On the role of PIII ligands in the conjugate addition of diorganozinc derivatives to enones

A mechanistic study on the conjugate addition of diethylzinc to cyclohexenone catalyzed by various chiral PIII ligands, provides new insights into its mechanism. Complete in situ conversion of the catalytic amount of Cu(OTf)2 into CuI species by excess ZnEt2 is demonstrated by EPR spectroscopy. Experimental evidence is presented in favor of a critical ternary 1:1:1 complex between enone, Et2Zn and catalyst, supporting a rate-limiting reductive elimination or carbocupration from a preformed mixed Cu/Zn cluster carrying one- or two-ligand molecules. A crystal structure has been obtained for a CuIL2 complex, which shows catalytic turnover on addition of reagent and substrate. NMR spectroscopic analyses and VT experiments reveal that steric hindrance may prevent complexation of a second ligand molecule on the CuI cation, leading to extremely fast precatalysts based on CuX.L species.     

Area-coded effective medium structures, a new type of grating design

We propose a new way to design gratings with desired diffraction properties by using subwavelength feature sizes perpendicular to the ordinary superwavelength grating period. This is different from well-known one-dimensional binary-blazed gratings that use a structuring along the grating period and thus opens new flexibility in generating arbitrary effective-index distributions in the direction of the grating period. Since the subwavelength features form contiguous areas, they are called area-coded effective medium structures (ACES). Compared with well-known binary subwavelength structures in two-dimensional arrangements consisting of pillars, ACES are more stable and have comparable efficiency properties. As an example we show how to design in principle a four-level area-coded effective medium grating, compare the efficiency of ACES with binary-blazed and échelette gratings, and optimize the subwavelength period of ACES.     

Electron-lattice interaction of Ag− and Cu− centers in alkali halides

Absorption, emission, and excitation spectra of Ag^? centers in KCl, RbCl, CsCl, and CsBr are measured at low temperatures. The positions of theA emission bands are slightly different afterC andA band excitation, respectively. This is believed to be due to the existence of two different types of minima in the adiabatic potential energy surface of the³ T _1u state. The symmetry of the energy minima in the¹ T _1u state is trigonal for KCl∶Ag^? and Cu^?, but tetragonal for CsBr∶Ag^?. This becomes evident from the polarization properties of the emission. The energy and temperature dependence of the polarization is discussed. Uniaxial stress causes polarized emission of Ag^? and Cu^? centers measured from LHeT to 100 K. This is due to a splitting and mixing of the relaxed excited states by the stress. The effects are used to calculate the coupling constants between thep electron and theE _g andT _2g lattice modes. They are compared with predictions from the point-charge model for different lattice structures. A new assignment of the absorption bands of KCl∶Cu^? to the excited states of Cu^? is established on measurements of emission spectra and lifetimes.     

(Sr,Mn)TiO3: a magnetoelectric multiglass

By close analogy with multiferroic materials with coexisting long-range electric and magnetic orders a "multiglass" scenario of two different glassy states is observed in Sr(0.98)Mn(0.02)TiO(3) ceramics. Sr-site substituted Mn2+ ions are at the origin of both a polar and a spin glass with glass temperatures T(g) approximately equal to 38 K and < or =34 K, respectively. The structural freezing triggers that of the spins, and both glassy systems show individual memory effects. Thanks to strong spin-phonon interaction within the incipient ferroelectric host crystal SrTiO3, large higher order magnetoelectric coupling occurs between both glass systems.