An insight into the intermolecular vibrational modes of dicationic ionic liquids through far-infrared spectroscopy and DFT calculations

Abstract

Dicationic ionic liquids (DILs) are a subclass of the ionic liquid (IL) family and are characterized by two cationic head groups linked by means of a spacer. While DILs are increasingly attracting interest due to their peculiar physico-chemical properties, there is still a lack of understanding of their intermolecular interactions. Herein, we report our investigations on the intermolecular vibrational modes of two bromide DILs and of a bistriflimide DIL. The minimal possible neutral cluster of ions was studied as a simplified model of these systems and was optimized at the DFT level. Normal modes of two sandwich-like conformers were then calculated using the harmonic approximation with analytical computation of the second derivatives of molecular energy with respect to the atomic coordinates. The calculated spectra were compared to far-infrared experimental spectra and two groups of peaks over three, for the two bromide DILs, and three over five, for the Tf₂N^- DIL, were described by the proposed neutral cluster model. Therefore, this model represents a reliable and computationally affordable model for the exploration of the intermolecular interactions of this kind of system.

Fig. 1. Structures of the dicationic ionic liquids (DILs) studied.

Fig. 2. Some possible conformers of a minimal neutral cluster of dicationic ionic liquids: syn (top left), anti (top right), open (bottom).

Fig. 3. Comparison between the experimental spectra and calculated normal modes frequencies and intensities. From top to bottom: [BIC₄IB]²⁺ + 2Br⁻, [BIC₅IB]²⁺ + 2Br⁻ and [MIC₆IM]²⁺ + 2Tf₂N⁻.

Fig. 4. Normal modes are represented by displacement vectors (light blue). Relative lengths of the vectors are in scale with respect to the displacement of the single atoms. The absolute scale is arbitrary (but is constant in all the normal modes represented in this paper). Full scale pictures are available as ESI. (a) [BIC₄IB]²⁺ + 2Br⁻syn: normal mode at 126.9 cm⁻¹. The ring rotates about a horizontal axis. (b) [BIC₄IB]²⁺ + 2Br⁻anti: normal mode at 138.6 cm⁻¹. Synchronous rotation of one ring with respect to the other. (c) [BIC₅IB]²⁺ + 2Br⁻syn: normal mode at 69.0 cm⁻¹. Parallel displacement of the top ring with respect to the two bromine anions for [BIC₅IB]²⁺. The distance between the two bromine atoms reduces while they approach the ring. (d) [BIC₅IB]²⁺ + 2Br⁻ anti: normal mode at 82.8 cm⁻¹. The two rings rotated about a vertical axis in opposite directions. This corresponds to a rotation of the bromide anions.

Fig. 5. Normal modes are represented by displacement vectors (light blue). Relative lengths of the vectors are in scale with respect to the displacement of the single atoms. The absolute scale is arbitrary (but is constant in all the normal modes represented in this paper). Full scale pictures are available as ESI. (a) [BIC₄IB]²⁺ + 2Br⁻syn: normal mode at 641.8 cm⁻¹. In phase out of plane bending of the C2–H bonds. (b) [BIC₄IB]²⁺ + 2Br⁻anti: normal mode at 639.8 cm⁻¹. In phase out of plane bending of the C2–H bonds. (c) [BIC₅IB]²⁺ + 2Br⁻syn: normal mode at 624.4 cm⁻¹. Deformation of the top ring. Synchronously the bottom ring shows a similar deformation with smaller amplitude. (d) [BIC₅IB]²⁺ + 2Br⁻anti: normal mode at 651.1 cm⁻¹. Concerted deformation of both rings.

Fig. 6. Normal modes are represented by displacement vectors (light blue). Relative lengths of the vectors are in scale with respect to the displacement of the single atoms. The absolute scale is arbitrary (but is constant in all the normal modes represented in this paper). Full scale pictures are available as ESI. (a) [MIC₆IM]²⁺ + 2Tf₂N⁻syn: normal mode at 498.2 cm⁻¹. Concerted Deformations of the coordinated anions in opposition of phase. There is also a similar, less intense, in phase normal mode at 499.7 cm⁻¹. The calculated intensities differ of about an order of magnitude. (b) [MIC₆IM]²⁺ + 2Tf₂N⁻anti: normal mode at 498.2 cm⁻¹. Deformation of a single coordinated anion. In a very similar normal mode at 500.9 cm⁻¹ the other anion deforms in a nearly identical way.