13C NMR study of dilute ternary solutions: Acetonitrile,silver nitrate and other electrolytes in water at 25°C

The ¹³C NMR chemical shifts have been measured for dilute aqueous solutions of acetonitrile in presence of various electrolytes including silver nitrate. The two formation constants of the silver ion/acetonitrile complexes have been calculated assuming an additive contribution of each possible complex configuration. Under these conditions the values obtained for the formation constants are very close to those deduced from vapour pressure or electromotive force techniques. The other systems studied are discussed in relation to the salting phenomenon in aqueous electrolyte solutions.     

Synthesis and Properties of 1, 1-Dihalogenocycloprop[b]Anthracenes

The synthesis of 1, 1-difluoro-1H-cycloprop[b]anthracene ( 3 ) is described. The key step of the synthesis is the cycloaddition of 1,2-dichloro-3,3-difluorocyclo-propene ( 6 ) to 2,3-dimethylidene-1,2,3,4-tetrahydronaphthalene ( 5 ). The ¹³C-NMR. spectrum of 3 is assigned on the grounds of C,F-coupling constants, selective H-decoupling and the resulting residual C,H-coupling. The 1, 1-dichloro derivative 4 was synthesized by the same route, but could not be isolated pure. Experiments for the reduction to 1H-cycloprop[b]anthracene ( 2 ) and for the ionization of 3 or 4 to the cation 16 failed.     

Anregung metastabiler Zustände von Barium und Thallium durch Atomstöße im Stoßwellenrohr

It is shown that relaxation times for the excitation of metastable levels of Ba and Tl by collisons of the ground state atoms with carrier gases Ne and Ar can be measured with good accuracy behind reflected shock waves (2700≦T≦3300 °K) by means of two-channel atomic line absorption spectroscopy. Quenching cross-sections, which thus can be derived, cover in principle the range 10^?14 cm²<Q<10^?21 cm². In collisions with Ar one finds for the deactivation of Ba(¹ D ₂)Q=1.2 · 10^?19cm², of Ba(³ D ₁)Q=0.95 · 10^?19 cm² and of Tl(² P _3/2)Q<5 · 10^?21 cm² without noticeable temperature dependence. From this follows a) (on the basis of the Landau-Zener formula) that the pseudo-crossing of the potential energy curves of Ba-Ar takes place within 0.25 eV of the Ba(D) levels and b) that spin-orbit coupling is not rate controlling in the Ba(³ D ₁ →¹ S ₀) transition. The ratio of cross-sections for quenching of Ba(¹ D ₂) by Ar and Ne is about 3, suggesting that the polarizabilities of the noble gas collision partners play a major rôle in the transition probabilities. The mean self-quenching cross-sectionQ=5 · 10^?16 cm² of Tl(² P _3/2) at 3000 °K is about the same as the one reported for 1000 °K and it exceeds the self-quenching cross-section of metastable Ba by at least one order of magnitude.     

Metallation des halogenures d'aryle par le tributylstannyllithium dans le tetrahydrofuranne

The metallation of aryl halides with tributylstannyllithium in THF proceeds via at least three different mechanisms whose relative rates depend on the nature of the halogen, the substrate and the experimental conditions. In the case of aryl bromides, a halogenmetal exchange (major process) is competing with a radical mechanism. With aryl chlorides, an arynic process, a halogenmetal exchange and a radical mechanism are involved. Finally, with aryl fluorides, the arynic process is usual, but it may be strongly competed by a radical mechanism whenever suitable experimental conditions are arranged.