Structure and Formation Mechanism of Iodine Complexes of Nylon Model Compounds as Revealed by X-ray Single-Crystal Structure Analysis and Density Functional Theory

Nylon 6 and nylon 66 are known to form crystalline complexes with the iodine ions. The aggregation structure of nylon chains and iodine rods has not yet been clarified in detail, unfortunately, because of the poor X-ray diffraction data. In the present study, the model compounds of nylon 66 ([CH3(CH2)4CONH(CH2)6NHCO(CH2)4CH3 and CH3(CH2)5NHCO(CH2)4CONH(CH2)5CH3]) were synthesized, and the single crystals of iodine complexes were grown successfully. The crystal structures were determined by X-ray diffraction data analysis. In the pristine state, the molecular chains adopt a fully extended trans-zigzag conformation and are combined with intermolecular hydrogen bonds. Herein, we have discovered that, in the iodine complexes, the molecular chains were contracted to the gauche form through the trans–gauche torsional rotations around the amide–CH2 bonds. This information should help us deduce the chain conformations of the mother nylon in the iodine complexes. The density functional theory calculations combined with experimentally obtained structural information revealed the charge transfer mechanism from iodine ions I3– to the amide groups.