The 1:1 crystal complex of salicylic acid and urea

The 1∶1 crystal complex of salicylic acid (C₇H₆O₃) and urea (CH₄N₂O), mp 121° C, is monoclinic, with space groupC2/c (C ₂ ⁶ h, No. 15) and unit cell dimensionsa=22.206(3),b=5.108(1),c=17.177(2) Å,β=106.18(1)°.d _calc=1.407 g cm^?3,d _meas=1.41 g cm^?3 forZ=8. The structure was determined by direct methods and refined by a full-matrix least-squares procedure to giveR=0.057 andR _w =0.050 for 1652 integrated intensities above 2σ(I). The structure contains a strong OH?O hydrogen bond with O?O distance 2.54 Å in which the carboxyl OH group is donor and urea oxygen atom is acceptor. There are two NH?O intermolecular hydrogen bonds with N?O distances of 2.90 and 2.96 Å. Additionally, the salicyclic acid contains an intramolecular OH?O hydrogen bond of 2.56 Å.     

Micropreparative system for enrichment of capillary GC effluents

An automated system for preparative gas chromatography with capillary columns is described. The effluet from the capillary column is switched to the FID detector or to the traps by means of a Live-T switching device. The pneumatics is controlled by a microprocessor so that repetitive sampling can be performed over a period of days in order to enrich sufficient amount of material for NMR or other spectroscopic methods. The effluent containing the compounds is collected in glass tubes filled with column packing material (e.g. Chromosorb coated with 3% OV - 101, crosslinked). The trap temperatue can be adjusted from + 20°C to ? 80°C, depending on the trapping material and volatility of trapped substances. The analysis of enriched substances or chromatographic fractions can be performed by thermal desorption of the same traps or by solven elution. The recovery of enriched substances is higher than 90%. High capacity and resolution for enrichment of trace components are obtained with the aid of a double column-double oven system. Examples of such applications are given.     

Depth-selective conversion-electron Mössbauer spectroscopy of the surface of Ba-M hexaferrite single crystals

The magnetic properties of a 50-to 2-nm-thick surface layer in hexagonal ferrite BaFe₁₂O₁₉ single crystals are investigated for the first time. Measurements are made on a conversion-electron Mössbauer spectrometer constructed on the basis of a unique ultrahigh-vacuum magnetostatic electron analyzer of the “orange” type. An analysis of the experimental Mössbauer spectra obtained from the surface layer of BaFe₁₂O₁₉ 50-to 2-nm-thick single crystals reveals that (1) the spectral line widths are close to the natural widths of Mössbauer lines and neither changes nor a set of the values of effective magnetic fields, as well as a paramagnetic state of iron ions, could appear due to a defect such as the “surface,” and (2) the experimental spectra are best described only under the assumption that the surface of a hexaferrite BaFe₁₂O₁₉ single crystal contains a 2-nm-thick layer, in which the magnetic moments of iron ions are deflected through ～20° from the crystallographic C axis along which the magnetic moments of ions located in the bulk of the crystal are oriented.     

Being a mathematics teacher in times of reform

Within research on mathematics teachers and/or their professional development, the concept of identity emerges as a critique of views of how teaching practice is related to teachers’ ‘internal states’ of knowledge and beliefs. Identity relates teachers’ professional lives to teaching practices and to the contexts in which the teaching and/or professional development occurs. However, what might count as the context still needs in-depth discussion. In order to contribute to the development of a theoretical framework for understanding mathematics teachers’ professional lives, we will draw on one remarkable teacher’s identity as a primary mathematics teacher in relation to one political, sociocultural, and pedagogical context. We use this teacher’s experience to discuss how education policies that create what Ball (2003) called ‘terrors of performativity’ tend to impede the formation of a balanced teacher identity.     

Metallacrown-based compartments: selective encapsulation of three isonicotinate anions in non-centrosymmetric solids

Upon crystallization in high yields, metallacrowns form compartments that are capable of asymmetrically encapsulating three isonicotinate guests.     

Mathematics and life: lessons from history

Developments in mathematics, technology, science, culture and society are closely interwoven with each other. Beyond that, these interrelations tend to be very complex due to the multiplicity of factors involved. In this contribution, three examples from the human story are analysed in order to sensitize for possible characteristics of the interrelation. For the historical cases of early Mesopotamia, the Inca state and Renaissance Italy, it is demonstrated how specific socio-cultural conditions affect the development of mathematics and technology as well as their use. These findings lead to questions about our use of mathematics and the ways in which mathematics, society, technology, culture and science may be linked, today.     

Influence of surface on the effective magnetic fields in α-Fe2O3 and FeBO3

First measurements of the effective magnetic fields as a function of the depth at which the iron ions are in the surface layer of α-Fe₂O₃ and FeBO₃ single crystals are reported. The method used is the depth-selective conversion-electron Mössbauer spectroscopy. An analysis of experimental spectra revealed that the magnetic properties of the crystal surface vary smoothly from the bulk to surface characteristics within a layer ～100 nm thick. The layers lying below ～100 nm from the surface are similar in properties to the bulk of the crystal, and their spectra consist of narrow lines. The spectral linewidths increase smoothly as one approaches the crystal surface. The spectra obtained from a ～10-nm thick surface layer consist of broad lines indicating a broad distribution of effective magnetic fields. Calculations show that the field distribution width in this layer is δ=2.1(3) T, for an average value H _eff=32.2(4) T. It has been experimentally established that, at room temperature (291 K), the effective magnetic fields smoothly decrease as one approaches the crystal surface. The effective fields in a 2.4(9)-nm surface layer of α-Fe₂O₃ crystals are lower by 0.7(2)% than the fields at the ion nuclei in the bulk of the sample. In the case of FeBO₃, the effective fields decrease by 1.2(3)% in a surface layer 4.9(9) nm thick.     

Selective total encapsulation of the sulfate anion by neutral nano-jars

Nano-sized toroidal copper(II)-hydroxide/pyrazolate assemblies, lined by H-bond donors on the inside and hydrophobic on the outside, selectively extract sulfate from mixtures with nitrate or perchlorate. Tetrabutylammonium "lids" seal the "nano-jars" and render the encapsulated sulfate anion completely buried and inaccessible, so that it is not precipitated by Ba(2+) ions.     

Harmonic ion acoustic waves in stimulated Brillouin scattering

Ion acoustic harmonic generation is a heavy damping mechanism of stimulated Brillouin scattering. The theory of first harmonic ion wave production is extended to frequencies four times the fundamental. Besides, the corresponding set of 6 differential equations is solved numerically for the steady state, and spatial solutions of the ion wave amplitudes are obtained. By varying typical parameters of laser plasma interaction experiments some characteristics properties are gained which might be of use for comparison with experiments.     

Characteristic properties of ion acoustic harmonics produced by stimulated brillouin scattering

The theory of ion acoustic harmonic production with stimulated Brillouin scattering is extended up to the third harmonic. Stationary spatial solutions are given for normalized ion acoustic wave amplitudes and characteristic properties derived, which may serve experimentalists for future detailed investigations.