Structural and Electronic Insights into 1-Ethyl-3-Methylimidazolium Bis(fluorosulfonyl)imide Ion Pair Conformers: Ab Initio DFT Study

Abstract

An understanding of the nature of molecular interactions among the ion pairs of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide [EMI[FSI]] can offer a starting point and significant insight into the more dynamic and multiple interactions within the bulk liquid state. In this context, close inspection of ion pair conformers can offer insight into the effects in bulk [EMI][FSI] liquid. The current work, therefore, gives a detailed analysis of the [EMI][FSI] ion pair conformers through analysis of the interaction energies, stabilization energies, and natural orbital of the ion pair conformers. The structures of the cations, anions, and cation-anion ion pairs of the conformers are optimized systematically by the ωB97X-D method with the DGDZVP basis sets, considering both the empirical dispersion corrections and the presence of a polar solvent, and the most stable geometries are obtained. The [FSI]^- anions, unlike [TFSI]^- anions, exist at the top position with respect to imidazolium rings. The presence of out-of-plane interactions between the [EMI]⁺ and [FSI]^- ions is in good agreement with the stronger interactions of the [FSI]^- anions with alkyl group hydrogens. The presence of out-of-plane conformers could also be related to the interaction of the anion with the π clouds of the [EMI]⁺ ring. In the [EMI]⁺ cation, the aromatic ring is π-acidic due to the presence of a positive charge in the N₁-C₁-N₂ ring, which leads to the presence of [FSI]^- anion donor [EMI]⁺ π-acceptor type interactions. The [EMI]⁺ cation and [FSI]^- anions tend to form multiple σ* and π* interactions but reduce the strength of the individual contributions from a potential (linear) maximum. For the ion pair [EMI][FSI], the absolute value of the interaction energies is lower than the normal hydrogen bond energy (50 kJ/mol), which indicates that there is a very weak electrostatic interaction between the [EMI]⁺ cations and [FSI]^- anions. The weaker attraction between the [EMI]⁺ and [FSI]^- ions is suggested to contribute to the larger diffusion coefficients of the ions.

Figure 1

[EMI]⁺ cations showing possible initial (input) structures for orientations/configurations that were considered during the optimization of [EMI][FSI] ion pairs.

Figure 2

Atomic numbering scheme employed in the present work: (a) [EMI]⁺ and (b) [FSI]⁻.

Figure 3

Structures of the fully optimized [FSI]⁻ anion and [EMI]⁺ cation: (a) trans-[FSI]⁻, (b) cis-[FSI]⁻, (c) staggered [EMI]⁺, and (d) planar [EMI]⁺.

Figure 4

Optimized geometrical structures of cation–anion pairs in order of their energy relative to the lowest energy conformer (kJ/mol). C₁ (planar cis[EMI]···cis[FSI]), C₂ (staggered nonplanar[EMI]···cis[FSI] anti), C₃ (staggered nonplanar[EMI]···cis[FSI] syn), C₄ (planar cis[EMI]···trans[FSI]), C₅ (staggered nonplanar[EMI]···trans[FSI]), and C₆ (staggered nonplanar[EMI](anti)···trans[FSI]).

Figure 5

[EMI]⁺ cation showing the possible H-bond interaction sites for the [FSI]⁻ anion.

Figure 6

Correlation between the interaction and relative conformer energies of different ion pair conformers of the [EMI][FSI] ion pairs.

Figure 7

Selected natural bond orbital (NBO) interactions between the different conformers of [EMI][FSI] ion pairs in a dielectric continuum medium using the ωB97X-D method and DGDZVP basis set. C₁ (planar cis[EMI]···cis[FSI]), C₂ (staggered nonplanar[EMI]···cis[FSI] anti), C₃ (staggered nonplanar[EMI]···cis[FSI] syn), and C₅ (staggered nonplanar[EMI]···trans[FSI]).