Non-covalent interactions in ionic liquid ion pairs and ion pair dimers: a quantum chemical calculation analysis

Abstract

Ionic liquids (ILs) being composed of bulky multiatomic ions reveal a plethora of non-covalent interactions which determine their microscopic structure. In order to establish the main peculiarities of these interactions in an IL-environment, we have performed quantum chemical calculations for a set of representative model molecular clusters. These calculations were coupled with advanced methods of analysis of the electron density distribution, namely, the quantum theory of atoms in molecules (QTAIM) and the non-covalent interaction (NCI; J. Am. Chem. Soc., 2010, 132, 6499) approaches. The former allows for profound quantitative characterization of non-covalent interactions between atoms while the latter gives an overview of spatial extent, delocalization, and relative strength of such interactions. The studied systems consist of 1-butyl-3-methylimidazolium (Bmim(+)) cations and different perfluorinated anions: tetrafluoroborate (BF4(-)), hexafluorophosphate (PF6(-)), trifluoromethanesulfonate (TfO(-)), and bis(trifluoromethanesulfonyl)imide (TFSI(-)). IL ion pairs and ion pair dimers were considered as model structures for the neat ILs and large aggregates. Weak electrostatic hydrogen bonding was found between the anions and the imidazolium ring hydrogen atoms of cations. Weaker but still appreciable hydrogen bonding was also noted for hydrogen atoms adjacent to the imidazolium ring alkyl groups of Bmim(+). The relative strength of the hydrogen bonding is higher in BmimTfO and BmimBF4 ILs than in BmimPF6 and BmimTFSI, whereas BmimTfO and BmimTFSI reveal higher sensitivity of hydrogen bonding at the different hydrogen atoms of the imidazolium ring.

Local Structure in Terms of Nearest-Neighbor Approach in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: MD Simulations

“…However, as it has been recently highlighted by us for a set of neat C 4 mim + -based ILs and for mixtures with acetonitrile, [17][18] as well as for a similar set of ion pairs considered in simulations of neat…”

“…19 However, as we have shown later in this paper, the mentioned potential energy profile is strikingly different between ILs bearing Cl − anion and multiatomic anions with delocalized charge distribution, such as BF 4 − . 18 All these findings highlight the need for detailed and systematic studies of ILs by means of molecular simulations. Here the term 'detailed' refers to the atomic-level description of the local structure around particularly relevant interaction sites, whereas 'systematic' implies a thoughtful selection of systems to be studied so that the eventual conclusions could be of use for the subsequent design of novel better performing ILs.…”

“…This point has mainly been studied in acetate-based systems, 56,58 however, numerous static quantum-chemical calculations on representative ion pair clusters suggest that this issue is also important in ILs with perfluorinated anions. 15,18,74 As we stick to the idea of the nearest neighbor approach, and try to avoid the use of any arbitrary H-bonding criteria, here we propose to follow the correlations between the distances from a given ring hydrogen atom to H-bond accepting atoms of the nearest neighboring anion. The nearest neighboring anion here was defined with respect to the distance between the ring hydrogen atoms H 2/4-5 and the central atom of the coordinating group of the anion (see above).…”

Influence of Anion on Physical and Electronic Property of Imidazolium Ionic Liquids: Role of Weak Interactions

“…Regardless of the approach employed, most research efforts are made on N, N‐dialkylimidazolium salts and focus primarily on interactions between anion and acidic protons of aromatic ring . The aliphatic C−H⋅⋅X (X= O, N, F etc.)…”

Determining the relative strengths of aromatic and aliphatic C―H⋅⋅⋅X hydrogen bonds in imidazolium ionic liquids through measurement of H/D isotope effects on 19F nuclear shielding

“…One involves the anion as the H‐bond acceptor and the C―H groups on the imidazolium ring as the H‐bond donors and is often referred to as aromatic or primary H‐bonds, while the other, known as aliphatic or secondary H‐bonds, is formed between the anion and C―H groups on the N ‐alkyl sidechains. While weaker, quantum theory of atoms in molecules (QTAIM) calculations have shown that the strengths of aliphatic C―H⋅⋅⋅X H‐bonds are in some cases comparable to those of their aromatic counterparts, suggesting that these interactions also play a central role in the ordering of the local structure of ILs . To date, however, there is virtually no experimental data that can corroborate these theoretical results.…”