Geometry of the guest molecules

In a review of the motions of guest molecules in hydrates, Davidson (1971) indicated that all molecules between the sizes of argon (3.8 A) and cyclobutanone (6.5 A) can form si and sll hydrates, if the above restrictions of chemical nature are obeyed. Ripmeester and coworkers note that the largest simple structure II former is tetrahydropyran (THP) (C5H10O) with a van der Waals diameter of 6.95 A (Udachin et al., 2002). Closely following THP are m- and p-dioxane and carbon tetrachloride, each with a molecular diameter of 6.8 A (Udachin et al., 2002). Molecules of size between around 7.1 and 9 A can occupy sH, provided that the below shape restrictions are obeyed and a help gas molecule such as methane is included. 

FIGURE 2.12 Lattice parameter vs. temperature plots for si (a) and sll (b) hydrates. 

As simple hydrates, methane, and hydrogen sulfide can stabilize the 512 cavities of structure I (size ratios of 0.86 and 0.90, respectively) and they can occupy all the large 51262 cavities of si (size ratios of 0.74 and 0.78, respectively). Ethane occupies the 51262 cavities of structure I with a ratio of 0.94. Propane and iso-butane each occupy the 51264 cavities of structure II with a size ratio of 0.94 and 0.98, respectively. 

Ratio of Molecular Diameters1 to Cavity Diameters0 for Natural Gas Hydrate Formers and a Few Others 

Molecular diameter/cavity diameter for cavity type 

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The carbon chains of these compounds were too short and, as a consequence, the sum of the van der Waals forces was too low to cause coiling. Another condition for the formation of the complex is the geometry of the guest molecule. For example, the size of the helix interior must be such that the guest molecule fits inside the helix. Several experiments have shown that the diameter of the helix cavity is variable and can expand from 4.5 to 6.0 A. This diameter also depends on the number of turns of the helix.653 Thus, the helix has a variable inclusion capacity and it can hold even seemingly bulky guest molecules. 

The thermal phase transition observed for silica was mimicked chemically by appropriately choosing the geometry of the guest molecules. Tetrahedral and pseu-dotetrahedral guests select the H-cristobalite-like host (space group However, guests that have a... 

A very interesting property of liquid crystals is their ability to orientate molecules of solute. The anisotropic solute-solvent interactions depend critically on the geometry of the guest molecule and the applications reported are based on this property. 

The anisotropic solute—solvent interaction depends critically on the geometry of the guest molecules and therefore provides a very sensitive physical parameter to distinguish between two geometrical isomers of a molecule. For this reason liquid crystals may be used most successfully as substrates in gas-liquid chromatography. This type of application is described in Section 5. 

Quantitative analysis of relaxation rates can also be used for the elucidation of inclusion complex geometry. Longitudinal and transverse H relaxation rates were exploited to determine the orientation of the camphor 4 molecule inside the CyD capsule in the (+)camphor-a-CyD 1 2 complexes applying the model of anisotropic tumbling of the guest molecules [10]. It is noteworthy that in this particular case the complex geometry could not be obtained from analysis of NOE correlations. 

In the gas phase or in isotropic hosts, such as liquids, polymers or glasses, the transition dipole moments of the guest molecules exhibit random orientations. In anisotropic systems, like crystals and stretched polymer films, there is usually a correlation between the orientation of the molecule and the geometry of the host matrix. The probability of electric dipole absorption depends on the square of the scalar product... 

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