EPR spectroscopy of poorly characterized systems: A historical and current view

Abstract

Modeling of the CW-EPR spectra due to transition metal ions in non-crystalline systems grew in sophistication from the early 1960's until fairly recently. A number of important effects have been introduced into the simulation of CW spectra over the past decade or so.These include allowance for local strain effects, the so-called "g-strain" and correlated "g-A strain", the latter especially important in multifrequency spectroscopy. Limits have been reached in what is reasonable by way of guesses regarding the underlying lineshape function and further information can only be sought via ENDOR or ESEEM experiments in favourable cases. A clear understanding of the simplest way to describe field swept EPR via a master equation derived in the frequency domain will be reviewed in which asymmetric field swept fines sometimes occur naturally as a consequence of the theory. The paper will provide a review of the history of computer simulations and various statistical approaches to correlation effects covering examples as diverse as proteins and glasses. It is hoped that sufficient progress will have been achieved with a new Bruker ESP 380 pulsed spectrometer that some new results can also be reported.

Application of the Genetic Algorithm Joint with the Powell Method to Nonlinear Least-Squares Fitting of Powder EPR Spectra

“…As a result, computer simulation combined with fitting appears to be practically the only way of reliable analysis of the experimental powder EPR spectra. A large number of suitable programs of various sophistication levels have been developed in past decades for this purpose. ,,, …”

“…With the advent of the spin Hamiltonian concept in the beginnings, the primary goal in much of EPR practice was a direct measurement of the spin Hamiltonian parameters from the positions of spectral lines in the magnetic field. Over the past two decades the focus has shifted toward computer simulation, the only method suitable for accurately extracting the EPR parameters, especially in the case of randomly oriented powders, glasses, or frozen solutions . Because usually the number of parameters to be optimized is large, single simulations are of rather limited practical value, and automated fitting has become an indispensable component in any proficient analysis of the powder EPR spectrum. , …”

First principles calculation of inhomogeneous broadening in solid-state cw-EPR spectroscopy

“…isotropic liquids or periodic solids. 20 Specifically, we have explicitly sampled the local strain effects, so-called ''g-strain'' and A-tensor anisotropy, by first principles calculations of these quantities from 30 different annealed ab initio MD trajectories. The latter were derived from conventional force-field MD simulations, which yielded a specific distribution of local water : methanol molar ratios around FS.…”

“…Such poorly characterized systems present special challenges in EPR spectroscopy. 20 In order to capture such heterogeneity, and its effects of the inhomogeneously broadened EPR lineshape we have designed a simulation scheme as depicted in Figure 2. 30 ab initio MD trajectories were started at regular time points from a classical MD trajectory.…”

Application of Genetic Algorithm for Extraction of the Parameters from Powder EPR Spectra

“…Paramagnetic samples in the form of randomly oriented powders, glasses or frozen solutions give rise to the EPR spectra that usually are not easily amenable to direct quantitative analysis. In addition, low symmetry environments often encountered in disordered heterogeneous systems can further complicate the spectra [1]. Under such circumstances direct measurements based on the resonance line positions may give incorrect values, and advanced computer analysis and computer simulation are the only method suitable for accurately extracting the parameters [2,3].…”