posted on 2024-03-18, 10:29authored byAbraham Molla Wagaye, Teketel Yohannes, Getachew Adam Workneh
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.