Understanding how a small, gas-phase molecule containing an actinide atom reacts with other molecules helps us better understand the chemistry of heavy elements. These elements are often available in quantities as small as one millionth of a gram. This study identified an extreme in the chemical behavior of curium. Curium lies at the center of the actinide series on the periodic table. It is at the transition between the rare heavy elements and the very rare, very heavy elements.
The periodic table of elements is key to how scientists understand and explain how chemicals behave. Chemists must understand the synthetic rare radioactive heavy elements in context, to understand the periodic table. These studies combined experiments with computation to examine a subset of the actinides, protactinium (Pa) through einsteinium (Es). The results revealed new, unexpected chemistry. These discoveries will help scientists better understand these elements’ role in the periodic table.
In this study, scientists synthesized the gas-phase curium dioxide cation containing the dominant natural oxygen-16 isotope, Cm16O2+. They then reacted it with water labeled with the rarer oxygen-18 isotope, H218O. The experiments were performed in a quadrupole ion trap mass spectrometer. This spectrometer had been modified to study gas-phase ion chemistry of radioactive actinide elements. The observed O-atom exchange of Cm16O2+ with H218O, yielding Cm16O18O+ and H216O, revealed a distinctive increase in actinide reactivity between Am (americium), which does not similarly exchange, and Cm. A parallel effort focused on theory employed coupled cluster CCSD(T) computations to rationalize the observations. It also identified extreme chemistry at Cm, indicated by very long Cm-O bonds in the results. The observed distinctive behavior of curium in its pentavalent oxidation state, Cm(+V), is attributed to the high stability of the half-filled 5f subshell. This result further illuminates the role of these characteristic electron orbitals in actinide chemistry. This union of experiment and theory in elementary gas-phase chemistry provides fundamental insights into the nature of the actinide period of elements and thus, the greater periodic table.
Lawrence Berkeley National Laboratory
The University of Alabama
Washington State University
The Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences, Heavy Element Chemistry Program. The isotopes used in this research were supplied by the U.S. Department of Energy Isotope Program, managed by the Office of Science for Nuclear Physics.
T. Jian, P. D. Dau, D. K. Shuh, M. Vasiliu, D. A. Dixon, K. A. Peterson, J. K. Gibson, “Activation of water by pentavalent actinide dioxide cations: Characteristic curium revealed by a reactivity turn after americium.” Inorg. Chem. 58, 14005 (2019) [DOI: 10.1021/acs.inorgchem.9b01997].
M. Vasiliu, J. K. Gibson, K. A. Peterson, D. A. Dixon, “Gas phase hydrolysis and oxo-exchange of actinide dioxide cations: Elucidating intrinsic chemistry from protactinium to einsteinium.” Chem. Eur. J. 25, 4245 (2019) [DOI: 10.1002/chem.201803932].