ENDOR pulses

Electron spin echo ENDOR 4.8 ESE-ENDOR pulsed MW and rf field Measurement of hf coupling constants... 

Figure 7 Davies ENDOR pulse sequence. The upper level indicates the microwave pulses and immediately helow delay times are indicated. The lowest level indicates the rf pulse. Typical times are given based on a 35 GHz spectrometer. The timing of the microwave and rf pulses is less commonly altered, hut the delay time T and the repetition time need significant adjustment to optimize spectral appearance of a given sample. Microwave phase cycling is also employed, hut is not indicated in the figure...

Fig. 2.26 (a) Q-band FSE EPR (EIE) spectrum of peridinin triplet (A) absorption, (E ) emission (b) Davies ENDOR pulse sequence (c) Q-band H ENDOR spectra recorded at the three canonical orientations Xn, Yn, Zn, which are marked with arrows in the ESR spectrum of panel (a) using the conditions in panel (b). At the proton Larmor frequency vh a narrow and intense line is visible resulting from nuclear transitions in the mg = 0 manifold. The frequency axis gives the deviation from Vh in the respective spectra. The excitation wavelength was 630 nm. Left numbering and spin density plot of peridinin in its excited triplet state. The orientation of the ZFS tensor axes X, Y, and Z is also given. The figure is adapted from [51] with permission from the American Chemical Society... 

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. 

Figure 14 shows the Davies [63] and Mims [64] ENDOR pulse sequences, both of which are based on the transfer of polarization between electron and nuclear transitions. 

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. 

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. 

Pulsed ENDOR offers several distinct advantages over conventional CW ENDOR spectroscopy. Since there is no MW power during the observation of the ESE, klystron noise is largely eliminated. Furthemiore, there is an additional advantage in that, unlike the case in conventional CW ENDOR spectroscopy, the detection of ENDOR spin echoes does not depend on a critical balance of the RE and MW powers and the various relaxation times. Consequently, the temperature is not such a critical parameter in pulsed ENDOR spectroscopy. Additionally the pulsed teclmique pemiits a study of transient radicals. 

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.

More advanced pulsed teclmiques have also been developed. For a review of pulsed ENDOR techniques the reader is referred to [43, 44 and 45]. 

Davies E R 1974 A new pulse ENDOR technique Phys. Lett. 47 1-2... 

Grupp A and Mehring M 1990 Pulsed ENDOR spectroscopy in solids Modern Pulsed and Continuous-Wave Electron Spin Resonance ed L Kevan and M K Bowman (New York Wiley) ch 4, pp 195-229... 

Dinse K P 1989 Pulsed ENDOR Advanced EPR in Biology and Biochemistry ed A J Hoff (Amsterdam Elsevier) ch 17, pp 615-30... 

Identification of nitrogen donor ligands was possible from the pulsed ENDOR and ESEEM spectra of Hred from M. capsulatus (Bath)... 

Main advances in ESR spectroscopy have recently come about by adding new dimensions to basic ID ESR [1015]. Dimensions such as time and radiofrequency radiation have either created new spectroscopies or enriched one-dimensional forms. Examples are pulsed ESR and ENDOR. 

The high-resolution EPR spectra of the 17-electron complex (446) have been studied in combination with pulse ENDOR and ESEEM techniques.750... 

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. 

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. 

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). 

Konovalova, T. A., S. A. Dikanov et al. (2001a). Detection of anisotropic hyperfine components of chemically prepared carotenoid radical cations ID and 2D ESEEM and pulsed ENDOR study.. /. Phys. Chem. B 105 8361-8368. 

Lawrence, J., A. L. Focsan et al. (2008). Pulsed ENDOR studies of carotenoid oxidation in Cu(II)-substituted MCM-41 molecular sieves. J. Phys. Chem. B 112 1806-1819. 

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