INTERCONVERSION OF COMPOUNDS WITH PENTA-AND HEXACOORDINATE PHOSPHORUS. CRYSTAL AND MOLECULAR STRUCTURE OF THE TRIALKYLAMMONIUM HEXAOXYPHOSPHORIDE, [(PO6)(C6H5)2(C6H4)(CF3C˭CCF3)]−[(C2H5)3NH]+

Abstract

The structure of triethylammonium diphenoxy-o-phenylenedioxy-1,2-bistrifluoromethylethenylenedioxyphosphoride, a compound with hexacoordinate phosphorus obtained by addition of triethylammonium phenoxide to the pentacoordinate phosphorus precursor, has been determined by x-ray crystallographic methods. The compound crystallizes from ether in space group P2_1/n of the monoclinic system. There are four formula units, (C₂₂H₁₄O₆P)-(NC₆H₁₅)⁺ in the unit cell (Z=4), with one ion-pair constituting the asymmetric unit of the crystal. The cell dimensions are a=10.787(5), b=16.604(6), c=16.668(4) Å; β=102.84°(3); D _ctlc=1.415 g cm^?3, D _meas=1.412 g cm^?3 (25°). Data were obtained on a CAD4 automatic diffractometer; 5310 unique non-zero reflections were collected with θ≤ 75° using (θ-2θ) scan, with a scan width of 1.0°. The phosphorus and oxygen atoms were located using the MULTAN program. All other non-hydrogen atoms were found in subsequent iterations of partial-structure phased Fourier maps. The structure was refined by full-matrix least-squares techniques to a final R _w value of 7.5% on F based on 4855 independent reflections. The phosphorus atom is hexacoordinate and at the center of a nearly regular octahedron, with the two phenoxy groups cis to each other. The two P–O (exocyclic) bonds are shorter (ave. 1.656(4) Å) than the four P–O (endocyclic) bonds (ave. 1.711(4) Å). The positively charged nitrogen is closer to two of the uncharged oxygen ligands (3.000 and 3.108(6) Å) than to the negatively charged phosphorus (4.063(6) Å) within the formula unit. The changes in molecular parameters when an oxyphosphoride is derived from an oxyphosphorane by addition of a sixth oxy-ligand are discussed.     

Synchrotron-based photoelectron spectroscopy provides evidence for a molecular bond between calcium and mineralizing organic phases in invertebrate calcareous skeletons

Organic compounds have been extracted from calcium carbonate skeletons produced by three invertebrate species belonging to distinct phyla. The soluble parts of these skeleton matrices were isolated and analysed by synchrotron-based X-ray spectroscopy (XPS). The presence of calcium associated with these organic materials was revealed in every sample studied, with important variations in Ca 2p binding energy from species to species. Measured Ca 2p binding energy values are more related to compositional diversity of the mineralizing matrices of the skeletons, whose taxonomic dependence has long been established, than to the Ca carbonate polymorph selected to build the skeletal units. This suggests a physical bond between species-specific mineralizing organic assemblages and the associated calcium. Remarkably, the binding energy of 2p electrons in calcium associated with mineralizing matrices is consistently higher than Ca 2p values obtained in purely mineral carbonate (both calcite and aragonite). The ability both to identify and measure the effect of organic matrices on their mineral counterpart in calcareous biominerals opens a new perspective for a functional approach to the biomineralization process.     

Photoexcitation in two-dimensional topological insulators

One of the most fascinating challenges in Physics is the realization of an electron-based counterpart of quantum optics, which requires the capability to generate and control single electron wave packets. The edge states of quantum spin Hall (QSH) systems, i.e., two-dimensional (2D) topological insulators realized in HgTe/CdTe and InAs/GaSb quantum wells, may turn the tide in the field, as they do not require the magnetic field that limits the implementations based on quantum Hall effect. However, the band structure of these topological states, described by a massless Dirac fermion Hamiltonian, prevents electron photoexcitation via the customary vertical electric dipole transitions of conventional optoelectronics. So far, proposals to overcome this problem are based on magnetic dipole transitions induced via Zeeman coupling by circularly polarised radiation, and are limited by the g-factor. Alternatively, optical transitions can be induced from the edge states to the bulk states, which are not topologically protected though.Here we show that an electric pulse, localized in space and/or time and applied at a QSH edge, can photoexcite electron wavepackets by intra-branch electrical transitions, without invoking the bulk states or the Zeeman coupling. Such wavepackets are spin-polarised and propagate in opposite directions, with a density profile that is independent of the initial equilibrium temperature and that does not exhibit dispersion, as a result of the linearity of the spectrum and of the chiral anomaly characterising massless Dirac electrons. We also investigate the photoexcited energy distribution and show how, under appropriate circumstances, minimal excitations (Levitons) are generated. Furthermore, we show that the presence of a Rashba spin–orbit coupling can be exploited to tailor the shape of photoexcited wavepackets. Possible experimental realizations are also discussed.     

Photokinetic methods: A mathematical analysis of the rate equations in photochromic systems

In this article the analytical integration of kinetic equations describing the dynamic behavior of reversible photoreactions has been undertaken. Photochemical systems giving rise to competing thermal and photochemical steps are kinetically analyzed; the rate law is set up and analytical solutions are obtained under precisely defined boundary conditions. More complicated kinetic systems, where several species interact both thermally and photochemically, are also investigated. The kinetic treatment leads to a system of coupled differential equations which are amenable to analytical solutions, under the appropriate experimental and boundary conditions. The usefulness of the equations developed is illustrated by their application to some spirooxazine and chromene photochromic systems: two examples are described in detail. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 303–313, 1999     

Volatiles Emission by Crotalaria nitens after Insect Attack

Plants are known to increase the emission of volatile organic compounds upon the damage of phytophagous insects. However, very little is known about the composition and temporal dynamics of volatiles released by wild plants of the genus Crotalaria (Fabaceae) attacked with the specialist lepidopteran caterpillar Utetheisa ornatrix (Linnaeus) (Erebidae). In this work, the herbivore-induced plant volatiles (HIPV) emitted by Crotalaria nitens Kunth plants were isolated with solid phase micro-extraction and the conventional purge and trap technique, and their identification was carried out by GC/MS. The poly-dimethylsiloxane/divinylbenzene fiber showed higher affinity for the extraction of apolar compounds (e.g., trans-β-caryophyllene) compared to the Porapak™-Q adsorbent from the purge & trap method that extracted more polar compounds (e.g., trans-nerolidol and indole). The compounds emitted by C. nitens were mainly green leaf volatile substances, terpenoids, aromatics, and aldoximes (isobutyraldoxime and 2-methylbutyraldoxime), whose maximum emission was six hours after the attack. The attack by caterpillars significantly increased the volatile compounds emission in the C. nitens leaves compared to those subjected to mechanical damage. This result indicated that the U. ornatrix caterpillar is responsible for generating a specific response in C. nitens plants. It was demonstrated that HIPVs repelled conspecific moths from attacked plants and favored oviposition in those without damage. The results showed the importance of volatiles in plant–insect interactions, as well as the choice of appropriate extraction and analytical methods for their study.     

Rotational isomers of methyl trans-cinnamate: A Raman study

The Raman and infrared spectra of methyl trans-cinnamate were measured as a function of temperature in the liquid and solid phases. The temperature dependence of the band intensities established the presence of two conformers in the liquid phase (the s-cis and s-trans forms, with CC CO dihedral angles equal to 0° and 180°, respectively; ΔH_{(s-trans)-(s-cis)} = 3.43 ± 0.84 kJ mol⁻¹) and led to the conclusion that the thermodynamically most stable s-cis form is the only form present in the solid.