Electron spin-echo modulation measurements

Long range dipolar interactions between an unpaired electron and nuclear spins on adjacent atoms will not normally be resolved in conventional powder EPR spectra.The pulse technique of electron spin echo modulation (ESEM) is in favourable cases able to detect very weak hyperfine interactions not seen in CW EPR. The method measures modulation of the electron spin echo signal by dipolar hyperfine coupling in the time domain at fixed magnetic field. Until recently,... 

S. A. Dikanov and Y. D. Tsvetkov, Electron Spin Echo Envelope Modulation Spectroscopy , CRC Press, Boca Raton, 1992. This book begins with the fundamental principles of ESEEM and develops the theory of the measurements with exquisite detail. 

The following sections provide a more detailed description of the hyperfine interaction as measured by ENDOR spectroscopy, a description of ENDOR instrumentation, and the types of ENDOR experiments that can be performed. Finally, examples of the application of ENDOR spectroscopy to a variety of biomolecules are described. In this brief review many statements are made without reference for details the reader is referred to the variety of more extensive works for the theory of EPR and hyperfine interactions and reviews of applications of continuous wave (cw) and pulsed ENDOR and ESEEM (electron spin echo envelope modulation) techniques. ... 

Larsen RG, Singel DJ (1993) Double electron-electron resonance spin-echo modulation -spectroscopic measurement of electron-spin pair separations in orientationally disordered solids. J Chem Phys 98 5134-5146... 

Abstract Multi-resonance involves ENDOR, TRIPLE and ELDOR in continuous-wave (CW) and pulsed modes. ENDOR is mainly used to increase the spectral resolution of weak hyperfine couplings (hfc). TRIPLE provides a method to determine the signs of the hfc. The ELDOR method uses two microwave (MW) frequencies to obtain distances between specific spin-labeled sites in pulsed experiments, PELDOR or DEER. The electron-spin-echo (ESE) technique involves radiation with two or more MW pulses. The electron-spin-echo-envelope-modulation (ESEEM) method is particularly used to resolve weak anisotropic hfc in disordered solids. HYSCORE (Hyperfine Sublevel Correlation Spectroscopy) is the most common two-dimensional ESEEM method to measure weak hfc after Fourier transformation of the echo decay signal. The ESEEM and HYSCORE methods are not applicable to liquid samples, in which case the FID (free induction decay) method finds some use. Pulsed ESR is also used to measure magnetic relaxation in a more direct way than with CW ESR. 

The pulsed EPR technique of Electron Spin Echo Envelope Modulation (ESEEM) is used to measure the nuclear transition frequencies of paramagnetic nuclei magnetically coupled to unpaired electron spins. We have employed this technique to study the Mn center of the Photosystem II oxygen-evolving complex. ESEEM measurements were performed on the multiline Mn EPR signal associated with the S2 state of the Kok cycle. 

Hyperfine splitting due to interaction with ligand nuclei with 7 > 0 reflects the extent of spin delocalization onto neighboring atoms and can be used to characterize the types and numbers of such nuclei. In cases where these couplings are too small to be resolved in the EPR spectra, electron nuclear double resonance (ENDOR) or electron spin echo envelope modulation (ESEEM) can be used to measure the couplings as discussed in Chapter 2.3. Modern calculational tools are approaching the capabilities required to calculate g and A values from electronic wave functions. However, much of the spectroscopy that has been performed to date has used empirical correlations to interpret g and A values. 

---