Absence of Dynamic Cohesion

Typically, values for the receptor carbons fall in the range of 200-3(X) ps, which is a reasonable value for a molecule of its size, with guest complexation not providing any perturbation, as might have been expected. For more rapidly tumbling alkylammonium ions of the type incorporated, values for CH2 and CH groups are typically 4-10 ps in the uncomplexed state. For a number of 

This indicates a lack of dynamic cohesion within the adducts i.e. the substrate has considerable freedom for reorientation within the receptor. The apparent reason for an absence of mechanical coupling is the nearly cylindrical symmetry of cucurbituril, which allows the guest an axis of rotational freedom when held within the cavity. Hence, the bound substrates show only a moderate increase in tc relative to that exhibited in solution. No relationship exists between values and the thermodynamic stability of the complexes as gauged by K (or K, cf. Tables 1 and 2). It must be concluded that the interior of cucurbituril is notably nonsticky . This reinforces previous conclusions that the thermodynamic affinity within adducts is chiefly governed by hydrophobic interactions affecting the solvated hydrocarbon components, plus electrostatic ion-dipole attractions between the carbonyls of the receptor and the ammonium cation of the ligands. 

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