Structural Chemistry of AlCl₃–<i>N</i>-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-<i>N</i>-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.