Spectra at x-band

In order to check further the correctness of this procedure, we used the deduced values for the distribution parameters and the residual linewidth to simulate the experimental spectra at X-band and S-band, using an expression which specifically includes the linewidths dependence on the distribution parameters. An orientation dependent linewidth was used, eq. 5, with Hj,= H. ... 

Figure 3. Values of 6An and 6gn for Cu(II) in polyacrylamide gels as a function of pore diameter, deduced from experimental spectra at X-band, 77 K and at S-band, 120 K.

Relaxation of the electron spins of the exchange coupled trimeric copper cluster octachlorodiadeniniumtricopper(II) was observed58 by measuring the EPR spectra at X-band and 190 GHz as a function of temperature. This was an example of an extra relaxation mechanism available to a cluster compared to an isolated ion. In this complex the three Cu(II) ions are linked in a linear fashion by the bridging chloride ions. The HFEPR spectra at room temperature showed three lines with Q = 2.056, g2 = 2.141 and g3 = 2.204. By 5 K the values were g — 2.027, g2 = 2.135 and g — 2.230. The shift in the values of g, and g3 with... 

Fig. 24. EPR spectra at x-band for some complexes of glutamine synthetase. Reprinted with permission of the American Chemical Society from Villafranca et at. 113).

If two radicals exist with different g-tensors, the spectrum may be partially resolved by going to higher magnetic fields. Dramatic separations have been shown in a paper by Hiittermann et al. [23] comparing spectra at X-band (9.5 GHz) and at 245 GHz using the high field EPR spectrometer at Grenoble. 

ESR spectra at X-band were measured with a RE-1006 spectrometer operating at 9,6 GHz (empty cavity at ambient temperature) and 50-lcHz magnetic field modulation. Spectra were recorded at a microwave power of 1 mW and a modulation amplitude 0.02 mT to avoid line shape distortions, which could arise from experimental conditions such as microwave saturation and overmodulation. Scan range was 150-420 mT, Calibration of g-values is based on diphenylpicrylhydracyl (DPPH, g=2.0036) and Cr (g= 1.9796) standards. The amounts of the paramagnetic species (PMS) were calculated by double integration of the resonance line areas. 

For organic aromatic radicals, the EPR spectra at X-band frequencies (9 GHz) are difficult to distinguish, all of them are in resonance approximately at the free electron g value (2.0023), which indicates that the angular momentum is strongly quenched for these molecules, and a typical line-width of 1-2 mT. They can be characterized by ENDOR spectroscopy but also very efficiently distinguished by their g-tensor anisotropy observable by means of high-field EPR spectroscopy. This is demonstrated (Fig. 4) for two radicals created transiently within the photocycle of photosynthetic purple bacteria... 

Fig. 1 Orientation dependence of nitroxide spin-probe EPR spectra and dynamical effects on line shapes (simulations). Maximum hyperfine splitting A, is observed along the lobes of the singly occupied molecular orbital (j direction). Within the xy plane, spectra at X band frequencies differ only slightly due to g anisotropy. Dynamic spectra are shown for rotational correlation times Tr corresponding to the fast limit (10 ps), slow tumbling (x, = 1/Av=5.25 ns), and the rigid limit (1 ys). (View this art in color at www.dekker.com.)...

Comparative Spin Label Spectra at X-band and W-band... 

The use of a remote-echo detector allows r values shorter flian the spectrometer deadtime to be employed [55]. This is important in two-pulse ESEEM experiments where the deadtime prevents the signal for times r < from being recorded. Also in the deadtime-free four-pulse experiments described in 3.3, a small T value is often needed to avoid blind spots. Bhnd spots are a particular concern for flie measurement of proton spectra at X-band, where flie signals typically extend from 5 to 25 MHz, and with a r = 100 ns blind spots occur at nh = 0, 10, 20,... MHz. 

Analysis of the spectra at different frequencies yielded the parameters D = — 2.20(5) cm-1, = 0.0(1) cm-1, and a nearly isotropic g-factor, g = 1.98(2), none of which could have been determined at X-band. Analysis was aided by the observation of different slopes of the B vs. v plots for Ams > 1 and Ams = 1 transitions. A review of advanced methods, including high-field EPR, is given in ref. 11. Various recent applications of high field and multi-frequency EPR are described in refs 19-31. 

Fig. 12. Electron spin resonance spectra recorded at X-band for powdered clay samples at ambient laboratory temperature (a) montmorillonite, (b) kaolin, (c) Bentonite (Aldrich Chemical), (d) Bentonite (SSP/NF), (e) Magnabrite, (f) Polargel, (g) Volclay, and (h) Fuller s Earth (68).

In this report we analyze the binding of hydrated Cu(ll) in pores of diameter in the range 0.7 to 5.8 nm, based on ESR spectra measured at X-band (9 GHz) and S-band (2.4 GHz), in the temperature range 77 K to 300 K. It will be shown that the results obtained in this study can best be rationalized in terms of cation solvation by water whose properties vary gradually, as a function of the distance from the polymer network. No evidence was observed for the presence of measurable amounts of bulk water in pores of diameter in the range studied. Some preliminary results have already been reported" 5. 

Figure 1. ESR spectra of Cu(II) at X-band and 77 K in chemically cross-linked polyacrylamide gels with pore diameters of 1.3 nm (A) and 4.0 nm (B). Solid lines are experimental spectra dotted lines are spectra calculated using the appropriate values of AH, and given in Table I and with 2.408, gj = 2.080, A,= 0.0134 cm, Aj,= 0.0009 cm, and AH = 30.0 Gauss.

The variation of the line width in one type of gel as a function of the mj value is however markedly different at S-band, compared to that observed at X-band the linewidth corresponding to mj = 1/2 is the narrowest line observed in this system. The linewidths corresponding to the mj = -3/2 and -1/2 transitions at S-band were measured directly from the spectra and are included in Table I, together with the widths measured at X-band. 

Figure 5. ESR spectra of Cu(II) at X-band in chemically cross-linked polyacrylamide gels with pore diameter of 0.7 nm as a function of temperature. The appearance of the quartet centered g-iso is visible above 245 K.

The ESR spectra of some copper complexes of benzotriazole, benzotriazolate, A-methyl-benzotriazole, and 5-nitrobenzotriazole have been described. They show either isotropic g values or indications of axial or rhombic splittings. In no cases are copper hyperfine splittings observed. This indicates that the copper ions in the solid compounds are exchange coupled. The fact that the g values observed at X-band frequency differ from those obtained at Q-band frequency also indicates... 

Applications to Biological Samples. - Methods of distance measurements were compared for four doubly spin-labelled derivatives of human carbonic anhydrase.53 The distances between the spin labels were obtained from continuous wave spectra by analysis of the relative intensity of the half-field transition, Fourier deconvolution of the line-shape broadening, and computer simulation of line-shape changes. For variants with interspin distances greater than 18 A, the DEER method also was used. For each variant, at least two methods were applicable and reasonable agreement between distances obtained by different methods was obtained. The useful distance ranges for the techniques employed at X-band with natural isotope abundance spin labels were estimated to be half-field transition (5-10 A), line-shape simulation (up to 15 A), Fourier deconvolution (8 - 20 A), and four-pulse DEER (> 18 A).53... 

An HFEPR study of polymeric and diamond-like a-C H powder samples16 observed a single resonance line at room temperature for all samples at frequencies up to 189 GHz. At X-band the 3-factor for both types of a-C H was found to be 2.0025(2). The linewidth of the polymeric (diamond-like) sample increased only from 0.9 (0.6) G to 1.65 (1.67) G between 9 and 189 GHz, a similar result to that noted above, but there was no indication of extra structure in the spectra of either type of sample at any frequency. It was suggested that the extra structure observed in films could arise from paramagnetic defects in the substrates. 

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