Compensation effects in the complexation reactions of alpha- and beta-cyclodextrin

Analysis of temperature dependent equilibrium constant data for the complexation of alpha-cyclodextrin with approximately 70 different substrates demonstrates the existence of a general compensation effect. Enthalpies (kJ mol(-1)) and entropies (J K-1 mol(-1)) of complexation are related by Delta H = -5.70(+/-0.05) + [421.1(+/-1.2)]Delta S. A few substrates deviate from the general effect, possibly owing to reorganization of internal hydrogen bonding interactions. Complexation reactions involving beta-cyclodextrin are found to divide into two groups. Eleven small molecule or ion substrates undergo isoentropic complexations with Delta S = -5.7 +/- 0.2 J K-1 mol(-1). Fourteen larger substrate species exhibit a compensation effect with Delta H = -23.5(+/-0.2) + [634(+/-26)]Delta S. The compensation behavior of the two cyclodextrins is discussed in terms of bonding mechanisms and cavity size.