Synthesis of CDE and BCDE molecular fragments of the limonoids havanensin and azadiradione

A new approach to the synthesis of CDE and BCDE molecular fragments of the limonoids havanensin and azadiradione has been achieved from cyclocitral and drimenal in seven steps in overall yields of 20 and 9%, respectively.     

A pricing policy for a computer facility with congestion externalities

The paper proposes an adaptive pricing mechanism which allocates computer resources among the different consumers in an economically efficient manner. Users are charged a unit price designed to reflect the marginal congestion load each imposes upon the system. Some of the more common allocation schemes are described briefly followed by a discussion of the desirable properties of an efficient allocative system. These properties are incorporated into a model of a computer facility, and a number of propositions regarding the behavior of this model are proposed. In order to test empirically the behavior of this system under conditions approximating those which would prevail in an actual computer center, the pricing mechanism was analyzed by a simulation model of a computer center, using data obtained from a survey of customers at the computer center of a university (Technion, Israel Institute of Technology at Haifa, Israel). A few suggestions regarding the implementation of the proposed pricing policy conclude the paper.     

Electron effective masses and lattice scattering in natural diamond

We report experimental evidence of the Si-like conduction band of natural diamond. A Monte Carlo analysis of electron drift velocity data enables for the first time the electron effective masses to be determined.     

Functional polymers and sequential copolymers by phase transfer catalysis. 25. Transformation of a monotropic mesophase into an enantiotropic one by increasing the molecular weight of the polymer and by copolymerization

The influence of molecular weight on thermal transitions and on the thermodynamic parameters was studied for two polymers based on 4,4′-dihydroxy-α-methylstilbene with either 1,9-dibromononane (HMS-C9 polyethers) or 1,11-dibromoundecane (HMS-C11 polyethers). HMS-C9 polyethers present an enantiotropic nematic mesophase over the entire range of molecular weights and a monotropic smectic mesophase for polymers of number average molecular weights higher than 17,000. The low molecular weight HMS-C11 polyethers are only crystalline. On increasing their molecular weight, the polymers become monotropic nematics, and at higher molecular weights, enantiotropic nematics. Up to a composition containing as little as 20 mol % nonane structural units, the random copolyethers based on 1,9-dibromononane, 1,11-dibromoundecane, and 4,4′-dihydroxy-α-methylstilbene (HMS-C9/11 copolyethers) exhibit on cooling a phase diagram resembling that of HMS-C9 polyether. HMS-C9/11 containing about a 1/1 mole ratio between the two spacers presents both smectic and nematic enantiotropic mesophases. These results suggest that the phase diagram of random liquid crystalline copolymers is controlled by the shorter spacer. The thermodynamic parameters of isotropization for both polyethers and copolyethers are compared and suggest that copolymerization does not significantly decrease the degree of order of the mesogenic units in the mesomorphic phase.     

Stereocontrolled synthesis of a complex library via elaboration of angular epoxyquinol scaffolds

We have accomplished the synthesis of a complex chemical library via elaboration of angular epoxyquinol scaffolds with distinct skeletal frameworks. The key strategy involves highly stereocontrolled [4 + 2] Diels-Alder cycloadditions of chiral, nonracemic epoxyquinol dienes to generate the scaffolds. Further scaffold diversification involves hydrogenation, epimerization, dehydration, and condensation of the carbonyl group with alkoxyamine and carbazate building blocks. Further elaboration of the scaffolds also provided new skeletal frameworks using hydroxyl-directed Diels-Alder cycloaddition and reductive N-N bond cleavage. The overall process afforded 244 highly complex and functionalized compounds. Preliminary biological screening of the library uncovered six compounds which showed significant inhibition of Hsp 72 induction.     

(1)H, (13)C NMR and X-ray crystallographic studies of highly polyhalogenated derivatives of costunolide lactone

The costunolide lactone, a sesquiterpene compound isolated from Zaluzania triiloba species, reacted with several dihalocarbene sources produced by trihaloform-NaOH under successive phase transfer reactions yielding mono-, bis- and tris-dihalocyclopropane adducts. The structures, as well as the configurational assignments of the different derivatives, were established by (1)H and (13)C NMR spectroscopy and assisted by X-ray crystallographic and molecular modelling studies. The specific shielding of protons in the neighbourhood of different halogens on the cyclopropane moieties was correlated to the pseudocontact interactions.     

Sonochemical removal of trihalomethanes from aqueous solutions

In this research, ultrasound irradiation was employed to degrade the trihalomethanes, THMs: CHCl₃, CHBrCl₂, CHBr₂Cl, CHBr₃, and CHI₃. The kinetics reaction rates and removal efficiencies of the THMs compounds, as a sole component in the aqueous solutions, were studied. Batch experiments were conducted at an ultrasonic frequency of 20 kHz and acoustic intensity of 3.75 W/cm². The first-order degradation rate constants and the sonolysis efficiencies followed the decreasing order of CHCl₃ > CHBrCl₂ > CHBr₂Cl > CHBr₃ > CHI₃. Up to 100% of the CHCl₃ was removed, while only 60% of the CHI₃ was sonodegraded, after 180 min sonication. The THMs vapor pressure was found to be the most important parameter affecting the sonodegradation kinetics and efficiency, while the bond dissociation energy and hydrophilic/hydrophobic characteristics of the THMs compounds were found to be of secondary importance.     

Graphitization of Carbon Particles in a Non-thermal Plasma Reactor

The production of nanomaterials using non-thermal plasmas remains the focus of ongoing investigations due to advantageous properties of this class of processes, most notably the intense plasma-induced heating arising from energetic recombination events occurring at the surface of nanoparticles, which allows for the tailored synthesis of crystalline nanoparticles. In this work, the authors discuss an in situ, in-flight Fourier Transform Infrared absorption spectroscopy technique to investigate the temperature variation of carbon nanoparticles during their synthesis in an acetylene–argon–hydrogen non-thermal RF plasma. Based on the FTIR measurements, decreasing hydrogenation levels and the progressive onset of an incandescence signal were observed at increasing RF input power. In the high RF power region, the carbon particle temperature, derived by fitting the corresponding FTIR spectra with a modified Planck’s law, shows values above 2000 K. The corresponding ex situ characterization of the synthesized materials by Transmission Electron Microscopy and Raman Spectroscopy displays the production of highly graphitic particles and loss of bonded hydrogen from the material, hence supporting the substantial nanoparticle heating derived from the FTIR measurements.