Nuclear and electronic spins as dynamic and kinetic probes

Nuclear and Electronic Spins as Dynamic and Kinetic Probes. Annual Meeting of the Deutsche Bunsen-Gesellschaft für Physikalische Chemie, Potsdam, Germany, 9–11 May, 2002

Scope of the meeting

With the award of the recent Nobel Prize in Chemistry to Kurt Wüthrich (Zürich) this special honor has again been conferred to a pioneer in magnetic resonance. Earlier recipients include Purcell and Bloch (1952), Bloembergen (1981), Dehmelt (1989), and Ernst (1991). This demonstrates the impact and the high recognition and importance of the achievements which have been made using magnetic resonance.

In much the same way as weather satellites are distributed over the surface of our planet, providing information about the development of weather conditions, magnetic nuclei pervade nearly any state matter and can be addressed selectively as local probes of their structural and dynamic environments. For any synthetic chemist, nuclear magnetic resonance (NMR) has become an essential tool for the determination of the connectivity and spatial arrangement of atoms. Its application to extended biomolecules, often termed as “NMR crystallography”, was honored with the most recent Nobel Prize. This accomplishment has opened the way for studying structure–function relationships in many biological processes. Today, NMR has become an essential tool to many physical chemists, materials scientists, and biologists to obtain important microscopic structural and dynamic information not accessible otherwise in condensed matter. In the same sense and well known as magnetic resonance imaging (MRI), it has become indispensable as a non-destructive routine diagnostic method to help physicians differentiate between healthy and pathological tissue.

To a much lesser extent but in a completely analogous fashion as magnetic nuclei, unpaired electrons can also serve as structural and magnetic probes. This method is called electron spin resonance (ESR). Paramagnetic species are generated in combustion, in photochemical, radiolytical or electrochemical reactions, but they are also important intermediates or products of normal metabolitic processes in living organisms, and in photosynthetic pathways. Since they are often very reactive and therefore short-lived they are present in far smaller concentrations than nuclear spins. Fortunately they carry a much larger magnetic moment which compensates for some of the loss of sensitivity due to their low concentrations.

Many variants of NMR and ESR have been devised to accommodate a wide range of different special requirements and circumstances of the systems under investigation. The development of pulsed excitation and Fourier transformation of the time-dependent response has revolutionized both methods, permitting the accumulation of multi-dimensional spectra. Modern electronics, fast computers, and superconducting magnets have become indispensable components which warrant high flexibility, resolution, and sensitivity. A relatively new twist for the enhancement of the signal to noise ratio is based on the use of non-thermal polarization, as with optically pumped, hyperpolarized noble gases, or with polarized particles from nuclear decay.

Hand in hand with the development of experimental techniques theoretical methods have been devised to handle the wealth of information and to support the interpretation. Complex multi-dimensional NMR spectra can be simulated, and very significant progress has been made in the quantum chemical calculation of the magnetic resonance parameters such as chemical shifts, nuclear J–J couplings, as well as free radical g- and hyperfine coupling tensors.

Nuclear and electronic spins as dynamic and kinetic probes have been the central topic of the 101st Annual Conference of the Deutsche Bunsen-Gesellschaft für Physikalische Chemie in Potsdam, Germany. The list of contributions included three plenary lectures, five invited and more than fifty contributed papers. The present Special Issue serves to showcase a selection of innovative applications presented at this meeting.

Hellmut Eckert

Westfälische-Wilhelms-Universität Institut für Physikalische Chemie

Hans-Heinrich Limbach

Freie Universität Berlin Institut für Organische Chemie und Instrumentelle Analytik

Emil Roduner

Universität Stuttgart Institut für Physikalische Chemie

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