Strongly cluster size dependent reaction behavior of CO with O2 on free silver cluster anions

Reactions of free silver anions Agn- (n = 1 - 13) with O2, CO, and their mixtures are investigated in a temperature controlled radio frequency ion trap setup. Cluster anions Agn- (n = 1 - 11) readily react with molecular oxygen to yield AgnOm- (m = 2, 4, or 6) oxide products. In contrast, no reaction of the silver cluster anions with carbon monoxide is detected. However, if silver cluster anions are exposed to the mixture of O2 and CO, new reaction products and a pronounced, discontinuous size dependence in the reaction behavior is observed. In particular, coadsorption complexes Agn(CO)O2- are detected for cluster sizes with n = 4 and 6 and, the most striking observation, in the case of the larger odd atom number clusters Ag7-, Ag9-, and Ag11-, the oxide product concentration decreases while a reappearance of the bare metal cluster signal is observed. This leads to the conclusion that carbon monoxide reacts with the activated oxygen on these silver clusters and indicates the prevalence of a catalytic reaction cycle.     

Synthesis of N,N-disubstituted selenoamides by O/Se-exchange with selenium-Lawesson's reagent

The selenium analogue of Lawesson's reagent, [PhP(Se)(μ-Se)]₂ is an effective reagent for synthesizing N,N-disubstituted selenoamides. The reaction is carried out under mild conditions (room temperature) and affords the selenoamide in higher yield than using other selenation reagents.     

Equilibrium and dynamical properties of gaussian stiff chain molecules

We present a novel analytically tractable model for stiff chain molecules. The equilibrium distribution function of the chain is derived using the maximum-entropy principle. For that purpose, we first formulate a discrete chain model, where the connections of the points and the restriction on bending are taken into account via constraints. We then perform the limit to a continuous chain and show that the mean-square end-to-end distance and the radius of gyration of the continuous chain are identical with the same quantities of the Kratky-Porod wormlike chain. The dynamics of our chain is investigated in dilute solution without hydrodynamic interactions. The linear dynamical equation is solved by a normal mode analysis. We discuss the dependence of the relaxation times on the single parameter of the model, the persistence length. For small persistence lengths we obtain the well known relaxation times of the Rouse model. In the stiff-chain limit, we find the pure bending relaxation times and, in addition, the rotational relaxation time.     

High-resolution cardiac imaging using an interleaved 3D double slab technique

A three-dimensional (3D) gradient-echo sequence with interleaved double-slab excitation was developed and optimized for the requirements in pediatric cardiac imaging. For this purpose high contrast between blood and myocardium signal should be obtained without the use of contrast agents. An acceptable measuring time for a large region examined with high spatial resolution should be achieved as well, especially with regard to the small structures of the heart and vessels of infants. The presented approach works with gradient moment nulling and a short echo time of 5.5 ms resulting in generally high signal intensity and only minor signal losses due to turbulent flow. The sequence allows simultaneous ECG-gated recording of two separately excited slabs with small thickness (10 mm) and with a distance of several centimeters between them. Thus, common effects of presaturation in 3D imaging can be avoided, although a relatively short measuring time is achievable. In order to get a 3D data set with good signal homogeneity of blood and of the other structures across a large volume of interest several double-slab measurements with suitable positions must be performed. The latter aspect is especially important for postprocessing techniques as multiple planar reconstruction and maximum intensity projection. Examples of applications of the new technique and appropriately postprocessed images are presented allowing demonstration even of subtle cardiac malformations.     

Influence of molecular stiffness on the dynamic structure factor

Characteristic features of the influence of molecular stiffness on the dynamic structure factor of macromolecules are briefly outlined. The relaxation times characterizing the internal dynamics of the macro‐molecules exhibit a crossover from Rouse‐Zimm to bending modes with increasing mode number. As a consequence the dynamic structure factor is strongly influenced by the molecular stiffness. In particular, a stretched exponential relaxation of the dynamic structure factor at scattering vectors larger than the inverse persistence length is predicted and confirmed by a comparison with experimental data. Moreover, the influence of polydispersity is discussed.     

Overview of Microstructure and Microtexture Development in Grain-oriented Silicon Steel

This paper outlines the development of the microstructure and microtexture of grain-oriented silicon steel during the industrial production process. In particular the evolution of the Goss orientation was studied in industrial material as well as in single crystal experiments.     

Highly substituted tetrahydropyrones from hetero-Diels-Alder reactions of 2-alkenals with stereochemical induction from chiral dienes

[reaction: see text] A new method for the stereoselective synthesis of libraries of 2,3,5-trisubstituted tetrahydro-gamma-pyrones and the corresponding tetrahydropyran-4-ols is reported. Dienes with a chiral moiety at position 5 were synthesized starting from (triphenylphosphoranylidene)acetone. In hetero-Diels-Alder (HDA) reactions, especially with alpha,beta-unsaturated aldehydes, they induce diastereomeric ratios from 4:1 to 14:1. Through selective epimerization and reduction, further building blocks are available. These constitute ideal starting points for their use in the total synthesis of complex polyketide macrocycles, especially with the vinyl group available for metathetic coupling.