Nuclear Spin Decoupling in ENDOR Spectroscopy

Nuclear spin decoupling in ENDOR may be used to facilitate the analysis of complicated ENDOR spectra. Some of the applications are40  

1) Assignment of ENDOR lines to the corresponding pair of nuclei which are coupled by a direct nuclear dipole-dipole interaction. 

3) Discrimination between splittings which originate from nuclear dipole-dipole couplings and splittings produced by slight deviations from magnetic equivalence. 

4) 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). 

Multiple quantum transitions (MQT) in ENDOR spectra may be observed for nuclei with I 1 if two or more (rf) photons of the same or of different frequencies combine to produce an ENDOR transition41,62,99 101). In a MQT the magnetic quantum number nij changes by An = n. The MQT should therefore be clearly distinguished from corresponding forbidden single quantum transitions (SQT) with Am] 0 discussed in Sect. 3.3. 

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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. 

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