Unravelling tautomeric mixtures

RIXS at BESSY II allows to see clearly

A team at HZB has developed a method of experimentally unravelling tautomeric mixtures. Based on resonant inelastic X-ray scattering (RIXS) at BESSY II, not only proportions of the tautomers can be deduced, but the properties of each individual tautomer can be studied selectively. This method could yield to detailed information on the properties of molecules and their biological function. In the present study, now advertised on the cover of “The Journal of Physical Chemistry Letters” the technique was applied to the prototypical keto-enol equilibrium.

Many (organic) molecules exist as a mixture of two almost identical molecules, with the same molecular formula but one important difference: A single hydrogen atom sits in a different position. The two isomeric forms transform into each other, creating a delicate equilibrium, a “tautomeric” mixture. Many amino acids are tautomeric mixtures, and since they are building blocks of proteins, they may influence their shape and function and thus their biological functions in organisms.

Until now: Mission impossible

Until now, it has been impossible to selectively investigate the electronic structure of such tautomeric mixtures experimentally: Classical spectroscopic methods “see” only the sum of the signals of each molecular forms – the details of the properties of the two individual tautomers cannot be determined.

Now at BESSY II: it works

A team led by HZB physicist Prof. Alexander Föhlisch has now succeeded in providing a method of experimentally unravelling tautomeric mixtures. Using inelastic X-ray scattering (RIXS) and a data processing/evaluation method newly developed at HZB, the individual proportions of the tautomers can be clearly deduced from the measured data. “We can experimentally separate the signal of each individual molecule in the mixture by X-ray scattering, which leads to a detailed insight into their functionality and chemical properties,” says Dr. Vinicíus Vaz Da Cruz, first author of the paper and postdoc in Föhlisch’s team.

Read more on the HZB website

Image: The illustration visualises the experimental method, here on the prototypical keto-enol equilibrium. It appears on the cover of “The Journal of Physical Chemistry Letters”.

Credit: © Martin Künsting / HZB

New discovery will have huge impact on the development of future battery cathodes

A new paper published today in Nature Energy reveals how a collaborative team of researchers have been able to fully identify the nature of oxidised oxygen in the important battery material – Li-rich NMC – using RIXS (Resonant Inelastic X-ray Scattering) at Diamond. This compound is being closely considered for implementation in next generation Li-ion batteries because it can deliver a higher energy density than the current state-of-the-art materials, which could translate to longer driving ranges for electric vehicles. They expect that their work will enable scientists to tackle issues like battery longevity and voltage fade with Li-rich materials.

The paper, ‘First cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk’ by a joint team from the University of Oxford, the Henry Royce and Faraday Institutions and Diamond, examines the results of their investigations to better understand the important compound known in the battery industry as Li-rich NMC (or Li1.2Ni0.13Co0.13Mn0.54O2).   

Principal Beamline Scientist on I21 RIXS at Diamond, Kejin Zhou,said:

Our work is much about understanding the mysterious first cycle voltage hysteresis in which the O-redox process cannot be fully recovered resulting in the loss of the voltage hence the energy density.

Read more on the Diamond website

Image: A previous study (Nature 577, 502–508 (2020)) into this process made by the same research team, at the I21 beamline at Diamond, reported that, in Na-ion battery cathodes, the voltage hysteresis is related to the formation of molecular O2 trapped inside of the particles due to the migration of transition metal ions during the charging process.

Classic double-slit experiment in a new light

An international research team led by physicists from Collaborative Research Centre 1238, ‘Control and Dynamics of Quantum Materials’ at the University of Cologne has implemented a new variant of the basic double-slit experiment using resonant inelastic X-ray scattering at the European Synchrotron ESRF in Grenoble. This new variant offers a deeper understanding of the electronic structure of solids. Writing in Science Advances, the research group have now presented their results under the title ‘Resonant inelastic x-ray incarnation of Young’s double-slit experiment’.

The double-slit experiment is of fundamental importance in physics. More than 200 years ago, Thomas Young diffracted light at two adjacent slits, thus generating interference patterns (images based on superposition) behind this double slit. That way, he demonstrated the wave character of light. In the 20th century, scientists have shown that electrons or molecules scattered on a double slit show the same interference pattern, which contradicts the classical expectation of particle behaviour, but can be explained in quantum-mechanical wave-particle dualism. In contrast, the researchers in Cologne investigated an iridium oxide crystal (Ba3CeIr2O9) by means of resonant inelastic X-ray scattering (RIXS).

>Read more on the European Synchrotron website

Image: Beamline ID20, where the experiments took place.
Credit: P. Jayet.