Cross-phase modulation instability in photonic crystal fibers

We report on the observation of cross-phase modulation instability in a highly nonlinear photonic crystal fiber. In such fibers the presence of higher orders of dispersion results in a complex phase-matching curve. We are able to observe this behavior experimentally and obtain excellent agreement between the measured and predicted shifts.     

Enantioselective surface chemistry of R-2-bromobutane on Cu(643)R&S and Cu(531)R&S

The enantioselective surface chemistry of chiral R-2-bromobutane was studied on the naturally chiral Cu(643)R&S and Cu(531)R&S surfaces by comparing relative product yields during temperature-programmed reaction spectroscopy. Molecularly adsorbed R-2-bromobutane can desorb molecularly or debrominate to form R-2-butyl groups on the surfaces. The R-2-butyl groups react further by beta-hydride elimination to form 1- or 2-butene or by hydrogenation to form butane. Temperature-programmed reaction spectroscopy was used to quantify the relative yields of the various reaction products. At low coverages of R-2-bromobutane on Cu(643)R&S and Cu(531)R&S, the surface chemistry is not enantioselective. At monolayer coverage, however, the product yields indicate that the R-2-bromobutane decomposition reaction rates are sensitive to the handedness of the two chiral surfaces. The impact of surface structure on enantioselectivity was examined by studying the chemistry of R-2-bromobutane on both Cu(643)R&S and Cu(531)R&S. The selectivity of R-2-bromobutane desorption versus debromination is enantiospecific and differs significantly from Cu(643) to Cu(531). The selectivity of the R-2-butyl reaction by beta-hydride elimination versus hydrogenation is only weakly enantiospecific and is similar on both the Cu(643) and Cu(531) surfaces. These results represent the first quantitative observations of enantioselectivity in reactions with well-known mechanisms probed using a simple adsorbate on naturally chiral metal surfaces.     

Synthesis and structural study of (8-phenylsulfanylnaphth-1-yl)diphenylphosphine metal complexes

A series of three platinum(II) halide complexes 2–4 [Pt(X)₂{Nap(PPh₂)(SPh)}] (Nap = naphthalene-1,8-diyl; X = Cl, Br, I) and a ruthenium(II) p-cymene complex 5 [Ru(η⁶-MeC₆H₄ⁱPr)(Cl){Nap(PPh₂)(SPh)}]⁺Cl^? of the sterically crowded peri-substituted naphthalene phosphine 1 have been prepared. The compounds were fully characterised by multinuclear NMR, IR and MS and X-ray data for 1–5 are compared. Molecular structures are analysed by naphthalene ring torsions, peri-atom displacement, splay angle magnitude, P···S interactions, aromatic ring orientations and geometry around the metal centre. Platinum adopts a strictly square planar geometry which increases the distortion of the naphthalene skeleton in 2–4. Conversely, the classical-piano stool conformation of 5 results in a pseudo-octahedral conformation around the ruthenium atom which influences the naphthalene geometry to a much lesser extent with distortion of a similar magnitude to the free ligand 1.     

Effect of energy addition on MR $\rightarrow$ RR transition

A combined computational and experimental study was performed to investigate the effect of a single laser energy pulse on the transition from a Mach Reflection (MR) to a Regular Reflection (RR) in the Dual Solution Domain (DSD). The freestream Mach number is 3.45 and two oblique shock waves are formed by two symmetric wedges. These conditions correspond to a point midway within the DSD wherein either an MR or an RR is possible. A steady MR was first obtained experimentally and numerically, then a single laser pulse was deposited above the horizontal center plane. In the experiment, the laser beam was focused resulting in a deposition volume of approximately 3 mm³, while in the simulation, the laser pulse was modeled as an initial variation of the temperature and pressure using Gaussian profile. A grid refinement study was conducted to assess the accuracy of the numerical simulations. For the steady MR, the simulation showed the variation of Mach stem height along the span due to side effects. The predicted spanwise averaged Mach stem height was 1.96 mm within 2% of the experimental value of 2 mm. The experiment showed that the Mach stem height decreased to 30% of its original height due to the interaction with the thermal spot generated by the laser pulse and then returned to its original height by s. That the Mach stem returned to its original height was most likely due to freestream turbulence in the wind tunnel. The numerical simulation successfully predicted the reverse transition from a stable MR to a stable RR and the stable RR persisted across the span. This study showed the capability of a laser energy pulse to control the reverse transition of MR RR within the Dual Solution Domain.     

A stereoselective synthesis of anti-gamma,delta-alkynyl- and -alkenyl-beta-hydroxy-alpha-amino esters from tin(II) enolates of glycinate

Condensations between the tin(II) enolate 11 of ethyl N-tosylglycinate and conjugated ynals 12 and ynones 14 are highly diastereoselective, in favour of the anti-isomers 13 and 15; similar reactions of enals and enones 17 show lower but still useful levels of anti-stereoselectivity.     

Computable Structure and Non-Structure Theorems

In a lecture in Kazan (1977), Goncharov dubbed a number of problems regarding the classification of computable members of various classes of structures. Some of the problems seemed likely to have nice answers, while others did not. At the end of the lecture, Shore asked what would be a convincing negative result. The goal of the present article is to consider some possible answers to Shore's question. We consider structures of some computable language, whose universes are computable sets of constants. In measuring complexity, we identify with its atomic diagram D(), which, via the Gödel numbering, may be treated as a subset of . In particular, is computable if D() is computable. If K is some class, then K^c denotes the set of computable members of K. A computable characterization for K should separate the computable members of K from other structures, that is, those that either are not in K or are not computable. A computable classification (structure theorem) should describe each member of K^c up to isomorphism, or other equivalence, in terms of relatively simple invariants. A computable non-structure theorem would assert that there is no computable structure theorem. We use three approaches. They all give the correct answer for vector spaces over Q, and for linear orderings. Under all of the approaches, both classes have a computable characterization, and there is a computable classification for vector spaces, but not for linear orderings. Finally, we formulate some open problems.     

Bimolecular extrusion of TeR2 from beta-diketiminato supported scandium bis-tellurolates

Reaction of the well-defined, base-free dialkyl scandium compounds LnSc(CH2SiMe3)2 supported by the beta-diketiminato ligands ArNC(R)CHC(R)NAr (Ar = 2,6-diisopropyl; R = Me, L1: R = tBu, L2) with two equivalents of nBuP = Te gives the bis-tellurolate complexes LnSc(TeCH2SiMe3)2, 1b (L1) and 2b (L2). Tellurolate 2b was isolated and fully characterized, including an X-ray structure analysis, and exhibits two chemically distinct tellurolate ligands in solution on the NMR time scale. Tellurolate 1b is only moderately stable and decomposes with elimination of two equivalents of Te(CH2SiMe3)2 to form the dimeric bis-telluride 1d. This compound was characterized by X-ray crystallography and elemental analysis. In contast, tellurolate 2b decomposes with loss of only one equivalent of Te(CH2SiMe3)2, leading to the formation of a dinuclear complex with one bridging telluride unit joining two L2Sc(TeCH2SiMe3) fragments. This compound was also structurally characterized. The solution behavior of this material is complex, as it exists as three conformational isomers that undergo slow exchange on the NMR time scale. The production of dimer 2c, along with the results of crossover experiments, suggest strongly that TeR2 elimination from these bis-tellurolates is bimolecular.     

Pyridinium chlorochromate (PCC) oxidation of bishomoallylic tertiary alcohols. A structure–reactivity study

The mechanism of the Cr(VI) oxidation of an alkene C=C is not known for certain. A particularly useful and novel example of this process is the intramolecular oxidative cyclization of bishomoallylic tertiary alcohols by pyridinium chlorochromate (PCC) to yield substituted tetrahydrofuran products via the tethered chromate ester. Several such tertiary alcohols were prepared in this study which varied in the number and position of alkyl groups attached to the C=C. The relative reactivity of these substrates toward PCC under standard conditions is dependent only on the number of R groups on the C=C, not on the degree of substitution on the most highly substituted alkene carbon. This observation suggests a symmetrical transition state in this intramolecular Cr(VI) alkene oxidation.     

Cyclopropane-derived peptidomimetics. Design, synthesis, and evaluation of novel enkephalin analogues

It is known that peptide mimics containing trans-substituted cyclopropanes stabilize extended conformations of oligopeptides, and molecular modeling studies now suggest that the corresponding cis-cyclopropane dipeptide isosteres could stabilize a reverse turn. To begin to assess this possibility, a series of cis-substituted cyclopropanes were incorporated as replacements of the Gly(2)-Gly(3) and Phe(4)-Leu(5) dipeptide subunits in Leu-enkephalin (H(2)N-Tyr-Gly-Gly-Phe-Leu-OH), which is believed to bind to opiod receptors in a conformation containing a beta-turn. General methods for the synthesis of the cyclopropane-containing dipeptide isosteres -XaaPsi[COcpCO]Yaa- and -XaaPsi[NHcpNH]Yaa-were developed by a sequence that featured the enantioselective cyclization of allylic diazoacetates catalyzed by the chiral rhodium complexes Rh(2)[(5S)-MEPY](4) and Rh(2)[(5R)-MEPY](4). A useful modification of the Weinreb amidation procedure was applied to the opening of the intermediate lactones with dipeptides, and a novel method for the synthesis of substituted diaminocyclopropanes was also developed. The Leu-enkephalin analogues were tested in a panel of binding and functional assays, and although those derivatives containing cyclopropane replacements of the Gly(2)-Gly(3) exhibited low micromolar affinity for the mu-receptor, analogues containing such replacements for the Phe(4)-Leu(5) subunit did not bind with significant affinity to any of the opioid receptors. These results are discussed.     

Depolymerization of Polyethylene Using Induction-Coupled Plasma Technology

A significant, valuable percentage of today's municipal solid wastestream consists of polymeric materials, for which almost no economicrecycling technology currently exists. This polymeric waste is incinerated,landfilled, or recycled via downgraded usage. Thermal plasma treatment is apotentially viable means of recycling these materials by converting themback into monomers or into other useful compounds. The technical, laboratoryscale, feasibility of using an induction-coupled RF plasma (ICP) heatedreactor for this purpose has been demonstrated in the presentstudy. Polyethylene powder was injected axially through the center of anICP torch. Results from the initial set of experiments, analyzed using astatistical design of experiment technique, showed that plasma plate power,central gas flow rate, probe gas flow rate, powder feed rate, and theinteraction between the quench gas flow rate and power input were the keyprocess parameters affecting the yield of ethylene in the product gasstream. The gaseous products obtained were mainly mixtures of ethylene andpropylene. The amount of propylene obtained was significantly higher thananticipated and was believed to be due to -scission reactionsoccurring at the higher plasma temperatures.