Dynamic buckling of discrete structural systems under combined step and static loads

The dynamic instability of discrete, elastic, multiple degree of freedom (d.o.f.) systems under a combination of static and step loads is studied. Conservative, autonomous and holonomic systems are considered, in which the associated static response is a bifurcation under one load parameter, and a limit point under the second parameter. A review of different criteria and algorithms obtained from them for the computation of dynamic buckling loads is first presented, followed by a procedure derived from previous investigations on one d.o.f. systems. The different procedures are applied to a two d.o.f. problem under axial and lateral load, with quadratic and cubic non-linearities. The response in time shows that the system oscillates about the static equilibrium position before dynamic buckling is reached, with the kinetic energy tending to zero as assumed in the static (energy) procedures of stability.     

Long-term effects of off-road vehicle traffic on tundra terrain

Traffic tests were conducted at two sites in northern Alaska with an air cushion vehicle, two light tracked vehicles, and three types of wheeled Rolligon vehicles. The traffic impact (surface depression, effect on thaw depth, damage to vegetation, traffic signature visibility) was monitored for periods of up to 10 years. Data show the immediate and long-term effects from the various types of vehicles for up to 50 traffic passes and the rates of recovery of the active layer. The air cushion vehicle produced the least impact. Multiple passes with the Rolligons caused longer-lasting damage than the light tracked vehicles because of their higher ground contact pressure and wider area of disturbance. Recovery occurs even if the initial depression of the tundra surface by a track or a wheel is quite deep (15 cm), as long as the organic mat is not sheared or destroyed.     

Hydrodynamic chromatography of latex blends

The ability to characterize the size and shape distributions of broadly polydisperse analytes is a driving force in particle size analysis. Multi-detector hydrodynamic chromatography (HDC), which has previously shown promise in its ability to characterize the size and shape of monodisperse, spherical polystyrene latex standards, is applied here to include the characterization of bi-, tri-, and tetramodal latex blends and their constituents varying in size, chemistry, and compactness. The ability of multi-detector HDC to distinguish between similar-sized particles of different chemistry is realized by the coupling of a concentration-sensitive detector and two different types of light scattering detectors. The use of both multi-angle static and quasi-elastic light scattering permits for determination of two different size parameters across the elution profiles of the blends. Combining the size information obtained from both light scattering methods provides a measure of how particle compactness changes as a function of size in a latex blend. Multi-detector HDC was shown to be a rapid and precise method for characterizing particle size, shape, and their distributions of broadly polydisperse analytes.     

Nonlinear optical properties of CNHP4 nanofibers: Molecular dipole orientations and two photon absorption cross-sections

Polarized second harmonic measurements are used to determine the in- and out-of plane orientations of molecular dipoles in optical active, elongated, surface bound nanoaggregates. As a specific example, nonlinear optical active nanofibers grown from CNHP4 molecular dipoles on muscovite mica are investigated. The orientation of the dipoles relative to the substrate is found to be 7 ± 5°, whereas that of the dipoles relative to the long nanofiber axes is ± 13 ± 5°. Following 780 nm femtosecond laser excitation, the two photon action potential ησ₂ for CNHP4 is determined to be 4.7 × 10⁻⁵⁴ cm⁴ s/photon. For comparison, ησ₂ for para-hexaphenylene nanofibers is measured to be 12 × 10⁻⁵⁴ cm⁴ s/photon.     

Evidence of two dimensionality in quasi-one-dimensional cobalt oxides

The quasi-one-dimensional (Q1D) cobalt oxides A(N + 2)Co(n + 1)O(3n + 3) (A = Ca, Sr, and Ba, n = 1 - infinity) were investigated by muon-spin spectroscopy under applied pressures of up to 1.1 GPa. The relationship between the onset Néel temperature T(on)(N) and the interchain distance (d(ic)), which increases monotonically with n, is well fitted by the formula T(N)/T(N,0) = (1 - d(ic)/d(ic,o)(beta), here for T(on)(N) approximately 100 K for Ca(3)Co(2)O(6) (n = 1) and approximately 15 for BaCoCoO(3) (n = infinity at ambient P. The T(on)(N) - d(ic) curve also predicts a large dependence of Y(N) for the compounds with n > or = 5, i.e., in the vicinity of , while the compounds show only a very small effect. Indeed, our high-pressure mu(+) results show that of BaCoO(3) is enhanced by with a slope of 2.2 K(Gpa), whereas no detectable changes by P for both Ca(3)Co(2)O(6) and Sr(4)Co(3)O(9) (n = 2). This clearly confirms the role of the 2D-antiferromagnetic interaction on T(on)(N) in the Q1D cobalt oxides.     

Synthetic approaches to bicyclic diazenium salts

Bicyclic diazenium salts have been prepared from α-chloroazo species via a Lewis acid-mediated intramolecular cycloaddition. An alternative, more direct, route to these salts by the reaction of hydrazones with dimethylsulfonium ditriflate is also described. Terminal olefins provided mixtures of fused and bridged bicyclic diazenium salts. The ratio of the fused and bridged species was observed to depend on the electronics of the N-aryl substituent, which is explained by considering a concerted asynchronous cycloaddition mechanism.     

Disposable pipette extraction for the analysis of pesticides in fruit and vegetables using gas chromatography/mass spectrometry

Organochlorine, organophosphate pesticides and fungicides in fruits and vegetables were analyzed using disposable pipette extraction (DPX) followed by gas chromatography–mass spectrometry-selective ion monitoring (GC/MS-SIM). The intrinsic rapid mixing capabilities of DPX result in fast and efficient extractions, and eluates are concentrated by using minimal elution solvent volumes rather than solvent evaporation methods. Matrix-matched calibrations were performed with reversed phase mechanisms (DPX-RP), and the limits of detection (LOD) were determined to be lower than 0.1 μg/mL for all targeted pesticides in carrot and orange sample matrices. Coefficients of determination (r²) were greater than 0.995 for most studied pesticides. DPX-RP exhibited recoveries between 72 and 116% for nonpolar and slightly polar pesticides (log P > 2) with most of the recoveries over 88%. Only very polar pesticides (e.g., acephate, mathamidophos) were not extracted well using DPX-RP.     

The conformational bias of aryl, arylsulfonyl geminally substituted tertiary carbon centers: applications in substrate-based stereocontrol

Intramolecular nitrile oxide-olefin cycloaddition to form hexahydrobenzisoxazole 14, which engenders a phenylsulfonyl, 2,5-difluorophenyl geminally substituted carbon substructure, proceeds with up to 99% ds. A rationalization of the high level of substrate-based stereo-induction observed in this and related ketone and acrylonitrile metallohydride reductions, supported by single crystal X-ray crystallography, is presented.