Host-Guest Inclusion Complexation of alpha-Cyclodextrin and Triiodide Examined Using UV-Vis Spectrophotometry

摘要

The historically relevant host-guest complexation of acyclodextrin (alpha-CD) and triiodide (I-3(-)) in aqueous solution was examined using a systematic UV-vis spectrophotometric approach. This particular system is experimentally challenging because of the coupled equilibria, namely, I-2 + I- reversible arrow I-3(-) and alpha-CD + I-3(-) reversible arrow alpha-CD.I-3(-). We therefore developed a unique experimental approach that allowed us to determine the concentration of all iodine species. This enabled us to unequivocally demonstrate that the large increase in the UV absorbance with added alpha-cyclodextrin is due to an increase in the overall triiodide concentration as alpha-CD essentially converts iodine to trliodide according to the coupled equilibria. Herein we report (a) the complexation stoichiometry is 1:1 (i.e., the host-guest complex is alpha-CD. (b) the binding constant is KH-G = (1.35 +/- 0.05) x 105 at room temperature, and (c) the binding constant is temperature dependent with OH =-31.0 +/- 0.9 kJ/mol.