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Scientists from Los Alamos National Laboratory and the University of California – Irvine are exploring a new oxidation state of plutonium. Credit Los Alamos National Laboratory

Researchers have identified the +2 oxidation state in a molecular system of plutonium.

The findings by researchers at Los Alamos National Laboratory in collaboration with the University of California- Irvine provide a significant step towards a more complete understanding of chemical trends across the actinide series and ultimately will provide knowledge about how to manipulate and control oxidation-state chemistry and electronic structure.

“This finding marks out plutonium, already known for its extremely complex chemistry, as the actinide element with the largest number of confirmed oxidation states,” Andrew Gaunt, lead Los Alamos investigator, said in a statement.

One of the most fundamental properties of an element’s chemical behavior is its oxidation state chemistry, which relates to the number of electrons that are removed from or added to a neutral element to form cations or anions.

Most accessible oxidation states are generally presumed to have already been identified over the last century for the elements of the periodic table. However, plutonium was revealed as among the most complex of all elements, with just six oxidation states previously known and verified.

“Over time, it's easy to get bogged down in the daily but important, details of more incremental experiments and analyzing data but this project was the kind of research experience and more profound fundamental discovery that excites,” Gaunt said. “This is exactly the kind of plutonium chemistry that the Laboratory is uniquely positioned to achieve in conjunction with academic collaborators.

The research built on prior work by UCI professors that showed the +2 oxidation state of the lanthanides, uranium and thorium could be generated using organometallic anions to facilitate reduction of molecules containing a +3 metal cation to molecules containing a +2 metal cation.

These +2 molecules were shown to be accessible on account of the organometallic framework around the metal ion, which allows the extra electron upon reduction of +3 to +2 to populate a ‘d’ rather than an ‘f’ orbital.

After successful isolating the molecule containing Pu +2, the research team revealed that unlike in the lanthanide, uranium and thorium analogs, plutonium appears to exhibit an intriguing electronic structure ‘crossover point’ with the extra electron primarily residing in an ‘f’ orbital, not a ‘d’ orbital.

“Elucidating the nature of this electronic structure 'break' in actinide +2 cations as the actinide series is traversed was previously inaccessible but we have now made the breakthrough and the area is brimming with potential for further exploration,” Los Alamos investigator Stosh Kozimor, said in a statement.

“Working with the highly radioactive elements beyond uranium (called the transuranics) is extremely technically challenging, given limited availability of isotopes of these elements and the safety protocols under which the research must be conducted, but we have now paved the way for future discovery of additional transuranic molecules containing the +2 oxidation state,” Kozimor added.

The study was published in the Journal of the American Chemical Society.  

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