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J Phys Chem A ; 117(23): 4988-95, 2013 Jun 13.
Artigo em Inglês | MEDLINE | ID: mdl-23675905


Enthalpies of formation for the ThX4 and UX4 (X = F, Cl, OH) species have been investigated with density functional theory and coupled-cluster methods. ThX4 molecules are all confirmed as tetrahedral, while all UX4 molecules are predicted to adopt D2d symmetry using density functional theory. Multireference coupled cluster approaches confirm the D2d symmetry for UF4. The bonding is mostly ionic, and predicted formation energies for the halogen species show good agreement with experiment. Our calculated hydration energy of UF4 (-54.0 kcal/mol) is in very good agreement with the experimental data (-54.8 kcal/mol). We predict CCSD(T) formation energies of ΔfG[U(OH)4(g)] = -286.3 kcal/mol and ΔfG[U(OH)4(aq)] = -318.7 kcal/mol. ΔfG[U(OH)4(aq)] is 21 kcal/mol less stable than the established experimental thermodynamic data.

Compostos Organometálicos/química , Teoria Quântica , Termodinâmica , Tório/química , Urânio/química
Inorg Chem ; 51(5): 3016-24, 2012 Mar 05.
Artigo em Inglês | MEDLINE | ID: mdl-22339109


Ab initio molecular dynamics simulations at 300 K, based on density functional theory, are performed to study the hydration shell geometries, solvent dipole, and first hydrolysis reaction of the uranium(IV) (U(4+)) and uranyl(V) (UO(2)(+)) ions in aqueous solution. The solvent dipole and first hydrolysis reaction of aqueous uranyl(VI) (UO(2)(2+)) are also probed. The first shell of U(4+) is coordinated by 8-9 water ligands, with an average U-O distance of 2.42 Å. The average first shell coordination number and distance are in agreement with experimental estimates of 8-11 and 2.40-2.44 Å, respectively. The simulated EXAFS of U(4+) matches well with recent experimental data. The first shell of UO(2)(+) is coordinated by five water ligands in the equatorial plane, with the average U═O(ax) and U-O distances being 1.85 Å and 2.54 Å, respectively. Overall, the hydration shell structure of UO(2)(+) closely matches that of UO(2)(2+), except for small expansions in the average U═O(ax) and U-O distances. Each ion strongly polarizes their respective first-shell water ligands. The computed acidity constants (pK(a)) of U(4+) and UO(2)(2+) are 0.93 and 4.95, in good agreement with the experimental values of 0.54 and 5.24, respectively. The predicted pK(a) value of UO(2)(+) is 8.5.

J Phys Chem A ; 113(16): 4367-73, 2009 Apr 23.
Artigo em Inglês | MEDLINE | ID: mdl-19284747


Ceramics such as SiC have the potential to act as protective coatings, primarily due to their high melting points and wear resistance. We use periodic density functional theory (DFT) within the generalized gradient approximation (GGA) to calculate the adhesion strength between SiC and Fe, for Si- and C-terminations of SiC(100) and two surfaces of Fe: (100) and (110). We predict a maximum ideal work of adhesion of 6.51 J/m(2) at the SiC(100)/Fe(110) interface for C-Fe interfacial bonding, which is stronger than the traditional chrome coating's adherence to Fe. We characterize the interfacial bonding via local densities of states and electron density difference analysis and find strong covalent bonding and some evidence of metallic bonding between Si (C) and Fe. Our results suggest that SiC might prove useful as a thin adhesion layer in a multilayer protective coating for steel.

J Chem Phys ; 128(10): 104703, 2008 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-18345915


Iron is known to undergo a pressure-induced phase transition from the ferromagnetic (FM) body-centered-cubic (bcc) alpha-phase to the nonmagnetic (NM) hexagonal-close-packed (hcp) epsilon-phase, with a large observed pressure hysteresis whose origin is still a matter of debate. Long ago, Burgers [Physica (Amsterdam) 1, 561 (1934)] proposed an adiabatic pathway for bcc to hcp transitions involving crystal shear followed by atom shuffles. However, a quantum mechanics search in six-dimensional stress-strain space reveals a much lower energy path, where the crystal smoothly shears along the entire path while the atoms shuffle only near the transition state (TS). The energy profile for this phase transition path exhibits a cusp at the TS and closely follows bcc and hcp diabatic energy wells. Both the cusp and the overlap with diabatic energy surfaces are hallmarks of nonadiabaticity, analogous to, e.g., electron transfer (ET) reactions in liquids. Fluctuations in the positions of FM bcc iron atoms near the TS induce magnetic quenching (akin to solvent fluctuations inducing ET), which then promotes NM hcp iron formation (akin to solvent reorganization after ET). We propose that the nonadiabatic nature of this transition at the atomic scale may contribute to the observed pressure hysteresis.