Raman spectroscopic investigation of N‐alkylurotropinium polyiodides

Abstract

N-Alkylurotropinium polyiodides, UrRI, , with different iodine contents and different sizes of alkyl group were investigated by Raman spectroscopy. The spectra were recorded for wavenumber shifts up to 500 cm-'. It was found that the triiodides, UrRI, , contain a symmetrical linear I,-ion. The pentaiodides, UrRI, , contain no triiodide unit; the anion structure can be described as nearly symmetrical, rectangular L-shaped I,-. An interpretation of the Raman spectra of the heptaiodides, UrRI, , is that the anion corresponds to an I,-. I, structure. This is the first proof of such a heptaiodide. A review of previously published data on the Raman spectra of polyiodides is also given. I N T R O D U C T I O NThe preparation of polyiodides is not very diffcult and a large number are known. However, it is not so easy to obtain correct information about their structures. The best way is, of course, to use x-ray methods, but it is often impossible or diffcult to grow suitable single crystals, especially of penta-and heptaiodides. Other methods that yield useful information about the structures of polyiodides are lz9I Mossbauer spectroscopy' and nuclear quadrupole resonance s p e c t r o s c~p y .~~~ However, both methods require expensive equipment and are not very commonly used. In this respect it is advantageous to use Raman spectroscopy for the investigation of polyiodides because it provides reliable information about anion structures in a simple manner.We recently reported the preparation of Nalkylurotropinium polyiodides, UrRI, , and their electrical and magnetic properties.495 Because we were unsuccessful in growing suitable single crystals for x-ray analysis, we decided to use Raman spectroscopy to obtain structural information on these compounds. The literature concerning the Raman spectra of polyiodides is reviewed here. The spectra of UrRI, were measured for shifts up to 500 cm-' in order to make elucidate the structures of the anions of the polyiodides. The region of the C-C, C-N and C-H vibrations of the cations was not measured because the urotropinium cation has been well characterized by vibration spectroscopy.6 EXPERIMENTALThe N-alkylurotropinium polyiodides were prepared as described in detail earlier4 by the reaction of the corre-*

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