TOT clathrates

The most common form of TOT clathrate crystallises as discrete C2 symmetric cavities in the chiral space group 1, implying that the (—)-(M) and (+)-(/") forms separate spontaneously as crystallisation occurs. This property of TOT has been used in an ambitious model experiment designed to test the theory that the parity-violating energy difference (the violation of parity or symmetry in elementary particles), with autocatalytic amplification (in the case of TOT during crystallisation) is responsible for the observed chirality of modern biomolecules. The experiment did not find any evidence to support the theory, with equal amounts of each enantiomeric crystal being isolated.26... 

TOT forms exclusively crystalline multimolecular inclusion compounds. The present section is mainly concerned with the stereochemical aspects of the following two types of TOT clathrates ... 

X-ray diffraction affords the basic method for investigating structural relationships in crystalline clathrates. Despite the large number of TOT elathrates mentioned in the literature, relatively few crystal structures have been completed. This sitnation is probably not fortuitous according to the current interest in chiral phenomena the attention has been mainly focused on enantiomorphous TOT clathrates which, within each of two distinct types, are isostructural. This feature might have detered further X-ray analyses because of the ensuing restricted prospect of making new structural discoveries. 

In their earlier investigation Lawton and Powell determined the unit cell dimensions of about 60 chiral TOT clathrates pertaining to either of two distinct... 

Table 1. Guest molecules in TOT clathrates structures determined by X-ray diffraction.

In most cases the precise recognition of the conformation of a chiral guest presents difficulties in X-ray structure analysis of cage-type TOT clathrates. The basic reason resides in the fact that in addition to any dynamic disorder two kinds of static disorders severely impair the resolution of atomic positions ... 

Other techniques than X-ray structure analysis, such as IR, NMR, NQR spectroscopy, enthalpies measurements, etc. can be applied to advantage to investigate the properties of clathrate compounds since in most cases microcrystalline samples are suitable for the measurements. To date very little use has been made of this wide range of methods for investigating TOT clathrates. 

The mode of mass spectral fragmentation of TOT has been investigated in connection with a study on the use of clathrate compounds for analytical separation. It appeared that the nature of the guest molecule had no effect on the mass spectrum of TOT at the inlet temperature of 180 that corresponds approximately to the melting point of most of the TOT clathrates. 

The practically important feature surveyed in this section is the concept of application of TOT clathrates to the separation of enantiomers. Broadly speaking, clathrate formation is assumed to rely upon a good fit of the guest molecular shape to the voids within the host lattice. With TOT, this type of complementarity offers two advantages ... 

It stands out that the photochemistry is also dependent of the crystalline phases where the topochemical relations between nearest-neighbor molecules are not the same in the pure guest crystals and in the TOT clathrates. When the TOT/cw-stilbene clathrate was irradiated, photoconversion to trans-stilbene occurred smoothly with the additional formation of small amounts of phenanthrene and of an unidentified product. On the contrary the TOT/tru j-stilbene clathrate remained unchanged when irradiated for long periods of time. In the majority of cases, crystalline cw-cinnamic acid derivatives are entirely converted to tran -isomers on irradiation and the trans-to-cis isomerization is not observed However both TOT clathrates with the trans- and cis-isomers of methyl cinnamate photoisomerized under similar conditions. It is noteworthy that these clathrates and those of the stilbenes (Sect. 2.1.4) are isomorphous. 

Fig. 14. Cis-trans photoisomerization of stilbene and methyl cinnamate in TOT clathrates. Reproduced with permission from J. Am. Chem. Soc. 101, 7529 (1979)...

The attraction forces between host and guest are necessary to the cohesion of the TOT clathrate lattice since crystal decomposition happens to be the inescapable fate met by the clathrates on desolvation. In a few instances the included guest molecule may assume a strained (unusual) conformation that is beneficial to the overall stability of the clathrate lattice. Conversely, the host lattices may be considered as media for stabilizing unstable guest species... 

Table 7. Characteristic IR absorption bands (cm ) in the region 850-600 cm for the pure guest and its TOT clathrate...

Attention has been mainly focused on the experimental and theoretical aspects of inclusion in chiral cage- and channel-type TOT clathrates, the former manifesting... 

The stable and selective inclusion of highly strained halocyclohexanes conformers or of monomeric carboxylic acids reveals the outstanding mutual fit of the TOT molecules constituting clathrate crystal, thus leading to a remarkable preservation of the cohesive van der Waals forces in the host lattice. But for a few exceptions, once formed the TOT clathrates prove to be thermally stable. 

The tri-ortho-thymotide (TOT) clathrates of dibromo- and dichlor-omethane have been characterized by single crystal X-ray diffraction at 200 K and solid-state NMR spectroscopy as a function of temperature. ... 

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