X-band frequency

The components of a typical EPR spectrometer operating at X-band frequencies [3, 4, 6] are shown in figure Bl.15,4. 

Lau K T, Bar-Chaim N, Ury I and Yariv A 1983 Direct amplitude modulation of semiconductor GaAs lasers up to X-band frequencies Appi. Phys. Lett. 43 11... 

This corresponds to an EPR-silent sample that gives no detectable ESR spectrum at X-band frequencies because it possesses a zero-field splitting larger than the Zeeman interaction (see Chapter 6), and the energy spacing between the two lowest levels is too large to be spanned by a microwave quantum at X-band. Nevertheless, higher frequencies are able to induce transitions. Since... 

For routine studies with the ESR spectrometer, it is most convenient to work at X-band frequencies ( 9.5 MHz or 3 cm). The sample is usually contained in a 4 or 5 mm diameter quartz tube having a sensitive region about 2 cm in length. An alternative frequency is at Q-band ( 35,000 MHz or 1 cm). Here, the cavity dimensions are much smaller and the diameter of the sample tube is less than 2 mm. This creates some problems in handling and degassing powder samples. By varying the frequency it is possible to determine which features in a spectrum are due to Zeeman interactions... 

By far the most important influence of a nuclear spin on the EPR spectrum is through the interaction between the electron spin S and the nuclear spin I. Usually, at X-band frequencies this interaction is weaker, by an order of magnitude or more, than the electronic Zeeman interaction, and so it introduces small changes in the EPR spectrum known as hyperfine structure. As a first orientation to these patterns, note that just like the electron spin S, also the nuclear spin / has a multiplicity ... 

Pairwise magnetic interactions in bioEPR spectroscopy (interactions given in typical decreasing order of strength at X-band frequencies)... 

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... 

Most ESR instruments use the fixed frequency of 9500 MHz (X-band frequency),886 but a number of ESR spectra are measured at 3500 MHz (Q-band frequency),892 which, in the case of misaligned concentrated copper(II) complexes, can resolve the local molecular g values of... 

ESR spectra were recorded on JEOL JES-SRE2X at X-band frequency (9.43 GHz), and g-value was determined by third and fourth signals of Mn02 (1.981 and 2.034) as a standard. Low temperature measurement down to 183 K was performed until the freezing temperature of the solution. 

Fig. 25. Polycrystalline powder ESR spectra of (Co, Zn)Cl2 (Ph3PO)2 recorded at X-band frequency at 4.2K...

As with NMR spectroscopy it is sometimes helpful to measure spectra over a range of frequencies. The most popular second choice is Q-band (ca. 35 000 MHz, i.e., ca. 3.5-times X-band frequencies). In principle, going to higher frequencies should increase the sensitivity, because of the Boltzmann factor, but in practice this is about compensated by the small size of the cavity resonator, and hence the need for small samples. This can, of course, be a great advantage if only small samples are available, as, for example, is often the case if isolated me-tallo enzymes are being studied. The next most popular frequency is S-band (ca. 3000 MHz, or ca. 1 /3 that of the X-band). Sensitivities are lower, but for aqueous systems, using loop-gap resonators, this may not be a serious factor. 

Rotational motion of a nitroxide modulates the anisotropic electron-nuclear magnetic dipolar interaction, giving rise to electron relaxation that affects the EPR spectral line shape. At X-band frequency, the spectra are sensitive to motions with correlation times in the range of 10-11 < rc < 10-7, but also reflect the anisotropy of the motion. Figure 5B shows simulated EPR spectra for isotropic motion in the fast, intermediate, and slow motional regimes and illustrates the high sensitivity of the line shape to motional rate. 

Accurate determination of the g tensors at X-band frequencies can only be achieved provided that a well defined surface is available, as opposed to a polycrystalline powder, and this requires EPR measurements to be performed under ultra high vacuum (UHV) conditions on thin films or single crystals. In this case, the sample can be preferentially aligned with the laboratory magnetic field, so that a given orientahon of 6 is obtained. The orientational dependence of the g tensor can then be systematically probed, and this approach can be far more informative... 

At 9.5 GHz, which is in the X-band frequency range, the magnetic held where the EPR signal should occur for a free... 

In common with all spectroscopic methods, ESR is characterized by a certain characteristic time-scale as determined by the typical energy involved in the transition. For ESR at X band frequency, this window lies between ca 10 and 10 ° s. 

Folybenzimidazole material of the common composition is a (helectric, typically possessing a relative permittivity of 3.5 and a loss tangent of 6.5 x 10 in the X-band frequency region (9.375 GHz). Room temperature resistivities generally are in the vicinity of 10 2 cm. A similar behavior h been reported for N-phenylated polybenz-... 

Both determined at X-band frequency (9.375 GEb ). From Ref. 74 Type/... 

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