Unit cell, guest molecules

A crystal structure in which ions or molecules pack in alternate ways in different unit cells is a disordered structure. Such disorder may be revealed by the presence of diffuse scattering, either as halos or streaks, around intense reflections. Guest molecules in crystals tend to be disordered, at times in a liquid-like state. Disorder is of two types. Static disorder occurs when a molecule is located in different orientations/conformations in symmetry-related positions in the unit cell. When molecules are fluxional at the same position in the crystal, the disorder is dynamic (e.g., perchlorate anion, tert-butyl group). Dynamic disorder can be dampened or frozen at low temperature, whereas static disorder persists even if x-ray data is collected at cryogenic conditions. 

Two helices are packed antiparallel in the orthorhombic unit cell. Association of the helices occurs through a series of periodic carboxylate potassium water - carboxylate interactions. An axial projection of the unit-cell contents (Fig. 23b) shows that the helices and guest molecules are closely packed. This is the first crystal structure of a polysaccharide in which all the guest molecules in the unit cell, consistent with the measured fiber density, have been experimentally located from difference electron-density maps. The final / -value is 0.26 for 54 reflections, of which 43 are observed, and it is based on normal scattering factors.15... 

Fic. 29. (continued)—(b) An axial projection of the unit cell shows that there is sufficient room for guest molecules in the middle along the short diagonal, such as the water molecule (open circle) bridging both chains. 

Three welan helices pass through the unit cell, as shown in Fig. 37b. Two of them, I and II, at (2A A0) and ( A2A0), are antiparallel as in gellan. A third helix, III, at (000) is new and parallel to the first. They are equilaterally 12.0 A apart from each other, 2.9 A farther than in 41. The unit cell contains a total of nine pentasaccharides surrounded by ordered guest molecules, which are accounted by six calcium ions and 75 water molecules. The two up and down helices interact via side chains, calcium ions, and/or water molecules. Specifically, helices I and II are linked only by side chain side chain hydrogen bonds... 

Vh-amylose shows an increase in water molecules from 4 to 16 per unit cell)51). Eight of these guests occupy interstitial sites and eight are present within the helical canals (Fig. 13). All unit cell dimensions are increased over the Va-form, thereby increasing the unit cell volume from 2304 to 2604 A3. The sixfold helical arrangement has been confirmed for Vh-amylose. 

It is now realised that amylose-polyiodide compounds can also involved Va- and Vh-forms 155 . Recent studies on the Vh-compound 154) have shown that the early structural ideas described above are essentially correct but that rather than P2l2i2i the monoclinic space group P2X is applicable. An almost linear arrangement of iodine atoms was present in the helical canals but the length of the iodide chain could not be determined. In addition to this guest, eight water molecules were present in the unit cell and once again these were present in the interstitial sites (see Fig. 14). 

The crystal structure of A-amylose is made up of parallel-stranded, ng/tr-handed double helices which pack in an antiparallel manner to give an orthorhombic unit celll67). About eight water molecules are present per unit cell and these guests are present in the interstitial sites (Fig. 15). 

Another polymorph of triethylamylose with dichloromethane or chloroform has also been studied 175). Here the number of guest molecules decreases from 8 to 4 per unit cell and the solvent molecules are now located in interstitial sites. The orthorhombic cell has space group P2l2l2l. 

The trimethylamylose-dichloromethane inclusion compound 176) has a pseudotetragonal unit cell and space group but this time all 8 guest molecules are... 

Equivalent v-sites i have the same probability p, to be occupied by a dye molecule. The occupation probability p is equal to the ratio between the occupied and the total number of equivalent sites. The number of unit cells I1C is controlled by the host while ns is determined by the length of the guest, which means that p relies on purely geometrical (space-filling) reasoning and that the dye concentration per unit volume of a zeolite crystal can be expressed as a function of p as follows ... 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

Figures 53 and 54 show the structure of the 3/98d complex as it exists in the unit cell [154, 303], Unlike the complexes with 98a-c, the 98d complex has both hydroxyl groups of one 3 hydrogen bonded to both carbonyl groups of one molecule of 98d. As a result, the diyne backbone is curved (Figure 53) [154, 303], There is no reason to believe that the walls of the reaction cavity experienced by 98d or by transients, lOld and 102d derived from it, in optically active 3 complexes are any more rigid or contain less free volume than do the other complexes. The enantiomeric purity of the product must result from specific attractive host-guest interactions retained along the...

However, it should be remembered that the fractional filling is a function of the product Cjjfj, rather than either factor in the product. Finally, in the original van der Waals and Platteeuw approach the Langmuir constants for both adsorption and enclathration were only functions of temperature for each molecule type retained at the individual site or cavity. In the modified approach below, the Langmuir constants are also a function of cage size, or the unit cell volume, which is a function of the hydrate guests, temperature, and pressure. 

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