α-cyclodextrin-I₃^- host-guest complex in aqueous solution: Theoretical and experimental studies

UV-vis spectroscopic studies on aqueous solutions of iodine, potassium iodide, and α-cyclodextrin (αCD), (C₆H₁₀O₅)₆, suggest that a complex forms with the composition of αCD₂I₃^-. Semiempirical quantum mechanical PM3 calculations suggest that two major host-guest isomer types exist for this complex. The most stable isomer has the I₃^- ion oriented almost parallel to the plane of one of the αCD molecules and sandwiched between two αCD molecules. The second sandwich isomer is less stable than the above isomer by about 5 kcal/mol (20.9 kJ/mol) and has the I₃^- ion oriented vertically (along the vertical axis) between two αCD molecules. The formation constant (K_f) of the complex is determined to be around 7.0 × 10⁸ M^-2 for both 15 and 25°C.