Crystallographic and magnetic order analysis of a banded limonite

Mössbauer and XRD studies of the light and dark brown parts of a banded limonite showed the presence of kaolinite in the light brown regions. Relaxation behaviour was very well described by a recently proposed model based on magnetic ordering of clusters. The difference in relaxation behaviour of the samples was attributed to different concentrations of non-magnetic impurities.     

Far infrared generation by CO2 lasers frequencies subtraction in a ZnGeP2 crystal

a pulsed radiation power of the order of 1W in the submillimeter range (102–110 µm) was reached for the first time, with the aid of a ZnGeP₂ crystal, at the difference frequency of CO₂ laser radiations.     

On the Brownian motion of a massive sphere suspended in a hard-sphere fluid. II. Molecular dynamics estimates of the friction coefficient

The friction coefficient exerted by a hard-sphere fluid on an infinitely massive Brownian sphere is calculated for several size ratios , where and are the diameters of the Brownian and fluid spheres, respectively. The exact microscopic expression derived in part I of this work from kinetic theory is transformed and shown to be proportional to the time integral of the autocorrelation function of the momentum transferred from the fluid to the Brownian sphere during instantaneous collisions. Three different methods are described to extract the friction coefficient from molecular dynamics simulations carried out onfinite systems. The three independent methods lead to estimates of which agree within statisticalerrors (typically 5%). The results are compared to the predictions of Enskog theory and of the hydrodynamic Stokes law. The former breaks down as the size ratio and/or the packing fraction of the fluid increase. Somewhat surprisingly, Stokes' law is found to hold withstick boundary conditions, in the range 1/4.5 explored in the present simulations, with a hydrodynamic diameterd=. The analysis of the moleuclar dynamics data on the basis of Stokes' law withslip boundary conditions is less conclusive, although the right trend is found as / increases.     

A novel extended covalent tripod for assembling nine-coordinate lanthanide(III) podates: a delicate balance between flexibility and rigidity

The introduction of long semirigid spacers between the capping carbon atom of the tripod and the unsymmetrical tridentate binding units provides the novel, extended covalent podand tris-[2-[2-(6-diethylcarbamoylpyridin-2-yl)-1-ethyl-1H-benzoimidazol-5-yl-methoxy]ethyl]methane (L(15)). Reaction of L(15) with lanthanide(III) in acetonitrile produces stable podates [Ln(L(15))](3+) (Ln=La-Lu) in which three tridentate binding units are facially organized. These wrap around the nine-coordinate pseudo-tricapped trigonal-prismatic metal ions. The crystal structure of [La(L(15))](ClO(4))(3) (18, LaC(67)H(82)N(12)O(18)Cl(3), trigonal, R3c, Z=6) reveals the formation of a C(3)-symmetrical triple-helical podate. Two slightly different arrangements of the flexible ethylenoxy parts of the spacer are observed in the solid state in agreement with the formation of two conformational isomers (M:m) in a 4:1 ratio. A qualitative analysis of the aromatic diamagnetic anisotropies affecting the NMR signals of [Ln(L(15))](3+) (Ln=La, Y, Lu) in solution, combined with the quantitative determination of electron-induced relaxation in the paramagnetic complex [Nd(L(15))](3+), demonstrate that the solid state structure is maintained in solution. This leads to a mixture of two triple-helical conformers of similar stabilities and that do not interconvert on the NMR timescale between 243 and 343 K. Particular attention has been given to the structural programming of extended covalent tripods for facially organizing unsymmetrical tridentate binding units around Ln(III). Photophysical measurements show that L(15) efficiently protects the metallic coordination spheres and sensitizes Eu(III) and Tb(III) upon UV irradiation.     

Ethene epoxidation selectivity inhibited by twisted oxametallacycle: a DFT study on Ag surface-oxide

Competitive ethene oxidation pathways are presented for a p(4 x 4) surface-oxide phase on Ag(111) obtained from density functional theory (DFT) calculations. Both parallel routes are found to proceed from a common oxametallacycle intermediate (OMME) in agreement with previous mechanistic studies on low coverage O adatom phase, although acetaldehyde (AcH) is favored by almost 2 kcal/mol. An even more striking difference with pure metal surface appears with the oxide regeneration pathways, which are found non-rate controlling. Furthermore, a kinetic model is developed on the basis of these DFT calculations and yields 96% selectivity in favor of AcH for a simulation in realistic catalytic conditions (600 K and respective partial pressures of 1 atm for ethene and oxygen reactants). As a key finding, this low ethene epoxide selectivity is proposed to be directly linked to the conformational barrier of the pivotal intermediate. In fact, the elasticity of the ultrathin oxide adlayer enables a twisted OMME structure as a true minimum, which agrees well with orbital prerequisite of the concerted H migration toward AcH. On the contrary, the desired selective ring closure forming ethene epoxide (EO) requires conformational inversion although the eclipsed form lies 2 kcal/mol above.     

Level structure of149Nd. The decay of149Pr

The decay of¹⁴⁹Pr (T _1/2=2.2 min) has been studied using the two fission product separators JOSEF and LOHENGRIN to produce the¹⁴⁹Pr nucleus. A level scheme for¹⁴⁹Nd has been established. Theβ-branching and logft values for the excited levels were deduced from the analysis ofγ-intensity balances. Furthermore, the spins and parities for most of the excited states observed were obtained from the comparison between the present work, the neutron capture results and the pick-up reactions. The positive parity levels have been described with the Nilsson model with Coriolis andΔN=2 interactions included. The properties of the negative parity states cannot be explained as easily; however, it has been attempted to extract the structure of the ground state \((I^\pi = \tfrac{5}{2}^ - )\) and the first excited states \((I^\pi = \tfrac{7}{2}^ - and\tfrac{3}{2}^ - )\) .     

Surface‐Guided Formation of an Organocobalt Complex

Organocobalt complexes represent a versatile tool in organic synthesis as they are important intermediates in Pauson–Khand, Friedel–Crafts, and Nicholas reactions. Herein, a single‐molecule‐level investigation addressing the formation of an organocobalt complex at a solid–vacuum interface is reported. Deposition of 4,4′‐(ethyne‐1,2‐diyl)dibenzonitrile and Co atoms on the Ag(111) surface followed by annealing resulted in genuine complexes in which single Co atoms laterally coordinated to two carbonitrile groups undergo organometallic bonding with the internal alkyne moiety of adjacent molecules. Alternative complexation scenarios involving fragmentation of the precursor were ruled out by complementary X‐ray photoelectron spectroscopy. According to density functional theory analysis, the complexation with the alkyne moiety follows the Dewar–Chatt–Duncanson model for a two‐electron‐donor ligand where an alkyne‐to‐Co donation occurs together with a strong metal‐to‐alkyne back‐donation.