In situ preparation of a highly active N-heterocyclic carbene-coordinated olefin metathesis catalyst

Highly active N-heterocyclic carbene-coordinated catalysts may be synthesized and used in situ, without requiring prior isolation of the catalyst. Activation of this in situ catalyst with ethereal HCl dramatically reduces the reaction times required for high conversions. A variety of alpha,beta-unsaturated carbonyl-containing substrates participate readily in cross and ring-closing metathesis reactions using this preparation.     

MiniMax: What has been learned thus far

Summary A small experiment, ?MiniMax?, has been set up in the C0 intersection region of the Fermilab Tevatron to seek evidence for disoriented chiral condensates and to study other forward physics phenomena. The experiment consists of a proportional wire chamber telescope accompanied by scintillation (trigger) counters, a lead converter, and followed by an electromagnetic calorimeter. The solid angle accepted is a cone centered at pseudorapidity (η) of 4.1 and of radius (in η-ϕ space) of about 0.6. Over 2.5 million events thus far have demonstrated the successful operation of the apparatus, however to date the analysis has not progressed sufficiently to permit any conclusions concerning disoriented chiral condensates. presented byL. W. Jones Paper presented at the Special Session on very high-energy cosmic-ray interactions (superfamilies) of the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Modelling flow over a backward–facing step using the F.E.M. and the two-equation model of turbulence

Flow over a downstream-facing step is predicted using the F.E.M. A two-equation model of turbulence is employed where the transport of turbulence kinetic energy and dissipation rate are depicted using transport-type equations, i.e. the two-equation model of turbulence. The results obtained are compared with other models and experimental results. Generally, the model was found to be under-predictive with regard to the reattachment length when previous empirical data was used in the transport equations.     

On the evaluation of ion source conditions for optimum sensitivity of chemical ionization

The best operating conditions that yield maximum sensitivity at minimum source pressure have been evaluated for the chemical ionization source in a Kratos MS-25 mass spectrometer. The best reagent gas pressure is shown to be different for each gas and is significantly lower than is usually reported in the literature.     

13C NMR spectra of 1,3,4-thiadiazole/thiadiazoline isomeric pairs

The ¹³C NMR spectra of four pairs of 1,3,4-thiadiazole/thiadiazoline isomers have been recorded using broad band and off-resonance proton decoupling techniques and in the coupled mode. Differences in the chemical shifts of the heterocyclic ring and the carbonyl carbons allow identification of the isomer present. Their IR and UV spectra are in agreement with literature reports for azole/azoline analogues.     

A linear algorithm for a core of a tree

A core of a graph G is a path P in G that is central with respect to the property of minimizing d(P) = Σ_υ?V(G)d(υ, P), where d(υ, P) is the distance from vertex υ to path P. We present a linear algorithm for finding a core of a tree. Since the core of a graph is not necessarily unique, we also output a list of all the vertices which are in some core.     

Interpretation of secondary ion mass spectra by means of fingerprint spectra and secondary ion imaging

A passive layer, of several thousand ? thickness, formed on a polycrystalline nickel electrode, has been examined using secondary ion mass spectrometry (SIMS) by spottering with a 5.5 keV, 13μA·cm⁻²,⁴⁰Ar⁺ primary beam. Concentration profiles were detived by monitoring the intensities of atomic and molecular mass peaks as a function of sputtering time (i.e. depth). Nickel was present throughout the layer but not as the element since the relative intensities of the Ni _n ⁺ (n=1, 2, 3, 4) peaks, constituting part of its fingerprint spectrum, differed from those in the fingerprint spectrum of elemental nickel. These values were eventually reached, signifying piercing of the layer and thus providing a means of estimating its thickness. Imaging of⁵⁸Ni⁺ showed the presence of nickel in at least two different modifications in the layer, both with higher Ni⁺ yields than the bulk nickel. Their fractional coverages were estimated from the images taken at various depths. The resulting profile of the Ni⁺ originating from one of these modifications was found to be proportional to the¹⁶O⁻ profile, indicating that these ions originate from the same molecule. This example demonstrates the advantage of combining different SIMS modes (viz. depth profiles, fingerprint spectra and imaging) in tackling certain analytical problems.     

Ethene adsorption, dehydrogenation and reaction with Pd(110): Pd as a carbon 'sponge'

The interaction of ethene with the Pd(110) surface has been investigated, mainly with a view to understanding the dehydrogenation reactions of the molecule and mainly using a molecular beam reactor. Ethene adsorbs with a high probability over the temperature range 130 to 800 K with the low-coverage sticking probability dropping from 0.8 at 130 K to 0.35 at 800 K. The adsorption is of the precursor type, with a weakly held form of ethene being the intermediate between the gas phase and strong chemisorption. Dehydrogenation begins at approximately 300 K and is fast above 350 K. If adsorption is carried out at temperatures up to approximately 380 K, adsorption saturates after about 0.25 monolayer have adsorbed, but above approximately 450 K, adsorption continues at a high rate with continuous hydrogen evolution and C deposition onto the surface. It appears that, in the intermediate temperature range, the carbonaceous species formed is located in the top layer and thus interferes with adsorption, whereas the C goes subsurface above 450 K, the adsorption is almost unaffected, and the C signal is significantly attenuated in XPS. However, the deposited carbon can easily be removed again by reaction with oxygen, thus implying that the carbon remains in the selvedge, that is, in the immediate subsurface region probably consisting of a few atomic layers. No well-ordered structures are identified in either LEED or STM, though some evidence of a c(2 x 2) structure can be seen. The Pd surface, at least above 450 K, appears to act as a "sponge" for carbon atoms, and this effect is also seen for the adsorption of other hydrocarbons such as acetaldehyde and acetic acid.