Comprehensive Conformational and Rotational Analyses of the Butyl Group in Cyclic Cations: DFT Calculations for Imidazolium, Pyridinium, Pyrrolidinium, and Piperidinium

Abstract

The flexibility and conformational variety of the butyl group in cations of ionic liquids (ILs) play an important role in dictating the macroscopic and microscopic properties of ILs. Here we calculate potential energy surfaces for the dihedral angles of the butyl group in four different types of cyclic cations, imidazolium, pyridinium, pyrrolidinium, and piperidinium, using the density functional theory method. The calculation results highlight the role of the butyl group in these cations by comparison of five-membered and six-membered rings, and of aromatic and alicyclic rings, in terms of stable conformations and rotational barriers. A striking result is that the butyl group rotation in pyrrolidinium induces pseudorotation of the ring whereas such a phenomenon does not occur in piperidinium. This difference is thought to be because of the relationship in rotational activation energy between the butyl group (10-40 kJ mol) and the ring (<6 kJ mol for pyrrolidinium and 40-50 kJ mol for piperidinium). The calculated stable conformers are compared with the ones observed in crystals recorded in the Cambridge Structural Database. Although conformers with lower calculated energy generally have higher chances to be experimentally observed, roughly independent of the cation structure, some calculated conformers deviate from this trend and show very low population. It is found that not only low energy but also high rotational activation energy (i.e., long lifetime) is required to observe conformers in crystalline states. In the last part of this article, to identify conformers in real systems, the applicability of the calculated Raman bands of cations with different butyl group conformations is discussed.

Evidence of a Fundamental Mechanism Governing Conductivity Relaxation in Room-Temperature Ionic Liquid

“…Nevertheless, the first type of motion can be immediately excluded as the origin of β-relaxation due to the very low activation energy (E a DFT = 12 kJ/mol). 18 On the other hand, to recognize the contribution of butyl chain rotation in secondary dynamics the high-pressure measurements are required. In general, τ β is pressure independent for intramolecular alkyl chain motions.…”

“…According to DFT calculations available in the literature there are at least two possible conformational changes within the [Pyr] + : motions of the nonaromatic ring and the butyl side group. Nevertheless, the first type of motion can be immediately excluded as the origin of β-relaxation due to the very low activation energy ( E a DFT = 12 kJ/mol) . On the other hand, to recognize the contribution of butyl chain rotation in secondary dynamics the high-pressure measurements are required.…”

“…In this work, we employed broad-band dielectric spectroscopy (BDS) to examine the ion dynamics in supercooled and glassy states of 1-butyl-1-methylpyrrolidinium BMPyr-based aprotic RTILs containing a charged nitrogen atom and butyl side group in the cation that provides a balance between Coulomb and van der Waals interactions . The four different anions, bis­(fluorosulfonyl)­imide (FSI), bis­(trifluoromethylsulfonyl)­imide (TFSI), tricyanomethanide (C­(CN) 3 ), and dicyanamide (N­(CN) 2 ), were chosen to vary the strength of the electrostatic interaction between the cation and the anion.…”

“…In this work, we employed broad-band dielectric spectroscopy (BDS) to examine the ion dynamics in supercooled and glassy states of 1-butyl-1-methylpyrrolidinium BMPyr-based aprotic RTILs containing a charged nitrogen atom and butyl side group in the cation that provides a balance between Coulomb and van der Waals interactions. 18 Methods. BDS.…”

Effect of the cation structure on the properties of homobaric imidazolium ionic liquids

“…A good example of this are the somewhat unexpected oddeven effects with increasing length of alkyl side chains. [23][24][25] However, if the energetic barriers that separate these conformers are too high, then the liquid essentially behaves like a mixture of differently shaped but rigid molecules. It is thus important to consider these energetic barriers as well.…”