Optical Spectra of Benzamide—Triiodide Ion Complexes : A Model of the Starch—Iodine Complex

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Abstract
Earlier work has shown that the chromophoric group in the HI3·2C6H5CONH2 complex is an infinite chain of triiodide ions linked end to end, and the close similarity of the spectra of the benzamide—triiodide complexes and the blue starch—iodine complex strongly suggests that such a chain is also the chromophore in the latter. The electronic spectrum of such a one‐dimensional chain of triiodide ions is explained by first assigning the transitions in the free triiodide ion and then assuming an excitonlike coupling of these transitions in the chain. The two well known transitions of the free triiodide ion at 28 300 and 34 800 cm—1 are assigned as antibonding pi to antibonding sigma and bonding pi to antibonding sigma, respectively. The high intensity of the pi to sigma bands results from a mixing of pi and sigma orbitals due to the supposed nonlinearity of the free ion. Electrostatic coupling of the strong, long‐axis pi to sigma transitions then leads to a semiquantitative understanding of the number of bands in the polymeric triiodide spectrum as well as their polarizations, intensities, and frequencies.
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