Anion and cation dynamics of sulfonylamide-based ionic liquids and the solid–liquid transitions

Abstract

Some of the important factors that characterise room-temperature ionic liquids (RTILs) are the variety of conformations adopted by the constituent ions and their flexibility. Using 1,3-dimethylimidazolium bis(fluorosulfonyl)amide ([C1mim][FSA]) and 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)amide ([C1mim][NTf2]) as samples, the longitudinal and transverse relaxation times (T1 and T2) for (19)F and (1)H were determined as a function of temperature and were correlated with the dynamics of the phase behaviours of the two RTILs. Because the anions and cations in the two compounds have (19)F and (1)H nuclei, respectively, their dynamics can be independently investigated and the relationships between them can be discussed. For [C1mim][FSA], the only observed phase changes included melting and crystallisation. The temperature dependences of T1 and T2 for (19)F were similar to those of T1 and T2 for (1)H, indicating similar dynamics due to the formation of strong anion-cation interactions. For [C1mim][NTf2], the T1 and T2 values for both (19)F and (1)H discontinuously changed at same temperatures, which were assigned to the crystallisation and melting points. However, the T1 curves for (19)F and (1)H were different in the crystalline region, suggesting independent dynamics for the anions and cations in [C1mim][NTf2]. In the crystalline state for each salt, the cation dynamics was distinctly separated into the framework movement of the imidazolium ring and the movement of the methyl groups, while the anion dynamics was characterised by the movement of the entire anion. The influence of the crystal structure on the dynamics of each salt was also considered.

Anions as Dynamic Probes for Ionic Liquid Mixtures

“…For instance, for [C 1 mim][TFSI], the E a rot estimated from the linear part of the proton T 1 curve (from 233 to 293 K) at low fields in the crystalline state was approximately 8.7 kJ mol −1 and was associated with the rotational motion of the methyl group. 43 An E a rot of 12.5−15.1 kJ mol −1 was also found for the crystalline states of [C 4 mim][PF 6 ] and assigned to the δ-CH 3 rotation. 59 In ILs composed of N-methyl-N-propyl-pyrrolidinium (P 13 ) with two anions, TFSI and FSI, the three-type protons have similar activation energies (ca.…”

“…However, the order of magnitude found here for the activation energies are in line with data from the literature. For instance, for [C 1 mim]­[TFSI], the E a rot estimated from the linear part of the proton T 1 curve (from 233 to 293 K) at low fields in the crystalline state was approximately 8.7 kJ mol –1 and was associated with the rotational motion of the methyl group . An E a rot of 12.5–15.1 kJ mol –1 was also found for the crystalline states of [C 4 mim]­[PF 6 ] and assigned to the δ-CH 3 rotation .…”

“…The spin–lattice (longitudinal, T 1 ) and spin–spin (transverse, T 2 ) relaxation times of nuclear spins are sensitive to both intra and intermolecular relaxation mechanisms and can be used to probe the rotational dynamics of the system. Several NMR active nuclei such as 1 H, 13 C, 19 F, 31 P, 14 N, 35 Cl, 2 H, and 7 Li have been used to characterize pure ILs as well as their mixtures with water and lithium salts. − ,…”

A review of NMR methods used in the study of the structure and dynamics of ionic liquids

“…It has been observed by the same research group that the magnitude of the T 1 and T 2 relaxation time are similar for both the 19 F and 1 H nuclei indicating the similar dynamics for both the ions of the [MMIM][FSA]. However, the relaxation times are different in the case of [MMIM][NTf 2 ] suggesting that their cation and anion dynamics are different from each other . The temperature dependent 14 N spin‐lattice (R 1 ) and spin‐spin (R 2 ) relaxation rates of a series of >cyclic pyrrolidonium and tetra‐alkyl acyclic ammonium‐based ILs has been studied by Alam et al .…”

“…[BMIM][Br] appears to exist in three different states that are the coagulation, liquefaction, and the crystallization state that has been detected from the calorimetric measurements and are supported by their temperature dependent relaxation studies . The temperature dependent dynamics of 1,3‐dimethylimidazolium bis(fluorosulfonyl)amide ([MMIM][FSA]) and 1,3‐dimethylimidazolium bis(trifluoromethylsulfonyl)amide ([MMIM][NTf 2 ]) has been investigated by using 19 F and 1 H NMR relaxation data . It has been observed by the same research group that the magnitude of the T 1 and T 2 relaxation time are similar for both the 19 F and 1 H nuclei indicating the similar dynamics for both the ions of the [MMIM][FSA].…”