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Published on: December 20, 2016
Rotation of a Charged Spin Probe in Room-Temperature Ionic Liquids
Jakov Slade1, Dalibor Merunka1, Ezequiel Huerta2
1Division of Physical Chemistry, Rud̵er Bošković Institute, Bijenička cesta 54, Zagreb HR-10000, Croatia.
Electron paramagnetic resonance reveals how a charged nitroxide probe (Cat-1) rotates in ionic liquids. Its motion is anisotropic and influenced by solvent structure, transitioning from globular to sponge-like at longer alkyl chains.
Area of Science:
- Physical Chemistry
- Materials Science
- Spectroscopy
Background:
- Understanding molecular dynamics in room-temperature ionic liquids (RTILs) is crucial for their application.
- Rotational diffusion of probe molecules provides insights into the microenvironment of RTILs.
- Charged nitroxide probes offer a unique perspective due to their electrostatic interactions.
Purpose of the Study:
- To investigate the rotational diffusion of a positively charged nitroxide (Cat-1) in various RTILs.
- To correlate probe rotation with the structural properties of RTILs based on alkyl chain length.
- To explore the applicability of free volume theories in describing molecular rotation within RTILs.
Main Methods:
- X-band electron paramagnetic resonance (EPR) spectroscopy was employed to study Cat-1.
- Measurements were conducted across a temperature range in 1-alkyl-3-methylimidazolium tetrafluoroborate RTILs (C2-C8 alkyl chains).
- Rotational correlation times were analyzed using Stokes-Einstein-Debye and Cohen-Turnbull free volume theories.
Main Results:
- Cat-1 exhibits anisotropic rotation, with the preferential axis along the NO radical.
- Rotational dynamics are solvent-dependent, showing slip boundary conditions and non-Arrhenius behavior.
- A transition to a sponge-like RTIL structure was observed with increasing alkyl chain length (≥4 carbons), affecting probe rotation.
Conclusions:
- The study demonstrates the utility of EPR spectroscopy for probing RTIL structure and dynamics.
- Cohen-Turnbull free volume theory successfully describes the rotational behavior of Cat-1 across all studied RTILs.
- The findings highlight the influence of RTIL microheterogeneity on solute rotational diffusion.
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