Interactions in 1-ethyl-3-methyl imidazolium tetracyanoborate ion pair: Spectroscopic and density functional study
James X. Mao, Anita S. Lee, John R. Kitchin, Hunaid B. Nulwala, David R. Luebke, and Krishnan Damodaran, “Interactions in 1-ethyl-3-methyl imidazolium tetracyanoborate ion pair: Spectroscopic and density functional study”, Journal of Molecular Structure 1038, 12-18 (2013). https://doi.org/10.1016/j.molstruc.2013.01.046
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Question
Room-temperature ionic liquids (RTILs) built on the tetracyanoborate anion [TCB]- (B(CN)4-) are hydrolytically stable alternatives to BF4- salts, and 1-ethyl-3-methylimidazolium tetracyanoborate ([Emim][TCB]) is attractive for electrolytes, aromatic/aliphatic separations and especially CO2 separation membranes, where it gives high CO2 permeance and CO2/N2 selectivity and roughly 30% higher CO2 solubility than [Emim][Tf2N]. A proposed reason is that weak cation-anion coordination leaves cavities that expand easily to hold CO2. The paper asks what the cation-anion interactions in [Emim][TCB] actually are, how strong they are compared with [Emim][BF4], and whether vibrational spectra reflect them (Mao et al. 2013).
Approach
Gas-phase ion pairs were optimized with B3LYP/6-31+G(d,p) in Gaussian 09, placing [TCB]- at various positions around both the planar (Cs) and non-planar (C1) conformers of [Emim]+. Minima were verified by frequency analysis, energies were corrected for zero-point energy, and interaction energies were corrected for basis set superposition error with the counterpoise method. Atoms-in-molecules (AIM) analysis located inter-ionic bond critical points, a difference electron density map showed charge reorganization on pairing, and Natural Energy Decomposition Analysis (NEDA, based on natural bond orbitals) split the interaction into electrical, core (repulsion) and charge transfer terms. Experimentally, commercial [Emim][TCB] was measured by ATR-FTIR and by Raman with 532 nm excitation, both from 400 to 3500 cm-1, and modes were assigned by visualizing displacements and by potential energy distribution analysis (Mao et al. 2013).
Key findings
The isolated [Emim]+ cation’s non-planar C1 form is 2.3 kJ/mol more stable than the planar Cs form. Four stable ion pair structures were found within about 6 kJ/mol of each other. In the most stable one the anion sits below the non-planar cation, away from the ethyl group, so it can approach the imidazolium ring and maximize hydrogen bonding; the only pair containing the planar cation was the least stable. BSSE-corrected interaction energies of the four pairs were about 280 to 287 kJ/mol in magnitude, much weaker than the roughly 364 kJ/mol reported for [Emim][BF4] pairs (the authors’ own [Emim][BF4] calculation gave about 353 kJ/mol). The authors suggest this weaker binding gives more free volume and hence better CO2 uptake (Mao et al. 2013).
AIM analysis of the most stable pair found four inter-ionic bond critical points with electron densities of 0.005 to 0.011 au, inside the 0.002 to 0.035 au range used as a hydrogen bond criterion, and all four lay in regions where the difference density increases on pairing. In NEDA the electrical (Coulomb-like) term dominated (about 350 to 361 kJ/mol), with charge transfer also important (about 62 to 68 kJ/mol for the non-planar-cation pairs, rising to about 100 kJ/mol for the planar-cation pair, although the total barely changed). Compared with [Emim][BF4] (electrical about 435 kJ/mol), the weaker interaction in [Emim][TCB] comes mainly from the electrical component, consistent with a larger cation-anion distance (Mao et al. 2013).
Scaled (factor 0.958) harmonic frequencies correlated with experiment with a coefficient of 0.9998, and a full table of IR and Raman assignments is given. The strongest Raman band, the anion CN stretch at 2225 cm-1, sits below that of Li, Na, K and NH4 tetracyanoborates (2233 to 2263 cm-1) and essentially matches [Bmim][TCB] (2224 cm-1), indicating weak interaction. The out-of-plane CH bend at 839 cm-1 is red-shifted by 11 cm-1 relative to 850 cm-1 in [Emim][BF4], again pointing to weaker interactions. Raman peaks near 598, 622 and 704 cm-1 show that planar and non-planar [Emim]+ conformers coexist in the liquid (Mao et al. 2013).
Significance
The work links the weakly coordinating character of [Emim][TCB], often invoked to explain its favorable CO2 solubility and conductivity, to computed interaction energies, their electrostatic origin, specific weak hydrogen bonds, and measurable vibrational shifts that can serve as spectroscopic probes of cation-anion strength. It also supplies a complete vibrational assignment for this RTIL (Mao et al. 2013).