Axial spectrum

Complex (77) has been obtained by electrolysis of the respective Ni11 species (i 1/2= —0.78 V vs. Cp2Fe/Cp2Fe+ in DMF) and represents the first Ni111 complex with aliphatic thiolato donors that could be characterized crystallographically.300 The overall square planar structure is retained, but the Ni—N and Ni—S distances shorten by 0.023 A and 0.057 A, respectively, upon oxidation. While (77) is EPR-silent, its pyridine adduct shows an axial spectrum (g = 2.313, 2.281, 2.000) with N-hyperfine coupling of the z-component, indicative of a (r )1 ground state. 

Figure 3.13 EPR absorption curves. (A) Isotropic spectrum, gx = gy = gz (B) Axial spectrum, gx = gy gz. (D) Rhombic spectrum, gx / gy / gz. (Reprinted with permission from Figure 4 of Palmer, G., in Que, L., ed. Physical Methods in Bioinorganic Chemistry Spectroscopy and Magnetism. University Science Books, Sausalito, CA, 2000, pp. 121-185. Copyright 2000, University Science Books.)...

The last class, called rhombic, occurs when all of the -factors differ gx gy gz)- As with the axial spectrum, the principal -factors can be determined from the derivative spectrum by noting the field positions for both the maximum and minimum absorption-shaped signals (gz and gx) at the extremes of the spectrum and the crossover (g,) of the center derivative-shaped signal. 

Because of the properties of PCA, the projection along each principal axis is not totally invariant as only direction is uniquely defined thus, changing the signs of the scores t -t, the reversed axial spectrum is equally valid. The principal axes being three, 2 different spectra of a molecule can be obtained. Therefore, to compare different spectra there are three possibilities ... 

Given two molecules a and b, each axial spectrum is compared by considering the two orientation possibilities for one of the two molecules then, for each orientation, a rigid shift of spectral signals of one molecule with respect to the other one is performed by a defined step (0.1). The maximum value of similarity found by this procedure is taken as the measure of similarity between the two molecules relative to the considered axial spectrum. 

Even more dramatic changes occur in the powder ESR spectra of Cs2Pb[Cu(N02)6] (251) at different temperatures (Figure 40a rmd b) from an isotropic g value at 460 K, to a reversed axial spectrum at 298 K, to a normal axial spectrum at 160 These changes... 

Figure 6 X-band EPR spectra of 6cLS ferric hemes, (a) Type I or large gniax spectrum typical of complexes with mutually perpendicular axial ligands, (b) T5 pe II or normal rhombic spectrum typical of complexes with mutually parallel axial ligands, (c) Type III axial spectrum with large, and (d) small difference between gi and gu. Type III spectra are typical of 6cLS complexes having axial 7r-acceptor ligands such as isocyanides. Figure adapted from reference 27.

Oxidation of the non-haem Fe= " " gives an epr spectrum with prominent peaks at g 8.0 and 5.6 [9]. Trypsin treatment altered the spectrum as shown in Fig.l to a more axial spectrum. The non-haem Fe is photoreduced by 77K illumination in untreated samples but in trypsin modified samples the extent of photoreduction was decreased. 

Calculations of EPR parameters were also performed on some of the complexes. Experimental EPR spectra are either axial (gx = gy-, axial type 1 copper proteins) or rhombic (other blue copper proteins). The results indicate that the geometry is more important than the electronic structure for the rhom-bicity of the spectrum the optimized trigonal structure of Cu(imidazole)2(SCH3)(S(CH3)2) and the crystal structure of plastocyanin both give an axial spectrum, while both the crystal structure of nitrite reductase and the other optimized model of Cu(imidazole)2(SCH3)(S(CH3)2)" give a rhombic spectrum, although the latter structure is mainly n bonded with... 

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