Multiple-magnetic field 139La NMR and density functional theory investigation of the solid lanthanum(III) halides

Results from a solid-state ¹³⁹La NMR spectroscopic investigation of the anhydrous lanthanum(III) halides (LaX₃; X=F, Cl, Br, I) at applied magnetic fields of 7.0, 9.4, 11.7, 14.1, and 17.6 T are presented and highlight the advantages of working at high applied magnetic field strengths. The ¹³⁹La quadrupolar coupling constants are found to range from 15.55 to 24.0 MHz for LaCl₃ and LaI₃, respectively. The lanthanum isotropic chemical shifts exhibit an inverse halogen dependence with values ranging from −135 ppm for LaF₃ to 700 ppm for LaI₃, which represents nearly half of the total lanthanum chemical shift range. The spans of the magnetic shielding tensors also vary widely, from 35 to 650 ppm for the solid LaF₃ through LaI₃. DFT calculations of the ¹³⁹La electric field gradient and magnetic shielding tensors have been performed and provide a qualitative interpretation of the trends observed experimentally.     

Isolation of swinholide A and related glycosylated derivatives from two field collections of marine cyanobacteria

[structure: see text] Chemical investigation of two field collections of marine cyanobacteria has led to the discovery of two new cytotoxic natural products, ankaraholides A (2) and B (3), along with the known compound swinholide A (1). Since swinholide-type compounds were previously localized to the heterotrophic bacteria of sponges, these findings raise intriguing questions about their true metabolic source.     

The thermal structure of a wind-driven Reynolds ridge

In this study, we investigate the nature of a Reynolds ridge formed by wind shear. We have simultaneously imaged the water surface, with a deposit of a monolayer of the surfactant, oleyl alcohol, subject to different wind shears, by using a high-resolution infrared (IR) detector and a high-speed (HS) digital camera. The results reveal that the regions around the wind-driven Reynolds ridge, which have subtle manifestations in visual imagery, possess surprisingly complex hydrodynamical and thermal structures when observed in the infrared. The IR measurements reveal a warm, clean region upstream of the ridge, which is composed of the so called fishscale structures observed in earlier investigations. The region downstream of the ridge is composed of colder fluid which forms two counter-rotating cells. A region of intermediate temperature, which we call the mixing (wake) region, forms immediately downstream of the ridge near the channel centerline. By measuring the velocity of the advected fishscales, we have determined a surface drift speed of about 2% of the wind speed. The spanwise length-scale of the structures has also been used to estimate the wind shear. In addition, a comparison of IR and visual imagery shows that the thermal field is a very sensitive indicator of the exact position of the ridge itself.     

Nondemolition measurements of a single quantum spin using Josephson oscillations

We consider a single localized spin-1/2 between the singlet superconducting leads of a Josephson junction (e.g., a superconducting STM). For the spin subject to a dc magnetic field B parallel z, we study the spin dynamics and the possibility to measure the spin state via transport through the junction embedded in a dissipative circuit. Turning on the tunneling or a voltage bias induces oscillations of the Josephson current, with an amplitude sensitive to the initial value of the z component of the spin, S(z)=+/-1/2. At low temperatures, when effects of quasiparticles are negligible, this procedure realizes a quantum nondemolition measurement of S(z).     

A molecular dynamics simulation study of the alpha-relaxation in a 1,4-polybutadiene melt as probed by the coherent dynamic structure factor

The dynamic coherent structure factor Scoh(q,t) for a 1,4-polybutadiene (PBD) melt has been investigated using atomistic molecular dynamics simulations. The relaxation of Scoh(q,t) at q = 1.44 angstroms(-1) and q = 2.72 angstroms(-1), corresponding to the first and second peaks in the static structure factor for PBD, was studied in detail over a wide range of temperature. It was found that time-temperature superposition holds for the alpha-relaxation for both q values over a wide temperature range and that the alpha-relaxation can be well described by a stretched (Kohlrauch-William-Watts) exponential with temperature independent but q dependent amplitude and stretching exponent. The alpha-relaxation times for both q values were found to exhibit the same non-Arrhenius temperature dependence, indicating that the same physical processes are responsible for relaxation on both length scales. The alpha-relaxation time was found to depend strongly upon the dynamical range of data utilized in determining the relaxation time, accounting for qualitative discrepancies between alpha-relaxation times reported here and those extracted for PBD from experimentally measured Scoh(q,t).     

Diastereoselective conjugate reduction with samarium diiodide: asymmetric synthesis of methyl (2S,3R)-N-acetyl-2-amino-2,3-dideuterio-3-phenylpropionate

A highly diastereoselective conjugate reduction using SmI2 and D2O has been demonstrated on a homochiral benzylidene diketopiperazine template, giving methyl (2S,3R)-N-acetyl-2-amino-2,3-dideuterio-3-phenylpropionate in 93% de and 90% ee after deprotection, hydrolysis and N-acetylation.     

In situ EPR spectroscopy of aromatic diyne cyclopolymerization

Electron paramagnetic resonance (EPR) spectroscopy was successfully used for the first time to follow the Bergman cyclization of bis-ortho-diynyl arene (BODA) compounds. Five BODA monomers with different spacer (X) and terminal groups (R) were compared. In situ polymerization via EPR spectroscopy yielded first-order rate expressions. Monomers with spacer -O- or -C(CF(3))(2) and terminal group R = Ph exhibited similar kinetic behavior upon thermal polymerization, whereas monomers with pyridine and thiophene terminal groups gave significantly higher rates of polymerization over phenyl-terminated derivatives. A model compound, 1,2-bis(phenylethynyl)benzene, was used to probe the polymerization mechanism, and radical intermediates were found to be stable indefinitely at room temperature.     

The influence of chiral auxiliaries and catalysts on the selectivity of intramolecular conjugate additions of pyrrole to N-tethered Michael acceptors

A series of pyrroles incorporating N-tethered acrylates and related groups has been prepared and examined for their capacity to undergo intramolecular Michael addition reactions to form, in a diastereo- or enantio-selective fashion, the corresponding 8-substituted tetrahydroindolizidine or homologues thereof.