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Thu Jul 16, 2020, 05:33 PM

The Effect of Pressure on Enhancing Covalent Bonding in Curium Complexes. [View all]

The paper I'll discuss in this post is this one: Compression of curium pyrrolidine-dithiocarbamate enhances covalency (Eva Zurek et al., Nature 583, pages 396–399(2020))

Sometime ago, in this space, I wrote about the critical masses of several americium isotopes: Critical Masses of the Three Accessible Americium Isotopes.

Perhaps in a smarter world than the one in which we live, the potentially very valuable properties of americium based nuclear fuels will be recognized, only one of which will be as a source of what is sure to be, in the not so distant future, a very, very, very rare yet useful gas, helium, which our generation is pissing away in typical disregard for all who come after us. (The technological importance of helium, seldom appreciated, far outstrips its importance in children's balloons.)


Because of the high energy to mass ratio of nuclear fuels - the very thing that makes them environmentally superior to all other forms of primary energy - the amount of helium produced by americium fueled reactors will be relatively small, but in the case where prices rise to extremes because of scarcity, may have significant economic import. Although Americium does release helium in its normal decay, in the case of Am-241 to neptunium, and in the case of Am-243 to plutonium, the long half-lives of these elements prevents this element from ever becoming an important source of helium when relying on ordinary nuclear decay. However as a nuclear fuel, necessarily in a fast neutron spectrum, owing to the high capture to fission ratio, significant quantities of helium will be produced from the nuclear decay of curium, specifically curium's isotopes 242 and 244, as well as the decay daughter of 242, Pu-238.

Thus the chemistry of curium is a worthy study, although one doesn't see as many explorations of curium chemistry as perhaps one would like.

Curium is a relatively pedestrian element in comparison to the earlier actinides, nothing like the wild and woolly chemistry of plutonium for example, and it is the first element among that actinides that behaves very much like an lanthanide in most respects, dominated by the 3+ oxidation state. The abstract, which should be open, describes why this is succinctly:

Curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chemical bonds that engage the 5f shell1,2,3. This is even more pronounced in gadolinium, curium’s lanthanide analogue, owing to the contraction of the 4f orbitals with respect to the 5f orbitals4...

...but the abstract continues...

owever, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons5. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms5,6. Therefore, the question arises of whether the frontier metal orbitals in curium(III)–ligand interactions can also be modified by applying pressure, and thus be induced to form metal–ligand bonds with a degree of covalency. Here we report experimental and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium–sulfur bonds in [Cm(pydtc)4]− (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals)...

Normal atmospheric pressure is about 100 kilopascals, so we're talking pressures that are thousands of times atmospheric pressure.

The authors worked with Cm-248 which is of (relatively) lower radioactivity owing to its very long half life, 348,000 years, as well as the even lower radioactivity of its daughter, plutonium-244, half-life 80 million years. Plutonium-244 is considered an extinct nuclei which most likely survived long enough to accrete with the earth; the existence of a xenon isotope in our atmosphere is believed to have resulted from the spontaneous fission of this isotope. (A few atoms of possibly primordial Pu-244 have reportedly identified in very old lanthanide ores in California.) There is also a curium isotope that is considered to be an extinct nuclide, Cm-247, which has a half-life of around 15.6 million years, but it is more difficult to obtain because of its high fission cross section, and in any case, its decay daughter Pu-243 is very radioactive, and leads to the fairly radioactive daughter Am-243.

Anyway, the authors made a pyrollidine thiocarbamate complex of curium to study the effects of pressure.

A picture:

The caption:

a, Thermal ellipsoid plot of the [Cm(pydtc)4]− anion in Cm-1, with the Cm3+ cation complexed by four bidentate pydtc− ligands, plotted at 50% probability. b, Structure of the Cm-1 anion, showing the ligand environment.

They then conducted spectral analysis of the ammonium salt of this complex at various pressures:

The caption:

a, Temperature-dependent spectra of Cm-1 showing the large splitting of the Stark levels of the emissive state. b, Pressure-dependent spectra of Cm-1 showing how the sharp features broaden upon the application of pressure. The excitation wavelength is λex = 420 nm for both spectra.

"Stark Levels" by the way, were discovered by Nobel Laureate Johannes Stark. There was on the shelf in Princeton's Firestone library in German written by him, a fascinating book through which I leafed one time - it's now been carried off to the Recap, and one would be embarrassed to request it. The guy would be right up Trump's ally except for the fact that Stark was a scientist, and Trump despises science, inasmuch as the book I found by Stark was written in the early 1930's and was all about how wonderful Adolf Hitler would be for German science, proving definitively that a criterion for winning the Nobel Prize does not generally include any requirement for possessing character.

Hitler destroyed German science, and Stark, a notably anti-semite, was arrested after World War II and subject to de-Nazification steps, whatever de-Nazification involved. I doubt he could be reformed any more than Bill Barr or Steven Miller or Donald Trump could be "De-nazified."

Sorry, I didn't mean to divert from the scientific subject.

Another complex of curium utilized is a metillate complex, that is a salt of benzene hexacarboxylic acid. It's called "Cm-2."

Here's a graphic comparing the pressure effects of the two complexes.

The caption:

a–d, Emission intensity (a) and λmax (b) of Cm-1, and emission intensity (c) and λmax (d) of Cm-2. The steady decrease in intensity as well as the steady increase in wavelength for Cm-1 demonstrates smooth changes upon pressurization, whereas Cm-2 does not show a consistent change in its intensity plot. The peak position of Cm-2 does increase as pressure is applied; however, it increases at a much slower rate and without the linear trend observed for Cm-1. Error bars represent the standard error in the pressure calculation.

The authors perform some in silico (DFT) analysis to generate this molecular orbital cartoon:

Example showing the greatest change in the Cm–S bond composition by the application of pressure. Comparing the selected NLMO at 0 and 11 GPa shows that the main difference is the increased Cm contribution at the higher pressure, as well as the 5f involvement. Further NLMO compositions can be found in Extended Data Fig. 3.

The authors conclude thus:

On the basis of the above experimental data and theory, we can conclude that the greater radial extension of 5f shells compared to 4f shells is not the sole factor dictating these pressure-induced changes in bonding. The nature of the ligands also affects the overlap of the f orbitals with the ligand orbitals, in addition to the extension of the metal electronic shells. In Cm-1, the increased polarizability of the pydtc− ligands compared to mellitate clearly plays an important role in its pressure response. These observations could provide guidance for the selection of actinide systems to study at high pressures.

How much practical import this work might have I can't say. It may be true that pressure changes may represent a tool in the difficult separation of curium from the lanthanides, allowing for the recovery and use of lanthanide fission products in the presence of large amounts of curium. Pressure plays an increasing role in separations chemistry, and it's conceivable at least, that this work may show the way.

Such separations may well be increasingly important if we are to save the world using nuclear energy.

I've been thinking about such separations quite a bit lately. I started to write a post about some papers I read on the separation of xenon and krypton - both of which are also fission products - which kind of caused my head to explode, and sent me off on tangents about chemistry about which I haven't thought in a long time, and even some I never knew. As I age, I rarely experience the emotions connected with an entirely new way of thinking about things, and so I haven't found time to write here all that much, since this rare thing happened, not that anything I write is of all that much general interest. I do learn things however when I write these posts, and that I suppose, is the selfish point.

Anyway. In spite of Covid, I hope you are managing to enjoy the summer season and the growing realization of how lucky we are to be alive in amazing, if hard, times.

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