ENDOR-Instrumentation

Contents Introduction. - ENDOR-Instrumentation. - Analysis of ENDOR Spectra. - Advances ENDOR Techniques. - Interpretation of Hyperfine and Quadrupole Data. - Discussion of the Literature. - Concluding Remarks. - Appendix A Abbreviations Used in this Paper. - Appendix B Second Order ENDOR Frequencies. - Appendix C Relations Between Nuclear Quadrupole Coupling Constants in Different Expressions of Hq (Sect.5.2). - References. - Subject Index. 

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. ... 

Fig. 9. Schematic diagrams of the major components of cw and pulsed EPR-ENDOR instruments. The sample is in a resonant microwave cavity, situated between poles of a magnet and surrounded by a temperature-control system (not shown). The structure of the circulator directs microwaves from the source to the cavity, and from the cavity to the detection system. A radio frequency synthesizer provides rf to coils situated around the cavity. Note that this diagram shows an arbitrary orientation of the rf coils. For convenience the magnetic field modulation coils are not shown for the cw spectrometer. For the pulsed EPR spectrometer (B), fast switches (ovals) are used to control pulse timing for the rf and microwave pulses, as well as to protect the detector. For simplicity, several features including the timing circuitry are not shown. The signal from the detector is sent to a boxcar integrator. Both spectrometers are computer-interfaced for data collection and storage. Further details may be found elsewhere.

Pulsed EPR experiments are typically performed with locally constructed instruments, although a commercially available X-band pulsed EPR/ENDOR instrument is now available from Bruker. A liquid helium immersion cryostat is generally employed. The pulsed EPR instrument creates short, high-power microwave and, for ENDOR, rf pulses, but the magnetization detected is of very small magnitude and requires a sensitive detector. This necessitates precise timing not only for creation and detec-... 

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. 

Varian Instrument Division Bull., E-700 High Power ENDOR System (1971)... 

The experimental methods in ENDOR spectroscopy have been extensively described by Kevan and Kispert4) in their monograph, Electron spin double resonance spectroscopy, and by Leniart18 in a recent paper. In this section we shall briefly review the instrumentation used in solid state ENDOR and describe the technical details of some new experimental methods. 

A large number of X-band ENDOR spectrometers have been discussed in the literature. Instruments operating in the less common K-19), Q-20,21 and V-22 frequency bands have also been described. Most of the ENDOR spectrometers are so-called low-power systems which produce rf fields = 0.1 mTrot (rot rotating frame). These field strengths are often sufficient to achieve nuclear saturation in transition metal complexes, i.e. to meet the condition (1.2). 

A technique related to EPR, electron nuclear double resonance (ENDOR), allows the assignment of the individual hfcs to particular nuclei and, with reasonable assumptions, will also identify the sign of the interaction. The only obvious drawback of this technique lies in the fact that it requires sophisticated instrumentation, which is, so far, available in only a few laboratories. Applications to strained ring systems, viz., cyclobutene, bicyclobutane, or a tricyclic derivative, have been reported. Howcvct, applications to simple cyclopropane systems have not been reported to date. 

The instrumental aspects of EPR have been summarized elsewhere, " although the text by Poole is especially noted for instrumental details and parameters. Obviously, to perform ENDOR experiments, one must have an EPR spectrometer, of which Bruker Biospin is the sole viable vendor at present (http //www.bruker-biospm.com/brukerepr/index.html accessed 03/01/2007). ENDOR accessories include the rf synthesizer and amplifiers and a microwave resonant cavity that includes rf cods in addition to the normal structure with field modulation coils. Locally constructed ENDOR spectrometers were more common in the past, with that of the late Clyde A. Hutchison Jr being a notable example, particularly for single-crystal studies. Such spectrometers still exist the author is familiar with those at Northwestern University. ... 

Electron spin resonance (ESR) is a well-established experimental method that has conventionally been limited to 35 GHz and lower in frequency. During the course of the last decade, workers in a number of laboratories (Grinberg et ai, 1983 Haindl et al., 1985 Lynch, et al., 1988 Barra et al., 1990 Wang et al., 1994) developed instruments that have pushed the maximum observation frequency up to nearly 1 THz (1000 GHz). Pulse methods at frequencies up to 604 GHz also have been developed (Weber et al., 1989 Bresgunov et al., 1991 Prisner et al., 1992 Moll, 1994), as well as Electron Nuclear Double Resonance (ENDOR) (Burghaus et al., 1988). 

In the above section the photodevices that can be employed to reach the appropriate reactant state were outlined. In the remainder of the chapter, the devices and instruments for monitoring the subsequent change and determining the rate characteristics will be discussed. By far the most frequently employed diagnostic techniques are optical absorption and emission spectrometry other less-used methodologies include light-scattering spectrometry, and the electron-spin-based spectroscopies, such as EPR, ENDOR, and CIDNP. These last three are not addressed in this review. 

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