Polymorphism and phase changes

An example of the application of NQR to polymorphism is provided by the 2 1 compounds of chloroform [22, 24, 25] with (3-methylpyridine)4MX2. Complexes in which M is either Ni, Co, Fe or Mn and where X = SCN or SeCN have been prepared. The structure of the 1 1 complex with (3-methylpyridine)4Ni(SCN)2 was already known 

The Werner complex (4-ethylpyridine)4Ni(SCN)2 forms 1 1 inclusion compounds with / am-dichlorobenzene when the method of preparation is direct recrystallisation of a mixture of the guest and host in 2-methoxyethanol. In an attempt to prepare crystals of high quality by vapour diffusion of pentane into a solution of a mixture of the host and guest in methanol, a compound showing a different NQR frequency was obtained [21]. This turned out to have a 4 1 host guest ratio and a triclinic crystal structure whereas the 1 1 complex is tetragonal. The results for several complexes in this series are summarised in Table 5. 

NQR of guest molecules in inclusion compounds rapidly provides a variety of structural information and, in addition, yields interesting information concerning the mobility of the guest molecule. Disordered structures do not usually yield NQR spectra and, while the absence of NQR transitions is evidence of either static or dynamic disorder, this is 

Das and E. L. Hahn, Nuclear Quadrupole Resonance Spectroscopy, Academic Press, New York (1958) 

Lucken, Molecular Spectroscopy, The Institute of Petroleum, London, pl57 (1971) 

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Crystal polymorphism and phase changes are valuable phenomena because they provide access to additional points on the supramolecular topology-energy surface [73]. It is unfortunate that interest in crystal and molecular structure by chemists bent on synthesis usually ceases after the first crystal structure is determined, and that other crystallizations are not investigated. Braga and Grepioni [79] comment on the limitations of a first come (crystallized) first served (put on a diffractometer) style of research. 

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