Effect of isopropyl side chain branching and different anions on electronic structure, vibrational spectra, and hydrogen bonding of isopropyl-imidazolium-based ionic liquids: Experimental and theoretical investigations.

[Display omitted] • Synthesis of 8 branched Isopropyl-Imidazolium-ILs with Br, BF 4 , PF 6 , NTf 2 anions. • Infrared and Raman spectra of the methylated and protonated ILs are reported. • Vibrational assignments performed and confirmed by DFT calculations. • Electronic structure and hydrogen bonding analyzed by employing DFT calculations and AIM analysis. • Good agreement between experiments and DFT calculations. In the present work, four branched methylated, 1,2-dimethyl-3-isopropyl-imidazolium (i-[C 3 Dmim+]) and protonated,1-methyl-3-isopropyl-imidazolium (i-[C 3 mim+])-based ionic liquids (ILs) with varying anion (Br–, BF 4 –, PF 6 –, and NTf 2 –) were synthesized and investigated by NMR, infrared (IR) and Raman spectroscopy. Based on infrared and Raman spectroscopy, complete vibrational assignments have been performed. The IR and Raman analysis revealed that the vibrational spectra are virtually unaffected upon methylation, while significant frequency changes were observed by changing anion. Furthermore, to determine the electronic structure, energetic stability, and vibrational properties of these i-[C 3 Dmim]Y, i-[C 3 mim]Y (Y = Br, BF 4 , PF 6 , and NTf 2) ion pairs, quantum chemical calculations including the dispersion correction method are performed both on single ions and on ionic couples. The calculated electron density was analyzed to expose non-covalent intra- and interionic interactions by the quantum theory of atoms in molecules (AIM) and interpreted in terms of both anion dependence and type of interaction. Computational results suggest that for all ionic couples the most energetically stable configuration is obtained with the anions located close to the C2 position of the imidazolium cation. However, in the case of i-[C 3 mim]NTf 2 and i-[C 3 Dmim]BF 4 , similar energies were obtained in configurations 2 and 3 where the anion is located above the imidazolium ring. For i-[C 3 mim]Br a stronger hydrogen bond is predicted than for other studied ILs. Calculations indicate that a red shift of the CH stretching bands should occur due to hydrogen bonding; indeed, such displacement of bands is experimentally observed. [ABSTRACT FROM AUTHOR]