Clathrates quinol

Class (d). These hydrates are distinguished on the grounds that there are no well defined polyhedral cavities, the nets being 3- or (3 + 4)-connected. The 3-connected framework in N4(CH2)g. 6 H2O (m.p. 13-5°C) is that of Fig. 3.31 (p. 97), the same as one of the two identical interpenetrating frameworks in /3-quinol clathrates. Since a framework of this kind built of H2O molecules has only 9 links (0—H—0 bonds) for every 6 H2O there are 3 H atoms available to form hydrogen bonds to the guest molecules. The latter are suspended bat-like in the cavities halfway between the 6-rings (Fig. 15.8), that is, they occupy the... 

In the quinol clathrates the molecules of quinol (p-dihydroxy-benzene) are again linked by hydrogen bonds between rings of six hydroxyl groups, but now such linkages take place at both ends of the molecule and bind the molecules into infinite three-dimensional frameworks of the form shown in fig. 14.33. This framework is very open, and in the crystal two such frameworks exist, completely interpenetrating... 

Fig. 14.33. Clinographic projection of the framework of quinol molecules in the structure of quinol clathrates. The plane hexagons represent rings of hydroxyl groups united by hydrogen bonds and the long lines represent the HO—OH axes of the quinol molecules. The framework extends indefinitely in three dimensions and the hexagons lie at the points of a rhombohedral lattice.

Fig. 14.34. Clinographic projection representing schematically the two interpenetrating frameworks in the structure of the quinol clathrates. The rhombohedra represent two independent frameworks, each of the form shown in fig. 14.33. Guest molecules occupy the interstices indicated between these frameworks. Both frameworks of course extend indefinitely but, for clarity, only one rhombohedron of each is shown.

One of the first clathrate systems to be investigated in detail, and still one of the best understood, comprises the substances in which the host is /J-quinol. Quinol clathrates have been prepared enclosing 02, N2, CO, NO, CH4, S02, HC1, HBr, Ar, Kr, Xe, HCOOH, HCN, H2S, CH3OH, or CH3CN. 

The p-quinol clathrate inclusion compounds are beautiful and early examples of solid-state catenation. In P-quinol each hydrogen-bonded network is single and three-dimensional two such networks interpenetrate, to use Powell s original term, but are linked neither by covalent nor by hydrogen bonding. We would describe them as being catenated. 

As noted in Section 2, Rapson, Shuttleworth and van Niekerk first reported the formation of 2 1 adducts of tetraphenylene with a variety of solvent molecules In their 1944 paper on the molecular structure determination of 7 , Karle and Brockway stated that the sample contained 10% benzene which was removed by placing the sample in the nozzle and heating it to 200° in a vacuum before electron diffraction photographs were taken. Understandably, the nature of molecular association in these adducts drew no attention at that time, since the concept of clathration was not clarified until Powell s pioneering study of the hydroquinone (quinol) clathrates in 1947 Nevertheless, it is surprising that the problem lay dormant for the next several decades. 

Matsui, S. Terao, T. Saika, A. Study of static and dynamic structure of (J-quinol clathrate by C high-resolution solid-state NMR and proton Tj measurement. J. Chem. Phys. 1982, 77, 1788. 

The Raman spectra of quinol clathrates unlike the infrared spectra [4], are not dominated in the 2700-3700 cm" region by the OH band of the host lattice. It is consequently quite easy to observe the methanol guest molecule bands in the Raman spectrum of the clathrate (Fig. 2a), and the difference spectrum (Fig 2c) merely highlights the intensity of the guest molecule bands. 

It is also possible to simulate spectra of clathrates simply by adding two spectra. Figure 5c is the synthesised spectrum of the acetonitrile clathrate obtained by adding the spectra of 6-quinol (Fig. 5a) and liquid CH CN (Fig. 5b). Note that the resulting spectrum (Fig. 5c) is not identi-caPto that of the clathrate (Fig. 4a), since the acetonitrile guest molecule has lost some of the rotational freedom possessed by the molecule in the liquid phase. This loss of rotational freedom leads to many characteristic features in the Raman spectra of the 3 quinol clathrates of CH CN and CDgCN [4J. 

The principal components o, v = a, 3, y, of the chemical shift tensor were determined from NMR spectra of NF3 trapped in 3-quinol clathrates at temperatures around 4.2 K (NF3 molecules become oriented under these conditions). Values in ppm are a =-60 30, Op = 260 40, o = -200 30. The a axis is in the plane of the fluorine triangle, and the 3 axis is along an N-F bond. The chemical shift tensor is defined as traceless, i.e., the mean chemical shift o = V32a = 0 [10]. 

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