DOUBLE ENDOR

Investigation of radicals in solutions with DOUBLE ENDOR was proposed by Freed89 and was first performed by Mobius and coworkers90 92. This field was recently covered by a review article93.  

Level populations when the (4-8) electron transition and the (3-4) nuclear transition ca(l/2), (double lines) are saturated. Full line ca(— 1/2), dashed lines cb( 1/2). b) Aa 0, 

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. 

Another type of DOUBLE ENDOR, called special TRIPLE , has been introduced by Dinse et al.90 to study proton hf interactions of free radicals in solution. In a special TRIPLE experiment two rf fields with frequencies vp + Av and vp — Av are swept simultaneously. For systems with Tln T,i this leads to a considerable signal-to-noise improvement and to TRIPLE line intensities which are directly proportional to the number of nuclei with the same hf coupling constant. It should be remembered, however, that in transition metal complexes in the solid state the resonance frequencies are not, in general, symmetrically placed about the free proton frequency vp and that the condition Tln Tj,i is not always fulfilled. 

The spin Hamiltonian for an anisotropic system is then given by 

Nuclear spin decoupling has been widely applied in NMR to simplify complicated spectra in liquid and solid phases . Recently, similar decoupling effects have been observed in single crystal ENDOR spectra with resolved nuclear dipole-dipole couplings between neighboring protons. This interaction can be partially decoupled by a strong second rf field. 

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Fig. 5. Schematic diagram of the DOUBLE ENDOR cavity system (X-band). (From Ref. 42)...

Recently the two techniques DOUBLE ENDOR and CP-ENDOR have been combined (CP-DOUBLE ENDOR) by introducing into the cavity a third half-loop which produces an additional linearly polarized rf field44. ... 

R.3 in (3.1) is the transpose of the third column of R, and represents a unit vector directed along the static field B0. More generally R j, i = 1,2,3, denotes the ith column of R. This notation proved to be convenient in the description of ENDOR spectra, especially in more complex cases, e.g. in CP ENDOR, PM-ENDOR, DOUBLE ENDOR as well as in spin decoupling experiments (Sect. 4). 

DOUBLE ENDOR 4.3 two rf fields 2.5 Separation of overlapped ENDOR spectra, determination of relative signs of hf splittings... 

DOUBLE ENDOR with circularly polarized rf fields 4.6 CP-DOUBLE- ENDOR one linearly pol. field one circ. pol. field 2.6 Reduction of the number of observed ENDOR lines... 

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)... 

Determination of relative signs of hyperfine interactions. In contrast to the sign determination by DOUBLE ENDOR, spin decoupling can also be applied if the ENDOR signals have no significant transient components (Sect. 4.3). 

Proton ENDOR and DOUBLE ENDOR data on VO(acac)2 in frozen solutions have been reported by van Willigen2895. Since the CH-protons exchange with deuterons in CD3OD they may easily be discriminated from the methyl protons. From the single crystal-like ENDOR spectrum for B0 along g and the two-dimensional powder spectrum at gi, the hf coupling constants Ap 5 = 0.7 MHz and AfHj = 0.7 MHz, A 3 = 1.5 MHz, respectively, have been found. The relative signs of the principal... 

During the last few years the versatility of ENDOR spectroscopy has been improved by a number of new techniques which make use either of special types of pumping fields (CP-ENDOR, PM-ENDOR), of more than one rf field (DOUBLE ENDOR, multiple quantum transitions, nuclear spin decoupling) or a different display of the spectrum (EI-EPR). In addition to these techniques, alternative methods have been developed (electron spin echo and electron spin echo ENDOR) which are able to supplement or to replace the ENDOR experiment under certain conditions. The utility of all these various advanced techniques, particularly in studies of transition metal compounds, has recently been demonstrated. 

Cook RJ, Whiffen DH (1964) Relative signs of hyperfine coupling constants by a double ENDOR experiment. Proc Phys Soc Lond 84(6) 845-845... 

In DOUBLE ENDOR (Sect. 4.3) the spin system is simultaneously irradiated with two rf fields. The frequencies of both fields have to be set independently from each other. Generation of rf fields by broadband DOUBLE ENDOR instruments are often strong enough for solid state studies at low temperature. In spin decoupling experiments (Sect. 4.4), however, the amplitude of one of the two fields (namely the decoupling field) should be as large as possible, whereas in studies of multiple quantum transitions ) (Sect. 4.5) two strong rf fields have to be applied. 

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