A temperature-dependent dynamic rearrangement of cis-(R,S)-[Pd(egta)]2– (egta4–=glycine,N,N′-(1,2-ethanediylbis(oxy-2,1-ethanediyl)bis[N-carboxymethyl]) in aqueous solution

The cis-(R,S)-[Pd(egta)]2– complex, egta4–=glycine, N,N-(1,2-ethanediylbis(oxy-2,1-ethanediyl)bis[N-carboxymethyl]), has been examined by 1H- and 13C-n.m.r. methods over the 18.0 to 95.0°C range in D2O. A dynamic process occurs above 65°C which makes the protons on the NCH2 functionalities of the egta tether become 1H-n.m.r. equivalent. The two states that interconvert coalesce at 81°C. Evidence from 13C-n.m.r. spectra obtained at 81°C show that the in-plane coordinated carboxylates are not lost, but rather a pendant carboxylate becomes attached with loss of the central imino donor. The resultant palladium(II)NO3 intermediate is able to reform cis-(R,S)-[Pd(egta)]2– or, presumably, give trans-(R,R)-[Pd(egta)]2–. The rate limiting step occurs with a rate constant of 178s–1 at 81°C and an activation energy of 20.5kJ/mol. However, competitive aquation of glycinato donors above 85°C prevents isolation of a stable trans-(R,R)-[Pd(egta)]2– isomer.     

Synthesis of polyketides via diastereoselective acetalization

[reaction: see text] Diastereoselective acetalization of pseudo-C(2)-symmetric 1,3,5-triol systems is a general strategy for the rapid generation of polyketides. The oxidative acetalization reaction shown above was studied under both kinetic and thermodynamic conditions, using synthetic 1,3,5-triol units. In addition, all possible stereochemical variants of a 1,3,5-triol system were prepared from the corresponding acetal to expand the synthetic versatility of this method.     

Novel fused-LEDs devices as optical sensors for colorimetric analysis

The development of a novel, low power optical sensing platform based on light emitting diodes (LEDs) is described. The sensor is constructed from a pair of LEDs fused together at an angle where one LED functions as the light source and the other LED is reverse biased to function as a light detector. Sensor function is based on the level of light received by the detector diode, which varies with the reflectance of the interface between the device and its environment, or the chemochromic membrane that covers the device. A simple microprocessor circuit is used to measure the time taken for the photon-induced current to discharge the detector LED from an initial 5 V (logic 1) to 1.7 V (logic zero). This sensing device has been successfully used for colour and colour-based pH measurements and offers extremely high sensitivity, enabling detection down to the sub micro molar level of dyes.     

Crystallographic analysis of thermochromic,unsolvated tetramesityldisilene at 173 K and 295 K

X-ray crystal structure determinations of tetramesityldisilene ( 1 ) at 295 K, where it is bright orange, and at 173 K, where it is pale yellow, were experimentally equivalent. The thermochromism of 1 , therefore, is not due to changes in conformation; a vibrational origin is considered. The crystal structure of unsolvated 1 differs substantially from that of the toluene solvate ( 1 ·C₇H₈) because of different crystal packing of the two forms. These two solid-state structures are compared with those of four other disilenes.     

Development of thermotropic liquid crystalline polymers

Wholly aromatic, thermotropic liquid crystalline (LC) terpolyesters made from 6-hydroxy-2-naphthoic acid (HNA) and 4,4′-biphenol (BP) as key components have been prepared and evaluated for utility as high performance fiber precursors. The processability and fiber properties of a specific composition (1) made from both HNA and BP were compared with those of a more well-known LC terpolyester made from p-hydroxybenzoic acid, tere- and isophthalic acids and BP (Ekonol^® type). The advantages of polymer 1 and its high-modulus fiber are described.     

Subdivisions and the chromatic index of r-graphs

Let T₂ be the graph obtained from the Petersen graph by first deleting a vertex and then contracting an edge incident to a vertex of degree two. We give a simple characterization of the graphs that contain no subdivision of T₂. This characterization is used to show that if every planar r-graph is r-edge colorable, then every r-graph with no subdivision of T₂ is r-edge colorable. © 1996 John Wiley & Sons, Inc.     

Quadrupole moment measurement of the highly deformed πg 9/2⊗νh 11/2 band in 130Pr

The quadrupole moment for the πg _9/2⊗νh _11/2 band in the ^130>Pr nucleus has been measured using the Doppler-shift attenuation method. A centroid shift analysis was carried out and a value of Q₀=6.1±0.4 eb, corresponding to an axial prolate deformation of β₂=0.35(3), has been determined. This is the first direct experimental confirmation of the deformation-driving character of the πg _9/2 orbital in an odd-odd nucleus in the A≃135 superdeformed region. Received: 23 April 1998     

A power-scaling strategy for longitudinally diode-pumped Tm:YLF lasers

We describe a power-scaling strategy for longitudinally diode-pumped Tm:YLF lasers based on optimisation of the thulium doping level and the gain geometry. Carefully designed laser experiments at lower powers are shown to allow a simple means for accurately predicting the thermal loading and hence the power-scaling properties of this system, including the effects of ground-state depletion, cross relaxation and energy-transfer upconversion. An optimised doping level of 2 at. % and a slab geometry are shown experimentally to allow scaling of a single gain unit to output powers of ∼70 W, limited by the available pump power, and a strategy for scaling well beyond 100-W output is discussed. PACS 42.55.-f; 42.55.Xi; 42.60.By