ENDOR spectra

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 3. ENDOR spectrum of radical cation, (a) obtained...

Note that the values of are orders of magnitude larger than the A values given for the hyperfine splitting of an EPR spectrum (or ENDOR spectrum) in units of the field sweep necessary to obtain electron spin resonance. These are usually on the order of only 10 mT or less ... 

ENDOR spectrum exhibited chlorine and nitrogen splittings indicating a carotenoid-quinone radical adduct formation. 

The narrow, resolved lines observed in the H-ENDOR spectrum of P-carotene indicated that the methyl protons at the 5 or 5 and the 9 or 9 carbon positions were undergoing rapid rotation. 

Adsorption of carotenoids on activated silica-alumina results in their chemical oxidation and carotenoid radical formation. Tumbling of carotenoid molecules adsorbed on solid support is restricted, but the methyl groups can rotate. This rotation is the only type of dynamic processes which is evident in the CW ENDOR spectrum. 

The Davies pulsed ENDOR spectrum of canthaxanthin oxidized on silica-alumina measured in the temperature range of 3.3-80K showed no lineshape changes, which is in agreement with previous 330 GHz EPR studies of canthaxanthin radical cations (Konovalova et al. 1999). This implies very rapid rotation of the methyl groups down to 3.3 K. 

CW ENDOR spectrum measurements carried out at 120 K (the optimum temperature for measuring resolved CW ENDOR powder spectra of carotenoid radicals) shows resolved lines from the P-methyl hfc (Piekara-Sady et al. 1991,1995, Wu et al. 1991, Jeevarajan et al. 1993b) (see Figure 9.5). The lines above 19 MHz are due to neutral radicals according to DFT calculations (Gao et al. 2006). 

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... 

FIGURE 9.5 CW ENDOR spectrum of 1-carotene radicals, (a) Experimental spectrum of Figure 9.4. (Reported in Wu, Y. et al., Chem. Phys. Lett., 180, 573, 1991.) (b) Simulated ENDOR powder pattern (using linewidth of 0.6MHz) for the sum of radical cation and neutral radicals in 5 3 1 1 ratio. (Reported in Gao, Y. et al., J. Phys. Chem. B, 110, 24750, 2006. With permission.)... 

For a nucleus with I = 1, the first order ENDOR spectrum consists of four transitions at frequencies... 

According to (3.3), 4 I ENDOR transitions will be observed for a nucleus with arbitrary spin I and with an unresolved hf structure in the EPR display. If the hf structure is resolved, however, each mrstate can be saturated individually and either a four-line ENDOR spectrum (EPR observer -1 < mi < I) or only a two-line ENDOR spectrum (EPR observer m, = I) will be observed. 

The cross-term described by (3.18) produces shifts or splittings in the ENDOR spectrum. Shifts are observed if the hfs of nucleus K is resolved in the EPR spectrum, i.e, if EPR transitions with different mg may be used as observers. This is often the case in... 

In second order, however, eight ENDOR frequencies are obtained for each ms-state. The transition frequencies tabulated in Appendix B, Eqs. (B 5) are again described by al5 a2 and a3 defined in (3.12). If the hfs is resolved in the EPR spectrum, the number of induced transitions depends on the mp-value of the saturated line in the EPR quintet. For mF = 0 six transitions, for mF = 1 four transitions, and for mF = 2 one transition are observed in the ENDOR spectrum of each ms-state62). 

A typical nitrogen ENDOR spectrum of a copper complex (Cu(sal)2) with two magnetically equivalent 14N nuclei and with the EPR observer at mF = 0 (two sets of six ENDOR lines) is shown in Fig. 9. The pronounced splitting of the lines into a doublet structure is described by the term 4/Jai. The splitting of the more intense lines by 4/ a3 is not resolved (see B5). 

Fig. 9. Second order splittings in spin systems with two magnetically equivalent 1=1 nuclei Single crystal nitrogen ENDOR spectrum of Cu(sal)2 diluted into Ni(sal)2. (Ref. 62)...

Fig. lOa-c. Higher order splittings in symmetry planes Single crystal nitrogen ENDOR spectrum of Cu(TPP) diluted into (H20)Zn(TPP) with Bo normal to the porphyrin plane B0 = 327.7 mT. a) Observed spectrum. (Adapted from Ref. 66) b) Transition frequencies obtained by numerical diagonalization of the full spin Hamiltonian matrix (Four nitrogen nuclei). (Ref. 68) c) First order frequencies, (Eq. (3.10))... 

It should be noted that for geometrically equivalent nuclei a complicated ENDOR spectrum may be observed for arbitrary orientations of B0, if the hfs tensors are nearly... 

Proper single crystal-like ENDOR spectra can best be obtained by saturating the low-and high-field flanks of an EPR spectrum. At these field positions the resolution of the ENDOR spectrum is increased and distortions of the ENDOR lines are minimized78- 79). 

In many planar metal complexes it is not possible to record an ENDOR spectrum which only contains contributions from Bo orientations in the complex plane. This is due to the fact that in the powder EPR spectrum the high- or low-field turning points may arise from extra absorption peakssl which do not correspond to directions of the principal axes. ENDOR spectra observed near the in-plane region of such a powder EPR spectrum are due to molecules oriented along a large number of B0 directions (in- and out-of-plane), so that the orientation selection technique is no longer effective. 

Fig. 12 a, b. Orientation selection in ENDOR. a) Powder EPR spectrum of Co(salen)py. Arrow indicates EPR observer b) Single crystal-like ENDOR spectrum of the pyridine nitrogen with B0 along g . (From Ref. 80)... 

Fig. 18 a, b. DOUBLE ENDOR spectrum of Cu(TPP) in a frozen nematic glass (Merck Phase 5). a) Two-dimensional nitrogen ENDOR spectrum with B0 in the complex plane. ENDOR observer frequency (v = 19 MHz) used in b) is marked by an arrow, b) DOUBLE ENDOR spectrum the corresponding ENDOR frequencies cN( 1/2) = AJ72 3/2 Qj vN obtained from single crystal work1 are marked by arrows. (Ref. 84)... 

This example demonstrates that the data Af1, Q, A2, Q2. evaluated from the nematic glass, together with the values A3, Q3, obtained from the single crystal-like ENDOR spectrum, allow the determination of the full nitrogen hfs and quadrupole tensors of Cu(TPP) without the use of a single crystal. 

Fig. 19 a, b. Nuclear spin decoupling in ENDOR. a) ENDOR spectrum of Cu(gly)2 in a-glycine vp free proton frequency, b) Decoupling sequence of the doublet structure of the proton H2 (I nucleus) for various pumping fields B2eff at H3 (K nucleus). (Ref. 40)... 

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