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

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Abstract
The vibrational spectra of [R3S]I3(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 I3 − of D∞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 I3 − in the LXS reduced radial distribution function. The centrosymmetric polyhalide ions Br3 − 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 2I3 − → I3(I2)− + I−, and a cation-induced, symmetry lowering of I3 −. The results from the theoretical study support this showing that I3 − and I5 − 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.

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