Conformation Control of Oligosilanes by Trimethylsilyl Groups: Dodecamethyl-, Undecamethyl-2-trimethylsilyl- and Decamethyl-2, 4-bis(trimethylsilyl)-n-pentasilane Studied by Raman Spectroscopy and Quantum Chemical Calculations

2,4-Bis(trimethylsilyl) substituted n-pentasilane Me₃SiSiMe(SiMe₃)SiMe₂SiMe(SiMe₃)SiMe₃ (1) has been prepared from Me₂SiCl₂ and (SiMe₃)₂SiMeK, and its solid state molecular structure has been determined by single crystal X-ray diffraction. By reaction of 1 with KO^tBu and subsequent treatment of the resulting silyl anion with (MeO)₂SO₂, the novel hexasilane Me₃SiSiMe(SiMe₃)SiMe₂SiMe₂SiMe₃ (2) has been synthesized. By again using the reaction sequence KO^tBu/(MeO)₂SO₂, known n-pentasilane Me₃SiSiMe₂SiMe₂SiMe₂SiMe₃ (3) has been prepared in a novel and very convenient way from 2. All silanes 1, 2, and 3 as well as the intermediate silyl anions obtained from 2 and 3 have been characterized by ²⁹Si and ¹H NMR spectroscopy, 1 and 2 by elemental analyses also. Raman spectra in the temperature range 210–370 K of the pure liquids 1, 2 and 3 as well as of solutions in toluene (1 and 2) or cyclopentane (3) reveal that all of the silanes exist as mixtures of conformers. The symmetric SiSi stretch observed in the range 341 to 363 cm^-1 and possessing by far the largest Raman intensity of all SiSi stretching vibrations splits into five bands for 3 and two bands for 1 and 3. Quantum chemical DFT B3LYP/6–311G(d) calculations located 15 conformational minima for 2 with relative energies up to 19.9 kJmol^-1, and five minima for 1 with relative energies up to 15.5 kJmol^-1. Calculated SiSiSiSi torsion angles vary between 26.2° (smallest value found) and 179.3°, demonstrating the flexibility of backbones composed of silicon.     

Steigerung der Softwareflexibilität bei Geschäftsprozessänderungen

In times of continuous change, companies need to adjust their business processes to gain sustainable competitive advantage. Resulting changes in the company’s IT currently require the involvement of developers from departments that are mostly not aligned with the business. These changes often result in high transaction and labor costs. The article presents a platform-based method to adjust business processes with the aim of increasing both efficiency and flexibility compared to current approaches. The core of our work is an evaluation against traditional component-based software development using a sound simulation model. Three real-world scenarios of business process change show that – despite a slight increase in transaction costs – our suggested method decreases labor costs while increasing operational flexibility.     

Evidence for Use of Chemical Cues by Male Horseshoe Crabs When Locating Nesting Females (Limulus polyphemus)

Horseshoe crabs come ashore in attached pairs during spring high tides to mate and nest on beaches of the Atlantic and Gulf coasts. Unattached males also come ashore and crowd around the nesting pairs as satellites and engage in sperm competition with the attached male. Females with no satellites and females with large numbers of satellites nest next to one another on the same tide. When females are removed and replaced by a cement model, satellite males continue to be attracted to the same location. Models over sites where females with many satellites had nested are more attractive to males than sites from which a female with no satellites had been removed or a site where no crab had been nesting recently. A second experiment demonstrated that males are responding to chemical cues. A sponge filled with seawater taken from below a female with many satellites and placed under a model female was more attractive to males than a sponge filled with seawater. This is the first demonstration that horseshoe crabs use chemical cues, in addition to visual cues, to locate mates.     

Nodal positioning analysis (NPA)—A numerical method for determining flow behavior of thermoplastic materials

A finite difference method is presented for the solution of two dimensional flow problems in polymer processing. The method is applicable to narrow gaps of any shape and variable thickness. NPA was developed for analyzing the filling stage of the injection molding cycle, but it could be used in extrusion, blown film, and other polymer processing operations. In NPA the position of the flow front is calculated at the end of each time increment, and an axial node is placed at the newest location of the flow front. Each axial node is then divided into a determined number of radial nodes. The velocity and temperature profiles are obtained from the simultaneous solution of the momentum and energy equations. The use of finite differences transforms the continuity, momentum, and energy equations into a system of linear equations which can be solved by any direct or iterative technique. The procedure is repeated until axial nodes have been placed throughout the whole flow channel or until the flow front stops due to polymer solidification. The main advantage of this technique, when compared to the use of a fixed finite difference grid, is that computation time is saved by considering only nodes filled with the fluid. Empty nodes are not considered and corrections for partially filled nodes are not needed. No complications due to the parabolic-shape of the flow front profile are introduced because the axial nodes are placed at average front locations determined by the average velocity at the particular time interval under consideration.     

Thermal stability and molecular weight of two new boron–nitrogen polymers

Molecular weight and thermal stability measurements on the presumed polymers of aminoborane, H₂BNH₂, and aminodifluoroborane, F₂BNH₂, were explored using a laser light scattering technique for the molecular weight measurements and both differential thermal analyses and thermogravimetric analyses for the thermal stability studies. Weight-average molecular weight of polymeric aminodifluoroborane in water was determined as 23,000 g/mole, whereas the molecular weight of polymeric aminoborane could not be determined since a suitable solvent was not found. Both DTA and TGA observations showed partial decomposition of polymeric aminoborane between 135° and 200°C, and the vaporization of polymeric aminodifluoroborane between 250° and 360°C.     

Heats of reaction and kinetics of a thermoset polyester

The curing reaction of a thermoset polyester was investigated by using the isothermal and dynamic techniques of differential scanning calorimetry (DSC). The heats of reaction (at different curing temperatures) and a kinetic expression of the crosslinking reaction are presented and compared to the results of the previous studies. The proposed kinetic model can be utilized to obtain the rates of heat generation and the extent of cure at different cure temperatures and time. The overall activation energy of the curing reaction was calculated on the basis of experimental data as 17.0 kcal/mole and the overall reaction rate constant as 2.60 × 10⁹ exp (?17,000/RT) min^?1.     

New Radioligands for Describing the Molecular Pharmacology of MT1 and MT2 Melatonin Receptors

Melatonin receptors have been studied for several decades. The low expression of the receptors in tissues led the scientific community to find a substitute for the natural hormone melatonin, the agonist 2-[¹²⁵I]-iodomelatonin. Using the agonist, several hundreds of studies were conducted, including the discovery of agonists and antagonists for the receptors and minute details about their molecular behavior. Recently, we attempted to expand the panel of radioligands available for studying the melatonin receptors by using the newly discovered compounds SD6, DIV880, and {"type":"entrez-nucleotide","attrs":{"text":"S70254","term_id":"546525","term_text":"S70254"}}S70254. These compounds were characterized for their affinities to the hMT₁ and hMT₂ recombinant receptors and their functionality in the classical GTPγS system. SD6 is a full agonist, equilibrated between the receptor isoforms, whereas {"type":"entrez-nucleotide","attrs":{"text":"S70254","term_id":"546525","term_text":"S70254"}}S70254 and DIV880 are only partial MT₂ agonists, with K_i in the low nanomolar range while they have no affinity to MT₁ receptors. These new tools will hopefully allow for additions to the current body of information on the native localization of the receptor isoforms in tissues.