This special issue of Molecular Physics on ‘Modern Electron Paramagnetic Resonance (EPR) Spectroscopy’ provides an overview of the most important recent developments in instrumentation, computational approaches, and experimental techniques and reports on ventures into new application fields. Until fairly recently continuous-wave (CW) EPR was considered as the most sensitive technique for measuring EPR spectra. The contribution of Mitchell et al., which is also highlighted on the cover page, demonstrates that under certain conditions rapid-scan EPR can lead to substantial improvements in concentration sensitivity. For small samples sensitivity can be enhanced by miniature resonators, an approach that is exemplified in this issue by the work of Twig et al. For special sample classes completely different detection schemes may improve sensitivity. The combination of electrical detection in semiconductors with pulsed methods has recently come of age, as is illustrated by Meier et al. and Suckert et al. For the very broad spectra associated with large zero-field splittings, frequency-domain Fourier transform terahertz spectroscopy opens new vistas (Schnegg et al.).
Despite these new developments conventional CW EPR measurements may continue to be the method of choice for weak signals on surfaces (Goncher and Risse), for complex systems where a large number of spectra have to be taken (Kattnig et al.), and for the detection of short-lived radicals by spin trapping, a technique that is shown by Schlick et al. to have great potential in materials applications. New computational approaches help in interpretation of EPR data (Zerbetto et al.) and in building of models from distances that were measured by EPR approaches (Hagelueken et al.).
Established pulsed EPR techniques continue to find new application fields, such as determination of electron–electron couplings by nutation spectroscopy (Ayabe et al.), detailed characterisation of moderate metal–ligand hyperfine couplings by high-field electron electron double resonance (ELDOR)-detected nuclear magnetic resonance (Cox et al.), and determination of the sign of hyperfine couplings by cross-polarisation-edited electron nuclear double resonance (ENDOR) (Bennati et al.).
During the past decade measurement of distances in the nanometre range between electron spins by pulsed EPR techniques has found widespread application in protein and nucleic acid studies and some applications in materials science. Relaxation-enhancement-based techniques can now be combined with spin labelling techniques (Lueders et al.), and intricacies of the more commonly applied pulsed ELDOR (PELDOR) (or double electron electron resonance [DEER]) technique at large electron spin polarisation (Marko et al.) or in multiple-spin systems (Giannoulis et al.) are studied. The technique can be applied to distances between low-spin iron centres and nitroxide spin labels (Ezhevskaya et al.) and can provide some information on the structure of protein complexes (Lovett et al.).
Accessibility of nitroxide spin labels by water has been probed by CW EPR techniques for a long time. More detailed insight can now be obtained by combining high-field CW EPR with deuterium electron spin echo envelope modulation (ESEEM) techniques (Urban and Steinhoff), and the deuterium ESEEM technique can also be applied to glycerol accessibility (Konov et al.) and to probe structural aspects of lipid bilayers (Goldfarb et al.).
EPR spectroscopy is also used to study prospective systems for quantum computing, as is exemplified here by the work of Lutz et al. on a trinuclear copper complex. Multi-electron spin systems are also of interest as molecular magnets (Drozdyuk et al.). Dynamic aspects of photophysical processes can be elucidated by pulsed EPR techniques (Domingo Köhler et al. and Di Valentin et al.). The influence of molecular dynamics on relaxation times remains an active area of research (Collauto et al.).
Fe(II) species are often deemed EPR silent because of their integer spin and large zero-field splitting. In a surprising case both an EPR-observable Fe(II) species and an unusual Fe(I) species are characterised in a catalytically active hydrogen-evolving complex by Stathi et al. Hydrogen storage in a metal organic framework is addressed by measurement of proton and deuterium hyperfine couplings by ENDOR and hyperfine sublevel correlation spectroscopy (HYSCORE) techniques (Jee et al.). In a similar vein adsorption of vanadyl ions on hydroxyapatite surfaces can be characterised by HYSCORE measurements of phosphorous hyperfine couplings (Dikanov et al.).
Applications of modern EPR and of the related chemically induced nuclear polarisation technique to biological problems are illustrated on a human copper transporter (Shenberger et al.) and in the context of oxidative stress and antioxidants (Gescheidt et al.), respectively.
This issue shows that current EPR spectroscopy is a field where diverse and mutually complementary new methods are being introduced and new application fields are being opened. Molecular physics aspects are a common theme to all these developments. The editors encourage the submission of new research articles in this field.