The complexation thermodynamics of a large number of guest molecules with natural alpha-, beta-, and gamma-cyclodextrins (CD) can be well-described within the framework of a recently introduced, unified, molecular size-based model for nonassociative liquids that also includes a modified hydration-shell hydrogen-bond model for water (Buchwald, P.; Bodor, N. J. Am. Chem. Soc. 2000, 122, 10671). With increasing guest size, 1:1 complex stability, as measured by In K or DeltaGdegrees, tends to increase linearly up to a size limit characteristic for each CD. For alpha- and beta-CD, the corresponding slopes and intercepts are in excellent agreement with those predicted by the model. For larger guest structures, values level off and are scattered around an average value depending on shape, goodness of fit, and possibly lipophilicity and some specific effects (e.g., such as those caused by presence of phenol functionality). It is an important achievement in the description of interactions in liquids that the very same interaction constants derived from boiling point and enthalpy of vaporization data can describe partition, and now complex stability data as well. Furthermore, for most molecules, heat capacity changes associated with complex formation are also in excellent agreement with those derived from the model based on hydrogen bonding changes in the hydration shell.
