Charge-changing cross sections of the neutron-rich isotopes8,9,11Li

Total charge-changing cross sections have been measured for⁸Li on C and Pb targets, for⁹Li on C, Al, Cu, Sn and Pb targets, as well as for¹¹Li on C, Sn and Pb targets at about 80 MeV/nucleon. These data are compared to measured total reaction cross sections and Glauber-type calculations using Hartree-Fock density distributions. These comparisons allow to draw conclusions on the proton density distribution of the neutronrich lithium isotopes. The results show that even for the most exotic nucleus¹¹Li the proton distribution is only very weakly influenced by the long tail in the neutron density distribution already established in several experiments.     

Thermal Analysis of the Conformational Disorder in SCLC Polymers with Rigid Backbone Glass transition studies

Bis [(ω-(4′-cyanobiphenyl)-4-yl)oxy-n-alkyl]norborn-5-ene-2,3-dicarboxylate was polymerised via ring opening metathesis polymerisation (ROMP). Two disubstituted polynorbornene derivatives both of cis configuration with different length of the side-chain were studied. Differential scanning calorimetry (DSC) was used to study the effect of thermal history on the assignment of the glass transition event associated with the biaxial orientation of a smectic phase. Glass transition temperatures, the change of isobaric specific heats at T_g and the enthalpies of isotropisation were calculated. The DSC traces only show the classic step-wise change in T_g in some cases, giving the evidence that the amorphous domains are constrained and highly restricted in movement due to the morphology developed as a result of the biaxial stretching. Based on the literature data of mono- and disubstituted polynorbornene derivatives and our calorimetric experiments, the shape of T_g dependence on number of (CH₂) units is interpreted. The origin of this shape is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparative investigation of ruthenium-based metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands

Exchange of one PCy3 unit of the classical Grubbs catalyst 1 by N-heterocyclic carbene (NHC) ligands leads to "second-generation" metathesis catalysts of superior reactivity and increased stability. Several complexes of this type have been prepared and fully characterized, six of them by X-ray crystallography. These include the unique chelate complexes 13 and 14 in which the NHC- and the Ru-CR entities are tethered to form a metallacycle. A particularly favorable design feature is that the reactivity of such catalysts can be easily adjusted by changing the electronic and steric properties of the NHC ligands. The catalytic activity also strongly depends on the solvent used; NMR investigations provide a tentative explanation of this effect. Applications of the "second-generation" catalysts to ring closing alkene metathesis and intramolecular enyne cycloisomerization reactions provide insights into their catalytic performance. From these comparative studies it is deduced that no single catalyst is optimal for different types of applications. The search for the most reactive catalyst for a specific transformation is facilitated by IR thermography allowing a rapid and semi-quantitative ranking among a given set of catalysts.     

Benchmarking of ruthenium initiators for the ROMP of a norbornenedicarboxylic acid ester

The kinetic study of ring-opening metathesis polymerization (ROMP) of a diester functionalised norbornene derivative, (±)-exo,endo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid diethyl ester, with a series of ruthenium benzylidene complexes revealed the applicability of these initiators for well defined polymerization reactions. Values for the rate of initiation as well as the rate of propagation of the initiators were determined and correlated to the molecular weight and polydispersity of the isolated polymers. As the only initiator providing an entry to virtually monodisperse polymers the classical “first generation Grubbs-catalyst” was identified, while N-heterocyclic carbene based initiators polymerized with a rate of propagation much higher than the rate of initiation yielding polymers with a broader molecular weight distribution.     

31P-NMR-Untersuchung an PhPH(CO)6Fe2(PPh)3Fe2(CO)6PPhH ? einem Vierkernkomplex mit einer ungewöhnlichen (PPh)3-Brücke

(CO)₆Fe₂(PhPH)₂ bildet mit PhPCl₂ den phosphidoverbrückten tetranuklearen Komplex [PhPHFe₂(CO)₆PPh]₂PPh. ³¹P{¹H}-NMR-Spektren der Titelverbindung werden als [AB]₂C-Systeme analysiert, Stereoisomere werden diskutiert. ³¹P-NMR Studies on PhPH(CO)₆Fe₂(PPh)₃Fe₂(CO)₆PPhH ? a Tetranuclear Complex with an Unusual (PPh)₃ Bridge (CO)₆Fe₂(PhPH)₂ and PhPCl₂ form [PhPHFe₂(CO)₆PPh]₂PPh, a tetranuclear complex with phosphido bridges. ³¹P{¹H}-NMR spectra are analysed as [AB]₂C systems, stereoisomers are discussed.