Chemical nature of guest molecules

Two classifications of the chemical nature of the guest molecule have been proposed. The first scheme by von Stackelberg (1956) was a combination of both size and chemical nature as discussed in Section 1.2.3. The second scheme by Jeffrey and McMullan (1967) characterized the guest molecules in one of the following four groups (1) hydrophobic compounds, (2) water-soluble acid gases, (3) water-soluble polar compounds, and (4) water-soluble ternary or quaternary alkylammonium salts. 

Restrictions on Guest Motions. The physicochemical nature of the guest molecule, once enclathrated, has been studied in some detail. Dielectric constant and NMR techniques have been successfully applied by Davidson (1971) and Davidson and Ripmeester (1984). The translational degree of freedom of the guest is limited. 

Davidson (1971) determined that the most important rotation inhibition interactions between guest molecules (in adjacent cages) were the dipole-dipole interactions, but even they were of minor importance. Within a single cage, both nonpolar and polar guest molecules such as EO, THF, and acetone have only small barriers to rotational freedom, which approximates that in the vapor phase. The rotational freedom is probably due to the fact that the sum of the cage water dipoles effectively cancel near the center of each cage, and even the quadrupolar fields are relatively small. 

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