Tetramethylammonium compounds have many applications in science and biology. It is extremely difficult, if not impossible, to prepare single crystals of tetramethylammonium salts suitable for diffraction studies; the available methods of preparation give microcrystalline powders that are not suitable for X-ray single crystal diffraction. Scientists' effort to find and use simple substituted methods for studying crystal habits of tetramethylammonium salts. One of the suggested methods is the use of infrared spectra and assigning of it by symmetry. The vibrational spectrum of the tetramethylammonium (Me 4 N + ) ion has been the subject of several publications during the last four decade 1-3 . Many studies had been done on the vibrational spectra of tetramethylammonium salts and ahowed that it possible to correlate of infrared spectral properties with C-H…X hydrogen bonding and crystal habit in tetramethylammonium salts

The synthesis of Triphenylphosphonium trichloroiodoaluminate (III) ionic liquids (ILs) is reported here. The product was characterized by spectroscopic and analytical methods such as 31P-NMR, FT-IR, cyclic voltammetry, CHN, TGA and differential thermal analysis (DTA). Thermal analyses were used to investigate the thermal behavior of this compound. The results show that this ILs has excellent thermal stability below 110•C. Along with the experimental study; this compound has been studied computationally at the B3LYP/LANL2DZ level of theory using the Gaussian 98 program package. From these calculations, optimized geometries, molecular parameters, and vibrational spectra of ILs have been calculated. In addition, calculated frequencies are compared with the experimental frequencies after correction by the appropriate scaling factor. This comparison shows that our theoretical data are in good agreement with the experimental results.

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