F?F coupling constants in dichlorotetrafluorocyclopropanes

The ¹⁹F NMR spectra of the cis- ( 1 ) and the trans-isomer ( 2 ) of the 1,2-dichlorotetrafluorocyclopropane and that of the 1,1-dichlorotetrafluorocyclopropane ( 3 ) have been investigated at different temperatures and in several solvents. From chemical shift calculations the two geminal fluorines in the cis-isomer ( 1 ) could be assigned and on this basis the two vicinal coupling constants of 1 , J_trans (ca. 140°) and J_cis (ca. 0°), were unequivocally distinguished. By frequency sweep double resonance J_trans has been shown to be of opposite sign to J_gem, whereas for J_cis the situation has been found to be reversed. Therefore J_trans is presumably negative and J_cis positive. Only the N(J_cis + J_trans) value could be extracted from the vicinal coupling constant in the fragment ? CFCl? CFCl? could be evaluated. It has been noted that J_cis is more sensitive to changes in temperature than is J_trans. The variations of J_cis and J_trans induced by solvents are, on the contrary, small and irregular and no correlation with the dielectric constant of the medium has been noted. The different temperature dependence of J_cis and J_trans can be useful for assigning the vicinal F? F coupling constants in cyclopropane derivatives and also for defining their signs. This method was applied to the coupling constants extracted from the ¹³C satellite spectrum of isomer 3 . The coupling constants results were compared with some literature data already known, and some rationalisation and correlations from the trends was attempted.     

Formamidines and triazenes as precursors for the preparation of rhenium imido-complexes

Summary The rhenium(V) imido-complexes [Re(NR)Cl₃(PPh₃)₂] have been obtained by heating the rhenium(V) derivative [ReOCl₃(PPh₃)₂] with RNHCHNR R = Ph (1a), p-MeC₆H₄ (1b), p-ClC₆H₄ (1c) andp-FC₆H₄ (1d) in THF under reflux. [Re(NR)Cl₃(PPh₃)₂] (R =p-MeQ₆H₄) has also been obtained by heating the rhenium(III)-triazenido complex [ReCl₂(RN N NR)(PPh₃)₂] in CCl₄ under reflux.     

19F NMR analysis of the fluorides of perfluorocyclohexane-sulphonic acid and of 3-trifluoromethylperfluorocyclopentane-1-sulphonic acid

From the electrochemical fluorination of the chloride and the fluoride of benzenesulphonic acid, in addition to the main product C₆F₁₁SO₂F, the side products C₅F₈(CF₃)SO₂F, a structural isomer of the main product and C₆F₁₀Cl? SO₂F are also obtained. The NMR analysis of these products was performed. Chemical shift and coupling constant values were obtained for all the compounds. The NMR parameters of the main product are compared with those of perfluoromethylcyclohexane according to the data of Jolley, Sutcliffe and Walker. From the NMR parameters of C₅F₈(CF₃)SO₂F it was possible to demonstrate that substitution on the ring for —SO₂F and CF₃— is of the 1,3 and not the 1,2 type.     

Free boundary enclosure in a three-dimensional filtration problem

We give a lower and an upper estimate for the level of a fluid filtrating through a porous, homogeneous and three-dimensional dam with variable cross-section.     

Laser heating of optically thin samples

Summary A mathematical model able to describe the temperature profiles generated in a thin film by the steady-state illumination by a Gaussian laser beam is presented. The film is supposed to be made by a weakly absorbing liquid sample bounded by two parallel transparent plates, the cell walls, whose thermal exchange to the surrounding ambient may be assumed to be linear with the temperature difference. An analytical solution of the problem is presented in form of Hankel transform and a matrix numerical approach to the computation of the temperature profiles is reported. It is particularly well suited to a computer implementation and allows one to get very accurate results in very short computing times. The influence of the heat exchange coefficient to the ambient is shown in an example. A check of the method accuracy is performed by comparison with literature results in the particular case of infinite heat exchange coefficient. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Chiral acetylenes as synthetic intermediates—II: Optically active 1,4-dialkylbuta-1,3-dienes: synthesis and conformational studies

The synthesis and the physical properties of some optically active 1,4-dialkylbuta-1,3-dienes are reported. The synthetic processes employed were essentially stereospecific. NMR, UV and CD spectra of these compounds provide information on the main conformation of the conjugated diene system, which was shown to be s-trans planar.     

Diisopropylethylamine: An effective catalyst for the introduction of the t-butyldimethylsilyl group

The rapid reaction of t-butyldimethylchlorosilane with alcohols catalysed by diisopropylethylamine at room temperature gave high isolated yields of the t-butyldimethylsilyl ethers.