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de Melo, Gabriel F.

Publications and source records attributed to de Melo, Gabriel F..

10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Structural chemistry of penta- and hexanitrato thorium($\tiny{IV}$) complexes isolated using N–H donors

In this study, a series of fifteen tetravalent thorium phases were prepared. The compounds were isolated from acidic aqueous nitrate solutions using protonated nitrogen heterocycles of varying hydrogen-bond donation strength. Structural analysis via single crystal X-ray diffraction showed that the structures are built from pentanitrato, [Th(NO 3 ) 5 (H 2 O) 2 ] 1- , and hexanitrato, [Th(NO 3 ) 6 ] 2- , molecular units, with the latter being far more prevalent in the solid state. The vibrational properties of the compounds were examined using Raman and IR spectroscopy; the spectra are dominated by stretches characteristic of nitrate and the organic ions. The relative energetics of nitrate complexation was examined using electronic structure theory. These results confirmed that there are clear thermodynamic sinks for the penta- and hexanitrato structural units that were observed experimentally. Additionally, electrostatic surface potentials (ESPs) were calculated in an effort to better understand the counterion stabilization of the complexes. The ESP surfaces showed that the position of the water and nitrate molecules and the coordination geometry of the metal complex had a clear effect on the polarizability of the two structural motifs. Despite limited speciation of the Th–nitrate structural units, the compounds exhibit rich supramolecular chemistry resulting from hydrogen bonding of the Th complexes with the organic N–H donors and π–π stacking interactions from the protonated N-heterocycles.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The Crucial Role of Sb 2 F 10 in the Chemical Synthesis of F 2

The purely chemical synthesis of fluorine is a spectacular reaction which for more than a century had been believed to be impossible. In 1986, it was finally experimentally achieved, but since then this important reaction has not been further studied and its detailed mechanism had been a mystery. The known thermal stability of MnF 4 casts serious doubts on the originally proposed hypothesis that MnF 4 is thermodynamically unstable and decomposes spontaneously to a lower manganese fluoride and F 2 . This apparent discrepancy has now been resolved experimentally and by electronic structure calculations. It is shown that the reductive elimination of F 2 requires a large excess of SbF 5 and occurs in the last reaction step when in the intermediate [SbF 6 ][MnF 2 ][Sb 2 F 11 ] the addition of one more SbF 5 molecule to the [SbF 6 ] - anion generates a second tridentate [Sb 2 F 11 ] - anion. The two tridentate [Sb 2 F 11 ] - anions then provide six fluorine bridges to the Mn atom thereby facilitating the reductive elimination of the two fluorine ligands as F 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Theoretical and Experimental Study of the Spectroscopy and Thermochemistry of UC +/0/–

A combination of high-level ab initio calculations and anion photoelectron detachment (PD) measurements is reported for the UC, UC – , and UC + molecules. To better compare the theoretical values with the experimental photoelectron spectrum (PES), a value of 1.493 eV for the adiabatic electron affinity (AEA) of UC was calculated at the Feller–Peterson–Dixon (FPD) level. The lowest vertical detachment energy (VDE) is predicted to be 1.500 eV compared to the experimental value of 1.487 ± 0.035 eV. A shoulder to lower energy in the experimental PD spectrum with the 355 nm laser can be assigned to a combination of low-lying excited states of UC – and excited vibrational states. The VDEs calculated for the low-lying excited electronic states of UC at the SO-CASPT2 level are consistent with the observed additional electron binding energies at 1.990, 2.112, 2.316, and 3.760 eV. Potential energy curves for the Ω states and the associated spectroscopic properties are also reported. Compared to UN and UN + , the bond dissociation energy (BDE) of UC (411.3 kJ/mol) is predicted to be considerably lower. The natural bond orbitals (NBO) calculations show that the UC 0/+/– molecules have a bond order of 2.5 with their ground-state configuration arising from changes in the oxidation state of the U atom in terms of the 7s orbital occupation: UC (5f 2 7s 1 ), UC – (5f 2 7s 2 ), and UC + (5f 2 7s 0 ). Furthermore, the behavior of the UN and UC sequence of molecules and anions differs from the corresponding sequences for UO and UF.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Electronic Properties of UN and UN – from Photoelectron Spectroscopy and Correlated Molecular Orbital Theory

In this work, the results of calculations of the properties of the anion UN – including electron detachment are described, which further expand our knowledge of this diatomic molecule. High-level electronic structure calculations were conducted for the UN and UN – diatomic molecules and compared to photoelectron spectroscopy measurements. The low-lying Ω states were obtained using multireference CASPT2 including spin-orbit effects up to ~20,000 cm –1 . At the Feller–Peterson–Dixon (FPD) level, the adiabatic electron affinity (AEA) of UN is estimated to be 1.402 eV and the vertical detachment energy (VDE) is 1.423 eV. The assignment of the UN excited states shows good agreement with the experimental results with a VDE of 1.424 eV. An Ω = 4 ground state was obtained for UN – which is mainly associated with the 3 H ΛS state. Thermochemical calculations estimate a bond dissociation energy (BDE) for UN – (U – + N) of 665.9 kJ/mol, ~15% larger than that of UN and UN + . The NBO analysis reveals U–N triple bonds for the UN, UN – , and UN + species.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Periodic Trends within Actinyl(VI) Nitrates and Their Structures, Vibrational Spectra, and Electronic Properties

In this work, a series of actinyl(VI) nitrate salts of the form MAnO 2 (NO 3 ) 3 , where M = NH 4 + K + , Rb + , Cs + , and Me 4 N + and AnO 2 2+ = U, Np, Pu, and AnO 2 (NO 3 ) 2 (H 2 O) 2 ·H 2 O, and the uranyl tetranitrates M 2 UO 2 (NO 3 ) 4 have been synthesized from aqueous solution and their structures determined using single-crystal X-ray diffraction. Together, these complexes represent an isostructural series of actinide complexes among the salts crystallized with the same charge-compensating cation and have been studied using vibrational spectroscopy including Raman and Fourier-transform infrared. Periodic trends in both the structural properties of these complexes and their vibrational spectra are presented and discussed, in particular the invariant nature of the O≡An≡O asymmetric stretching frequencies observed across the actinyl series. Electronic structure calculations were performed at a variety of levels of theory to aid in the interpretation of the vibrational data and to correlate trends in the data with the underlying electronic properties of these molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Experimental and Computational Description of the Interaction of H and H – with U

In this work, the results of ab initio correlated molecular orbital theory electronic structure calculations for low-lying electronic states are presented for UH and UH – and compared to photoelectron spectroscopy measurements. The calculations were performed at the CCSD(T)/CBS and multireference CASPT2 including spin–orbit effects by the state interacting approach levels. The ground states of UH and UH – are predicted to be 4 Ι 9/2 and 5 Λ 6 , respectively. The spectroscopic parameters T e , r e , ω e , ω e x e , and Be were obtained, and potential energy curves were calculated for the low energy Ω states of UH. The calculated adiabatic electron affinity is 0.468 eV in excellent agreement with an experimental value of 0.462 ± 0.013 eV. The lowest vertical detachment energy was predicted to be 0.506 eV for the ground state, and the adiabatic ionization energy (IE) is predicted to be 6.116 eV. The bond dissociation energy (BDE) and heat of formation values of UH were obtained using the IE calculated at the Feller–Peterson–Dixon level. For UH, UH – , and UH + , the BDEs were predicted to be 225.5, 197.9, and 235.5 kJ/mol, respectively. The BDE for UH is predicted to be ~20% lower in energy than that for ThH. The analysis of the natural bond orbitals shows a significant U + H – ionic component in the bond of UH.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗