Magnetic Ramanspectroscopy

Raman and resonance Raman spectroscopy are well established methods to investigate the vibrational frequencies of molecular systems. In particular, in the area of catalysis, one of the focal points of the MPI für Kohlenforschung, this spectroscopy is a valuable tool for acquiring experimental information about the electronic structure of catalysts and thereby of the catalytic mechanism. This can be achieved by either measuring the vibrational fingerprint of the final products, the educts, the metalorganic catalyst, or even of reaction intermediates, provided their lifetime is sufficiently large so that they can be trapped with freeze quench methods.A second spectroscopy, equally relevant in particular for the elucidation of catalytic mechanisms that involve one-electron redox steps, is electron paramagnetic resonance (EPR) spectroscopy. When one-electron redox chemistry occurs, paramagnetic reaction intermediates will occur.The energy window accessible by conventional Raman spectroscopy roughly spans a range from 100 cm^-1 to several 1000 cm^-1, although present-day monochromators allow detection of bands down to 5 cm^-1 relative to the Rayleigh line. On the other side, EPR spectroscopy, including the latest generation of high-field spectrometers, is typically used in the range from 0.1 cm^-1 to 9 cm^-1. Yet, oftentimes, the magnetic interactions in the system are larger than those accessible with even a state-of-the-art high-frequency EPR spectrometer. Although such systems are intrinsically magnetic, they are generally qualified as EPR silent systems and little experimental information is typically be retrieved. A prime example is high-spin Fe^II, 3d⁶, with a total spin S =2. In this integer spin system, the interaction between the unpaired electrons, including second-order spin-orbit contributions, known together as zero-field splitting, is notoriously difficult to measure directly by EPR spectroscopy. A second example of a field of research where EPR cannot be used to its fullest extent is the field of molecular magnetism, where magnetic interactions on the order of 20 cm^-1 to 150 cm^-1 occur. This energy window has to the best of our knowledge been dark for any spectroscopy, and has become generally known as the “Terahertz gap”.

The extractable information from the THz gap is manifold. First and foremost THz spectroscopy is relevant for catalysis: besides being able to for the first time directly measure electronic structure parameters in this frequency regime and analyzing these parameters with the aim to determine the reaction mechanisms together with state-of-the-art quantum chemical methods, for which the Neese group has established itself as one of the world-leading groups, the energy window of the THz gap is also the window where electron-phonon coupling occurs. Thus, the ability to directly measure frequencies that correspond to electron-phonon couplings allows direct information about the onset of energy transfer processes themselves. Thirdly, by building a magnetic Resonance-Raman spectrometer for operation in the THz regime, one could even envision actively steering energy transfer in situ, since the magnetic field allows for a coupling or decoupling of the electron-phonon interactions. Lastly, recent developments in the field of FTIR spectroscopy resulted in the construction of a narrowband (<0.0007 cm^-1) THz source which can go down to 3 cm^-1 in energy. As such, the development of a magnetic Resonance-Raman spectrometer combined with a THz source is timely and allows a full flexibility of experiments in terms of Raman-detected magnetic resonance, as well as employing the strength of the underlying Raman and EPR methods, e.g., in terms of studying crystalline materials under polarized-light conditions.

These experiments have never been systematically attempted before. As such, even the theory of magnetic Resonance-Raman spectroscopy is in its infancy. As part of this project, the theory of magnetic Resonance-Raman spectroscopy will be established and a magnetic Raman spectrometer will be built.