HYSCORE techniques

Use of CW ENDOR techniques to detect P-proton hyperfine couplings and matrix nuclei Pulsed ENDOR techniques to detect P-proton hyperfine couplings and matrix nuclei HYSCORE techniques to detect a-proton anisotropic coupling tensors... 

Advanced EPR techniques such as CW and pulsed ENDOR, electron spin-echo envelope modulation (ESEEM), and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) have been successfully used to examine complexation and electron transfer between carotenoids and the surrounding media in which the carotenoid is located. 

HYSCORE, is a 2D four-pulse ESEEM technique which provides correlation between nuclear frequencies originating from different electron manifolds. The sequence of four microwave pulses is tx/2—x—tx/2—/tx— t2-nl2-x-echo where the echo amplitude is measured as a function of tx and t2 at fixed x. The a-proton anisotropic couplings can be detected by this technique (Konovalova et al. 2001a, Focsan et al. 2008). 

Carotenoid radical intermediates generated electrochemically, chemically, and photochemically in solutions, on oxide surfaces, and in mesoporous materials have been studied by a variety of advanced EPR techniques such as pulsed EPR, ESEEM, ENDOR, HYSCORE, and a multifrequency high-held EPR combined with EPR spin trapping and DFT calculations. EPR spectroscopy is a powerful tool to characterize carotenoid radicals to resolve -anisotropy (HF-EPR), anisotropic coupling constants due to a-protons (CW, pulsed ENDOR, HYSCORE), to determine distances between carotenoid radical and electron acceptor site (ESEEM, relaxation enhancement). 

Despite the important role that EPR plays in the characterisation of the surface properties of these transition metal oxides, virtually all of the published papers have continued to utilise traditional cw X-band EPR even though high frequencies and/or additional EPR techniques (such as ENDOR, HYSCORE or ESEEM) could offer enormous advantages. 

The isotropic g and a values are now replaced by two 3x3 matrices representing the g and A tensors and which arise from the anisotropic electron Zeeman and hyperfine interaction. Other energy terms may also be included in the spin Hamiltonian, including the anisotropic fine term D, for electron-electron interactions, and the anisotropic nuclear quadrupolar interaction Q, depending on the nucleus. Usually the quadrupolar interachons are very small, compared to A and D, are generally less than the inherent linewidth of the EPR signal and are therefore invisible by EPR. They are readily detected in hyperfine techniques such as ENDOR and HYSCORE. All these terms (g. A, D) are anisotropic in the solid state, and must therefore be defined in terms of a tensor, which will be explained in this section. 

Other pulse sequences are in use such as the three-pulse sequence (Figure 3.16) and hyperfine sublevel correlation (HYSCORE) spectroscopy, the latter being a two-dimensional technique.P ]... 

Hyperfine techniques also allow for the probing of the accessibility of the paramagnetic transition metal sites to gases and small molecules. When ammonia is adsorbed to vanadium (VO )-exchanged ZSM-5, HYSCORE features typical of equatorial ammonia ligation to the vanadyl site are observed [141],... 

Almost naturally, CW EPR spectroscopy can also contribute to rmderstanding the electronic properties of polymers that are envisioned for application in the fields of polymer electronics (e.g., [98,99]) and photovoltaics (e.g., [100-102]). Unlike in the smdies highlighted before, in these cases aU materials are EPR active and paramagnetic probes do not have to be added. Going beyond the conventional use of simple CW EPR spectroscopy to study electronic defects, Van Doorslaer, Goovaerts, Groenen, and coworkers have used multifrequency (X-, Q-, and W-band) EPR techniques such as HYSCORE and pulse ENDOR (see Sect. 2.1) to elucidate the extension of polarons in films of electro-active polymers [103]. 

Abstract Multi-resonance involves ENDOR, TRIPLE and ELDOR in continuous-wave (CW) and pulsed modes. ENDOR is mainly used to increase the spectral resolution of weak hyperfine couplings (hfc). TRIPLE provides a method to determine the signs of the hfc. The ELDOR method uses two microwave (MW) frequencies to obtain distances between specific spin-labeled sites in pulsed experiments, PELDOR or DEER. The electron-spin-echo (ESE) technique involves radiation with two or more MW pulses. The electron-spin-echo-envelope-modulation (ESEEM) method is particularly used to resolve weak anisotropic hfc in disordered solids. HYSCORE (Hyperfine Sublevel Correlation Spectroscopy) is the most common two-dimensional ESEEM method to measure weak hfc after Fourier transformation of the echo decay signal. The ESEEM and HYSCORE methods are not applicable to liquid samples, in which case the FID (free induction decay) method finds some use. Pulsed ESR is also used to measure magnetic relaxation in a more direct way than with CW ESR. 

Overlap of lines can make analysis difficult when several nuclei contribute in the one-dimensional (ID) two- and three-pulse ESEEM spectra. Eollowing the development in NMR, methods to simplify the analysis involving two-dimensional (2D) techniques have therefore been designed. The Hyperfine Sublevel Correlation Spectroscopy, or HYSCORE method proposed in 1986 [14] is at present the most commonly used 2D ESEEM technique. The HYSCORE experiment has been applied successfully to study single crystals, but is more often applied to orienta-tionally disordered systems. It is a four-pulse experiment (Fig. 2.23(a)) with a k pulse inserted between the second and the third k/2 pulse of the three-pulse stimulated echo sequence. This causes a mixing of the signals due to the two nuclear transitions with m.s = Vi of an 5 = Vi species. For a particular nucleus two lines appear at (v , V ) and (V ", v ) in the 2D spectrum as shown most clearly in the contour map (d) of Fig. 2.23. The lines of a nucleus with a nuclear Zeeman frequency... 

HYSCORE spectra take longer to record than 2- and 3-pulse ESEEM. As mentioned above, the technique is therefore more commonly applied to orientationally disordered systems, e.g. frozen solutions in chemical and biochemical applications, heterogeneous samples in applications to catalysis and environmental sciences. 

As mentioned in Chapter 2 overlap of lines can make analysis difficult when several nuclei contribute in the one-dimensional (ID) ESEEM spectra. The HYSCORE method is at present the most commonly used two-dimensional (2D) ESEEM technique to simplify the analysis. Contour maps obtained after 2D Eourier transformation of the echo decay signal followed by projection on the frequency plane are mainly employed for visual or computer analysis to obtain the anisotropic hyperfine couplings. Software for the data processing to obtain the contour is often provided with commercial instruments. Tools for ID and 2D Eourier transforms are also available in commercial software like Matlab. 

Separation of interactions allows for precise measurements of the small interactions of the observed electron spin with remote spins in the presence of line broadening due to larger contributions. Such techniques are therefore most useful for solid materials or soft matter, where ESR spectra are usually poorly resolved. The most selective techniques for isolating one type of interaction from all the others are pulsed double resonance experiments, such as ENDOR and electron-electron double resonance (ELDOR), which are discussed in more detail in Chapter 2. If the hyper-fine couplings are of the same order of magnitude as the nuclear Zeeman frequency, ESEEM techniques may provide higher sensitivity than ENDOR techniques. In particular, the two-dimensional hyperfme sublevel correlation (HYSCORE) experiment provides additional information that aids in the assignment of ESEEM spectra. These experiments are also discussed in Chapter 2. 

ENDOR techniques work rather poorly if the hyperfine interaction and the nuclear Zeeman interaction are of the same order of magnitude. In this situation, electron and nuclear spin states are mixed and formally forbidden transitions, in which both the electron and nuclear spin flip, become partially allowed. Oscillations with the frequency of nuclear transitions then show up in simple electron spin echo experiments. Although such electron spin echo envelope modulation (ESEEM) experiments are not strictly double-resonance techniques, they are treated in this chapter (Section 5) because of their close relation and complementarity to ENDOR. The ESEEM experiments allow for extensive manipulations of the nuclear spins and thus for a more detailed separation of interactions. From the multitude of such experiments, we select here combination-peak ESEEM and hyperfine sublevel correlation spectroscopy (HYSCORE), which can separate the anisotropic dipole-dipole part of the hyperfine coupling from the isotropic Fermi contact interaction. 

In addition to CW ESR, ENDOR, and pulsed techniques (HYSCORE spectra, 2D-ESTN, and ESEEM) were also used for dendrimer analysis. The ESR techniques have provided useful information on the dendrimers structure, properties, and applications, and their interactions with selected species. 

The stated aim of this review is to demonstrate that elassical analyses of physieal organie ehemistry are feasible with respect to complex systems such as supported metal catalysts through the application of advanced EMR spectroscopic techniques and determining the relevant spin Hamiltonian parameters via the Zeeman-dependent hyperfine spectrum. The principles of analysis were outlined in the preceding section and entail replicate collection of ESEEM or ENDOR spectra by incremental steps and mapping the trajectory of peak positions. Deconvolution of peaks may be made either by traditional tau-suppression in the stimulated echo pulse sequence or via advanced pulse sequences such as HYSCORE (2-D ESEEM, Hofer, 1994). Mapping of spectral peak position as it varies depending on the Zeeman field is very important to the accurate determination of hyperfine terms. 

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