Electron spin echo envelope modulation studies of mixed valence manganese complexes: applications to the catalytic manganese cluster of photosynthetic oxygen evolution

Abstract

Abstract Professor Erwin Hahn’s 1950 discovery of spin echoes has opened a number of areas in experimental physics and chemistry [ 1] . Among these is the use of the electron spin echo envelope modulation (ESEEM) technique, such as described by Rowanet al.[2], which has become an important tool for measuring the spin transitions of magnetic nuclei magnetically coupled to EPRvisible electron paramagnets. In our applications, we use the ESEEM technique to study the coordination of ligands containing magnetic nuclei to metal centres active in biological catalysis. Details of experimental and theoretical aspects of the use of the ESEEM technique are provided in a number of good references [3-11] . In a brief review, the ESEEM technique requires the use of a pulsed EPR spectrometer, in which powerful microwave pulses are employed to affect large rotations of the net electron spin magnetization on a time-scale short compared to the relaxation processes which return the electron spin magnetization to equilibrium. For very narrow EPR signals, the EPR spectrum can be obtained by Fourier analysis of the Free Induction Decay (FID) following a single microwave pulse. However, the large bandwidths of most EPR signals, in particular those associated with transition metals, preclude such simple pulsed EPR experiments. In such cases, only a small fraction of an inhomogeneously broadened EPR line is excited by the microwave pulse.