Experimental tests of quantum nonlinear dynamics in atom optics

Cold atoms in optical potentials provide an ideal test bed to explore quantum nonlinear dynamics. Atoms are prepared in a magneto-optic trap or as a dilute Bose-Einstein condensate and subjected to a far detuned optical standing wave that is modulated. They exhibit a wide range of dynamics, some of which can be explained by classical theory while other aspects show the underlying quantum nature of the system. The atoms have a mixed phase space containing regions of regular motion which appear as distinct peaks in the atomic momentum distribution embedded in a sea of chaos. The action of the atoms is of the order of Planck's constant, making quantum effects significant. This tutorial presents a detailed description of experiments measuring the evolution of atoms in time-dependent optical potentials. Experimental methods are developed providing means for the observation and selective loading of regions of regular motion. The dependence of the atomic dynamics on the system parameters is explored and distinct changes in the atomic momentum distribution are observed which are explained by the applicable quantum and classical theory. The observation of a bifurcation sequence is reported and explained using classical perturbation theory. Experimental methods for the accurate control of the momentum of an ensemble of atoms are developed. They use phase space resonances and chaotic transients providing novel ensemble atomic beamsplitters. The divergence between quantum and classical nonlinear dynamics is manifest in the experimental observation of dynamical tunnelling. It involves no potential barrier. However a constant of motion other than energy still forbids classically this quantum allowed motion. Atoms coherently tunnel back and forth between their initial state of oscillatory motion and the state 180 out of phase with the initial state.

Nitrogen transfer within and between plants through common mycorrhizal networks (CMNs)

Mycorthizae play a critical role in nutrient capture from soils. Arbuscular mycorrhizae (AM) and ectomycorrhizae (EM) are the most important mycorrhizae in agricultural and natural ecosystems. AM and EM fungi use inorganic NH4+ and NO3-, and most EM fungi are capable of using organic nitrogen. The heavier stable isotope N-15 is discriminated against during biogeochemical and biochemical processes. Differences in N-15 (atom%) or delta(15)N (parts per thousand) provide nitrogen movement information in an experimental system. A range of 20 to 50% of one-way N-transfer has been observed from legumes to nonlegumes. Mycorrhizal fungal mycelia can extend from one plant's roots to another plant's roots to form common mycorrhizal networks (CMNs). Individual species, genera, even families of plants can be interconnected by CMNs. They are capable of facilitating nutrient uptake and flux. Nutrients such as carbon, nitrogen and phosphorus and other elements may then move via either AM or EM networks from plant to plant. Both N-15 labeling and N-15 natural abundance techniques have been employed to trace N movement between plants interconnected by AM or EM networks. Fine mesh (25similar to45 mum) has been used to separate root systems and allow only hyphal penetration and linkages but no root contact between plants. In many studies, nitrogen from N-2-fixing mycorrhizal plants transferred to non-N-2-fixing mycorrhizal plants (one-way N-transfer). In a few studies, N is also transferred from non-N-2-fixing mycorrhizal plants to N-2-fixing mycorrhizal plants (two-way N-transfer). There is controversy about whether N-transfer is direct through CMNs, or indirect through the soil. The lack of convincing data underlines the need for creative, careful experimental manipulations. Nitrogen is crucial to productivity in most terrestrial ecosystems, and there are potential benefits of management in soil-plant systems to enhance N-transfer. Thus, two-way N-transfer warrants further investigation with many species and under field conditions.

Exact solution, scaling behaviour and quantum dynamics of a model of an atom-molecule Bose-Einstein condensate

0We study the exact solution for a two-mode model describing coherent coupling between atomic and molecular Bose-Einstein condensates (BEC), in the context of the Bethe ansatz. By combining an asymptotic and numerical analysis, we identify the scaling behaviour of the model and determine the zero temperature expectation value for the coherence and average atomic occupation. The threshold coupling for production of the molecular BEC is identified as the point at which the energy gap is minimum. Our numerical results indicate a parity effect for the energy gap between ground and first excited state depending on whether the total atomic number is odd or even. The numerical calculations for the quantum dynamics reveals a smooth transition from the atomic to the molecular BEC.

Accessing chain length dependent termination rate coefficients of methyl methacrylate (MMA) via the reversible addition fragmentation chain transfer (RAFT) process

The RAFT-CLD-T methodology is demonstrated to be not only applicable to 1-substituted monomers such as styrene and acrylates, but also to 1,1-disubstituted monomers such as MMA. The chain length of the terminating macromolecules is controlled by CPDB in MMA bulk free radical polymerization at 80 degrees C. The evolution of the chain length dependent termination rate coefficient, k(t)(i,i), was constructed in a step-wise fashion, since the MMA/CPDB system displays hybrid behavior (between conventional and living free radical polymerization) resulting in initial high molecular weight polymers formed at low RAFT agent concentrations. The obtained CLD of k(t) in MMA polymerizations is compatible with the composite model for chain length dependent termination. For the initial chain-length regime, up to a degree of polymerization of 100, k(t) decreases with alpha (in the expression k(t)(i,i) = k(t)(0) . i(-alpha)) being close to 0.65 at 80 degrees C. At chain lengths exceeding 100, the decrease is less pronounced (affording an alpha of 0.15 at 80 degrees C). However, the data are best represented by a continuously decreasing nonlinear functionality implying a chain length dependent alpha.

Challenges for simultaneous nitrification, denitrification, and phosphorus removal in microbial aggregates: mass transfer limitation and nitrous oxide production

The microbial community composition and activity was investigated in aggregates from a lab-scale bioreactor, in which nitrification, denitrification and phosphorus removal occurred simultaneously. The biomass was highly enriched for polyphosphate accumulating organisms facilitating complete removal of phosphorus from the bulk liquid; however, some inorganic nitrogen still remained at the end of the reactor cycle. This was ascribed to incomplete coupling of nitrification and denitrification causing NO3- accumulation. After 2 h of aeration, denitrification was dependent on the activity of nitrifying bacteria facilitating the formation of anoxic zones in the aggregates; hence, denitrification could not occur without simultaneous nitrification towards the end of the reactor cycle. Nitrous oxide was identified as a product of denitrification, when based on stored PHA as carbon source. This observation is of critical importance to the outlook of applying PHA-driven denitrification in activated sludge processes. (c) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.

Synthesis and aggregation behavior of four-arm star amphiphilic block copolymers in water

We report the first synthesis of amphiphilic four-arm star diblock copolymers consisting of styrene (STY) and acrylic acid (AA) made using reversible addition-fragmentation chain transfer (RAFT; Z group approach with no star-star coupling). The polymerization proceeded in an ideal living manner. The size of the poly(AA(132)-STYm)(4) stars in DMF were small and close to 7 nm, suggesting no star aggregation. Slow addition of water (pH = 6.8) to this mixture resulted in aggregates of 15 stars per micelle with core-shell morphology. Calculations showed that the polyAA blocks were slightly extended with a shell thickness of 15 nm. Treatment of these micelles with piperidine to cleave the block arms from the core resulted in little or no change on micelle size or morphology, but the polyAA shell thickness was close to 29 nm (33 nm is the maximum at full extension) suggesting a release of entropy when the arms are detached from the core molecule. In this work we showed through the use of star amphiphilic polymers that the micelle size, aggregation number, and morphology could be controlled.

The mechanochemical modification of polyolefins using reactive modifier systems

The principal objective of this work was to improve the mechanical properties of glass fibre reinforced polypropylene (PP) composites by the mechanochemical modification of the PP. The modification of the PP was carried out by reactive processing of the PP with a modifier in a Buss Ko-Kneader. Two main types of modifier were evaluated one type based on N-substituted maleimides the others based on 2-allylamino-4,6-dichloro-1,3,5-triazine (ACCT). The modification of the PP was carried out in two stages. Firstly the PP was reactively processed with the modifier and a free radical initiator. The objective of this stage was to bind the modifier to the PP. In the second stage the modified PP was reactively processed with the glass fibre. The objective in this stage was to form a chemical bond between the bound modifier and the silane coupling agent on the surface of the glass. Two silane coupling agents were evaluated these had a aliphatic chloro group and an aliphatic amino group respectively available for reaction with the modifier. The modifiers synthesised for this work had two main functional groups. The first was a double bond for free radical addition to the PP. The second was an organic group chosen for its potential reactivity to the silane coupling agent. A preliminary investigation was carried out using maleic anhydride (MA) as the modifier, this is reactive to the amino silane coupled glass. Studies of a commercially available system were also carried out for comparison purposes. During the work it was found that the amino silane coupled glass fibres produced, without any modification being made to the PP, mechanical properties comparable to the commercial system. Further any modification added to the amino silane system failed to improve the mechanical performance and in some cases acted in the opposite fashion. This failure was evident even when a chemical bond between glass fibre and PP could be shown. In the case of the chloro silane coupled glass fibres the mechanical properties of the composite without modification were poorer than those of the commercial system. It was found that the mechanical properties of these systems could be enhanced by the modifiers, however, no system tested significantly out performed the commercial system. Of the two modifier systems tested those based on the n-substituted maleimides were more successful at enhancing mechanical properties than those based on ACCT. This was attributed to the Poor chemical binding of the ACCT based modifiers to the PP. During the work it was found that several of the modifiers improved the properties of the PP when no glass fibres were present, particularly the % elongation and impact strength. It is possible that these modifiers could be used to improve the impact performance of PP, this may be of particular interest in recycling. These modifiers have only been tested for improving the properties of glass fibre composites. The N-substituted maleimide based modifiers could be used as compatibleisers for alloys of PP and other polymers. These could function by the formation of the bond with PP via the double bond whilst the group attached to the nitrogen atom could react with the alloying polymer.

Coupling between counterpropagating cladding modes in fiber Bragg gratings

We present an experimental demonstration of energy transfer between counterpropagating cladding modes in a fiber Bragg grating (FBG). A strong FBG written in a standard photosensitive optical fiber is illuminated with a single cladding mode, and the power transferred between the forward propagating cladding mode and different backward propagating cladding modes is measured by using two auxiliary long period gratings. Resonances between cladding modes having 30 pm bandwidth and 8 dB rejection have been observed.

Fundamental Pd0/PdII redox steps in cross-coupling reactions:homogeneous, hybrid homogeneous-heterogeneous to heterogeneous mechanistic pathways for C-C couplings

C–C bond-forming, cross-coupling reactions of organohalides with nucleophilic compounds, catalysed by palladium, are amongst the most important chemical reactions available to the synthetic chemist. The intimate mechanisms of these reactions, involving Pd0/PdII redox steps, have been of great historical interest and continue to be so. The myriad of possible mechanisms is reviewed in this chapter. The interplay of mononuclear Pd species with higher order Pd species, e.g. nanoclusters/nanoparticles are considered as being equally important in cross-coupling reaction mechanisms. A focus is placed on trichotomic behaviour of cross-coupling catalytic manifolds, from homogeneous to hybrid homogeneous–heterogeneous to truly heterogeneous behaviour. For the latter, surface chemistry and metal atom leaching (and various experimental techniques) are broadly discussed. It is now clear that mechanism for general cross‐coupling reactions, that is as presented to undergraduate students studying Chemistry degrees across the world, is undoubtedly more complex than first thought. New opportunities for catalyst design have therefore emerged in the area of Pd nanoparticles and nanocatalysis, with some wonderful applications especially in chemical biology, providing a snapshot of what the future might hold.

Radical intermediates in chloroform reactions over triphenylphosphine-protected Au nanoparticles

Reactions of chloroform over triphenylphosphine-protected Au nanoparticles have been studied using electron paramagnetic resonance (EPR) spectroscopy and a spin trapping technique. Two competing reactions, abstraction of hydrogen and halogen atoms, were identified. The hydrogen abstraction reaction showed an inverse kinetic isotope effect. Treatment of nanoparticles with oxidizing or reducing reagents made it possible to tune the selectivity of radical formation from halogen to hydrogen (deuterium) abstraction. Treatment with PbO2 promoted the deuterium abstraction reaction followed by the loss of nanoparticle activity, whereas treatment with NaBH4 regenerated the nanoparticle activity towards Cl atom abstraction. X-ray photoelectron spectroscopy showed an increased Au:P ratio upon treatment with oxidizing reagents. This is likely due to the oxidation of some phosphine ligands to phosphine oxides which then desorb from the nanoparticle surface. © 2009 The Royal Societ of Chemistry.

Ag alloyed Pd single-atom catalysts for efficient selective hydrogenation of acetylene to ethylene in excess ethylene

Semihydrogenation of acetylene in an ethylene-rich stream is an industrially important process. Conventional supported monometallic Pd catalysts offer high acetylene conversion, but they suffer from very low selectivity to ethylene due to overhydrogenation and the formation of carbonaceous deposits. Herein, a series of Ag alloyed Pd single-atom catalysts, possessing only ppm levels of Pd, supported on silica gel were prepared by a simple incipient wetness coimpregnation method and applied to the selective hydrogenation of acetylene in an ethylene-rich stream under conditions close to the front-end employed by industry. High acetylene conversion and simultaneous selectivity to ethylene was attained over a wide temperature window, surpassing an analogous Au alloyed Pd single-atom system we previously reported. Restructuring of AgPd nanoparticles and electron transfer from Ag to Pd were evidenced by in situ FTIR and in situ XPS as a function of increasing reduction temperature. Microcalorimetry and XANES measurements support both geometric and electronic synergetic effects between the alloyed Pd and Ag. Kinetic studies provide valuable insight into the nature of the active sites within these AgPd/SiO2 catalysts, and hence, they provide evidence for the key factors underpinning the excellent performance of these bimetallic catalysts toward the selective hydrogenation of acetylene under ethylene-rich conditions while minimizing precious metal usage.

Stereoselective synthesis of vinyl germanes and silanes. Applications of tris(trimethyl)germanes and silanes in Pd-catalyzed cross -coupling reactions

Although group 14 organometallic compounds (Si, Sn) have been well developed as transmetallation reagents in cross-coupling reactions, the application of organogermanium compounds as cross-coupling reagents is still a relatively new area with few papers published. This study aimed to develop methods for the synthesis of new classes of vinyl germane and vinyl silane compounds, mainly Z and E tris(trimethylsilyl)germanes and silanes, which were then applied to Pd-catalyzed cross-couplings with aryl and alkenyl halides. The stereoselective radical-mediated desulfonylation of vinyl sulfones with tris(trimethyl)germanium or silane hydrides provided access to the synthesis of trans vinyl germanes or silanes. Alternatively hydrogermylation or hydrosilylation of terminal alkynes gave cis vinyl germanes or silanes. The application of these new classes of organometallic compounds in cross-coupling reactions with various aryl and alkenyl halides under aqueous [NaOH/H2O2/Pd(PPh 3)4] and anhydrous [KH/t-BuOOH/Pd(PPh 3)4] oxidative conditions were investigated. ^ It was found that the vinyl tris(trimethylsilyl)germanes successfully underwent Pd-catalyzed cross-couplings with aryl and alkenyl halides and aryl triflates under aqueous and anhydrous oxidative conditions. These procedures provided examples of "ligand-free" Pd-catalyzed coupling of organogermanes with aryl and alkenyl halides. Interestingly, couplings with fluorinated vinyl germanes appeared to occur more easily than with the corresponding (α-fluoro)vinyl stannanes and silanes since neither addition of an extra ligand nor activation with fluoride was necessary. The vinyl tris(trimethyl)silanes were found to be alternative substrates for the Hiyama reaction. The coupling of TTMS-silanes with various aryl, heteroaryl as well as alkenyl halides proceeded smoothly upon treatment with hydrogen peroxide in the presence of sodium hydroxide and fluoride ion. ^

Synthesis of multisubstituted halo -olefins via Pd-catalyzed cross -coupling reactions. Applications in nucleoside chemistry

The enzyme S-adenosyl-L-homocysteine (AdoHcy) hydrolase effects hydrolytic cleavage of AdoHcy to adenosine (Ado) and L-homocysteine (Hcy). The cellular levels of AdoHcy and Hcy are critical because AdoHcy is a potent feedback inhibitor of crucial transmethylation enzymes. Also, elevated plasma levels of Hcy in humans have been shown to be a risk factor in coronary artery disease. ^ On the basis of the previous finding that AdoHcy hydrolase is able to add the enzyme-sequestered water molecule across the 5',6'-double bond of (halo or dihalohomovinyl)-adenosines causing covalent binding inhibition, we designed and synthesized AdoHcy analogues with the 5',6'-olefin motif incorporated in place of the carbon-5' and sulfur atoms. From the available synthetic methods we chose two independent approaches: the first approach was based on the construction of a new C5'-C6' double bond via metathesis reactions, and the second approach was based on the formation of a new C6'-C7' single bond via Pd-catalyzed cross-couplings. Cross-metathesis of the suitably protected 5'-deoxy-5'-methyleneadenosine with racemic 2-amino-5-hexenoate in the presence of Hoveyda-Grubb's catalyst followed by standard deprotection afforded the desired analogue as 5' E isomer of the inseparable mixture of 9'R/S diastereomers. Metathesis of chiral homoallylglycine [(2S)-amino-5-hexenoate] produced AdoHcy analogue with established stereochemistry E at C5'atom and S at C9' atom. The 5'-bromovinyl analogue was synthesized using the bromination-dehydrobromination strategy with pyridinium tribromide and DBU. ^ Since literature reports on the Pd-catalyzed monoalkylation of dihaloalkenes (Csp2-Csp3 coupling) were scarce, we were prompted to undertake model studies on Pd-catalyzed coupling between vinyl dihalides and alkyl organometallics. The 1-fluoro-1-haloalkenes were found to undergo Negishi couplings with alkylzinc bromides to give multisubstituted fluoroalkenes. The alkylation was trans-selective affording pure Z-fluoroalkenes. The highest yields were obtained with PdCl 2(dppb) catalyst, but the best stereochemical outcome was obtained with less reactive Pd(PPh3)4. Couplings of 1,1-dichloro-and 1,1-dibromoalkenes with organozinc reagents resulted in the formation of monocoupled 1-halovinyl product. ^

Synthesis of multisubstituted halo-olefins via Pd-catalyzed cross-coupling reactions : applications in nucleoside chemistry

The enzyme S-adenosyl-L-homocysteine (AdoHey) hydrolase effects hydrolytic cleavage of AdoHcy to adenosine (Ado) and L-homocysteine (Hcy). The cellular levels of AdoHcy and Hcy are critical because AdoHcy is a potent feedback inhibitor of crucial transmethylation enzymes. Also, elevated plasma levels of Hcy in humans have been shown to be a risk factor in coronary artery disease. On the basis of the previous finding that AdoHcy hydrolase is able to add the enzyme-sequestered water molecule across the 5',6'-double bond of (halo or dihalohomovinyl)-adenosines causing covalent binding inhibition, we designed and synthesized AdoHcy analogues with the 5',6'-olefin motif incorporated in place of the carbon-5' and sulfur atoms. From the available synthetic methods we chose two independent approaches: the first approach was based on the construction of a new C5'- C6' double bond via metathesis reactions, and the second approach was based on the formation of a new C6'-C7' single bond via Pd-catalyzed cross-couplings. Cross-metathesis of the suitably protected 5'-deoxy-5'-methyleneadenosine with racemic 2-amino-5-hexenoate in the presence of Hoveyda-Grubb's catalyst followed by standard deprotection afforded the desired analogue as 5'E isomer of the inseparable mixture of 9'RIS diastereomers. Metathesis of chiral homoallylglycine [(2S)-amino-5-hexenoate] produced AdoHcy analogue with established stereochemistry E at C5'atom and S at C9' atom. The 5'-bromovinyl analogue was synthesized using the brominationdehydrobromination strategy with pyridinium tribromide and DBU. Since literature reports on the Pd-catalyzed monoalkylation of dihaloalkenes (Csp2-Csp3 coupling) were scarce, we were prompted to undertake model studies on Pdcatalyzed coupling between vinyl dihalides and alkyl organometallics. The 1-fluoro-1- haloalkenes were found to undergo Negishi couplings with alkylzinc bromides to give multisubstituted fluoroalkenes. The alkylation was trans-selective affording pure Zfluoroalkenes. The highest yields were obtained with PdCl 2(dppb) catalyst, but the best stereochemical outcome was obtained with less reactive Pd(PPh3)4 . Couplings of 1,1- dichloro-and 1,1-dibromoalkenes with organozinc reagents resulted in the formation of monocoupled 1-halovinyl product.