1,150 research outputs found
Directed carbonylative (3+1+2) cycloadditions of amino-substituted cyclopropanes and alkynes: reaction development and increased efficiencies using a cationic rhodium system
AbstractUrea-directed carbonylative insertion of Rh(I)-catalysts into one of the two proximal C–C bonds of aminocyclopropanes generates rhodacyclopentanone intermediates. These are trapped by N-tethered alkynes to provide a (3+1+2) cycloaddition protocol that accesses N-heterobicyclic enones. Stoichiometric studies on a series of model rhodacyclopentanone complexes outline key structural features and provide a rationale for the efficacy of urea directing groups. A comprehensive evaluation of cycloaddition scope and a ‘second generation’ cationic Rh(I)-system, which provides enhanced yields and reaction rates for challenging substrates, are presented
Factorization fits to charmless strangeless B decays
We present fits to charmless strangeless hadronic B decay data for mean
branching ratios and CP-violating asymmetries using the QCD factorization model
of Beneke et al. Apart from one CP-violating parameter, the model gives a very
good representation of 26 measured data. We find the CKM angle alpha = (93.5
+/- 8.4 -1.3) degrees and to be quite stable to plausible "charming penguin"
corrections.Comment: 4 pages, LaTeX, Minor changes to text, references adde
Modular access to substituted azocanes via a rhodium-catalyzed cycloaddition-fragmentation strategy
A short entry to
substituted azocanes by a Rh-catalyzed cycloaddition–fragmentation
process is described. Specifically, exposure of diverse <i>N</i>-cyclopropylacrylamides to phosphine-ligated cationic RhÂ(I) catalyst
systems under a CO atmosphere enables the directed generation of rhodacyclopentanone
intermediates. Subsequent insertion of the alkene component is followed
by fragmentation to give the heterocyclic target. Stereochemical studies
show, for the first time, that alkene insertion into rhodacyclopentanones
can be reversible
The Effects of Nitrogen Fertilisation on Plant Species Richness in European Permanent Grasslands: A Systematic Review and Meta-Analysis
Nitrogen fertilisation is a common form of agricultural intensification, aimed at increasing biomass, which can affect plant species diversity and ecosystem functioning. Using a systematic review and meta-analysis of nitrogen fertilisation studies in European permanent grasslands, we asked: (i) what relationship form exists between nitrogen application rate and change in plant diversity, compared to zero fertilisation controls; and (ii) how grassland, management and study characteristics affect this relationship. Meta-analysis of 34 control-treatment effects from 14 studies conducted across nine European countries revealed a negative linear relationship between nitrogen fertilisation rate and change in plant species richness, equivalent to approximately 1.5 species/m(2) lost for every 100 Kg ha(-1) yr(-1) of nitrogen added. Fertilisation induced reductions in plant species richness were greater when defoliation rates were lower. We found some evidence that grasslands with a higher baseline plant diversity lost more species when fertilised compared to more species poor grasslands, although uncertainty was high. Due to the diverse grassland types included in the analysis, the variability in fertilisation-driven changes in plant diversity was high. We identified several remaining limitations to our understanding, including uncertainty about non-linear effects, which could aid efforts to optimise the trade-off of plant diversity and increasing grassland yields
Laser Intensity Dependence of Photoassociation in Ultracold Metastable Helium
Photoassociation of spin-polarized metastable helium to the three lowest
rovibrational levels of the J=1, state asymptoting to 2SP is studied using a second-order perturbative
treatment of the line shifts valid for low laser intensities, and two variants
of a non-perturbative close-coupled treatment, one based upon dressed states of
the matter plus laser system, and the other on a modified radiative coupling
which vanishes asymptotically, thus simulating experimental conditions. These
non-perturbative treatments are valid for arbitrary laser intensities and yield
the complete photoassociation resonance profile. Both variants give nearly
identical results for the line shifts and widths of the resonances and show
that their dependence upon laser intensity is very close to linear and
quadratic respectively for the two lowest levels. The resonance profiles are
superimposed upon a significant background loss, a feature for this metastable
helium system not present in studies of photoassociation in other systems,
which is due to the very shallow nature of the excited state potential.
The results for the line shifts from the close-coupled and perturbative
calculations agree very closely at low laser intensities.Comment: 14 pages, 7 figures, title altered, text reduce
Do biodiversity-ecosystem functioning experiments inform stakeholders how to simultaneously conserve biodiversity and increase ecosystem service provisioning in grasslands?
Two key stakeholders primarily important for nature conservation are farmers (and their lobby groups) and conservationists. Both have substantial inputs into environmental strategies and policies calling for biodiversity conservation aimed to directly increase ecosystem services. The scientific literature concurs that as biological diversity increases so do ecosystem functions and services in grasslands. While the evidence for this is strong, the majority comes from controlled small-scale biodiversity-ecosystem functioning (BEF) experiments. Thus, it is unclear whether the scientific basis for implementing BEF relationships into practice is sufficiently evidenced. Here we explore the applicability of findings from BEF experiments to the conservation and management of temperate grassland, a widespread and potentially highly biodiverse habitat. While we acknowledge that BEF research can reveal insights into fundamental mechanisms, the saturation of biodiversity effects at low levels and unrealistic (management) treatments widely impede the applicability of these experimental results to permanent grasslands. Additionally, the integration of BEF research results into practice is considerably hampered by experimental studies not answering stakeholders' crucial questions, e.g. is there evidence of biodiversity conservation potentials? Thus, stakeholders do not have a strong evidence base for taking decisions for the addressed management goals, except intensive production in (species-poor) temporary grasslands. If BEF work is to inform stakeholders future research needs to overcome unrealistic management, missing stakeholder involvement and ineffective communication. A new generation of applied BEF experiments employing applied, multi-actor approaches is needed to facilitate the relevance of BEF research for nature conservation, agriculture and land management
Hadronic B decays: Supersymmetric enhancement and a simple spectator model
Two aspects of hadronic B decays are investigated. Firstly, the
supersymmetric enhancement of hadronic b decays by gluino penguin processes is
studied through their effect on the Wilson coefficients of the effective
Hamiltonian. Secondly, hadronization of the final state quarks is studied
through a simple phase space spectator model.Comment: 24 pages, REVTEX, minor additional text and some references adde
Suppression of Phase Separation in LiFePO4 Nanoparticles During Battery Discharge
Using a novel electrochemical phase-field model, we question the common
belief that LixFePO4 nanoparticles separate into Li-rich and Li-poor phases
during battery discharge. For small currents, spinodal decomposition or
nucleation leads to moving phase boundaries. Above a critical current density
(in the Tafel regime), the spinodal disappears, and particles fill
homogeneously, which may explain the superior rate capability and long cycle
life of nano-LiFePO4 cathodes.Comment: 27 pages, 8 figure
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