Electronic and diffusion-induced spin-lattice relaxation and the lattice expansion about interstitial12B in Al

The spin-lattice relaxation of¹²B in single-crystal aluminum was measured by NMR in the temperature range 290 to 700 K. The total relaxation rate was separated into two well understood contributions caused by the interaction with conduction electrons and by¹²B diffusion, and into a third unexplained part at high temperatures. From the comparison of the diffusional relaxation rate with theoretical calculations it was concluded that the nearest neighbours of¹²B, trapped in the octahedral interstitial site, are displaced away by 8.8 (30)%.     

Influence of the side chain protecting group on the stereoselectivity of the 1,3-dipolar cycloaddition of d-talo-configured nitrones

Two new five-membered cyclic d-talo-configured nitrones were synthesized from d-mannose, and examined in 1,3-dipolar cycloadditions with vinyl acetate and vinylene carbonate. The stereoselectivity of the cycloadditions depends greatly on the protecting group of the vicinal diol attached to the nitrone C-5 carbon atom. Methyl protection resulted in limited syn/anti-selectivity, giving mixtures of the two isomeric exo-syn and exo-anti isoxazolidines in comparable amounts. On the other hand, the cyclohexylidene-substituted nitrone reacted more selectively in favour of the syn isomer. The difference in the diastereoselectivity was attributed to the specific spatial orientation of the nitrone C-5 substituent.     

Experimental investigation of the muon transfer reaction from deuterium to helium isotopes

Muon transfer from the ground state of muonic deuterium to a helium atom proceeds mainly via the formation of a muonic molecule in an excited state. A large number of decay X rays ( 6.8 keV) from these (dµHe)^* molecules were observed for the⁴He as well as for the³He case. The time distributions of these X rays allow the determination of the ground state transfer rate. The simultaneous employment of Ge/Si-detectors and CCDs for the same target conditions allows the determination of the branching ratio of radiative to nonradiative decay for the first time.     

First measurement of the polarisation transfer on the proton in the reactionsH(\vec e,e'\vec p) andD(\vec e,e'\vec p)

The measurement of the polarisation transfer to the proton in the reactions $H(\vec e,e'\vec p)$ and $D(\vec e,e'\vec p)$ performed with longitudinally polarised electrons in quasi-free kinematics is presented. The coincidence measurement was executed atQ ²≈8fm ^?2 using the 855 MeV, c.w. beam of the Mainz Microtron MAMI. The recoil polarisation was determined by means of a carbon analyser. The experiment shows that the binding of the nucleon does not modify the polarisationP _x of the recoil proton within an error ofΔ P _x/P_x≈10%. The measured polarisation agrees with recent theoretical predictions. Implications for the measurement of the electric form factor of the neutron using the $D(\vec e,e'\vec n)$ reaction are discussed.     

Effect of a Phosphazene Base on the Diastereoselectivity of Addition of alpha-Sulfonyl Carbanions to Butyraldehyde and Isopropylideneglyceraldehyde

The effect of tBuP4, a strong and cation-free base, on the yield and diastereoselectivity of additions of thus formed "naked" alpha-sulfonyl carbanions to achiral butyraldehyde and chiral isopropylideneglyceraldehyde was studied. It has been found that with tBuP4 a reasonable yield ( approximately 55%) and a slightly better diastereoselectivity (72% of the anti diastreomer) are obtained with achiral and nonfunctionalized butyraldehyde while with isopropylideneglyceraldehyde the use of tBuP4 allowed us to greatly increase the yields (up to 95-100%) and the diastereoselectivities (83-89% of a single diastereomer over the four possible diastereomers). It was also shown that the extra oxygen atom in the alpha-position plays a determinant role in this effect.