X-band spectrum

When one of the Fe-coordinating Ns of the porphyrin is made inequivalent to the others, for example, by pulling on it, or by putting a protein structure around the cofactor, then the molecular x axis and y axis become inequivalent, and the axial EPR spectrum turns into the rhombic spectrum in trace d with derivative trace e (see also Table 5.4). There are now three features in the spectrum a peak, a zero crossing, and a negative peak, and their field positions closely (exactly for zero linewidth) correspond to those of the g-values, gx, gy, and gz. Finally, in trace f of Figure 5.4, which is the experimental X-band spectrum of cytochrome c, it can be seen that not only the g-value (peak position) but also the linewidth is frequently found to be anisotropic. This extra complication will be discussed extensively in Chapter 9. 

FIGURE 5.8 Complex hyperfine patterns due to axes noncolinearity in a low-symmetry prosthetic group. The X-band spectrum is from 65Cu(II)-bicarbonate in human serum transferrin (a,b) experimental spectrum (c,e) simulation assuming axial symmetry (d, f) simulation assuming triclinic symmetry with the A-axes rotated with respect to the g-axes over 15° about the gz-axis and then 60° about the new y -axis. Traces b, e, and f are 5x blow-ups of traces a, c, d, respectively (Hagen 2006). (Reproduced by permisson of The Royal Society of Chemistry.)... 

FIGURE 5.12 Manganese as a common contaminant in protein EPR. This X-band spectrum is characteristic for high-spin Mn(II) aspecifically bound to proteins. 

To find out what the X-band spectrum of such a system will look like, let us now complete the energy matrix with the Zeeman interaction using all the spin-operations written out in Equations 7.48a to 7.48m ... 

The very first EPR pattern ever analyzed in terms of exchange interaction is the X-band spectrum of a crystal of copper acetate hydrate (Bleaney and Bowers 1952), whose stoichiometry is written as Cu2(CH3C00)4 2H20 because it forms dimers of Cu(II) bridged by four carboxylato ligands and with a water molecule at each end of the dimer (van Niekerk and Schoening 1953). The powder of copper acetate is a cheap, stable, and easy to measure example compound for exchange interaction. 

Fe—2S] clusters cysteines, 38 294 P-band spectrum, 38 191-192 X-band spectrum, 38 189 [2Fe—2S] clusters EPR spectroscopy, 38 187 excess electron localization, 38 179-180 [3Fe-4S] clusters, 38 16-24, 119-120 conversion reactions, 38 17-18 D. africanus Fd III, 38 138-144 electronic structure, 38 19 electron transfer series, 38 18-19 formation, oxidation and spin states, 38 17-22... 

Frozen-solution ESR spectra of Tc2G in mixed aqueous hydrochloric acid and ethanol provided data consistent with equal coupling of the unpaired electron to both technetium nuclei (101). IsotopicaUy pure "Tc (/ = 9/2) in 99Tc2Cl leads to a large number of lines in the X-band spectrum owing to second-order effects, in addition to the hyperfine lines presence for this dimeric axially symmetric system. The Q-band spectrum obtained at 77°K with a microwave frequency of 35.56 GHz exhibited fewer lines, and computer-simulated spectra were generated to correspond to the experimental spectrum withgit = 1.912, gi = 2.096, An = 166 x 10 4 cm"1, IAL = 67.2 x 10 4 cm 1, and gav = 2.035. 

The changes of the X-band spectrum over the temperature range 4-120 K at the stationary magnetic field HII (1 0 0) (C4) axes are shown in Fig. 1. The parameters of the spin Hamiltonian describing the spectrum are provided in Table 1. 

Fig. 1. Temperature dependence of the single crystal X-band spectrum of ZnZrF6 6H20 Cu2+ with //II (1 0 0)(C4).

Molybdenum is present in many enzymes and is often EPR detectable (except for Mo-nitrogenase, where it exists in a complex MoFeySg homocitrate cluster). Shown in Figure 9 is the Mo " " (f) X-band spectrum of one form of xanthine oxidase, called the Very Rapid form. Xanthine oxidase is a homodimer containing one Mo center, two Fe-S, centers and one molecule of flavin adenine dinucleotide per subunit. This enzyme catalyzes the oxidation of xanthine at the Mo center with subsequent reduction of O2 at the flavin center. In this enzyme Mo + exhibits a slightly rhombic EPR spectrum (g = 2.0252, gj, = 1.9550, g = 1.9495 gave < 2). 

Fig. 3.15 X-band spectrum (—) of an X-irradiated tooth measured at X-band, v = 9,762.1 MHz. The simulated spectrum (+) was obtained with gj = 2.0023 and g = 1.9970...

The X-band spectrum of the NO/Na-LTA zeolite system is mainly due to the NO mono-radical when the pressure is low (Pno <0.1 kPa), while the (NO)2 bi-radical becomes dominant at higher NO pressure (Pno > 10 kPa) [24, 36]. The ESR signals due to the NO mono-radical (NO-Na" complex) and the (NO)2 bi-radical are superimposed at intermediate pressures. The 2-band ESR spectrum helped very much to resolve the individual spectrum and to evaluate the accurate ESR parameters of the (N0)2 bi-radical. As shown in Fig. 6.5, the Q-band spectral line-shape is well simulated using the following g tensor and the D and E parameters of the zero field splitting (ZFS) tensor for the (N0)2 bi-radical (gxx, gyy, gzz) = (1.9120, 2.0042,... 

Fig. 6.15 Experimental (a-c) and simulated (a -c ) ESR spectra of the N-containing paramagnetic species in N-TiOi. experimental and simulation, (a, a ) X-band spectrum of the species containing and isotopes (70 and 30%, respectively), (b, b ) X-band spectrum of the species containing (99.63% in natural abundance), (c, c ) Q-band spectmm of the species containing The spectra are adapted from [104] with permission from the American Chemical Society...

The local structure of iron sites in Fe-mazzite and Fe-ZSM-5, in which iron was incorporated during zeolite synthesis, was studied by X- and Q-band ESR, electron spin echo detected ESR (ED-ESR), electron spin echo envelope modulation (ESEEM), and diffuse reflectance UV-vis [94G1]. The X-band ESR spectra of Fe-MAZ (100 Fe/(Fe + A1 + Si) = 1.20) render three signals at g = 4.3, g = 2.3, and g = 2.0 - Table 14a. The Q-band spectra testifies only the signal atg = 2.0. The linewidths of the g = 2.0 signals are smaller in the Q-band spectra - Table 14. This narrowing indicates that the linewidth is at least partially due to the second-order broadening of the -l/2> l/2> transition. The X-band spectrum of Fe-MAZ with 100 Fe/(Fe + A1 + Si) = 0.07 exhibits the... 

Figure 10. HYSCORE spectra of the remote " N of Cu(II)NCTPP diluted in ZnTPP powder, measured at the field positions indicated by the arrows in the field-swept spectra (upper part of figure), (a) X-band spectrum m.w. fi-equency, 9.7 GHz Vhn = 0.9 MHz i = 100 ns. (b) Q-band spectrum m.w. frequency, 35.6 GHz vi4n = 3.6 MHz i = 100 ns. All interactions are given in MHz. Modified with permissitm from [42]. Copyright 2005, Wiley-VCH.

Figure 15. EPR spectra of the high-spin Fe-NO complex of soybean LOX-1 at two frequencies. The soybean lipoxygenase (LOX-l)-nitric oxide complex (3.2 mM in 0.1 M potassium phosphate, pH 7.0) was sealed under argon in a quartz EPR tube of 0.7 mm ID. Spectra were recorded 9.26 GHz and 94 GHz, and the temperature was 6 K. The original 94 GHz spectrum was scaled to the same g-value scale as the 9.26 GHz spectrum, and the magnetic field units are shown for the X-band spectrum. Because of the scaling, the high-fiequency spectrum is labeled in the figure as if it was recorded at 92.6 GHz, and the corresponding magnetic field scale would be ten times that shown.

Ni (31)(R2acac)]+ gives an X-band spectrum at 4 K which is simulated as an S = 3/2 state, thus indicating ferromagnetic coupling between metal and ligand. The g(-values found are consistent with those expected from ... 

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