Electronic Structure and Function by Electron Paramagnetic Resonance (EPR)

The Leiden Spin Group concentrates on the study of the electronic and geometric structure and the dynamics of (transient) paramagnetic molecules and centers in the condensed phase. Metal centers in bio-inorganic complexes and in proteins are being studied, spin-labeled proteins, organic radicals and triplet states, and defects in semi-conductor nano-particles. Spin-spin interactions of electrons are exploited, between spin labels and between a paramagnetic metal center and a spin label, as well as electron-nuclear hyperfine interactions.

 

Use is made of advanced EPR techniques, which largely concern in-house instrumental developments. The group is a pioneer of high-frequency EPR, with the first pulsed EPR spectrometer at 95 GHz (W-band) and the recent development of a continuous-wave and pulsed EPR spectrometer at 275 GHz (J-band). Both cw and pulsed EPR experiments are performed as well as Electron-Nuclear DOuble Resonance (ENDOR) experiments, from room temperature to liquid helium temperatures, on solutions, solids and crystals.

 

As compared to the classical EPR frequency of 9 GHz (X-band), the application of higher microwave frequencies provides enhanced electron Zeeman resolution, enhanced absolute sensitivity, enhanced nuclear Zeeman resolution, the possibility to study nuclei with small magnetic moment by ENDOR, and the possibility to study paramagnetic centers with a substantial zero-field splitting. Even more important is the fact that the availability of higher frequencies allows a multi-frequency approach of a research question.

 

The EPR observables concern the g tensor, the dipolar- and exchange tensors, and the nuclear hyperfine and quadrupole tensors. These observables provide a fingerprint of the electronic wave function and distance information to determine structure. In addition, line shape analysis of EPR spectra enables the study of reorientation dynamics of molecules or spin-carrying groups.

 

When necessary, complementary techniques are being used: optical absorption and emission spectroscopy, optically detected magnetic resonance, and quantum-chemical calculations.