Ring‐opening polymerization of cyclic ethers initiated by benzazaphosphole‐W(CO)5/silver hexafluoroantimonate

Attempts to prepare mixed M(0)/Ag⁺ complexes with µ‐bridging P ligands by reaction of benzazaphosphole M(CO)₅ complexes 1a–e (M?W, Mo, Cr) with AgSbF₆ in THF lead to rapid ring‐opening polymerization of this ether at room temperature and, as shown for 1a /AgSbF₆, even at low temperature. Oxetane and epoxides (styrene oxide and cyclohexene oxide) polymerize even more vigorously in the presence of this initiator and require dilution with toluene to control the strongly exothermic reaction. Related P(III)W(CO)₅/AgSbF₆ systems with Ph₃P, (EtO)₃P or 2,4,6‐triphenyl‐phosphinine ligands also initiate the THF polymerization, but less efficiently. Efforts to isolate the initiator complex in other solvents failed because of its high sensitivity to nucleophiles and provided 2a ,characterized by crystal structure analysis as the addition product of methanol at 1a , although 1a itself is stable towards MeOH. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 664–670     

Autoinduced Catalysis and Inverse Equilibrium Isotope Effect in the Frustrated Lewis Pair Catalyzed Hydrogenation of Imines

The frustrated Lewis pair (FLP)‐catalyzed hydrogenation and deuteration of N‐benzylidene‐tert‐butylamine ( 2 ) was kinetically investigated by using the three boranes B(C₆F₅)₃ ( 1 ), B(2,4,6‐F₃‐C₆H₂)₃ ( 4 ), and B(2,6‐F₂‐C₆H₃)₃ ( 5 ) and the free activation energies for the H₂ activation by FLP were determined. Reactions catalyzed by the weaker Lewis acids 4 and 5 displayed autoinductive catalysis arising from a higher free activation energy (2 kcal mol^?1) for the H₂ activation by the imine compared to the amine. Surprisingly, the imine reduction using D₂ proceeded with higher rates. This phenomenon is unprecedented for FLP and resulted from a primary inverse equilibrium isotope effect.     

Current status of quark-gluon plasma signals

Compelling evidence for the creation of a new form of matter has been claimed to be found in Pb+Pb collisions at SPS. We discuss the uniqueness of often proposed experimental signatures for quark matter formation in relativistic heavy ion collisions. It is demonstrated that so far none of the proposed signals like J/? meson production/suppression, strangeness enhancement, dileptons, and directed flow unambiguously show that a phase of deconfined matter has been formed in SPS Pb+Pb collisions. We emphasize the need for systematic future measurements to search for simultaneous irregularities in the excitation functions of several observables in order to come close to pinning the properties of hot, dense QCD matter from data.     

A microscopic calculation of secondary Drell-Yan production in heavy ion collisions

A study of secondary Drell-Yan production in nuclear collisions is presented for SPS energies. In addition to the lepton pairs produced in the initial collisions of the projectile and target nucleons, we consider the potentially high dilepton yield from hard valence antiquarks in produced mesons and antibaryons. We calculate the secondary Drell-Yan contributions taking the collision spectrum of hadrons from the microscopic model URQMD. The contributions from meson-baryon interactions, small in hadron-nucleus interactions, are found to be substantial in nucleus-nucleus collisions at low dilepton masses. Preresonance collisions of partons may further increase the yields. Received: 9 July 1997 / Revised version: 17 December 1997 / Published online: 10 March 1998     

Ultrasensitive 3He magnetometer for measurements of high magnetic fields

We describe a ³He magnetometer capable to measure high magnetic fields (B> 0.1 T) with a relative accuracy of better than 10^-12. Our approach is based on the measurement of the free induction decay of gaseous, nuclear spin polarized ³He following a resonant radio frequency pulse excitation. The measurement sensitivity can be attributed to the long coherent spin precession time T₂ ^? being of order minutes which is achieved for spherical sample cells in the regime of “motional narrowing” where the disturbing influence of field inhomogeneities is strongly suppressed. The ³He gas is spin polarized in situ using a new, non-standard variant of the metastability exchange optical pumping. We show that miniaturization helps to increase T₂ ^? further and that the measurement sensitivity is not significantly affected by temporal field fluctuations of order 10^-4.