical Elements, See also Mineralogy,... 

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