A reduced model for shock and detonation waves. II. The reactive case

We present a mesoscopic model for reactive shock waves, which extends a previous model proposed in [G. Stoltz, Europhys. Lett. 76 (2006), 849]. A complex molecule (or a group of molecules) is replaced by a single mesoparticle, evolving according to some Dissipative Particle Dynamics. Chemical reactions can be handled in a mean way by considering an additional variable per particle describing a rate of reaction. The evolution of this rate is governed by the kinetics of a reversible exothermic reaction. Numerical results give profiles in qualitative agreement with all-atom studies

The Total Synthesis of (–)-Scabrolide A

The first total synthesis of the norcembranoid diterpenoid scabrolide A is disclosed. The route begins with the synthesis of two chiral pool-derived fragments, which undergo a convergent coupling to expediently introduce all 19 carbon atoms of the natural product. An intramolecular Diels–Alder reaction and an enone–olefin cycloaddition/fragmentation sequence are then employed to construct the fused [5–6–7] linear carbocyclic core of the molecule and complete the total synthesis

A Computational Model Relating Structure and Reactivity in Enantioselective Oxidations of Secondary Alcohols by (−)-Sparteine−Pd^(II) Complexes

The key interactions responsible for the unique reactivity of (−)-sparteine−PdX_2 complexes (X = chloride, acetate) in the enantioselective oxidation of secondary alcohols have been elucidated using quantum mechanics (B3LYP DFT with the PBF polarizable continuum solvent model). From examining many possible pathways, we find the mechanism involves:  (1) substitution of the alcohol in place of an X-group, (2) deprotonation of the bound alcohol by the deposed anion and free sparteine, (3) β-hydride elimination through a four-coordinate transition state in which the second anion is displaced but tightly associated, (4) replacement of the ketone product with the associated anion. The enantioselectivities observed under base-rich reaction conditions follow directly from calculated energies of diastereomeric β-hydride elimination transition states incorporating (R) and (S) substrates. This relationship reveals an important role of the anion, namely to communicate the steric interaction of the ligand on one side of the PdII square plane and the substrate on the other side. When no anion is included, no enantioselectivity is predicted. Locating these transition states in different solvents shows that higher dielectrics stabilize the charge separation between the anion and metal and draw the anion farther into solution. Thus, the solvent influences the barrier height (rate) and selectivity of the oxidation

Three-Dimensional Analysis of Wakefields Generated by Flat Electron Beams in Planar Dielectric-Loaded Structures

An electron bunch passing through dielectric-lined waveguide generates $\check{C}$erenkov radiation that can result in high-peak axial electric field suitable for acceleration of a subsequent bunch. Axial field beyond Gigavolt-per-meter are attainable in structures with sub-mm sizes depending on the achievement of suitable electron bunch parameters. A promising configuration consists of using planar dielectric structure driven by flat electron bunches. In this paper we present a three-dimensional analysis of wakefields produced by flat beams in planar dielectric structures thereby extending the work of Reference [A. Tremaine, J. Rosenzweig, and P. Schoessow, Phys. Rev. E 56, No. 6, 7204 (1997)] on the topic. We especially provide closed-form expressions for the normal frequencies and field amplitudes of the excited modes and benchmark these analytical results with finite-difference time-domain particle-in-cell numerical simulations. Finally, we implement a semi-analytical algorithm into a popular particle tracking program thereby enabling start-to-end high-fidelity modeling of linear accelerators based on dielectric-lined planar waveguides.Comment: 12 pages, 2 tables, 10 figure

Anomalous diffusion for a class of systems with two conserved quantities

We introduce a class of one dimensional deterministic models of energy-volume conserving interfaces. Numerical simulations show that these dynamics are genuinely super-diffusive. We then modify the dynamics by adding a conservative stochastic noise so that it becomes ergodic. System of conservation laws are derived as hydrodynamic limits of the modified dynamics. Numerical evidence shows these models are still super-diffusive. This is proven rigorously for harmonic potentials

High nuclear polarization of helium-3 at low and high pressure by metastability exchange optical pumping at 1.5 Tesla

We perform metastability exchange optical pumping of helium-3 in a strong magnetic field of 1.5 T. The achieved nuclear polarization, from 80% at 1.33 mbar to 25% at 67 mbar, shows a substantial improvement at high pressures with respect to standard low-field optical pumping. The specific mechanisms of metastability exchange optical pumping at high field are investigated, advantages and intrinsic limitations are discussed. From a practical point of view, our results open the way to alternative technological solutions for polarized helium-3 applications and in particular for magnetic resonance imaging of human lungs.Comment: accepted for publication in Europhysics Letter