Charge-Transfer Spectra of Iodine with Hydrogen Sulfide and Benzene in Low-Temperature Matrices

The absorption spectrum of benzene–iodine and hydrogen sulfide–iodine was studied at 20°K in argon, krypton, xenon, methane, nitrogen, and sulfur hexafluoride in the spectral range of 2300 to 4500 Å. In addition to the component spectra, strong bands were observed around 2800 Å in all solvents. They are assigned to the electronic charge‐transfer transitions of the electron donor‐acceptor systems benzene–iodine and hydrogen sulfide–iodine, respectively. The bands are broad and exhibit coarse as well as fine structure. The latter corresponds closely to the ground‐state vibrational frequency of iodine. While the solvent shifts in the benzene–iodine system follow the predictions for the heavy‐atom effect, the shifts of the hydrogen sulfide–iodine bands appear reversed. The half‐width of the bands increases, for benzene–iodine, with increasing polarizability, but decreases for the hydrogen sulfide–iodine system. The absorbance of the charge‐transfer bands of the hydrogen sulfide system are stronger than those of the benzene system, and both are greater than that of the $\mathrm{a–X}$ system of benzene. The electron‐donor spectra seem little perturbed by the presence of the charge‐transfer complex. However, a great increase of transition energy and absorbance is noted for the visible transition of iodine. The fine structure of the 3600‐Å band of iodine in matrices containing hydrogen sulfide–iodine is 130 cm⁻¹, corresponding well to the vibrational frequency of the $B$ state of iodine.