Publisher Summary There have been many reviews and books on electron spin resonance (ESR) and several on transition metal ions. Many of these publications have been written by physicists or theoreticians and are very comprehensive. This review is aimed at inorganic chemists, who have had no experience relating to electron spin resonance and who are becoming more and more likely either to use the technique or to need to appreciate the significance of the information available from the technique. The first part of the review gives an elementary account of the relevant theory. There are many good books for further reading for the inorganic chemist, who wishes to go deeper into the subject. The second part of the review is a comprehensive survey of the results of ESR studies on compounds of transition elements. Inevitably, there has had to be some selection of material, and the criteria for inclusion or rejection has been based upon the likely interest of an inorganic chemist might have in the species. This is particularly in the areas, where there is a lot of related work, e.g., the Mn2+ host-lattice data. In the case of copper dg complexes, there has been much duplication of work and papers devoted to the spectra of complexes where a minor substitutive change has been made to a bulky organic ligand.

Electron Spin Resonance of Transition Metal Complexes

Preface. Part One: Optical Methods. 1. Developments in Classical Optical Spectroscopy J. Amesz. 2. Linear and Circular Dichroism G. Garab. 3. Fluorescence K. Sauer, M. Debreczeny. 4. Ultrafast Spectroscopy of Photosynthetic Systems R. Jimenez, G.R. Fleming. 5. Data Analysis of Time-Resolved Measurements A.R. Holzwarth. 6. Photosynthetic Thermoluminescence as a Simple Probe of Photosystem II Electron Transport Y. Inoue. 7. Accumulated Photon Echo Measurements of Excited State Dynamics in Pigment-Protein Complexes T.J. Aartsma, R.J.W. Louwe, P. Schellenberg. 8. Spectral Hole Burning: Methods and Applications to Photosynthesis N. Raja, S. Reddy, G.J. Small. 9. Infrared and Fourier-Transform Infrared Spectroscopy W. Mantele. 10. Resonance Raman Studies in Photosynthesis - Chlorophyll and Carotenoid Molecules B. Robert. 11. Stark Spectroscopy of Photosynthetic Systems S.G. Boxer. 12. The Photoacoustic Method in Photosynthesis - Monitoring and Analysis of Phenomena which Lead to Pressure Changes Following Light Excitation S. Malkin. Part Two: Magnetic Resonance. 13. Magnetic Resonance: an Introduction A.J. Hoff. 14. Time-Resolved Electron Paramagnetic Resonance Spectroscopy - Principles and Applications H. Levanon. 15. Electron Spin Echo Methods in Photosynthesis Research R.D. Britt. 16. ENDOR Spectroscopy W. Lubitz, F. Lendzian. 17. Optically Detected Magnetic Resonance (ODMR) of Triplet States in Photosynthesis A.J. Hoff. 18. MagicAngle Spinning Nuclear Magnetic Resonance of Photosynthetic Components H.J.M. de Groot. Part Three: Structure and Oxygen. 19. Structure Determination of Proteins by X-Ray Diffraction M. Schiffer. 20. Electron Microscopy E.J. Boekema, M. Roegner. 21. X-Ray Absorption Spectroscopy:Determination of Transition Metal Site Structures in Photosynthesis V.K. Yachandra, M.P. Klein. 22. Moessbauer Spectroscopy P.G. Debrunner. 23. Characterization of Photosynthetic Supramolecular Assemblies Using Small Angle Neutron Scattering D.M. Tiede, P. Thiyagarajan. 24. Measurements of Photosynthetic Oxygen Evolution H.J. van Gorkom, P. Gast.

Biophysical techniques in photosynthesis

Vanadium in Biological Systems

copper proteins systems containing the “Blue” copper center

A DEER (double electron–electron resonance) spin–echo technique was applied to measure the electron–electron dipolar spectrum of a frozen toluene solution of the biradical, 2,6‐bis[(((2,2,5, 5‐tetramethyl‐1‐oxypyrrolin‐3‐yl)carbonyl)oxy)]‐anthracene. Modulation of the DEER spin–echo envelope was observed and identified as originating from the dipolar coupling between the two nitroxide spins of the biradical. Fourier transformation of the modulated components of the echo envelope yielded a dipolar spectrum from which a spin–pair separation of 19.73±0.14 A was calculated. Constraints on the relative orientation of the two nitroxide spin moieties were obtained by analysis of the effect of the microwave pulse orientational selectivity on the DEER modulation amplitudes. Molecular models of the studied compound exhibit structures that correspond well with the structural information deduced by DEER spectroscopy.

Double electron–electron resonance spin–echo modulation: Spectroscopic measurement of electron spin pair separations in orientationally disordered solids

Magnetically dilute crystalline powders of the following metal complex systems have been prepared by coprecipitation: copper salicylaldoxime in palladium salicylaldoxime, copper dimethylglyoxime in nickel dimethylglyoxime, copper picolinate in phthalic acid and also in zinc picolinate, copper 2‐methylpicolinate in zinc 2‐methylpicolinate, copper quinaldinate in zinc quinaldinate, copper 8‐hydroxyquinolinate in phthalimide and also in zinc 8‐hydroxyquinolinate, and silver picolinate in phthalic acid and also in zinc picolinate. These samples have been investigated with electron–nuclear double‐resonance (ENDOR) spectroscopy. At some settings of the magnetic field, single‐crystal‐type ENDOR spectra are obtained which arise substantially from molecules in a single orientation with respect to the magnetic field. At other settings of the field, powder‐type ENDOR spectra are found which arise from molecules in many orientations. In either case the spectra are simpler and easier to interpret than the EPR powder s...

Ligand ENDOR of Metal Complexes in Powders

Instrumentation for frequency‐swept electron–electron double resonance (ELDOR) at X band is described. Reliable control of the sweeping (pump) microwave frequency to 1 part in 106 (10 kHz) has been achieved. The technique has been applied to liquid and frozen solutions of DPPH in toluene. The isotropic nitrogen coupling constants are Aα = 7.933 ± 0.007 and Aβ = 9.739 ± 0.014 G including second‐order corrections. The ratio Aα / Aβ = 0.815. In the solid phase the largest hyperfine coupling to the α‐nitrogen | A + 2B |α and the largest hyperfine coupling to the β‐nitrogen | A + 2B |β are each 20.1 G. The literature on this molecule is reviewed and the present experiments discussed in this context. Much of the paper is devoted to the physical basis of ELDOR spectroscopy. An analysis of ELDOR line shapes is given in an appendix.

Frequency‐Swept Electron–Electron Double Resonance: DPPH in Liquid and Frozen Solution

Ligand ENDOR of Cu‐8‐hydroxyquin‐olinate Substituted into a Single Crystal and a Powder of Phthalimide

The technique of electron–nuclear double resonance (ENDOR) has been applied to copper‐8‐hydroxyquinolinate substituted into single crystals of phthalimide and of 8‐hydroxyquinoline. This method has permitted determination of the nitrogen hyperfine tensor, with sufficient accuracy to detect small orthorhombic deviations from axial symmetry, and the nitrogen quadrupole tensor. These tensors have the same principal axes and the coupling constants of the two interactions have opposite sign. The axis corresponding to the largest quadrupole splitting is oriented along the nitrogen lone pair in the plane of the metal complex. One g axis of the slightly monoclinic copper‐8‐hydroxyquinolinate complex is oriented approximately along the oxygen–copper axis, and the other in‐plane g axis is at a small angle with respect to the nitrogen–copper direction. The delocalization of the unpaired electron density to the ligand protons depends on the host lattice. The quadrupole tensor and its dependence on the host lattice are discussed. It is shown that the unpaired electron in the aromatic rings of 8‐hydroxyquinoline resides mainly in the σ system, in accordance with the symmetry of the ground state. The isotropic and anisotropic coupling constants of the proton adjacent to the nitrogen and some coupling components of other protons in the aromatic ligands and in the neighbouring molecules of the host lattice have been determined. The spin densities in the aromatic rings are positive.The technique of electron–nuclear double resonance (ENDOR) has been applied to copper‐8‐hydroxyquinolinate substituted into single crystals of phthalimide and of 8‐hydroxyquinoline. This method has permitted determination of the nitrogen hyperfine tensor, with sufficient accuracy to detect small orthorhombic deviations from axial symmetry, and the nitrogen quadrupole tensor. These tensors have the same principal axes and the coupling constants of the two interactions have opposite sign. The axis corresponding to the largest quadrupole splitting is oriented along the nitrogen lone pair in the plane of the metal complex. One g axis of the slightly monoclinic copper‐8‐hydroxyquinolinate complex is oriented approximately along the oxygen–copper axis, and the other in‐plane g axis is at a small angle with respect to the nitrogen–copper direction. The delocalization of the unpaired electron density to the ligand protons depends on the host lattice. The quadrupole tensor and its dependence on the host lattice ar...

Ligand ENDOR of Cu‐8‐Hydroxyquinolinate Substituted into Organic Single Crystals

Electron-nuclear double resonance has been used to study ligand hyperfine interactions of the copper (II) complex in a frozen solution of the blue protein stellacyanin. It is shown that the copper ion coordinates with at least one nitrogen ligand, and probably more than one, and that the copper ion is in a hydrophobic environment.

Günther H. Rist

Papers

Ligand ENDOR of Metal Complexes in Powders

Frequency‐Swept Electron–Electron Double Resonance: DPPH in Liquid and Frozen Solution

Ligand ENDOR of Cu‐8‐hydroxyquin‐olinate Substituted into a Single Crystal and a Powder of Phthalimide

Ligand ENDOR of Cu‐8‐Hydroxyquinolinate Substituted into Organic Single Crystals

Electron-Nuclear Double Resonance of a Protein That Contains Copper: Evidence for Nitrogen Coordination to Cu(II) in Stellacyanin.