A simple phenomenological approach to the thermal behavior of glasses during uniform heating or cooling

Thermal behavior of glasses, as observed from the isobaric variations of volume, v, and enthalpy, H, is analyzed in terms of retardation kinetics. A phenomenological theory involving a single retardation time, τ, is developed, assuming that molecular mobility is controlled essentially by the actual free volume, or configurational entropy of the glassy specimen. The characteristic features of the v and H isobars, as derived from the theory, are examined as a function of the thermal history of a typical glassy specimen. The respective contributions of temperature and structural parameters to τ, are also discussed in terms of the characteristic parameters of the isobars. The theoretical predictions are compared with some dilatometric data obtained with an atactic polystyrene. The comparison reveals the limitations of the theoretical treatment and suggests that glass-transition phenomena involve more than one retardation mechanism.     

The microwave spectrum, structure, and barrier to internal rotation of selenoacetaldehyde, CH3CHSe

The rotational spectrum of the unstable molecule selenoacetaldehyde, CH₃CHSe, has been studied by microwave spectroscopy between 26.5 and 40 GHz. Transitions have been measured for five abundant selenium isotopic variants. These measurements have, together with structural information from the related molecules CH₃CHS and CH₃CHO, allowed reliable data on the C=Se bond length (1.758 ± 0.01 Å) and the e angle (125.7 ± 0.3°) to be derived. The spectral lines show splittings due to hindered internal rotation and using these together with the derived structure, barrier heights of 1602 cal mole⁻¹ (6703 J mole⁻¹) and 1648 cal mole⁻¹ (6859 J mole⁻¹) have been determined for the ground and first torsionally excited states, respectively.     

Transformations of cyclic nonaketides by Aspergillus terreus mutants blocked for lovastatin biosynthesis at the lovA and lovC genes

Two mutants of Aspergillus terreus with either the lovC or lovA genes disrupted were examined for their ability to transform nonaketides into lovastatin 1, a cholesterol-lowering drug. The lovC disruptant was able to efficiently convert dihydromonacolin L 5 or monacolin J 9 into 1, and could also transform desmethylmonacolin J 15 into compactin 3. In contrast, the lovA mutant has an unexpectedly active beta-oxidation system and gives only small amounts of 1 upon addition of the immediate precursor 9, with most of the added nonaketide being degraded to heptaketide 22. Similarly, the lovA mutant does not accumulate the polyketide synthase product 5 and rapidly degrades any 5 added as a precursor via two cycles of beta-oxidation and hydroxylation at C-6 to give 20. The possible involvement of epoxides 21a and 21b in the biosynthesis of 1 was also examined, but their instability in fermentation media and fungal cells will require purified enzymes to establish their role.     

Explicit time-reversible orbit integration in Particle In Cell codes with static homogeneous magnetic field

A new explicit time-reversible orbit integrator for the equations of motion in a static homogeneous magnetic field – called Cyclotronic integrator – is presented. Like Spreiter and Walter’s Taylor expansion algorithm, for sufficiently weak electric field gradients this second order method does not require a fine resolution of the Larmor motion; it has however the essential advantage of being symplectic, hence time-reversible. The Cyclotronic integrator is only subject to a linear stability constraint (ΩΔt < π, Ω being the Larmor angular frequency), and is therefore particularly suitable to electrostatic Particle In Cell codes with uniform magnetic field where Ω is larger than any other characteristic frequency, yet a resolution of the particles’ gyromotion is required. Application examples and a detailed comparison with the well-known (time-reversible) Boris algorithm are presented; it is in particular shown that implementation of the Cyclotronic integrator in the kinetic codes SCEPTIC and Democritus can reduce the cost of orbit integration by up to a factor of ten.     

Magnetic normalization of particle size effects in a heavy metal pollution study of intertidal sediments from the Yangzte Estuary

Magnetic, granulometric and geochemical analyses were conducted on an intertidal sediment core from the Yangtze Estuary to evaluate the possibility of normalizing samples for particle size effects in a heavy metal pollution study by means of magnetic proxies. It has been found that the magnetic parameter_ϰARM, indicating fine grained ferrimagnetic minerals, correlates well with the clay content and organic matter concentration of the sediments._ϰARM also shows significant relationship with heavy metals. Therefore_ϰARM is proposed as a proxy for clay content in the sediments, and can be used to compensate for the particle size effect in sedimentary heavy metal records, where magnetic minerals are not subject to significant post-depositional alteration.

     

Atomgewichte

Ohne Zusammenfassung     

Ein Verfahren zur Bestimmung des Kalkbedarfs des Bodens

Ohne Zusammenfassung     

Die Trennung des Kupfers vom Cadmium

Ohne Zusammenfassung