Electronic properties of the magnetocaloric compound GdAl2: A Compton scattering study

We present Compton profiles of the GdAl₂ compound and its constituents using a 20Ci ¹³⁷Cs Compton spectrometer. The experimental Compton data have been analysed using theoretical data obtained from the spin polarised relativistic Korringa–Kohn–Rostoker (SPR-KKR) method and also the charge transfer on the formation of the compound. Both the experimental and the SPR-KKR theoretical Compton data support a charge transfer from Al→Gd in GdAl₂, which is in accordance with the conclusions drawn from the partial, total and integrated density of states of GdAl₂ and its constituents.     

Structural change of liquid Si15Te85 and Si20Te80 with temperature: Ab initio molecular dynamics simulations

Using ab initio molecular dynamics simulations, the structural and electronic properties of liquid Si₁₅Te₈₅ and Si₂₀Te₈₀ at two temperatures were studied respectively. Compared with available experimental data, the calculated structure factors are acceptable. From symmetry arguments, the calculated partial bond-angle distribution functions suggest that with increasing temperature the extensive tetrahedral network structures persist longer in liquid Si₂₀Te₈₀ than those do in liquid Si₁₅Te₈₅. Our results indicate that the local tetrahedral structure around Si atoms and the Peierls-like distorted local atomic structure around Te atoms both play important roles in the structural change of liquid Si₂₀Te₈₀ and Si₁₅Te₈₅, which also suggest that the mechanisms of the structural change upon cooling in liquid Si₂₀Te₈₀ and Si₁₅Te₈₅ are of no essential difference. The results of DOS and LDOS indicate that the variation of the dip in DOS at E_F mainly results from the change of Te p orbitals.     

Phosphorous disulfide and its ions in the electronic ground state: a gas phase theoretical study

Ab initio molecular orbital theory and density functional theory calculations were performed on the electronic ground states of the open-shell PS₂ molecule and its singly charged ions. A comparison of the optimized molecular structures indicates as the stepwise one-electron reduction of the PS₂⁺ ion, to yield PS₂ and PS₂⁻, provokes a symmetric elongation of both PS bonds along with a bending of its linear equilibrium geometry. The ionization potential (IP), adiabatic electron affinity (EA_ad), and atomization energy (AE) of the open-shell PS₂ molecule were calculated at different levels of theory. The following values were obtained at the more realistic UMP4SDTQ/6-311+G(3df)//UHF/6-311+G(3df) level of theory: IP=8.32 eV, EA_ad=3.03 eV and AE=12.40 eV. At the same level of theory, the calculated vertical detachment energy (VDE) of the PS₂⁻ anion is 3.22 eV. The donor–acceptor complexes formed in the gas-phase upon interaction of either one or two ammonia molecules with PS₂⁺ were also investigated. The calculated gas-phase binding energies indicate that the formation of the bis-adduct is favored over that of the mono-adduct by a binding energy gain of about 20 kcal/mol.     

α‐Tetraphosphorus Trichalcogenide Diamides and Imides containing both Sulfur and Selenium

Reaction of a mixture of bicyclic phosphorus sulfide selenide iodides α‐P₄S_nSe_3−nI₂ (n = 0–3) with PrⁱNH₂ and Et₃N gave corresponding diamides α‐P₄S_nSe_3−n(NHPrⁱ)₂ (n = 0–3) and imides α‐P₄S_nSe_3−n(μ‐NPrⁱ) (n = 2–3), identified in solution by ³¹P NMR. In one isomer of α‐P₄S₂Se(μ‐NPrⁱ), the C₂ symmetry of imides such as α‐P₄S₃(μ‐NPrⁱ) was broken, allowing relative assignment of ²J NMR couplings to the PNP bridge and the PSP bridge opposite to it. The coupling through the sulfur bridge was found to be reduced to ca. zero, in contrast to previous assumptions for this class of compounds. Ab initio models were calculated at the MPW1PW91/svp level for the sulfide selenide imides and for a selection of bond rotamers of the diamides, and at the MPW1PW91/LanL2DZ(d) level for the sulfide selenide diiodides. Different skeletal isomers were prevalent for the mixed chalcogenide diamides than for the diiodides, showing that exchange of chalcogen between skeletal positions took place in the amination reaction even at room temperature. Similar differences to those observed were predicted by the models, suggesting that equilibrium was attained.     

Ab initio and density functional theory calculations of molecular structure and vibrational spectrum of ethyl azidoacetate

Here we report ab initio and density functional results for molecular properties of ethyl azidoacetate (N₃CH₂COOC₂H₅) and for the corresponding singly ionized structure (N₃CH₂COOC₂H₅⁺). Ab initio ionization energies based on Koopmans’ theorem are in excellent agreement with the experimental data from ultraviolet photoelectron spectroscopy. DFT adiabatic energy differences between neutral and ionized structures are very sensitive to electronic correlation effects and are not in very good agreement with experiment. The results for the structure and vibrational frequencies are compared with the experimental data of related molecular structures.     

Density functional study of the adsorption of K on the Cu(111) surface

The adsorption of potassium on the Cu(111) surface in a (2×2) pattern has been simulated with all-electron full-potential density functional calculations. The top site is found to be the preferred adsorption site, with the other highly symmetric adsorption sites being nearly degenerate. The bond length from potassium to the nearest copper atom is computed to be 2.83 ?. Population analysis and density of states indicate that there is no evidence for covalent bonding so that the binding mechanism appears to be a metallic bond. Received 11 April 2001