1,2-xylene guest molecules

The family of true clathrates based on hydrocarbons only is further enriched by the inclusion compounds of 48 with benzene (1 1) and p-xylene (2 1) 90> (Table 19). Figure 28 illustrates the structure of 48 benzene (1 1). The structure of the p-xylene clathrate shows intercalated guest molecules at centers of symmetry in the crystal lattice. Both clathrates are rather unstable at ambient temperatures and decompose easily, e.g. on exposure to X-rays (even in the presences of mother liquor). The 48 p-xylene clathrate is unstable to such a degree that decomposition occurs at low temperature. 

Figure 12 Cross-sections of CA host channels sliced parallel to the direction of the channel (carbon and hydrogen atoms are represented by gray and white, respectively) with arrays of included guest molecules (hydrogen atoms are omitted for clarity, and carbon and oxygen atoms are represented by open and filled circles, respectively), (a) toluene, (b) w-propylbenzene, (c) n-butylbenzene, (d) w-hexylbenzene, (e) p-xylene, and (f) methyl methacrylate.

This selectivity is completely different from that in the case of CA as host. The difference between the two hosts is ascribed to the different shapes of their cavities. Figure 14 shows typical cross-sections of host cavities with arrays of o-xylene, where the guest molecules are arrayed in cavities that do not possess grooves. In the case of CA, the grooves play an important role in selective enclathration, while the DCA cavity has no grooves. This means that the mechanism of separation differs between the CA and the DCA hosts. 

Figure 7.5 Packing of two-dimensional layers in (left) [Ni(N03)2(bpbp)2] 4(o-xylene) and (right) [Ni(N03)2(bpbp)2] 1.7mesitylene . Guest molecules are omitted for clarity.112 (Reprinted with permission from K. Biradha et al., Angew. Chem. Int. Ed. 2002, 41, 3395-3398. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

Apparently, in both zeolite structures the pore deformation that has to be achieved to adsorb guest molecules is caused by the collisions of these molecules with the pore entrances. In the case of the MFI-type zeolite, p-xylene as well as naphthalene were loaded to the maximum population, which is 8 and 4 molecules/u.c., respectively, in the pores. 

A single C label at the methyl group of p-xylene was used to estabhsh 13c 13c distances between guest molecules and hosts with naturally abundant C, using the 2D RFDR approach [91]. Other Dianin ICs, with -hexane and n-pentanol guest molecules, have been reported [92]. Eckhardt and coworkers investigated Dianin clathrates containing two ethanol molecules and a heptanol, [93] as well as 1,6 hexanediol, 1,5 pentenediol and 1-heptanol [94]. 

Selective complexation has been demonstrated 132) by crystallizing p-tert-butyl-ealix[4]arene from 50 50 mixtures of two guest molecules such as benzene and p-xylene. It was found that anisole and p-xylene are complexed in preference to most other simple aromatic hydrocarbons. p-ter/-Butylcalix[4]arene has been shown to have some selectivity for mixtures of aromatic hydrocarbons when used as the immobile phase in a chromatographic column 133). 

Host/guest interactions of benzene, p-xylene,p-chlorotoluene and chlorocyclo-hexane with ZSM-5 were investigated by Huang et al. [832]. These authors used FT-Raman spectroscopy and utilized the excitation by an NIR laser (A=1064 nm), which considerably reduced the fluorescence background. The effect of loading on the framework and OH stretching vibrations was determined as well as sor-bate-induced phase transitions, conformational properties of the guest molecules, their location and sorbate/cation interactions. 

As described before, the p-xylene inclusion compound of 7 c shows the guest molecule fixed by the surrounding phenyl groups of 7 c only, and hydrogen bonding plays no role This result prompted us to design new hosts without hydroxyl groups. 

Another problem that is still under discussion concerns the size of the guest molecule. Researchers mostly use the kinetic diameters for guidance (see Table 2.22). However, it has been remarked (574,580) that the kinetic diameter is not the best criterion for the molecular dimension. The flexibility and the shape of the molecule in relation to the shape of the pore openings should be taken into account. Consideration of these properties explains some results that indicated penetration of relatively large molecules into smaller pores. Armaroh et al. (331) observed differing behavior of m-xylene and 2,6-lutidine (similar in size) in accessing the ZSM-5 pores and su ested that chemical effects also play a role for accessibihty. 

Many calixarenes form complexes in the solid state, this property having been observed even before the structure of compounds was established. Among the simple ca-lix[n]arenes, for example, / >-rer/-butylcalix[4]arene forms complexes with chloroform, benzene, toluene, xylene, nitrobenzene, nitromethane, acetonitrile, and many others, while /r-rer/-butylcalix[5]arene complexes were studied with lower alcohols, ethyl acetate, toluene, and tetraline. More flexible / -terr-butylcalix[6]arene was found to form complexes with acetonitrile and / -rerr-butylcalix[8]arene with chloroform. The tenacity with which the guest molecule is held by the calixarene hosts varies widely with the size of macrocycle. Whereas tetramers to hex-... 

Fig. 3 CD inclusion complex formation. p-Xylene is the guest molecule, and the small circles represent the water molecules.

Fig. 1 Dianin s compound with p-xylene guest. (From Ref. [10].) (Top) Packing of six Dianin s host molecules around a p-xylene molecule, looking down the sixfold axis of the x-ray diffraction model (only part of each Dianin s inolecule is shown). (Bottom) Room-temperature C-CP/MAS spectrum showing the multiple splittings for the methyl groups (18 and 19. pointing into the cavity) that arise because the p-xylene molecule is static, and the sixfold symmetry is lost.

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