Hydrogen Bond‐Mediated Local Structure and Far‐Infrared Vibrational Frequencies in Imidazolium‐Based Ionic Liquid

This study investigates the intermolecular interactions and far-infrared (FIR) vibrational spectra of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsul-fonyl)imide ([Emim] <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msup><mml:mrow></mml:mrow> <mml:mo>+</mml:mo></mml:msup> </mml:mrow> <mml:annotation>$^{&#x00026;amp;amp;amp;amp;amp;amp;plus;}$</mml:annotation></mml:semantics> </mml:math> [NTf <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mrow></mml:mrow> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> <mml:annotation>$_{2}$</mml:annotation></mml:semantics> </mml:math> ] <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msup><mml:mrow></mml:mrow> <mml:mo>-</mml:mo></mml:msup> </mml:mrow> <mml:annotation>$^{-}$</mml:annotation></mml:semantics> </mml:math> ). Density functional theory calculations are used to optimize 24 single ion pair structures, while classical molecular dynamics simulations explore the liquid's cluster structure. The results highlight the role of CH···N, CH···O, and CH···F hydrogen bonding in stabilizing the system. Experimental FIR spectra reveal absorption bands associated with hindered translational modes of cations and anions, as well as intramolecular wagging modes of anions, aligning well with computational predictions. The multiple ion pair structures in the solvent model broaden the calculated peaks of the hindered translational modes and separate the two peaks of the wagging modes for the cis- and trans-anions. These findings provide valuable insights into the interaction of IL, enhancing our understanding of their structure for future applications.