Frequently, there are reports of non-symmetric, anisotropic EPR spectra in fluid solution. Basically, if the molecules tumble perfectly in the solution any molecular magnetic anisotropy will be averaged out and a symmetric spectrum should result. Nevertheless, people pretend that they obtained non-symmetric spectra in solution. They claim e.g. non-homogeneous line broadening to be the reason for this.
What do you think, dear colleagues? Are these EPR spectra flawed through low viscosity, non-homogeneous line-broadening, high concentrations (and precipitates), ... or are there real cases of anisotropic spectra in solution?
Guessing that fluid solution is at room temperature. So assuming that if there is anisotropic spectra in solution, viscosity does really matter, e.g. NO radicals in viscous solution such as glycerol. Tumbling of NO radicals in glycerol is still in slow-motion regime, and if solution concentration is increased, the spectrum is further broadened inhomogeneously (I know this from experiments). Hyperfine lines of 14N of nitroxide is quite anisotropic.
Hope this will helpful.
Acquisition parameters matter, too. I've seen many examples of asymmetric EPR spectra "fixed" by changing scan rate, etc.
As the question is very general - I would say yes. Just imagine the case where the paramagnetic electron is situated in large anisotropic, dipolar molecule. Then nematic or columar order of the molecules could occur partially, depending on outer parameters - just like LCDs. Perhaps even the standing wave in the resonator could partially order dipolar molecules. At high temperatures, this should not really be a strong effect but perhaps at lower.
Dear all, I am delighted to get such qualified answers. If I may summarize:
1) there might be examples of non-symmetric = anisotropic EPR spectra in fluid solution, provided that the spin resides on a molecule with a highly anisotropic shape, which orientates in the magnetic field (and can thus not freely tumble in solution). Martin, any examples?
2) the idea by Andrei is interesting. However, also here, the ligand exchange (strong case) or ligand solvatation (weaker case) must lead to molecular (!) anisotropic behaviour, which prevents the free tumbling of the molecules. I don't see at the moment how such behaviour of individual molecules could be translated into a bulk phenomenon of a bunch of molecules (an ensemble of molecules). Also here, an example would be helpful.
3) probably for most examples a better parameter set, lower viscosity (higher fluidity) of the solvent or surrounding, or lower concentration of the paramagnetic species would have rendered the spectra isotropic. Thanks, Amgalanbaatar and James. Of course, for NO radicals this inhomogeneous line-broadening in viscous solutions or surroundings can be used to study dynamics, e.g. in biological material. However, if the same paramagnetic molecules were measured under ideal conditions, the spectra were isotropic, right?
best
AXEL
Yes, the spectra would be definitely isotropic, for example, TEMPO in toluene.
the point (1) is obviously interesting. Like spin liquids in frustrated magnetism? But such that requires a large spin-orbit coupling, so is broadened spin-lattice interaction? Spin-lattice interaction is much larger in liquids so relaxation is shorter.
What about the measurements with Q-band or W-band at diferent temperatures ?
Very good remark Ana. I guess the rotational correlation times depend not only on the shere size but largely on the shape of a large molecule. From this I conclude that it should be possible to use this effect to measure the shape of molecules, comparable to the use of diffusion NMR. Do you know an example for this?
Saludos a Santa Fé
It's good that you are listening to Dr. Genarro. The whole question revolves around the rotational correlation time, which depends on viscosity and on the size of the probe. And you are correct, Alex; the correlation time itself becomes anisotropic when either the molecule is grossly non spherical, or the environment is anisotropic. See for example the many studies of correlation times of spin labeled proteins in lipid bilayers.
Potential artifacts include binding of magnetic species to the walls of the sample tube.
In principle, even for isotropic rotation the anisotropic parameters can be determined by measuring the relaxation times at different wavebands. Of course this measurement is not easy. The basics mathematics is given our paper:
A. Rockenbauer and P. Simon
Ambiguity Problems of the Spin Hamiltonian
J. Magn. Resonance 22, 243-263 (1976)
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