Determination of the conformation of 1,4-diheterocyclanes. I—The 1H n.m.r. spectrum and ring conformation of 3-methyl-2-oxo-1,4-dioxepane

The ¹H n.m.r. spectrum of 3-methyl-2-oxo-1,4-dioxepane was recorded at 300 and 60 MHz. The ring conformation was considered in terms of the spectral parameters and shown to be a twisted boat where the lactone grouping is approximately planar.     

Motion of a spin probe in hexagonal and lamellar potassium palmitate

Summary Nitroxyl radical Tempol dissolved in the hexagonal liquid-crystalline phase of potassium palmitate experienced a homogeneous time averaged environment. In the lamellar phase the radicals were distributed in two environments of different solvent polarities, which was demonstrated by simulation of the spectra.In both phases the nitroxyl probes were able to tumble comparatively freely (the rotational correlation time a was 0.15–0.9 ns).

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Zusammenfassung Das Nitroxylradikal Tempol wurde zur Untersuchung der Molekülbeweglichkeit in den verschiedenen Phasen im System Kaliumpalmitat — H₂O benutzt. Die hexagonale, flüssig-kristalline Phase erscheint homogen; in der lamellaren Phase wurden zwei verschiedene Umgebungen mit unterschiedlicher Lösungsmittelpolarität festgestellt. In beiden Phasen waren die Nitroxylradikale relativ frei beweglich ( = 0,15–9,9 nsec).

     

1H NMR spectra and stereochemistry of cis- and trans-5,6-dimethyl-2-oxo-1,4-dioxans

The ¹H NMR spectra of isomeric 5,6-dimethyl-2-oxo-1,4-dioxans have been recorded and the pertinent chemical shifts and coupling constants determined. The parameters indicate that both isomers exist as an equilibrium mixture of interconverting half chair ring conformations.     

On the stereochemistry of 1,4-diheterocyclanes. IV—carbon-13 NMR spectra and structural properties of 1,4-dioxane-2,3-diols and their methyl-substituted derivatives

The 1,4-dioxane-2,3-diols and a number of their methyl-substituted derivatives were synthesized and their ¹³C NMR spectra measured. The configurational properties were studied on the basis of the spectral data and chemical equilibration. For the parent compound the trans configuration is slightly more stable. The hydroxyl groups are predominantly axial. Introduction of a methyl group at the 2 position causes the cis configuration of the hydroxyl substituents to become more stable. The substituent effects on the chemical shifts were calculated and used to test the conformational homogeneity of the compounds. The results indicate a biased (chair) conformation for all the methyl-substituted derivatives which were studied. Methyl groups have strongly predominating equatorial orientation in each case studied.     

Screening the Thermodynamics of Trimethylaluminum‐Hydrolysis Products and Their Co‐Catalytic Performance in Olefin‐Polymerization Catalysis

Herein, we introduce an approach for the computational screening of stoichiometric reactions between trimethylaluminum (TMA) and water. The thermodynamic products of these reactions are methylaluminoxanes (MAOs) with different compositions, which have the general formula (AlOMe)_n(AlMe₃)_m, in which n describes the degree of oligomerization and m is the number of associated TMA molecules. These reaction products were thoroughly explored up to n=4, thus demonstrating the thermodynamically preferable association of up to four AlMe₃ molecules, that is, TMA molecules in their monomeric form. The relative Lewis acidities of the Al sites in these MAOs were systematically explored and we found that the associated TMA molecules were a key ingredient for co‐catalytic activity in olefin‐polymerization catalysis. This conclusion was supported by computational studies on catalyst activation, which revealed an exergonic insertion of ethene into the metallocene/MAO complex.     

Modification of Lignins. I. Reaction of Lignins with Chlorophosphazenes

Lignin is a potential raw material for several applications. The immediate future should result in the development of lignin modifications yielding new polymers with special properties such as flame and/or thermal resistance. In this report a new reaction of lignins with chlorophosphazenes is discussed. The effect of different types of lignin and chlorophosphazene oligomers on the properties of the reaction product was investigated. The reactions were found to result in products with flame as well as with a base and acid hydrolitic resistance.     

Compact middle-wave infrared Fabry-Perot interferometer with double metallic subwavelength gratings

We demonstrate a compact middle-wave IR (MWIR) Fabry-Perot (FP) interferometer by replacing the traditional distributed Bragg reflectors consisting of thick multilayer films with two identical metallic subwavelength gratings. The transmission efficiency, resonance peak position, and finesse of the FP interferometer can be controlled by tuning the structural parameters. The performance of the MWIR FP interferometer is verified by experiment, which shows its potential application as, for example, narrowband MWIR filter for gas detection.     

Emerging methods and tools for environmental risk assessment, decision-making, and policy for nanomaterials: summary of NATO Advanced Research Workshop

Nanomaterials and their associated technologies hold promising opportunities for the development of new materials and applications in a wide variety of disciplines, including medicine, environmental remediation, waste treatment, and energy conservation. However, current information regarding the environmental effects and health risks associated with nanomaterials is limited and sometimes contradictory. This article summarizes the conclusions of a 2008 NATO workshop designed to evaluate the wide-scale implications (e.g., benefits, risks, and costs) of the use of nanomaterials on human health and the environment. A unique feature of this workshop was its interdisciplinary nature and focus on the practical needs of policy decision makers. Workshop presentations and discussion panels were structured along four main themes: technology and benefits, human health risk, environmental risk, and policy implications. Four corresponding working groups (WGs) were formed to develop detailed summaries of the state-of-the-science in their respective areas and to discuss emerging gaps and research needs. The WGs identified gaps between the rapid advances in the types and applications of nanomaterials and the slower pace of human health and environmental risk science, along with strategies to reduce the uncertainties associated with calculating these risks.