D-strain modeled as a rhombicity distribution

The zero-field spin Hamiltonian parameters, D and E, are assumed to be distributed according to a normal distribution with standard deviations oD and aE, which we will express as a percentage of the average values (D) and (E). -Strain itself is not expected to be of significance, because the shape of high-spin spectra in the weak-field limit is dominated by the zero-field interaction. 

If the distributions in D and E would be fully positively correlated, then no broadening of the EPR spectrum whatsoever would ensue, because their ratio (i.e., the rhombicity q = E/D) would remain constant, and so the effective -values would not be distributed. On the other hand, a maximal broadening effect would occur in case of full negative correlation oD = -aE. This definition reduces the description of /7-strain to the fitting of a single parameter. 

FIGURE 12.4 S = 7/2 EPR of the [8Fe-7S] P-cluster in Azotobacter vinelandii nitrogenase. The experimental spectrum (trace A) has been simulated in the absence (trace B) and the presence (trace C) of D-strain modeled as a correlated distribution in the zero-field parameters D and E. 

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