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The far-infrared and Raman spectra of (CH3)4NI5, (TMA.H2O)10HI5 (TMA = trimesic acid), (phen)2HI5 (phen = phenacetin), (CH3)4NBrI4, (C2H5)4NI7, and (CH3)4NI9 have been recorded and analyzed. The first three compounds contain bent I5- ions, linear I5- ions, and (I3-).(I2) species, respectively, whereas the last two may be described as (I3-)(2I2) and (I5-)(2I2), respectively. Approximate stretching frequencies for the various moieties are 180 cm-1 for I2, 110 and 140 cm-1 for I3-, 145 and 155 cm-1 for the outer stretches of bent I5-, 76 and 113 cm-1 for the inner stretches of bent I5-, 163 cm-1 for the outer stretches of linear I5-, and 75 and 90 cm-1 for the inner stretches of I5-. A partial force constant calculation shows the outer I-I bonds of linear I5- to be stronger than those of the bent anion, in agreement with previous X-ray results.
Nour et al. (Sun,) studied this question.