Experimental and DFT studies on structure, spectroscopic and thermal properties of N-Methyl-N,N,N-trioctylammonium chloride ionic liquid

The [Aliquat+][Cl-] ionic liquid (IL) has been structural and vibrationally characterized combining experimental FT-IR, FT-Raman and 1H- and 13C-NMR spectroscopies with theoretical studies based on the hybrid B3LYP/6-31G* method. The theoretical determination of structure of IL in gas phase and aqueous solution by using that level of theory shows three ionic C-H···Cl bonds, as supported by atomic Merz-Kollman (MK) charges, bond orders, natural bond orbital (NBO) and atoms in molecules (AIM) calculations. Hence, a monodentate coordination between the [Aliquat+] cation and Cl anion has been proposed for IL because only one of three ionic C-H···Cl bonds presents higher energy and lower distance. The optimized structure confirmed by NMR measurements has allowed the assignments of 237 normal vibration modes to the bands observed in the infrared and Raman spectra with the aid of scaled quantum mechanical force field (SQMFF) methodology and the normal internal coordinates. The effect of Cl in the IL is the shifting of vibration modes corresponding to the CH2 and CH3 groups, as compared with the cation. The mapped MEP surface evidence a strong concentration of charges around the chloride anion compatible with nucleophilic sites in these regions while the frontier orbital analyses suggest that IL is most reactive than the cation probably due to the low values of both global electrophilicity and nucleophilicity indexes. The thermal stability shows that [Aliquat+][Cl-] start to decompose just above 200°C. Comparisons between [Aliquat+][Cl-] and [Aliquat+][NTf2-] ILs show that the IL containing chloride is most reactive than the other one while [Aliquat+] [NTf2-] is thermally more stable than [Aliquat+][Cl-]. These results suggest that the properties of an IL containing the [Aliquat+] cation are strongly dependent of anion. In addition, the scaled force constants for the [Aliquat+][Cl-] IL are also calculated and compared with the reported for the cation. The Cl anion modifies in notable form the properties of cation.