A Mild and Efficient CH2‐Extrusion Reaction for the Enantiospecific Synthesis of Highly Configurationally Stable Tröger Bases

A novel CH₂‐extrusion reaction leading to the transformation of ethano‐Tröger bases into disubstituted methano derivatives is reported (yields up to 93 %). Under mild and metal‐free oxidative conditions, a loss of CH₂ and a ring contraction are provoked. Despite two bond cleavages at stereogenic nitrogen and carbon centers and a temporary rupture of the bicyclic structure, a very high enantiospecificity (es≥98 %) is observed for this unusual reaction.     

Overriding subsuming rules

This paper is concerned with intelligent agents that are able to perform nonmonotonic reasoning, not only with, but also about general rules with exceptions. More precisely, the focus is on enriching a knowledge base Γ with a general rule that is subsumed by other rules already there. Such a problem is important because evolving knowledge needs not follow logic as it is well-known from e.g. the belief revision paradigm. However, belief revision is mainly concerned with the case that the extra information logically conflicts with Γ. Otherwise, the extra knowledge is simply doomed to extend Γ with no change altogether. The problem here is different and may require a change in Γ even though no inconsistency arises. The idea is that when a rule is to be added, it might need to override any rule that subsumes it: preemption must take place. A formalism dedicated to reasoning with and about rules with exceptions is introduced. An approach to dealing with preemption over such rules is then developed. Interestingly, it leads us to introduce several implicants concepts for rules that are possibly defeasible.     

Injection locking properties of a microchip laser

An injected microchip laser is theoretically studied, with the use of two models: in the first model, which is traditional, the field is represented by a single frequency component in the slowly-varying envelope approximation. In the second model, referred to as the Fabry-Perot model, two field components are considered which are respectively centered around the frequency of an eigenmode of the injected laser and the frequency of the injected field. Computation of locking ranges, bistability do mains are performed and the results compared. They show not only an improved precision of the second model but also a necessity to use it to describe some effects such as the bistable domains in the limits of the locking domains. Received: 1st April 1998 / Accepted: 21 August 1998     

Synthesis, crystal structure, and transistor performance of tetracene derivatives

The substitution of chloro or bromo groups in tetracene gives rise to the change of crystal structure, having a substantial effect on carrier transport. Halogenated tetracene derivatives were synthesized and grown into single crystals. Monosubstituted 5-bromo- and 5-chlorotetracenes have the herringbone-type structure, while 5,11-dichlorotetracene has the slipped pi stacking structure. Mobility of 5,11-dichlorotetracene was measured to be as high as 1.6 cm2/V.s in single-crystal transistors. The pi stacking structure, which enhances pi orbital overlap and facilitates carrier transport, may thus be responsible for this high mobility.     

Erbium-doped fiber laser simultaneously mode locked on more than 24 wavelengths at room temperature

We demonstrate simultaneous mode locking of more than 24 wavelengths at 3 GHz in an actively mode-locked erbium-doped fiber laser operating at room temperature. The multiwavelength operation is achieved when a frequency shifter and an all-fiber 50-GHz periodic filter are inserted into a ring cavity. Active mode locking is performed with an amplitude modulator, and pulses with a FWHM of 30 ps are obtained.     

Purete radionucleidique du solute injectable d'eau tritiee

Conclusion Nous avons mis au point une méthode simple permettent de détecter les impuretés β^-, à l'aide d'un compteur à scintillation liquide. La courbe établie en mesurant la variation du taux de comptage en fonction du seuil de discriminateur pour la préparation à l'essai, doit être parallèle à celle obtenue, dans les mêmes conditions, avec une solution étalon d'eau tritiée. Le mélange scintillant dioxane-naphtalène, 2,5-diphényloxazole (PPO), 1,4-bis-2-(4-méthyl-5-phényloxazolyl)-benzène (diméthyl POPOP) [1 litre/100 g/7 g/0,3 g] permet de détecter la présence d'au moins 0,005% de³⁵S, 0,004% de¹⁴C et 0,04% de⁴⁵Ca.

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The radionuclidic purity of tritiated water is checked by an easy and rapid method using a liquid scintillation counter. The liquid scintillator: dioxane (1 liter)/naphtalene (100 g)/(PPO (7 g)/dimethyl POPOP (0.3 g) allows the detection of, at least, 0.005% of sulfur—³⁵S, 0.004% of carbon—¹⁴C and 0.004% of⁴⁵Ca.

     

Canonical quantization and the spectral action; a nice example

We study the canonical quantization of the theory given by Chamseddine–Connes spectral action on a particular finite spectral triple with algebra _M2(C)⊕C$M_2\left(\mathbb{C}\right)\oplus \mathbb{C}$. We define a quantization of the natural distance associated with this noncommutative space and show that the quantum distance operator has a discrete spectrum. We also show that it would be the same for any other geometric quantity. Finally we propose a physical Hilbert space for the quantum theory. This spectral triple had been previously considered by Rovelli as a toy model, but with a different action which was not gauge invariant. The results are similar in the two cases, but the gauge invariance of the spectral action manifests itself by the presence of a non-trivial degeneracy structure for our distance operator.     

Local density enhancement in supercritical carbon dioxide studied by Raman spectroscopy

The polarized IVV and depolarized IVH Raman profiles of the Fermi dyad (1285 cm(-1) and 1388 cm(-1)) of supercritical (SC) CO2 have been measured along the isotherms 307, 309, 313, and 323 K in the reduced density range 0.04相似文献