This Special Issue of Magnetic Resonance in Chemistry is devoted to high-field/high-frequency EPR spectroscopy. In the last 15 years, this area of electron magnetic resonance spectroscopy is growing rapidly and is one of the reasons, together with advances in the technology of fast microwave pulsing, for the currently observed renaissance of EPR spectroscopy. Application of higher and higher magnetic fields and microwave frequencies leads to dramatic improvements of spectral and time resolution. Thereby, more complex spin systems can be studied by EPR, very much in analogy to what has happened in modern NMR spectroscopy more than one decade ago. The reasons for the time lag are obvious: EPR has to cope with resonance frequencies three orders of magnitude larger than in NMR, and relaxation times are correspondingly shorter. It was only after the end of the cold war that low-noise mm and sub-mm microwave sources and detectors became available for unclassified research. These devices deliver and process microwave power in the milliwatt range necessary for fast pulsed high-field EPR at frequencies up to several 100 GHz. Nowadays we witness multi-frequency and multidimensional EPR benchmark experiments at the technical limits of sweepable wide-bore cryomagnets and quasioptical sub-mm microwave bridges.
The unique potential of high-field/high-frequency EPR spectroscopy for studying complex spin systems was recognized already 35 years ago, and first publications on this subject appeared around 1970 (1969: E.F. Slade and D.J.E. Ingram in Keele; 1970: I. Amity in Jerusalem; 1973: Y. Alpert et al. in Paris). Nevertheless, it is fair to state that Yacov Lebedev (Moscow) was the first to start a dedicated high-field/high-frequency research program and, in 1976, completed a versatile continuous-wave (cw) high-field EPR spectrometer employing a superconducting 5 T magnet and 2 mm microwaves. The history of pulsed high-field EPR starts with Jan Schmidt (Leiden), who built the first 3 mm pulsed spectrometer in 1989, using a Fabry-Perot microwave resonator placed in a split-coil superconducting magnet. Since then, a small number of laboratories were courageous enough to build their own high-field EPR and ENDOR spectrometers, cw or pulsed, at different microwave frequencies ranging from 95 to about 400 GHz. In the early 90's a broader application of this advanced EPR technology to fundamental and applied research projects became feasible with the introduction of a commercial high-field EPR spectrometer operating at 94 GHz by BRUKER (Karlsruhe).
Fortunately, the scientific potential of high-field EPR has been recognized by several national science foundations around the world. The Deutsche Forschungsgemeinschaft (DFG), for example, substantially supported this research area by initiating the installation of three 94 GHz spectrometers and a 360 GHz machine in 1996 and, subsequently, by providing continuing support in the frame of the DFG Priority Program ''High-field EPR in Biology, Chemistry and Physics'' (SPP 1051). Dedicated research grants were available from 1998 to 2004 and, in fact, most articles in this Special Issue document recent achievements in high-field EPR that were made possible by this DFG program. Naturally, the DFG program primarily supported research groups in Germany, but also funded high-field EPR activities in Rehovot (Israel) and Pecs (Hungary). The European Union supports high-field EPR research in large-scale facilities in Grenoble (France) and Pisa (Italy), and some results from the respective groups are contained in this Issue. This also holds for the high-field/high-frequency EPR results from research groups in Paris (France) and Kazan (Russia). In the USA, two national research centres focus on the development and application of high-field/high-frequency EPR (Tallahassee, FL, and Ithaca, NY), and it is gratifying to note that contributions from these centres are also included in this Issue.
No exhaustive coverage of the rapidly expanding field of high-frequency/high field EPR spectroscopy could be attempted in this Special Issue. We have rather restricted ourselves to include a representative cross section of EPR activities of dedicated laboratories around the world. They profit from distinct advantages of this technique in comparison with conventional X-band EPR. Examples of such advantages are: enhanced spectral resolution of small g-matrix anisotropies even for disordered samples; separation of radicals with only slightly different g-values; enhanced detection sensitivity for small samples with low spin concentration; detection of EPR transitions in high-spin systems with large zero-field splittings which are 'EPR silent' at Xband; increased sensitivity towards relaxation effects by transforming motionally narrowed EPR spectra into the slow-motion regime; and increased orientation selection by high-field ENDOR on disordered samples to obtain single-crystal like spectra. The Special Issue combines recent instrumental developments with typical applications in biology, chemistry and physics. The articles are arranged in this order. The Issue also includes state-of-the-art quantum-mechanical approaches to calculate the g-and hyperfine-tensor components with high accuracy that is adapted to the precision of the measured parameters of the spin Hamiltonian. We hope that the balanced mixture of invited papers providing either mini-reviews of the activities in the respective