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Mobility of spin probes in ionic liquids
Author(s) -
Strehmel Veronika,
Laschewsky André,
Stoesser Reinhard,
Zehl Andrea,
Herrmann Werner
Publication year - 2006
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1072
Subject(s) - ionic liquid , chemistry , microviscosity , hexafluorophosphate , substituent , tetrafluoroborate , ionic bonding , alkyl , inorganic chemistry , crystallography , ion , organic chemistry , biochemistry , membrane , catalysis
The spin probes TEMPO, TEMPOL, and CAT‐1 were used to investigate microviscosity and micropolarity of imidazolium based ionic liquids bearing either tetrafluoroborate or hexafluorophosphate as anions and a variation of the substitution at the imidazolium ion. The average rotational correlation times ( τ ) obtained by complete simulation of the X‐band ESR spectra of TEMPO, TEMPOL, and CAT‐1 increase with increasing viscosity of the ionic liquid although no Stokes Einstein behavior is observed. This is caused by microviscosity effects of the ionic liquids shown by application of the Gierer–Wirtz theory. Interestingly, the jump of the probe molecule into the free volume of the ionic liquids is a nonactivated process. The hyperfine coupling constants ( A iso ( 14 N)) of TEMPO and TEMPOL dissolved in the ionic liquids do not depend on the structure of the ionic liquids. The A iso ( 14 N) values show a micropolarity of the ionic liquids that is comparable with methylenchloride in case of TEMPO and with dimethylsulfoxide in case of TEMPOL. Micropolarity monitored by CAT‐1 strongly depends on structural variation of the ionic liquid. CAT‐1 dissolved in imidazolium salts substituted with shorter alkyl chains at the nitrogen atom exhibits a micropolarity comparable with dimethylsulfoxide. A significant lower micropolarity is found for imidazolium salts bearing a longer alkyl substituent at the nitrogen atom or a methyl substituent at C‐2. Copyright © 2006 John Wiley & Sons, Ltd.