Introduction

During the last decade, the chemistry, biology and physics communities have apparently witnessed a boost of new EPR (electron paramagnetic resonance) applications. This is largely due to technological breakthroughs in the development of pulsed microwave sources and components, sweepable cryomagnet design and fast data-acquisition instrumentation. They enable the EPR spectroscopists to introduce multiple-pulse microwave irradiation schemes, very much in analogy to what is common practice in modern NMR (nuclear magnetic resonance), and to apply advanced multifrequency high-field EPR techniques as powerful spectroscopic tools with unique potential for the elucidation of structure and dynamics of complex systems, for example membrane proteins in biological action.

This assessment is corroborated by the substantial increase of publications related to high-field/high-frequency EPR since the last 15 years (see Figure 1.1). The growing appreciation is mirrored also by the rising number of research groups in Europe, the US and Japan dedicated to the development and/or application of high-field EPR spectroscopy. This was made possible by increased financial support from national and international funding agencies. The European Union, for example, supported the Human Capital and Mobility (HCM) project "High-Field EPR: Technology and Applications" (coordinator J. Schmidt, Leiden, 1993–1996) and the EU network project "SENTINEL" ("Service Enhancement through Infrastructure Networking for Electron Paramagnetic Resonance Spectroscopy with Large Fields", coordinator M. Martinelli, Pisa, 2001–2005). Exceptionally strong support was granted by the DFG (Deutsche Forschungsgemeinschaft) through the Priority Program "High-Field EPR in Biology, Chemistry and Physics" (coordinator K. Möbius, Berlin, 1998–2004). These initiatives acted like seeding programs for the rapid development of high-field EPR spectroscopy in Europe, including Israel and Russia. In Figure 1.2 the present distribution of high-field EPR groups in Europe is shown. There is a noticeable congestion of such groups in Germany, apparently as a benefit from the sustaining support by the DFG.

In the US, there is a strong representation of high-field EPR spectroscopy with about ten research groups throughout the country. Particularly renowned high-field EPR groups are concentrated in dedicated national facilities located in Ithaca, NY, at Cornell University (ACERT, the "National Biomedical Center for Advanced ESR Technology", headed by J.H. Freed,) in Tallahassee, FL, (National High Magnetic Field Laboratory, the EPR group headed until recently by L.-C. Brunel), in Milwaukee, WI, at the Medical College of Wisconsin (National Biomedical EPR Center, headed by J.S. Hyde) and in Cambridge, MA, at MIT (Francis Bitter Magnet Laboratory, headed by R.G. Griffin).

In Japan, high-field EPR research is traditionally devoted to physics with the focus on novel magnetic materials. There exist about ten mm and sub-mm high-field EPR facilities in Japanese universities and national institutes working in the field of physical sciences. They cover broad field ranges up to 150 T using superconducting and hybrid magnets, either in constant-field or repetitive pulse-field mode of operation. But also in chemical and biological sciences a growing interest is noticeable in Japan, both in terms of instrument developments and scientific applications. For example, in Sendai at the Tohoku University, S. Yamauchi and coworkers have demonstrated the advantages of W-band high-field EPR for their studies of organic excited multiplet states in fluid solution over recent years.

1.1 Why EPR at High Magnetic Fields?

Induced chemical reactions in condensed phases often proceed via the formation of radical pairs as reaction intermediates. Radical pairs are formed, for example, after appropriate excitation of donor molecules, by one-electron transfer to acceptor molecules leading to ionic radical pairs. Such electron-transfer reactions are found in many fields of chemistry and biology, for example in tribochemistry, i.e. during chemical reactions initiated by mechanical activation of solid mixtures of compounds by pressure, in photochemistry, i.e. during chemical reactions initiated by the absorption of infrared, visible and ultraviolet light, and in photosynthesis, i.e. during photoinduced chemical reactions in green plants, algae and certain bacteria by which carbon dioxide and water are converted to carbohydrates and oxygen.

In principle, EPR appears to be a promising spectroscopic technique to study both stable and transient radical-pair intermediates. In practice, however, for large spin systems in solid-state reactions, standard EPR – similar to other types of spectroscopy – soon reaches its limits of useful information content, unless single-crystal samples are available. Unfortunately, large molecular complexes are often available only as disordered samples. Their standard X- band (9.5 GHz) EPR spectra are poorly resolved, and the information on magnetic parameters and molecular orientations is hidden under the broad lines. By going to higher and higher magnetic fields and microwave frequencies, for example to EPR at W-band (95 GHz) or even at 360 GHz, at least five important features, (i)–(v), are emerging from the EPR spectra: (i) enhanced spectral resolution; (ii) enhanced orientational selectivity in disordered samples; (iii) enhanced low-temperature electron-spin polarization; (iv) enhanced detection sensitivity for restricted-volume samples such as small single crystals of proteins or fullerenes, and (v) last but not least enhanced sensitivity for probing fast motional dynamics, i.e. high-frequency EPR acts as a faster "snapshot" for molecular motion.

Ad (i): The strategy for spectral resolution enhancement is similar in EPR and NMR: With increasing external Zeeman field the field-dependent spin interactions in the spin Hamiltonian are separated from the field-independent ones (see Figure 1.3). In high-field EPR, the g-factor resolution is increased in relation to the hyperfine couplings, in high-field NMR the chemical-shift resolution is increased in relation to the spin–spin couplings.

Ad (ii): The important feature of enhanced orientation selectivity by high-field EPR on randomly oriented spin systems becomes essential for organic radicals with only small g-anisotropy (see Figure 1.4) Well below room temperature, the overall rotation of, for example, a protein complex becomes so slow that powder-type EPR spectra are obtained. If the anisotropy of the leading interaction in the spin Hamiltonian is larger than the inhomogeneous linewidth, even from disordered powder-type EPR spectra the...