Polyiodine units in starch–iodine complex: INDO CI study of spectra and comparison with experiments

The starch–iodine blue complex formation does not involve negatively charged iodine species like I 3−, I 5−, or I 7−; rather, neutral iodine units are involved. The heat of reaction is determined to be about −110 kJ for every mole of I‐I unit in the amylose helix, which suggests that the dissociation of I₂ (binding energy 149 kJ/mol) does not take place during the complex formation. Quantum mechanical (INDO CI) calculations indicate that the linear as well as nonlinear polyiodine units, I₆, with interiodine distance of 3.0 Å are responsible for characteristic absorbance bands of the starch–iodine complex. Based on our previous article [(1989) J. Polym. Sci. A 27, 4161] and the present studies we identify (C₆H₁₀O₅)_16.5I₆ to be the polymeric unit responsible for the characteristic blue color of the complex.