Pulse sequence spin-echo ENDOR

This limitation is not there for pulsed ENDOR methods, which can be used at all microwave frequencies. In ENDOR, the sample is irradiated with a combination of microwaves and radio waves. Continuous-wave ENDOR was already introduced in 1956 by Feher [1] and for a long time remained an important tool to determine the hyperfine and nuclear quadrupole interactions. However, nowadays this technique is largely replaced by the pulsed counterparts, which are more versatile. Two of the most commonly used ENDOR pulse sequences are Davies ENDOR [18] and Mims ENDOR [ 19]. In these techniques a combination of microwave pulses and a n radio frequency (RF) pulse with variable RF is used. A first set of microwave pulses creates electron polarization. When the RF matches one of the nuclear transitions, the populations of the different energy levels will be affected. This will change the electron polarization that is read out by a last sequence of microwave pulses, usually via electron spin echo detection as a function of the radio frequency. In this way, the nuclear frequencies can be directly detected. 

ESEEM is a pulsed EPR technique which is complementary to both conventional EPR and ENDOR spectroscopy(74.75). In the ESEEM experiment, one selects a field (effective g value) in the EPR spectrum and through a sequence of microwave pulses generates a spin echo whose intensity is monitored as a function of the delay time between the pulses. This resulting echo envelope decay pattern is amplitude modulated due to the magnetic interaction of nuclear spins that are coupled to the electron spin. Cosine Fourier transformation of this envelope yields an ENDOR-like spectrum from which nuclear hyperfine and quadrupole splittings can be determined. 

The key feature of ReMims (Doan) ENDOR is that Ti can be less than the deadtime of the spectrometer. This is because, in contrast to Mims ENDOR, the stimulated spin echo (which would be distorted by cavity ringdown) is not detected. Instead, an additional tt pulse is applied after time tz, which leads to a standard spin echo at time tz (Hahn echo, since it results from the original Hahn sequence here from the third tt/2 pulse and the following tt pulse). More important, there are two additional spin echoes formed one at time (tz + x ), which is observable for all values of x and xz, and is denoted the RME, which is detected, and one at time (xz — x ), which is observable only for values of t2 > ti, and is denoted the refocused stimulated echo (RSE). The deadtime for the ReMims (Doan) sequence is thus the minimum feasible value of (t2 + Ti), rather than the minimum value of x in a Mims sequence (Figure 6). As mentioned earlier (Section 3.3.4), the maximum undistorted Aiso (MHz) is - 1/(2ti (jls)). The deadtime in the X-band pulsed spectrometer is /d 0.1 J,s... 

Fig. 7. Pulse sequences for (A) Mims ENDOR and (B) Davies ENDOR. The top line in each sequence indicates pulses at microwave frequencies, whereas the lower line indicates pulses at rf frequencies. In both cases the microwave pulse sequence will be repeated at constant frequency, and the intensity of the resulting electron spin echo is recorded as a function of changing rf frequency.

An intensity change of the electron spin echo signal occurs when a RF pulse with a frequency corresponding to a nuclear spin transition is applied. An ENDOR spectrum is obtained by sweeping the RF frequency. Some well-established methods are named after their inventors. Mims-ENDOR [46] is a stimulated echo sequence with a RF pulse inserted between the second and third mw-pulses. The method is particularly used for measurements of small hyperfine couplings. Another pulse technique for performing ENDOR devised by Davies [47] is also commonly employed. The pulse sequence for this method is shown in Fig. 2.26. 

Pulsed ENDOR avoids some of the difficulties encountered with CW ENDOR in that the entire pulse sequence can usually be made short enough to exclude unwanted relaxation effects. This means that the pulsed ENDOR method can be used at any temperature, provided that an electron spin echo can be detected. There are a variety of pulse ENDOR techniques that have been applied to the investigation of free porphyrin bases and of nitrogen-14 nuclei in a disordered system. 

More recently, pulsed ENDOR methods have been introduced. There are two commonly used ENDOR pulse sequences, both of which are based on the impact of RF pulses on the intensity of a spin echo that is formed from a series of three pulses at the microwave frequency. These techniques are sometimes called ESE-ENDOR. In Mims ENDOR the pulse at the RF frequency is applied between the second and third microwave pulses whereas in Davies ENDOR the RF pulse is applied between the first and second microwave pulses. The Mims ENDOR experiment is particularly effective for weakly coupled nuclei, but has some blind spots (frequencies that cannot be observed). It is often advantageous to combine data from both ENDOR methods. 

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. 

Pulsed ENDOR is an alternative technique, in which radiofrequency pulses at NMR frequencies are used to perturb the spin-echo sequence (Figure 9). Hyperfine interactions lead to loss of coherence of the electron spins during the mixing period, so the amplitude of the echo is decreased. A plot of echo amplitude against RE frequency yields a spectrum analogous to that of a CW ENDOR spectrometer. 

Pulsed ENDOR has also become a more common technique. This is achieved by adding a radio frequency pulse within a spin-echo pulse sequence. Then, by detecting the echo intensity while the radio frequency is swept, one can obtain a pulsed ENDOR spectrum which directly reveals electron-nuclear hyperfine frequencies. 

Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

Davies ENDOR. The second pulsed ENDOR technique is called Davies ENDOR. In this experiment (Fig. 7B) a preparation v microwave pulse of duration Tp inverts spin packets, burning a hole of approximate width 1 /Tp in a broad EPR line. The resulting magnetization is subsequently detected by a vll-v two-pulse echo sequence. The application of an on-resonance rf pulse after the preparation phase increases the magnetization measured by the two-pulse detection sequence. 

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