On Closed Sets with Convex Projections

A shadow of a subset A of Rⁿ is the image of A under a projectiononto a hyperplane. Let C be a closed nonconvex set in Rⁿ suchthat the closures of all its shadows are convex. If, moreover,there are n independent directions such that the closures ofthe shadows of C in those directions are proper subsets of therespective hyperplanes then it is shown that C contains a copyof R^n–2. Also for every closed convex set B ‘minimalimitations’ C of B are constructed, that is, closed subsetsC of B that have the same shadows as B and that are minimalwith respect to dimension.     

Glycine/Glycolic acid based copolymers

Glycine/glycolic acid based biodegradable copolymers have been prepared by ring-opening homopolymerization of morpholine-2,5-dione, and ring-opening copolymerization of morpholine-2,5-dione and glycolide. The homopolymerization of morpholine-2,5-dione was carried out in the melt at 200°C for 3 min using stannous octoate as an initiator, and continued at lower reaction temperatures (100–160°C) for 2–48 h. The highest yields (60%) and intrinsic viscosities ([η] = 0.50 dL/g; DMSO, 25°C) were obtained after 3 min reaction at 200°C and 17 h at 130°C using a molar ratio of monomer and initiator of 1000. The polymer prepared by homopolymerization of morpholine-2,5-dione was composed of alternating glycine and glycolic acid residues, and had a glass transition temperature of 67°C and a melting temperature of 199°C. Random copolymers of glycine and glycolic acid were synthesized by copolymerization of morpholine-2,5-dione and glycolide in the melt at 200°C, followed by 17 h reaction at 130°C using stannous octoate as an initiator. The morphology of the copolymers varied from semi-crystalline to amorphous, depending on the mole fraction of glycolic acid residues incorporated. © 1994 John Wiley & Sons, Inc.     

Transcyclometalation,a versatile methodology for multiple metal-carbon bond formation with multisite ligands

Transcyclometalation has been successfully applied for the multi-platination and -ruthenation of 'chartwheel'-type ligand systems containing six potential metal binding sites, thus providing a method for multiple metal-carbon bond formation via C-H bond activation which is superior to established cyclometalation protocols.     

A-dependence study of inclusive ϑ production

TheA-dependence of the inclusive ? meson cross-section is measured using two target materials, beryllium and tantalum, in the kinematic range 0F<0.3 andp _T ² <1 GeV². Parametrizing the cross-section with δ(A)=δ(A=1)A ^α, yields α=0.90 ±0.02 and α=0.86±0.02 for production of ?'s with 120 GeVπ ⁺ andp-beams respectively.     

Information theory applied to feature selection of binary-coded infrared spectra for automated interpretation by retrieval of reference data

A method is described for feature selection from infrared spectra, intended for identification of organic compounds by computer-aided retrieval of reference data contained in small files. Complete discrimination of the binary-coded spectra is achieved by selecting a minimum number of spectral features; the information content is used as the selection criterion. The selection procedure is applied to five data sets (saturated and unsaturated hydrocarbons, alcohols, ethers and aldehydes/ketones) involving some 400 spectra. Each spectrum is uniquely coded by using about 10% of the 140 spectral features (binary-coded peak positions) available originally. For the intensity, a threshold of 50% appears to be applicable in some cases. For coding the frequency or wavelength parameter, wavenumbers (cm^-1) are preferred to wavelengths (mm). The method takes into account the a priori probabilities of spectral features and their correlations. Results of a retrieval program for a few “unknown” spectra are given.     

Liquid-core waveguide technology for coupling column liquid chromatography and Raman spectroscopy

The on-line coupling of liquid chromatography (LC) and Raman spectroscopy (RS) via an entirely plastic liquid-core waveguide (LCW) was optimized in terms of excitation wavelength of the laser, especially in relation to the fluorescence background, and the length of the LCW. Excitation at 632.8 nm (He-Ne laser) was found to be a good compromise between a wavelength long enough to strongly reduce the fluorescence background and, on the other hand, short enough to avoid (re)-absorption of laser light and Raman signals by H2O in LCWs of considerable length. This conclusion is supported by a theoretical discussion on the optimization of LCW lengths as function of the excitation wavelength for H2O and 2H2O. When using the He-Ne laser the optimum length is approximately 50 cm for H2O; this corresponds to a detection cell volume of 19 microl for an LCW of 220 microm I.D., which is fully compatible with conventional-size LC. The influence of an organic modifier, usually necessary for reversed-phase LC, on the free spectral window was evaluated. The potential applicability of LC-LCW-RS was shown for a mixture of adenosine 5'-monophosphate (AMP), guanosine 5'-monophosphate (GMP) and uridine 5'-monophosphate (UMP), utilizing an aqueous eluent without the addition of a modifier. Improved detectability was achieved by using the stopped-flow mode and applying a large-volume-injection procedure (injection volume: 200 microl). Under these conditions, the limit of identification for AMP, GMP and UMP was in the 0.1-0.5-mg/ml range.     

Temperature-dependent fracture mechanisms in ultra-high strength polyethylene fibers

The influence of the temperature on the mechanical properties of gel-spun hot-drawn ultra-high molecular weight polyethylene fibers has been investigated.From these experiments two different fracture mechanisms could be distinguished. The results indicate that above 20C a stress-induced orthorhombic-hexagonal phase transition is responsible for fiber failure. In the hexagonal or rotator phase the chains can easily slip past one another and fiber fracture is initiated by creep. Below 20C the phase transition cannot be introduced because the stress needed for the phase transition would exceed the covalent-bond strength in the polyethylene chain. The strength temperature data of the low temperature region was treated with Zhurkov's kinetic concept, leading to a bond-fracture activation energy of 160 kj/mol and an activation volume of 0.01 nm³. These values, together with the data from irradiation and shrinkage experiments, indicated that in the low temperature region fiber failure might be initiated by the fracture of trapped entanglements instead of that by overstressed, taut tie molecules.     

Structure and formation of [C4H6N]+ ions: 1—Ion structures derived from aliphatic nitriles

Structures and formation of the [C₄H₆N]⁺ ions present in the mass spectra of eleven α-substituted and eleven α-unsubstituted nitriles have been investigated from collisional activation and metastable ion spectra. Collisional activation spectra lead to identification of six structures. The [C₄H₆N]⁺ ions from some branched compounds prove to be mixtures. This, as well as the identity of all metastable ion parameters and certain spectral data, shows that energy differences between all structures are small. This is corroborated by MINDO/3 calculations showing a spread from 724 to 891 kJ mol^?1 over the structures. Earlier proposals for two different [C₄H₆N]⁺ ion structures, based on the mass spectra of deuterium labelled compounds, appeared to be correct. A computer program to calculate the contribution of standard spectra in a measured spectrum has been developed.     

The loss of CH3⋅ from some unsaturated dithioesters

The mechanism for the loss of CH₃? (and C₂H₅?) from the molecular ions of some unsaturated dithioesters of the type with n=0, 1, 2, has been studied. Based on first field free region metastable ion characteristics it is proposed that 1,3-dithiolium type product ions are generated. Deuterium labelling experiments indicated that the molecular ions of the 2-alkenyl alkanedithioates undergo a rearrangement prior to fragmentation which resembles the [3,3]sigmatropic rearrangement in solution chemistry.