An Efficient Procedure for the Diastereoselective Dehydration of beta-Hydroxy Carbonyl Compounds by CeCl(3).7H(2)O/NaI System

The dehydration of beta-hydroxy ketones and beta-hydroxy esters is a synthetically useful method for the conversion of these compounds to the corresponding alpha,beta-unsaturated derivatives. Cerium(III) chloride heptahydrate in combination with sodium iodide in refluxing acetonitrile acts as an efficient reagent for this conversion. The present procedure, which utilizes cheap and "friendly" reagents, offers the corresponding (E)-enones in good yields as the only isolable products.     

Dichlorocarbene Addition to

In contrast to the terminal phosphinidene complex PhPW(CO)(5) (2), which adds to [5]metacyclophane (1) in a 1,4-fashion, dichlorocarbene preferentially adds in a 1,2-fashion to the formal "anti-Bredt" type double bond of the aromatic ring of 1 to afford the norcaradiene 11b, which immediately rearranges to the bridged cycloheptatriene 12b and further by a [1,5] sigmatropic chlorine migration to the isomeric 13b as the first observable product. More slowly, the latter isomerizes via a dissociative mechanism to give 15b. A computational study supports the notion that the [1,5] chlorine migration in the rearrangement 12b --> 13b, for which an activation barrier of 70.2 kJ mol(-)(1) was calculated, is essentially concerted with minor charge separation. In contrast, the analogous [1,5] chlorine migration in the flat model compound 7,7-dichlorocycloheptatriene (12a) displays features of a dissociative pathway.     

Search for narrow resonances in e+e− annihilation into hadrons at adone

A search for narrow resonances in the reaction e⁺e^? → hadrons in the mass regions 1915–2345 MeV and 2970–3090 MeV has been perforned at ADONE, the Frascati storage ring. With 90% confidence level our data exclude the production of narrow resonances with integrated cross section larger than 20% of the integrated cross section for production of the J/Ψ (3100 MeV).     

Selective covers for natural cooling devices

Summary Extra-atmospheric space is practically a pure sink of radiation, and can be used as a nonconventional energy source. In previous papers we have shown that surfaces with an emissivity matched with the atmospheric (8÷13) μm ?transparency window? (natural emitters) interact with cold space when exposed to clear sky at night, and undergo a sizable cooling effect. In this paper, starting from experimental results concerning the diurnal performances of natural emitters, we discuss the problem of their interaction with solar radiation, and propose the use of selective covers which shade the emitter from solar radiation, without preventing the interaction with cold space via emission of infra-red radiation.

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Riassunto Lo spazio extraatmosferico può essere considerato un pozzo ideale per la radiazione e può essere usato come una sorgente di energia non convenzionale. In precedenti articoli abbiamo mostrato che superfici dotate di potere emissivo nella regione, di trasparenza atmosferica compresa fra 8 e 13 μm (emettitori naturali), se esposte di notte al cielo sereno, interagiscono con lo spazio freddo extraatmosferico raffreddandosi sensibilmente. Nel presente articolo, partendo da risultati sperimentali riguardanti le prestazioni diurne degli emettitori naturali, si discute il problema della loro interazione con la radiazione solare e si propone l'uso di rivestimenti selettivi che schermano l'emettitore della radiazione solare stessa, senza impedire l'interazione con lo spazio freddo mediante l'emissione di radiazione infrarossa.

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Резюме Внеатмосферное пространство представляет практически чистый сток излучения и может быть использовано, как нестандартный энергетический источник. В предыдущей статье мы показали, что поверхности с отнсительной излучательной способностью, согласованной с атмосферным (8÷13) μм ?окном прозрачности? (естественные эмиттеры)взаимодействуют с холодным пространством, когда экспонируются на чистое ночное небо, и испытывают заметный эффект охлаждения. В этой работе, исходя из экспериментальных результатов, касающихся суточного поведения естественных эмиттеров, мы обсуждаем проблему их взаимодействия с солнечной радиацией. Мы предлагаем использовать селективные покрытия, которые зкранируют эмиттер от солнечной радиации, не препятствуя взаимодействнт с холодным пространством через эмиссию инфракрасного излучения.

     

Transport properties of photo-excited carriers in slightly compensated Hg0.785Cd0.215Te

Photo-Hall measurements are presented for slightly compensated n-type Hg_0.785Cd_0.215Te at 4 K where the electron mobility is limited by charged scattering centers. Using a 10.6 μm laser as the optical source, we observe that the electron mobility passes through a maximum as a function of photo-excited carrier density. Mobility expressions based on the Kane-band model are modified to incorporate neutralization of charged scattering centers by photo-excited carriers. Calculated mobility values are found to be in satisfactory agreement with the data. The observed decrease in mobility for high carrier densities is attributed to electron-hole scattering. The mobility enhancement at lower carrier densities is explained in terms of neutralization of charged acceptors.     

Small Complete Caps from Singular Cubics

New families of complete caps in finite Galois spaces are obtained. For most pairs with and , they turn out to be the smallest known complete caps in . Our constructions rely on the bicovering properties of certain plane arcs contained in plane cubic curves with a cusp.