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A theoretical investigation of the hydrolysis of uranium hexafluoride: the initiation mechanism and vibrational spectroscopy

Journal Article · · Physical Chemistry Chemical Physics. PCCP
DOI:https://doi.org/10.1039/d1cp05268c· OSTI ID:1888555
 [1];  [2];  [2];  [1];  [3];  [1]
  1. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
  2. ENSCO, Inc., Melbourne, FL (United States)
  3. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
+ Show Author Affiliations
Depleted uranium hexafluoride (UF6), a stockpiled byproduct of the nuclear fuel cycle, reacts readily with atmospheric humidity, but the mechanism is poorly understood. Here we compare several potential initiation steps at a consistent level of theory, generating underlying structures and vibrational modes using hybrid density functional theory (DFT) and computing relative energies of stationary points with double-hybrid (DH) DFT. A benchmark comparison is performed to assess the quality of DH-DFT data using reference energy differences obtained using a complete-basis-limit coupled-cluster (CC) composite method. The associated large-basis CC computations were enabled by a new general-purpose pseudopotential capability implemented as part of this work. Dispersion-corrected parameter-free DH-DFT methods, namely PBE0-DH-D3(BJ) and PBE-QIDH-D3(BJ), provided mean unsigned errors within chemical accuracy (1 kcal mol-1) for a set of barrier heights corresponding to the most energetically favorable initiation steps. The hydrolysis mechanism is found to proceed via intermolecular hydrogen transfer within van der Waals complexes involving UF6, UF5OH, and UOF4, in agreement with previous studies, followed by the formation of a previously unappreciated dihydroxide intermediate, UF4(OH)2. The dihydroxide is predicted to form under both kinetic and thermodynamic control, and, unlike the alternate pathway leading to the UO2F2 monomer, its reaction energy is exothermic, in agreement with observation. Finally, harmonic and anharmonic vibrational simulations are performed to reinterpret literature infrared spectroscopy in light of this newly identified species.
Research Organization:
Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States)
Sponsoring Organization:
USDOE National Nuclear Security Administration (NNSA)
Grant/Contract Number:
NA0003525
OSTI ID:
1888555
Report Number(s):
SAND2022-3458J; 704440
Journal Information:
Physical Chemistry Chemical Physics. PCCP, Journal Name: Physical Chemistry Chemical Physics. PCCP Journal Issue: 16 Vol. 24; ISSN 1463-9076
Publisher:
Royal Society of ChemistryCopyright Statement
Country of Publication:
United States
Language:
English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY