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

Boumediene Haddad; Deepak K Pandey; Dheeraj K Singh; Annalisa Paolone; Mokhtar Drai; Didier Villemin; Serge Bresson

doi:10.1016/j.saa.2023.122325

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

Spectrochim Acta A Mol Biomol Spectrosc. 2023 Apr 15:291:122325. doi: 10.1016/j.saa.2023.122325. Epub 2023 Jan 6.

Authors

Boumediene Haddad¹, Deepak K Pandey², Dheeraj K Singh², Annalisa Paolone³, Mokhtar Drai⁴, Didier Villemin⁵, Serge Bresson⁶

Affiliations

¹ Department of Chemistry, Dr. Moulay Tahar University of Saida, 20000 Saida, Algeria; Chemistry Laboratory of Synthesis, Properties, and Applications (CLSPA-Saida), 20000 Saida, Algeria; Laboratoire de Chimie Moléculaire et Thio-organique, ENSICAEN, University of Caen, 6 Boulevard Maréchal Juin, 14050 Caen, France. Electronic address: haddadboumediene@yahoo.com.
² Department of Basic Sciences, Institute of Infrastructure Technology Research and Management, Ahmedabad 380026, India.
³ Consiglio Nazionale delle Ricerche, Istituto dei SistemiComplessi, U.O.S. La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy.
⁴ Chemistry Laboratory of Synthesis, Properties, and Applications (CLSPA-Saida), 20000 Saida, Algeria; Université DjillaliLiabes, BP 89, 22000 Sidi-Bel-Abbes, Algeria.
⁵ Laboratoire de Chimie Moléculaire et Thio-organique, ENSICAEN, University of Caen, 6 Boulevard Maréchal Juin, 14050 Caen, France.
⁶ Laboratoire de Physique des Systèmes Complexes, Université Picardie Jules Verne, 33 rue St Leu, 80039 Amiens cedex, France.

PMID: 36634492
DOI: 10.1016/j.saa.2023.122325

Abstract

In the present work, four branched methylated, 1,2-dimethyl-3-isopropyl-imidazolium (i-[C₃Dmim⁺]) and protonated,1-methyl-3-isopropyl-imidazolium (i-[C₃mim⁺])-based ionic liquids (ILs) with varying anion (Br^-, BF₄^-, PF₆^-, and NTf₂^-) 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-[C₃Dmim]Y, i-[C₃mim]Y (Y = Br, BF₄, PF₆, and NTf₂) 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-[C₃mim]NTf₂ and i-[C₃Dmim]BF₄, similar energies were obtained in configurations 2 and 3 where the anion is located above the imidazolium ring. For i-[C₃mim]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.

Keywords: Branched ionicliquid; Density functional theory; H-bonding Interaction; Ion pair; Vibrational spectra.