Simulation of Strain Effects in EPR Spectra
Strain-induced line broadening is often observed in EPR spectra. This line broadening is due to
distortions in the local structure around the radical center resulting in a distribution of spin
Hamiltonian parameters. The X-band EPR spectrum of Rhenium(VI) in the polyoxoanion,
[P2ReW17O62]-6, shown in the top figure , shows remarkable changes in linewidth. The
linewidth varies from 25 Gauss at lowest fields to more than 120 Gauss at highest fields. Clearly
for this rhenium system, the linewidth is nearly completely dominated by strain effects, and
simple MI and MI2 dependant linewidth expressions are deemed inappropriate for a spectrum
which is not first-order. As can be seen our simulation reproduces extremely well both the line
positions and linewidths in the EPR spectrum. The two peaks marked with asterisks are
"forbidden" mI = 5/2 to 3/2 and mI = 3/2 to 5/2 transitions. The spacing between these two peaks
is direct measure of the nuclear quadrupole coupling constant and is equal to 8QD/geff. The
lower figure, which compares the simulation with and without the strain broadening,
demonstrates graphically the marked field dependence of the strain-induced line-broadening.
This linewidth variation can be readily ascribed to random strains or distortions of the rhenium
ion complex with a resultant distribution of spin Hamiltonian parameters, particularly in the g-tensor. Strain parameters were determined not only for g and A, but also for the nuclear
quadrupole tensor, which is a direct measure of the change in electric field gradients at the metal
nucleus due to strain. Simulation and fitting of electron paramagnetic resonance spectra used the
automated simulation program SIMPIPM which is a special version of SIMPIP that includes
strain-broadening. The spin Hamiltonian
= {ßBgS - ßngnBI} + hSAI + hIPI + hSDS
is solved by exact diagonalization, followed by a fourth-order field-frequency perturbation to
transform the energy spectrum into a field-swept spectrum. The spin Hamiltonian parameters are
varied using the SIMPLEX method to minimize the RMS deviation between experimental and
calculated spectra. Strain effects were included by calculating exact analytical gradients of the
eigenvalues of the spin Hamiltonian. The fourth-order frequency perturbation (was used to
transform the frequency-swept spectrum to a field sweep one and is also used applied to the
calculation of the analytical gradients.
All images and text copyright 2002 M. J. Nilges
Last Updated on March 27, 2002 by M. Nilges