MIMS ENDOR

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. 

Figure Bl.15.13. Pulsed ENDOR spectroscopy. (A) Top energy level diagram of an. S-/=i spin system (see also figure Bl,15,8(A)). The size of the filled circles represents the relative population of the four levels at different times during the (3+1) Davies ENDOR sequence (bottom). (B) The Mims ENDOR sequence.

Carotenoid neutral radicals are also formed under irradiation of carotenoids inside molecular sieves. Davies and Mims ENDOR spectra of lutein (Lut) radicals in Cu-MCM-41 were recorded and then compared with the simulated spectra using the isotropic and anisotropic hfcs predicted by DFT. The simulation of lutein radical cation, Lut +, generated the Mims ENDOR spectrum in Figure 9.7a. Its features at B through E could not account for the experimental spectrum by themselves, so contribution from different neutral radicals whose features coincided with those of the experimental... 

The above point is related to the Larmor-centered frequencies at which Mims ENDOR signals appear. It must be made clear, however, that this technique is not applicable irrespective of hyperfine coupling. Mims ENDOR is not... 

As a result of equation (9), the Mims ENDOR signal is minimal ( blind ) at t = (n/Aiso), n = 0 (which is experimentally meaningless), 1,2,..., and is maximum at T = [(In -F l)/2Aiso]. Mims ENDOR spectra of a broad H pattern can clearly show the oscUlating blind spot pattern... 

For readers interested in the quantitative description of the time-dependent magnetization processes, the magnetization grating , that occurs in Mims ENDOR using the density operator formalism, which leads to the experimental behavior described earlier, the reader is referred to other sources by authors infinitely more qualified than the present... 

The other major form of pulsed ENDOR spectroscopy is known as Davies ENDOR, after its discoverer, E. R. Davies at the Clarendon Laboratory, Oxford, UK. Davies ENDOR is in many ways complementary to Mims ENDOR in that the former is more suited for larger couplings and the latter for smaller couplings, as described earlier (Section 3.3.4). 

Figure 8 ReMims (Doan) ENDOR pulse sequence. The upper level indicates the microwave pulses and immediately below delay times are indicated. The lowest level indicates the rf pulse. SE is the stimulated echo that is not detected, RSE is the refocused stimulated echo that can be detected (for T2 > ti as shown here), and RME is the refocused Mims echo that is normally detected (at time T2 + ti). HE is the Hahn echo that is not detected and normally eliminated by microwave phase cycling. Typical times are given based on an X-band ( 9.6 GHz) spectrometer, but are also applicable at 35 GHz. The timing of both delay times ti and T2 are adjusted to optimize spectral appearance as desired. Other aspects are similar to standard Mims ENDOR...

The above description is of standard TRIPLE, i.e., exphcit TRIPLE, employing two rf energies. Pnlsed ENDOR can also exhibit relative sign information, in the absence of an additional rf energy. This phenomenon has been called implicit TRIPLE , and was demonstrated for nitrile hydratase, a nonheme iron enzyme (low-spin Fe , S = 1/2). Standard Mims ENDOR experiments were performed and the intensities of signals for H were compared for natural... 

Figure 10 35 GHz CW C ENDOR (except for Mims ENDOR as radicated) of lo-CO and hi-CO inhibitor forms of nitrogenase FeMo-cofactor, prepared with both CO and CO. The spectra were recorded at g ax of each species, as indicated. The spectra are centered...

T = 2n + l)/2Aiso]. Mims ENDOR spectra of a broad H pattern can clearly show the oscillating blind spot pattern... 

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... 

---