Quantifying hydrogen-bonding strength: the measurement of 2hJNN couplings in self-assembled guanosines by solid-state 15N spin-echo MAS NMR

(2h)J(NN) hydrogen-bond mediated J couplings are measured in the solid state for two synthetic deoxyguanosine derivatives by (15)N MAS NMR spin-echo experiments. The use of rotor-synchronised Hahn-echo pulse train (RS-HEPT) (1)H decoupling, with a duty cycle of 6%, allows spin-echo durations out to 200 ms, hence enabling the accurate determination of J couplings as small as 3.8 Hz. A single-crystal X-ray diffraction structure exists for the shorter alkyl chain derivative dG(C(3))(2): the observation of significantly different (2h)J(NN) couplings, 6.2 +/- 0.4 and 7.4 +/- 0.4 Hz, for the two resolved N7 resonances is to be expected given the NH...N hydrogen-bonding distances of 2.91 and 2.83 A for the two distinct molecules in the asymmetric unit cell. For the longer alkyl chain derivative, dG(C(10))(2), for which there is no single-crystal diffraction structure, a (15)N refocused INADEQUATE spectrum (Pham et al., J. Am. Chem. Soc., 2005, 127, 16018-16019) has demonstrated the presence of N2-H...N7 intermolecular hydrogen-bonds indicative of a quartet-like structure. The (2h)J(NN) hydrogen-bond mediated J coupling of 5.9 +/- 0.2 Hz is at the lower end of the range (5.9-8.2 Hz) of (2h)J(NN) couplings determined from solution-state NMR of guanosine quartets in quadruplex DNA. A full discussion of the determination of error bars on the fitted parameters is given; specifically, error bars determined by a non-linear fitting (using the covariance matrix) or in a Monte-Carlo fashion are found to give effectively identical results.     

Continuous-Wave Operation of a 460-GHz Second Harmonic Gyrotron Oscillator

We report the regulated continuous-wave (CW) operation of a second harmonic gyrotron oscillator at output power levels of over 8 W (12.4 kV and 135 mA beam voltage and current) in the TE(0,6,1) mode near 460 GHz. The gyrotron also operates in the second harmonic TE(2,6,1) mode at 456 GHz and in the TE(2,3,1) fundamental mode at 233 GHz. CW operation was demonstrated for a one-hour period in the TE(0,6,1) mode with better than 1% power stability, where the power was regulated using feedback control. Nonlinear simulations of the gyrotron operation agree with the experimentally measured output power and radio-frequency (RF) efficiency when cavity ohmic losses are included in the analysis. The output radiation pattern was measured using a pyroelectric camera and is highly Gaussian, with an ellipticity of 4%. The 460-GHz gyrotron will serve as a millimeter-wave source for sensitivity-enhanced nuclear magnetic resonance (dynamic nuclear polarization) experiments at a magnetic field of 16.4 T.     

Finite Element Analysis of Dynamically Loaded Homogeneous Viscous Beams

The paper is concerned with impulsively loaded beams in which the material is treated as homogeneous viscous as an approximation of a rigid-viscoplastic constitutive relation. As opposed to the standard displacement method finite element formulation, where interpolation functions describing the velocity field across elements is given, a mixed formulation is used in which nodal velocities and nodal moments are carried as parameters. At each instant the accelerations (by the Tamuzh principle) and the rates of change of moment (by a virtual velocities formulation) are found, and velocities and moments are integrated forward independently. The properties of the mode solution are also introduced, and the forward integration is carried through only for the difference between the mode solution and the actual solution. This leads to a very efficient scheme for the numerical solution of a cantilever beam problem shown as an illustration.     

Levels of mineral nutrients in fresh- and frozen bulk hydrated biological specimens: A comparison of EDS data collected in the environmental SEM and a conventional cryo-SEM

Energy-dispersive X-ray microanalysis (EDS) was compared in fresh- and frozen bulk hydrated tissues using the Environmental SEM (ESEM) and conventional cryo-SEM, respectively. Analyses of globoid inclusions of Eucalyptus calophylla seed from two soil types demontsrated that higher levels of cations (K, Ca, Mg, Al, Fe, Mn, Cu, Zn) occurred in seeds from soils containing higher levels of Al, while EDS-detectable levels of S and P were dependent upon the techniques utilised. Cumulative changes in ESEM-EDS-detectable levels of S and P were characterised by collecting cumulative spectra from nutrient standards and compared with those for K. Progressive increases in K occurred and were consistent with an enriching effect. Levels of S and P increased during early analysis (40–60 sec live time) and decreased thereafter. The semi-conductive nature of biological samples, the loss of anions and gain of cations from the net negatively-charged electron interaction volume contributed to an electrochemical bias. These local modifications in fluid chemistry were reversible. Dehydration effects also occurred in stable, “wet‘ samples. These differences indicated that EDS in ESEM may be limited to cations rather than anions, and that changes in fluid electrochemistry and dehydration may affect the level and distribution of elements.     

A comparison of quasi-static uniaxial-strain and hugoniot tests for quartz-phenolic composite

The purpose of this paper is to show that, within experimental uncertainty, the change in volume with stress obtained by quasi-static uniaxial-strain tests matches that obtained by hugoniot experiments over the same pressure range for quartz phenolic. The result of these tests shows that comparing the data by both techniques is meaningful. In addition, the use of the relatively simple and inexpensive quasi-static uniaxial-strain test (strain rates of 10^?4/sec) may provide designers and materials engineers a method for rapid surveying of materials for their hugoniot properties.