The chemistry of actinides plays an important role in the nuclear fuel cycle. Limited experimental data exists, however, for transuranium molecules arising from the difficulty in production and safe handling of these species. This is particularly true for those in the gas phase. For instance, the only gaseous actinide molecule with an experimental heat of formation with an uncertainty within 1 kcal/mol is UF6. Accurate gas phase thermochemistry, however, can be essential to understanding the stability of actinide species in more complex environments. High level quantum chemical calculations are now capable of filling these gaps, but like all actinides, transuranium species can be very challenging to investigate with ab initio methods due to the large number of electrons, strong relativistic effects, and partially filled 6d and 5f shells, leading to a large range of available oxidation states. In the present work, the Feller-Peterson-Dixon composite approach is employed to calculate the atomization energies, heats of formation, and bond dissociation energies for a series of actinide (An) halides, AnXm (X=F, Cl, Br, m=4-6), with An=U, Np, and Pu. The largest component to the total energies is from a complete basis set (CBS) extrapolation using recently developed eXact 2-component (X2C) relativistic correlation consistent basis sets (cc-pVnZ-X2C) for U, Np, and Pu at the CCSD(T) level of theory. Corrections are then included for contributions from correlation effects from the outer-core (5s, 5p, 5d) electrons, the Lamb shift, zero-point vibrational effects, and spin-orbit coupling. Scalar relativistic effects are included from the onset by utilizing the X2C Hamiltonian. Spin-orbit calculations used the full 4-component(4c) Dirac-Coulomb-Gaunt Hamiltonian.

Ab initio thermochemistry of transuranium molecules: Halides of U, Np, and Pu

255th National Meeting (2018)

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