Structural Chemistry of AlCl3–N-Methylformamide Deep Eutectic Solvent: Study on Spectroscopy Techniques and Quantum Chemical Calculations

The application of AlCl3-amide deep eutectic solvent (DES) prompted research into its physicochemical properties and ionic structures. In the present study, first, the viscosity, density, conductivity, and electrochemical behavior of AlCl3–N-methylformamide (AlCl3–NMF) DES were determined at 303–363 K. Then, types of complex ions and the bonding form in AlCl3–NMF were elucidated by Raman spectroscopy, nuclear magnetic resonance spectroscopy, and quantum chemical calculations. Finally, four chemical reactions between Al species and their thermodynamic data were clarified at 303–373 K. The results indicated that AlCl3–NMF exhibited satisfactory physical properties. Thus, AlCl3–NMF DES could be applied in various fields. The electrochemical study showed the reduction or oxidation behavior of Al (III) or Al. The complex species existing in AlCl3–NMF were identified as AlCl4–, Al2Cl7–, AlCl3L, and Al2Cl6L (“L” stood for NMF). Especially, Al species coordinated with NMF through carboxyl oxygen to generate AlCl2L2+, with an energy of −6.16 eV. Furthermore, [AlCl2NMF2]+[Al2Cl7]− exhibited weaker hydrogen bonding compared with [AlCl2NMF2]+[AlCl4]− in the system. Finally, thermodynamic calculations showed that the formation of the above Al species in AlCl3–NMF was exothermic within the temperature range of 303–373 K.