Temperature Dependence of Interactions between Stable Piperidine‐1‐yloxyl Derivatives and an Ionic Liquid

Abstract

2,2,6,6-Tetramethylpiperidine-1-yloxyl derivatives substituted with either hydrogen bonding [-OH, -OSO(3)H] or ionic [-OSO(3) (-)Na(+), -OSO(3) (-)K(+), N(+)(CH(3))(3)I(-), N(+)(CH(3))(3) N(-)(SO(2)-CF(3))(2)] substituents are investigated in 1-butyl-3-methylimidazolium tetrafluoroborate over a wide temperature range covering both glassy and viscous states. The Vogel-Fulcher-Tammann equation describes the temperature dependence of the ionic liquid viscosity. Quantum chemical calculations of the spin probes at the UB3LYP/6-311(2d,p++) level are done to describe the dependence of the spin density on nitrogen on the substitution pattern of the 4-position of the probe. The results of these calculations are also used to understand the experimental results obtained by applying the Spernol-Gierer-Wirtz theory to analyze the viscosity dependence of the rotational correlation time of the spin probes. Significant differences are found between 2,2,6,6-tetramethylpiperidine-1-yloxyl and its derivatives containing substituents that are able to form hydrogen bonds with the ionic liquid. Moreover, derivatives substituted with ionic groups at the 4-position have a large effect on temperature-induced solvent viscosity, as this is particularly dependent on the nature of the substituent at the 4-position. These dependencies include the temperature region that can be used to describe interactions between the spin probes and the ionic liquid, diffusion into the free volume during non-activated (neutral spin probes) and activated (charged spin probes) processes. Additional parameters are the radii of the ionic liquid and the spin probes, which are calculated and measured approximately. In addition, the temperature dependence of the isotropic hyperfine coupling constants of the spin probes results in information about the micropolarity of the ionic liquid. At room temperature, this is comparable to that of the solvent dimethylsulfoxide.

Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets

“…The mobility of stable nitroxide radical provides information on local viscosity and environment, and this approach is widely used in various fields of science, including 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 studies of ILs. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] However, in most cases previous CW EPR data were obtained at temperatures close to ambient and well above melting temperatures of corresponding ILs, and only a few works addressed the low temperature regions between glass transition (T g ) and melting (T m ) temperatures.…”

“…A number of EPR studies in ILs using spin probes have been accomplished, both in neat ILs and in IL/water mixtures. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] In particular, it was shown that nanostructuring of ILs in water can be monitored via polaritydependent hyperfine splitting. However, in general, the shape of the nitroxide spectrum was found to be not very sensitive to the effects of nanostructuring, therefore developing other types of probes for heterogeneities in ILs would be a topical task.…”

2,2,6,6-Tetramethylpiperidine-1-yloxyl bound to the imidazolium ion by an acetamido group for investigation of ionic liquids

“…Stable radicals, so called spin probes, have been successfully used as model compounds to describe the mobility of reactive species in the ionic liquids. [8][9][10][11][12][13][14][15][16][17][18][19][20] Furthermore, the isotropic hyperfine coupling constants of the stable radicals give information about micropolarity of the surrounding matrix. 13,16,20-22 2,2,6,6-Tetramethylpiperidine-1-yloxyl derivatives are examples for spin probes, which have been successfully applied for the investigation of ionic liquids.…”

“…It is shown that the ionic substituent at the 4 position with respect to the nitroxyl group significantly reduces the mobility of these spin probes in the ionic liquids caused by the additional ionic interactions between the ionic substituent and the individual ions of the ionic liquids. 13,14,16,20 Recently, a pyrrolidine-1-yloxyl derivative covalently bound via a spacer group to the imidazolium ion was described in the literature as further new spin probe for the investigation of ionic liquids. 21 In this Letter, we describe the synthesis of a 2,2,6,6-tetramethylpiperidine-1-yloxyl derivative that is covalently bound to the imidazolium ion.…”

“…13,16,20-22 2,2,6,6-Tetramethylpiperidine-1-yloxyl derivatives are examples for spin probes, which have been successfully applied for the investigation of ionic liquids. 13,14,[16][17][18][19][20] Recently, 2,2,6,6-tetramethylpiperidine-1-yloxyl derivatives have been modified by various substituents to investigate the interactions between model radicals and ionic liquids. [17][18][19][20] Thus, cationic or anionic substituents are bound at the 4 position with respect to the nitroxyl group resulting in additional ionic interactions with either the anion or the cation of the ionic liquid.…”