UV-vis spectroscopic studies on aqueous solutions of iodine, potassium iodide, and alpha-cyclodextrin (alphaCD), (C6H10O5)(6), suggest that a complex forms with the composition of alphaCD(2)I(3)(-). Semiempirical quantum mechanical PM3 calculations suggest that two major host-guest isomer types exist for this complex. The most stable isomer has the 13 ion oriented almost parallel to the plane of one of the alphaCD molecules and sandwiched between two alphaCD molecules. The second sandwich isomer is less stable than the above isomer by about 5 kcal/mol (20.9 kJ/mol) and has the I-3(-) ion oriented vertically (along the vertical axis) between two alphaCD molecules. The formation constant (K-f) of the complex is determined to be around 7.0 x 10(8) M-2 for both 15 and 25 degreesC.