Normal modes and structure of iodine in the starch–iodine complex

Normal modes of a linear array of iodine atoms with various repeat units, (I ), (I I ), (I I ), and (I ), have been calculated. The four resonance Raman bands (158, 107, 57, and 27 cm −1 ) observed with the starch–iodine complex can be assigned to the symmetric modes of either the (I I ) n or (I ) n linear chain. However, the nature of the valence force fields of these two structures is very different: the former consists of a weakly coupled linear chain of (I ) structure, while the latter, (I ) n , is a tightly bound chain approaching equidistant iodine spacings. The nearly equivalent I‐I stretching force constants obtained in the present normal mode analysis of the (I ) n structure is supported by the results of previous x‐ray and I‐129 Mössbauer effect measurements. A numerical method of calculating the frequency dispersions of one‐ and two‐dimensional molecular arrays is given.