A vibrational spectroscopic, structural and quantum chemical study of the triiodide ion

The vibrational spectra of [R3S]I-3(l) (R=Me or Et) showed extra spectral features in the 145 and 170 cm(-1) region, in contrast to what is predicted by the selection rules for I-3(-) of D-infinity h symmetry. Depolarisation experiments showed that the 170 cm(-1) peak originates from a symmetric vibration mode. The addition of an excess of I- obliterates the 170 cm(-1) peak, whereas the peak at 145 cm(-1) is invariant. The excess of I- also introduces a decrease in intensity of the structural effects from I-3(-) in the LXS reduced radial distribution function. The centrosymmetric polyhalide ions Br-3(-) and IBr2- do not exhibit analogous spectral features to the triiodide ion. The results suggest that the extra spectral features observed for the triiodide systems are caused by the presence of higher polyiodides, viz. pentaiodides formed by the disproportionation reaction 2I(3)(-) --> I-3(I-2)(-)+I-, and a cation-induced, symmetry lowering of I-3(-). The results from the theoretical study support this showing that I-3(-) and I-5(-) have vibrational features in the 145 and 170 cm(-1) region. The energy barrier to linear deformation is also shown to be very small both for linear and asymmetric triiodide ions, thus supporting the ideas about a Grotthus type of mechanism of conduction in polyiodides, where the triiodide ions act as iodide donors.