A model-free
approach for simulation of EPR spectra of nitroxide
spin probes in liquid-crystalline materials was suggested and used
to obtain parameters of molecular orientation and rotational mobility.
The developed method is based on experimental recording and numerical
simulation of the angular dependence of EPR spectra, which is shown
to be much more informative in comparison with a single EPR spectrum.
Quantitative spectral simulations considering both local orientational
ordering and distribution of local directors in the sample were used
for discrimination of models of rotational mobility and orientational
alignment. The method was applied for detailed quantitative characterization
of axial, orthorhombic, and low-symmetry non-orthorhombic molecular
orientation distributions. It is shown that the ordinarily used model
of rotational diffusion in a mean-field potential is suitable for
the description of molecular mobility and orientational ordering only
for relatively low sample temperatures and low-mobility probe molecules
with large sizes. In cases of high molecular mobility, the more realistic
jump mechanism of molecular moves can be approximately described as
quasi-librations. For ordered liquid crystals it was found that mostly
the order parameters up to rank 12–14 are essential and easily
determined. When well-aligned materials are described, the order parameters
up to 18th rank or even higher become meaningful. Both molecular and
sample biaxiality is analyzed and quantitatively characterized. The
local molecular ordering and sample orientational alignment are quantitatively
characterized separately.