Porphyrin-LEGO®: synthesis of a hexafullereno-diporphyrin using porphyrins programmed for [4+2]-cycloaddition

A hexafullereno-diporphyrinoid was obtained in a sequence of cycloaddition steps using porphyrins programmed for [4+2]-cycloaddition reactions and C(60)-fullerene.     

Figure of merit for spectrometers for EDXRF

How should one select the best detector for a particular measurement in energy dispersive X‐ray fluorescence (EDXRF)? How should one select the optimum system configuration, i.e. the best shaping time and beam current? Manufacturers provide a variety of specifications, such as energy resolution and maximum count rate, but these are indirectly related to the end use of an EDXRF instrument, the measurement and detection limit of the measured elemental concentrations. We suggest in this paper using the time required to achieve a given statistical uncertainty as a figure of merit. We derive scaling rules for this figure of merit based on conventional specifications, including energy resolution, peaking time, maximum count rate, detector area, and intrinsic efficiency. These scaling rules also include the peak to background ratio of a photopeak and the number of overlapping peaks. We then show how this figure of merit can be used to select the optimum detector and spectrometer configuration for specific applications and compare the results to data obtained with typical systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Quantitative anomalous small-angle X-ray scattering — The determination of chemical concentrations in nano-scaled phases

In the last years Anomalous Small-Angle X-ray Scattering became a precise quantitative method resolving scattering contributions two or three orders of magnitude smaller compared to the overall small-angle scattering, which are related to the so-called pure-resonant scattering contribution. Additionally to the structural information precise quantitative information about the different constituents of multi-component systems like the fraction of a chemical component implemented into the materials nanostructures are obtained from these scattering contributions. The application of the Gauss elimination algorithm to the vector equation established by ASAXS measurements at three X-ray energies is demonstrated for three examples from chemistry and solid state physics. All examples deal with the quantitative analysis of the Resonant Invariant (RI-analysis). From the integrals of the pure-resonant scattering contribution the chemical concentrations in nano-scaled phases are determined. In one example the correlated analysis of the Resonant Invariant and the Non-resonant Invariant (NI-analysis) is employed.     

Unexpected trends in halogen-bond based noncovalent adducts

Unexpected trends in the strengths of halogen-bond based adducts of CY(3)I (Y = F, Cl, Br, I) with two typical Lewis bases (chloride and trimethylamine) show that the halogen-bond donor strength (Lewis acidity) of a compound R-X is not necessarily increased with higher electronegativity of the (carbon-based) group R.     

Spectroscopic selection of distance measurements in a protein dimer with mixed nitroxide and Gd3+ spin labels

The pulse DEER (Double Electron-Electron Resonance) technique is frequently applied for measuring nanometer distances between specific sites in biological macromolecules. In this work we extend the applicability of this method to high field distance measurements in a protein assembly with mixed spin labels, i.e. a nitroxide spin label and a Gd(3+) tag. We demonstrate the possibility of spectroscopic selection of distance distributions between two nitroxide spin labels, a nitroxide spin label and a Gd(3+) ion, and two Gd(3+) ions. Gd(3+)-nitroxide DEER measurements possess high potential for W-band long range distance measurements (6 nm) by combining high sensitivity with ease of data analysis, subject to some instrumental improvements.     

Nuclear electric quadrupole moments of Rb from the hyperfine spectrum of RbF

The molecular beam electric resonance technique has been used to examine the hyperfine spectrum of RbF. The Rb nuclear electric quadrupole interaction, the spin-rotation interactions, and tensor and scalar spin-spin interactions have been measured for both Rb isotopes, including their dependence on vibrational and rotational states. Transition frequencies have been determined to a precision of better than 1 Hz in many cases. The magnetic interactions in the two isotopomers are consistent with what is expected from the known masses and magnetic dipole moments. In the case of the Rb nuclear electric quadrupole interaction, adjustments have been made for a small isotopomer shift, and for the ratio of the effective nuclear electric quadrupole moments, Q(87Rb)Q(85Rb) = 0.483 830 1+/-0.000 001 8. The effective quadrupole interaction includes a pseudoquadrupole interaction that may be significant at this level of precision, but cannot be distinguished experimentally.     

A trapped-ion local field probe

We introduce a measurement scheme that utilizes a single ion as a local field probe. The ion is confined in a segmented Paul trap and shuttled around to reach different probing sites. By the use of a single atom probe, it becomes possible characterizing fields with spatial resolution of a few nm within an extensive region of millimeters. We demonstrate the scheme by accurately investigating the electric fields providing the confinement for the ion. For this we present all theoretical and practical methods necessary to generate these potentials. We find sub-percent agreement between measured and calculated electric field values.     

Nd:YAG waveguide laser with 1.3 W output power, fabricated by direct femtosecond laser writing

Tracks of modified material were written with femtosecond-laser pulses in neodymium-doped YAG crystals. Due to a stress-induced change of the refractive index, waveguiding beside the tracks and between two adjacent tracks with a distance of approximately 25 μm was observed. Loss measurements resulted in guiding losses of about 1.6 dB/cm for the double track waveguide. Spectroscopic investigations of the ⁴ F _3/2→⁴ F _11/2 transmission lines of the neodymium ions, which are close to the modified region, revealed a small stress-induced red shift of the lines. Laser oscillation of single-track waveguides and double-track waveguides was demonstrated with Ti:Sapphire laser pumping at a wavelength of 808 nm. Best laser performance with about 1.3 W output power at 2.25 W launched pump power was achieved using a double-track waveguide with a separation of 27 μm at an outcoupling transmission of 95%.     

Size-dependent freezing of n-alcohols in silicon nanochannels

We present a study on the phase behavior of several linear n-alcohols (heptanol, nonanol and undecanol) in their bulk state as well as confined in mesoporous silicon. We were able to vary the mean pore radii of the nanochannels from r = 3.5  nm to 7 nm and to determine the respective temperatures of the freezing and melting transitions using infrared and dielectric spectroscopy. The smaller the chain length the lower the freezing point, both in the bulk and in the confined state. Under confinement the freezing temperature decreases by up to 28 K compared to the bulk value. In accordance with the Gibbs-Thompson model the lowering is proportional to the inverse pore radius, ΔT _fr ∝ 1/r. Moreover, the ratio of freezing temperature depression to melting temperature depression is close to the theoretical value of ΔT _fr /ΔT _melt = 3/2. The spectra also indicate a structural change: while the solid bulk alcohols are a polycrystalline mixture of the orthorhombic β- and monoclinic γ-form, geometrical confinement forces the alcohol-chains into the more simple orthorhombic structure. In addition, a part of the material does not crystallize. Such an additional amorphous phase seems to be a logical consequence of the size mismatch between molecular crystals and irregular shaped pores.     

A lattice Boltzmann model for coupled diffusion

Diffusion coupling between different chemical components can have significant effects on the distribution of chemical species and can affect the physico-chemical properties of their supporting medium. The coupling can arise from local electric charge conservation for ions or from bound components forming compounds. We present a new lattice Boltzmann model to account for the diffusive coupling between different chemical species. In this model each coupling is added as an extra relaxation term in the collision operator. The model is tested on a simple diffusion problem with two coupled components and is in excellent agreement with the results obtained through a finite difference method. Our model is observed to be numerically very stable and unconditional stability is shown for a class of diffusion matrices. We further develop the model to account for advection and show an example of application to flow in porous media in two dimensions and an example of convection due to salinity differences. We show that our model with advection loses the unconditional stability, but offers a straight-forward approach to complicated two-dimensional advection and coupled diffusion problems.