First-order isostructural Mott transition in highly compressed MnO

We present evidence for an isostructural, first-order Mott transition in MnO at 105+/-5 GPa, based on high-resolution x-ray emission spectroscopy and angle-resolved x-ray diffraction data. The pressure-induced structural and spectral changes provide a coherent picture of MnO phase transitions from paramagnetic B1 to antiferromagnetic distorted B1 at 30 GPa, to paramagnetic B8 at 90 GPa, and to diamagnetic B8 at 105+/-5 GPa. The last is the Mott transition, accompanied by a significant loss of magnetic moment, an approximately 6.6% volume collapse and the insulator-metal transition as demonstrated by recent resistance measurements.     

A study on the primary and secondary nucleation of ice by power ultrasound

Several different investigations have been carried out to study the primary and secondary nucleation of ice by sonocrystallisation. Firstly, the primary nucleation of discrete ice crystals in a supercooled sucrose solution has been observed. For increasing concentrations of sucrose solutions from 0 to 45 wt%, the nucleation temperature consistently occurs at a higher nucleation temperature in the presence of ultrasound. The nucleation temperature also increases as the power output and duty cycle of a commercial ultrasonic horn are increased. Snap shot images of the bubble clouds obtained from the ultrasonic horn also show that the number of bubbles appears to increase as the ultrasonic output is increased. This suggests that the nucleation of ice is related to the power output and number of cavitation bubbles. The effect of a single bubble on the sonocrystallisation of ice is discussed. High-speed movies (1120 fps) have shown that the crystallisation appears to occur in the immediate vicinity of the single bubble. In most cases, many crystals are observed and it is not known whether a single ice crystal is being fragmented by the bubble or whether many crystals are being initiated. The bubble appears to undergo a dancing regime, frequently splitting and rejoining and also emitting some small microbubbles. A study on the secondary nucleation of ice in sucrose solutions has been carried out using a unique ultrasonic cold stage device. Images taken using a microscope system show that the pre-existing ice dendrite crystals can be broken up into smaller fragments by an ultrasonic field. Cavitation bubbles appear to be important during the fragmentation process, possibly melting any ice crystals in their path. Flow patterns around cavitation bubbles have also been observed, and these may be responsible for the fragmentation of ice crystals.     

A new class of protein mimics: preparation and electrophoretic properties of polycationic β-alanine-based dendrimers

The synthesis and characterization of a series of polyammonium β-alanine containing dendrimers 4-6 were reported. Such polycationic species were shown to exhibit electrophoretic behavior similar to that of protein molecules such as lysozyme and aprotinin in acidic polyacrylamide gel electrophoresis. It was also found that their electromobility was directly related to their molecular size, and hence such biomimetic dendrimers are potential molecular weight markers for protein analysis.     

Axial chirality in 1,4-disubstituted (ZZ)-1,3-dienes. Surprisingly low energies of activation for the enantiomerization in synthetically useful fluxional molecules

Trialkylsilyltrialkylstannes (R(3)Si-SnR'(3)) add to 1,6-diynes in the presence of Pd(0) and tris-pentaflurophenylphosphine to give 1,2-dialkylidenecyclopentanes with terminal silicon and tin substituents. The (ZZ)-geometry of these s-cis-1,3-dienes, resulting from the organometallic reaction mechanisms involved, forces the silicon and tin groups to be nonplanar, thus making the molecules axially chiral. There is rapid equilibration between the two helical forms at room-temperature irrespective of the size of the Si and Sn substituents. However, the two forms can be observed by 1H, 13C, and 119Sn NMR spectroscopy at low temperature. The rates of enantiomerization, which depend on the Si and Sn substituents, and the substitution pattern of the cylopentane ring can be studied by dynamic NMR spectroscopy using line shape analysis. The surprisingly low energies of activation (DeltaG++ = 52-57 kJ mol(-1)) for even the bulky Si and Sn derivatives may be attributed to a widening of the exo-cyclic bond-angles of the diene carbons.     

4,6,8,10,16-penta- and 4,6,8,10,16,18-hexamethyldocosanes from the cane beetle antitrogusparvulus--cuticular hydrocarbons with unprecedented structure and stereochemistry

The major cuticular hydrocarbons from the cane beetle species Antitrogus parvulus were deduced to be 4,6,8,10,16,18-hexa- and 4,6,8,10,16-pentamethyldocosanes 2 and 3, respectively. Isomers of 2,4,6,8-tetramethylundecanal 27, 36, and 37, derived from 2,4,6-trimethylphenol, were coupled with the phosphoranes 28 and 29 to furnish alkenes and, by reduction, diastereomers of 2 and 3. Chromatographic and spectroscopic comparisons confirmed 2 as either 6a or 6b and 3 as either 34a or 34b. [structure: see text]     

Space-time formulation of Harnack inequalities for curvature flows of hypersurfaces

We consider the motion of hypersurfaces in Riemannian manifolds by their curvature vectors. We show that the Harnack quadratic is an affine second fundamental form of the space-time track of the hypersurface. Given a solution to the Ricci flow, we show that with respect to an appropriate metric on space-time, the space-slices evolve by mean curvature flow. This enables us to identify the Harnack quadratic for the mean curvature flow with the trace Harnack quadratic for the Ricci flow.     

The lattice anharmonicity-induced broadenings of defect spectral lines

Summary The effect of the anharmonicity of the host lattice to the defect optical absorption and emission is studied. Anharmonicity contributes to the defect optical linewidths via phonon lifetime effect and defect-phonon scattering mediated by a second phonon. Numerical estimates of these contributions indicate that the anharmonic broadenings may not be negligible in some insulating crystalline hosts.     

The boundedness and extendibality of differential systems under integral perturbation

Summary Necessary and sufficient conditions for integral boundedness are given in terms of a Liapunov function are given the construction of the Liapunov function is a modification of the Okamura function. Similar results are also given for the extendability of solutions under integral perturbations. Partially supported by a Summer Research Grant from the University of Missouri. Partially supported under National Science Foundation GU-2648. Entrata in Redazione il 10 novembre 1971.     

Instabilities of Waves on a Free Surface

This paper is concerned with instabilities which develop on a class of parallel shear flows in the presence of a free surface. Implicit in this type of study is the existence of three dimensional instabilities on slightly perturbed two dimensional spatially periodic shear flows.