A Tale of Two Ions:  The Conformational Landscapes of Bis(trifluoromethanesulfonyl)amide and N,N-Dialkylpyrrolidinium

Abstract

The conformational landscapes of two commonly used ionic liquid ions, the anion bis(trifluoromethanesulfonyl)amide (Ntf2) and the cations N-propyl- and N-butyl-N-methylpyrrolidinium, were investigated using data obtained from Raman spectroscopy, molecular dynamics, and ab initio techniques. In the case of Ntf2, the plotting of three-dimensional potential energy surfaces (PES) and the corresponding molecular dynamics (MD) simulations confirmed the existence of two stable isomers (each existing as a pair of enantiomers) and evidenced the nature of the anion as a flexible, albeit hindered, molecule capable of interconversion between conformers in the liquid state, a result confirmed by the Raman data. In the case of the N,N-dialkylpyrrolidinium cations, the PES show a much more limited conformational behavior of the pyrrolidinium ring (pseudorotation). Nevertheless, such pseudorotation produces two stable isomers with the propyl and butyl side chains in completely different positions (axial-envelope and equatorial-envelope conformations). This result was also confirmed by Raman spectra analyses and MD simulations in the liquid phase. The implications of the conformational behavior of the two types of ions are discussed in terms of the solvation properties of the corresponding ionic liquids.

Microscopic Diffusion Dynamics of Silver Complex‐Based Room‐Temperature Ionic Liquids Probed by Quasielastic Neutron Scattering

“…These features can be attributed to the low lattice energy of [Tf 2 N À ] salts because of the highly delocalized anionic structure, as detailed by a number of groups. [16][17][18] The expected structures of these RTILs were confirmed using 1 H/ 13 C NMR and FTIR spectroscopy. The results of the characterization of the RTILs and the details of the QENS experiment are reported in the Experimental Section.…”

Do TFSA Anions Slither? Pressure Exposes the Role of TFSA Conformational Exchange in Self-Diffusion

“…N -butyl- N -methylpyrrolidimium TFSA (BMpyrr TFSA) is the only non-imidazolium IL for which pressure-dependent self-diffusion constants are available, 59 and it is distinguished from the imidazolium salts by the fact that the saturated pyrrolidine ring undergoes its own conformational exchange (pseudorotation) 47 while the planar aromatic imidazolium cations are rigid. (Calculated activation volumes for BMpyrr TFSA are presented in Supporting Information Section 2 Table S22.)…”

Studies on the translational and rotational motions of ionic liquids composed of N-methyl-N-propyl-pyrrolidinium (P13) cation and bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amide anions and their binary systems including lithium salts

“…Although the differences were small, ␣-CH 2 gave the largest c values followed by the ␤-CH 2 in the propyl chain and the ␣-CH 2 ͑4H͒ in pyrrolidinium ring showed slightly faster motions reflected by the pyrrolidinium ring inversion studied by Raman spectroscopy. 43 The activation energies for the three-type protons in four samples given in Table V Fig. 8 and the rates of the motion ͑ϳ1 / c ͑H͒͒ are in the order P 13 -FSAϾ P 13 -FSA-Li ӷ P 13 -TFSAϾ P 13 -TFSA-Li.…”

“…The investigations of the so-called pseudorotation for the five-membered pyrrolidinium ring gave activation energies 2-4 kJ/mol. [42][43][44] Also MD simulations were performed for P 13 -TFSA between 303 and 393 K. 45,46 Various modes of molecular motions coexist in a pyrrolidinium ring. Liquid structures of P 14 -TFSA were studied by the large-angle x-ray scattering technique with the aid of MD simulations.…”