Polyiodides in room-temperature ionic liquids

Abstract

Polyiodides, I_m⁻, were formed in room-temperature ionic liquids (RTILs). The RTILs were N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium iodide, [DEME][I], and 1-alkyl-3-methylimidazolium iodide, [C_nmim][I] (n = 1, 2, 3, 4, 6, and 12). In [C_nmim][I_m], prepeak and intensity modulation of X-ray diffraction were caused by polyiodides. Above m = 4, polyiodide-mediated local structures were formed, although little alkyl side-chain length, n, dependence was seen in the X-ray diffraction patterns. The viscosity of the mixtures was measured as a function of m to obtain their dynamic properties. The m tendencies of the viscosity and density changed at around m = 4 when fixing n = 4. By ¹²⁷I-NMR, the local environments of only polyiodides were extracted by changing n, m, and the temperature. Doublet/triplet peak splitting of pure [C_nmim][I] (n = 3, 4, and 6) occurred at low temperature. In [C_nmim][I_m] (n = 2 and 4), the effect of polyiodides was clarified from the chemical shifts and peak splitting in the NMR spectra. The experimental results support a Grotthuss-type exchange mechanism of iodide between polyiodides.