Magnetic ENDOR

Figure Bl.15.8. (A) Left side energy levels for an electron spin coupled to one nuclear spin in a magnetic field, S= I =, gj >0, a<0, and a l 2h)<(a. Right side schematic representation of the four energy levels with )= Mg= , Mj= ). +-)=1, ++)=2, -)=3 and -+)=4. The possible relaxation paths are characterized by the respective relaxation rates W. The energy levels are separated horizontally to distinguish between the two electron spin transitions. Bottom ENDOR spectra shown when a /(21j)< ca (B) and when co < a /(2fj) (C).

In Chapter 2, ENDOR (electron-nuclear double resonance) was briefly described. To perform an ENDOR experiment it is necessary to apply both a radiofrequency and a microwave frequency, effectively performing simultaneous NMR and ESR, respectively, on the sample. The experiment is performed at a fixed magnetic field, with the ESR saturating frequency centered on a... 

N.D. Chasteen and P.A. Snetsinger, ESEEM and ENDOR spectroscopy, in Physical Methods in Bioinorganic Chemistry, Spectroscopy and Magnetism, ed. L. Que, Jr, University Science Books, Sausalito, CA, 2000. 

O.G. Poluektov, L.M. Utschig, A.A. Dubinskij and M. Thurnauer, ENDOR of spin-correlated radical pairs in photosynthesis at high magnetic field A tool for mapping electron transfer pathways, J. Am. Chem. Soc., 2004, 126, 1644. 

As mentioned above, in an ENDOR experiment the rf field is swept while the static magnetic field is held at a constant position in the EPR spectrum. For slow sweep rates and narrow EPR lines a device would be desirable which is able to stabilize the ratio of the microwave frequency to the static magnetic field. The applicaiton of a commercially available field/frequency lock system is restricted to a region of 6 mT about the DPPH resonance field33). In metal complexes with strongly anisotropic EPR spectra, however,... 

First order ENDOR frequencies of nonequivalent nuclei or of pairs of magnetically equivalent nuclei are given by Eq. (3.3) which is derived from the direct product spin base. To obtain correct second order shifts and splittings, however, adequate base functions have to be used. We start the discussion of second order contributions with the most simple case of a single nucleus and will then proceed to more complex nuclear spin systems. 

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)...

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