Uranium-bearing phases in Hanford nuclear waste

Journal of Radioanalytical and Nuclear Chemistry - Uranium is the most prevalent radioactive element in the 210 million liters of nuclear waste at the Hanford Site but the uranium speciation has...     

Influence of broken-pair excitations on the exact pair wavefunction

Doubly occupied configuration interaction (DOCI), the exact diagonalisation of the Hamiltonian in the paired (seniority zero) sector of the Hilbert space, is a combinatorial cost wave function that can be very efficiently approximated by pair coupled cluster doubles (pCCD) at mean-field computational cost. As such, it is a very interesting candidate as a starting point for building the full configuration interaction (FCI) ground state eigenfunction belonging to all (not just paired) seniority sectors. The true seniority zero sector of FCI (referred to here as FCI₀) includes the effect of coupling between all seniority sectors rather than just seniority zero, and is, in principle, different from DOCI. We here study the accuracy with which DOCI approximates FCI₀. Using a set of small Hubbard lattices, where FCI is possible, we show that DOCI ～ FCI₀ under weak correlation. However, in the strong correlation regime, the nature of the FCI₀ wavefunction can change significantly, rendering DOCI and pCCD a less than ideal starting point for approximating FCI.     

Through‐space 19F–19F spin–spin coupling in ortho‐fluoro Z‐azobenzene

We report through‐space (TS) ¹⁹F–¹⁹F coupling for ortho‐fluoro‐substituted Z ‐azobenzenes. The magnitude of the TS‐coupling constant (^TSJ_FF) ranged from 2.2–5.9 Hz. Using empirical formulas reported in the literature, these coupling constants correspond to non‐bonded F–F distances (d_FF) of 3.0–3.5 Å. These non‐bonded distances are significantly smaller than those determined by X‐ray crystallography or density functional theory, which argues that simple models of ¹⁹F–¹⁹F TS spin–spin coupling solely based d_FF are not applicable. ¹H, ¹³C and ¹⁹F data are reported for both the E and Z isomers of ten fluorinated azobenzenes. Density functional theory [B3YLP/6‐311++G(d,p)] was used to calculate ¹⁹F chemical shifts, and the calculated values deviated 0.3–10.0 ppm compared with experimental values. Copyright © 2015 John Wiley & Sons, Ltd.