Q-band frequency

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... 

The -tensor anisotropy of the quinone radical-anions is much larger than that of P+ or BChl a +. It can thus often be fully resolved at W-band or even at Q-band frequencies when deuterated samples are employed. An overview of the measured 0-tensor components has already been given by Weber45, and an interpretation of 0-tensors and their usefulness can be found in ref. 131. Significant progress in the calculation of g values of quinone radical-anions has been made, see references 134-139. 

The g-tensor components are usually not resolved for a triplet state of a porphyrin or chlorin molecule at X and Q band frequencies and only the zero field splitting (ZFS) parameters are obtained. This situation changed with the advent of high field/high frequency EPR techniques. The first data of 3P700 and 3Chl a were published by Poluektov et a/. 194 the spectra obtained at D-band are shown in Fig. 5. 

Lee et al used pulsed ENDOR (both Davies and Mims protocols) and ESEEM studies, at both X- and Q-band frequencies, of wild type and altered... 

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... 

The ESR spectrum of a copper(H) complex measured at (a) X-band and (b) Q-band frequency, showing the resolution of the g values of the local molecular Cu chromophore in (b) and (c) in solution29 ... 

For Re2(02CC Hs)4Cl2 the first reduction wave near —0.3 V is quasi-reversible. The product of this one-electron oxidation was not isolated as a solid, but ESR spectra were obtained on frozen dichloromethane solutions at 77°K at both X- and Q-band frequencies. Both naturally occurring rhenium isotopes have a nuclear spin of 5/2 with slightly different nuclear magnetic moments. For dimeric rhenium species there are three distinct isotopic isomers and each generates a unique ESR spectrum as a result of differing hyperfine interactions. The observed ESR spectrum is a superposition of these lines with the intensity... 

Figure 1.14 EPR spectra at X- and Q-band frequencies of surface adsorbed NO2 and NO2 at low temperatures.

In the above description, it was assumed that the nucleus lines up with the applied field in all orientations of the orbital, i.e., the applied field Hq is much stronger than the field due to the electron at the nucleus. In practice, however, this strong field approximation is only a good one when (a) the applied field Po is large (e.g., at Q-band frequencies) (b) the anisotropic hyperfine coupling is small (and so the field at the nucleus is still relatively small) and (c) the isotropic hyperfine coupling is large (since the field due to the electron at the nucleus reinforces the applied field). Often when electrons are confined to p or d orbitals, the... 

ESR measurements are usually recorded at X-band frequencies because of the convenience of the electronic components. Q-band frequency spectrometers are now becoming more widely used for special applications. Other frequencies, e.g., K- and S-band frequencies are also sometimes used. The frequencies, wavelength, and magnetic field of the more common spectrometers are given in Table XVII. The following are recommended volumes on instrumentation (567, 562, 681, 711). 

The protein cytochrome c oxidase, with a monomeric molecular weight of about 100,000, contains two a-type hemes and two copper atoms. A low-spin ferric heme signal and a so-called intrinsic copper signal near g = 2.0 are observed in the frozen solution EPR spectrum of the fully oxidized oxidase. The latter signal shows EPR features which one could assign either to a thiyl radical (R-S ) or to a cupric ion center with an unusually low g value Since no hf structure from copper is resolved at X- or Q-band frequencies, the controversy could not be solved unambiguously from EPR data alone. 

The levels involved in the transition are referred to as a Kramers doublet having effective spin S = 1/2 and an effective g-factor that depends on the fs. The splitting can be obtained by measurements at two microwave frequencies, applying an analytic procedure developed by Aasa et al. several years ago [30]. Values of D for the Fe(III) ESR spectrum of lipoxygenase estimated by measurements of the effective g-values at X- and Q-band frequencies and by analysis of the temperature variation of the signal strength in the range 0.6 < D (cm ) < 3 were reported.The asymmetry... 

Fig, 3. Gd-ESR in a Au single crystal at Q-band frequency. The Gd concentration used in these experiments was 20 ppm. With respect to the externa magnetic field the crystal is rotated in the (101) plane. From the spectrum a positive CF parameter b = 30G can be deduced. From Baber-schke and von Spalden (1979). 

Fig. 23. Gd ESR (2000 ppm) in single crystalline LaSb at Q-band frequency and at various temperatures between liquid helium and room temperature. The spectra were taken with the external magnetic field along the (111] direction. From Urban et al. (1975).

Fig. 57. Magnetic susceptibility, g-shift and Korringa slope as a function of Sc concentration x in Cei jS( Pd3 Gd (a) molar susceptibility of the f-electron at 150 K as a function ofx (b, c) g-shift and Korringa rate vs. Sc concentration as determined at Q-band frequencies. From Gambke et al. (1980).

Figure 10 shows HYSCORE spectra from the Cu(II) N-confused tetra-phenylporphyrin (NCTPP) complex measured at X- and Q-band frequencies [42], The correlation peaks observed in the single-crystal like spectra, measured at g, are assigned to the remote nucleus of the inverted pyrrole. In the Q-band spectrum (Fig. 10b, weak-coupling case) the stronger peaks appear in the first quadrant. 

Numerous studies have appeared in the past two years on the eharacterisation of supported vanadium oxides under statie eonditions. " EPR eharaeterisation at X- and Q-band frequencies was earned out on a fresh and used V205-W03/Ti02 catalyst. In both cases, evidence was found for the existenee of ions in eluster-like arrangements in two different sites, eorresponding to VO + in axially distorted octahedral symmetry ( j = 1.923, = 1.983, Aj =... 

---