Crystal Structure of HI3·2C6H5CONH2: A Model of the Starch—Iodine Complex

The lattice of the HI3·2C6H5CONH2 inclusion complex is triclinic (a=20.828 Å, b=9.885 Å, c=9.588 Å, α=95°46′, β=101°54′, γ=94°44′) with four molecules per unit cell. The host structure consists of dimerized benzamide molecules stakced in such a way as to form long channels in which the guest tri-iodide ions are aligned. Each channel is filled with two chains of tri-iodide ions polymerized end to end, the chains being separated by the van der Waals distance, 4.6 Å. Within the chains, the tri-iodide ions are bent about 3° from linear and are asymmetric with one bond being 0.02 Å longer than the other. The distance between ends of adjacent tri-iodides within a chain is only 3.8 Å, indicating a strong attractive interaction between the ions. It is postulated that a single chain of tri-iodide ions of the above description is the chromophore in the blue starch—iodine complex.