The role of the C2 position in interionic interactions of imidazolium based ionic liquids: a vibrational and NMR spectroscopic study

Abstract

Methylation of the C2 position of 1,3-dialkylimidazolium based ionic liquids disrupts the predominant hydrogen-bonding interaction between cation and anion leading to unexpected changes of the physicochemical properties. We found the viscosity of 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [C(2)C(1)C(1)Im][Tf(2)N], for example, to be about three times higher than that of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C(2)C(1)Im][Tf(2)N]. In order to explain these macroscopic changes upon methylation we investigated the vibrational as well as the magnetic resonance structure of [Tf(2)N](-) salts involving the cations 1-ethyl-3-methylimidazolium [C(2)C(1)Im](+), 1-ethyl-2,3-dimethylimidazolium [C(2)C(1)C(1)Im](+), 1-butyl-3-methylimidazolium [C(4)C(1)Im](+), and 1-butyl-2,3-dimethylimidazolium [C(4)C(1)C(1)Im](+) by means of Fourier-transform infrared (FTIR), Raman and (13)C NMR as well as (1)H NMR spectroscopy aiming a better microscopic understanding of the cation-anion interaction. To reveal the impact of methylating the C2 position and changing the alkyl side chain length of the imidazolium a detailed assignment of the individual peaks is followed by a comparative discussion of the spectral features also considering already published work. Our spectroscopic findings deduce electron density changes leading to changes in the position and strength of interionic interactions and reduced configurational variations. Both facts are represented on a macroscopic level by the viscosity and melting point. Therefore changes on a macroscopic level clearly express molecular alterations which in turn can be observed using spectroscopic methods as Raman, IR and NMR.

Palladium–poly(ionic liquid) membranes for permselective sonochemical flow catalysis

“…Based on these two Cl(2p 3/2 ) binding energy environments, the level of surface quaternisation was calculated to be 52 ± 9% (this is most likely to be an underestimate due to the XPS sampling depth (2-5 nm) also probing the sub-surface [55,56]). Infrared spectroscopy of the quaternised membranes did not detect any contributions from characteristic positively charged imidazolium ring absorbances at 1350 cm −1 and 1180 cm −1 [57], thereby indicating that only near-surface quaternisation had occurred (i.e. very low concentration relative to the bulk underlying pulsed plasma deposited poly(vinylbenzyl chloride) layer), Fig.…”

Effects of C(2) Methylation on Thermal Behavior and Interionic Interactions in Imidazolium-Based Ionic Liquids with Highly Symmetric Anions

“…Deionized H 2 O was obtained with a Millipore ion-exchange resin deionizer. MPrImX and MMPrImX (with X = [I − ], [BF 4 − ], and [PF 6 − ]) were synthesized as follows. A mixture of 1-methylimidazole (7.78 mL, 100 mmol) and propyl iodide (9.74 mL, 50 mmol) was heated at 120 • C for 3 min.…”

“…The list of investigated anions includes bis(trifluoromethylsulfonyl) imide [4][5][6][7], nitrate [8], alkyl sulfates [5,9], dicyanamide [5,10], thiocyanate [5], acetate [11,12], hydrogen sulfate [13,14], and trifluoromethanesulfate [15,16]. On the other hand, interactions with highly symmetrical anions have been studied less.…”

“…On the other hand, Hunt's reduced entropy theory [28] argues that the methyl group at the C(2) position blocks low energy configurations. Noack et al [4] showed that electron density changes can alter the position and strength of interionic interactions and lead to reduced configurational variations. Izgorodina's dynamic theory [29] suggests that the free-energy barrier for the anion to move relative to the imidazolium cation increases the viscosity.…”

“…To study the effects of the C(2) position, it has commonly been analyzed in its protonated and methylated forms [4,8,19,[25][26][27]. The methylation at C(2) basically removes the possibility to form hydrogen bonds with the anion at this site.…”