Ionic conductivity, infrared and Raman spectroscopic studies of 1-methyl-3-propylimidazolium iodide ionic liquid with added iodine

Polyiodides (Ix−, x = 3 and 5) and 2I−…I2 adducts were established from the Raman spectra study of 1-methyl-3-propylimidazolium iodide (MPIm+Ix−; 1 ≤ x ≤ 5) ionic liquids containing various amounts of iodine (0 mol ≤ I2 ≤ 2 mol). The existence of I3− and 2I−…I2 was established for 1 ≤ x ≤ 2.5, symmetric I3− ions for x = 3, while linear and discrete I5− was substantiated for 3 ≤ x ≤ 5. The presence of polyiodide species in MPIm+Ix− (1 ≤ x ≤ 5) was correlated with an enhanced ionic conductivity, attributed to the established relay-type Grotthus mechanism. Two-step conductivity increase was also reflected in decrease of the hydrogen bond interactions between the CH ring groups and polyiodides. While in the concentration range 1 ≤ x ≤ 3 (triiodides and tetraiodides) IR bands changed only slightly in intensity, in the concentration range x > 3 the CH stretching bands (3040–3170 cm−1) split and the new band at 1585 cm−1 appeared in the IR spectra beside the already existing Im+ ring stretching mode at 1566 cm−1.