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31.
On the basis of a pyrrolidine tweezer 1, a library of peptidosulfonamide tweezers (15a-e, 16a-e) was synthesized on the solid phase. This library was screened in a simultaneous substrate screening procedure for the ability to enantioselectively catalyze the Ti(O-i-Pr)(4)-mediated addition of diethylzinc to aldehydes. One of the best solid-phase tweezer catalyst (i.e., 16d, giving an ee of 32% in solid-phase catalysis) was resynthesized in solution (compounds 20 and 21). The now homogeneous solution-phase catalysis showed even better enantioselectivity (i.e., up to 66%). 相似文献
32.
L. Monz R. Hohmann H.-J. Kluge S. Kunze J. Lantzsch E.W. Otten G. Passler P. Senne J. Stenner K. Stratmann K. Swendt K. Zimmer G. Herrmann N. Trautmann K. Walter 《Spectrochimica Acta Part B: Atomic Spectroscopy》1993,48(14):1655-1671
Environmental assessment in the wake of a nuclear accident requires the rapid determination of the radiotoxic isotopes 89Sr and 90Sr. Useful measurements must be able to detect 108 atoms in the presence of about 1018 atoms of the stable, naturally occurring isotopes. This paper describes a new approach to this problem using resonance ionization spectroscopy in collinear geometry, combined with classical mass separation. After collection and chemical separation, the strontium from a sample is surface-ionized and the ions are accelerated to an energy of about 30 keV. Initially, a magnetic mass separator provides an isotopic selectivity of about 106. The ions are then neutralized by charge exchange and the resulting fast strontium atoms are selectively excited into high-lying atomic Rydberg states by narrow-band cw laser light in collinear geometry. The Rydberg atoms are then field-ionized and detected. Thus far, a total isotopic selectivity of S > 1010 and an overall efficiency of ξ = 5 × 10−6 have been achieved. The desired detection limit of 108 atoms 90Sr has been demonstrated with synthetic samples. 相似文献
33.
It was investigated, how a chemical reaction was affected by diffusion processes. The analytical solution of the problem is presented for three sample geometries (plate, cylinder, sphere). Furthermore we perform an analysis of the dimensions and get the result, that four dimensionless parameters are essential for the reaction rate. The influence of these parameters on the thermogravimetric curves is discussed and the results are compared with experimental findings. 相似文献
34.
M. Della Negra D. Drijard H.G. Fischer G. Fontaine H. Frehse P. Frenkiel C. Chesquiere R. Gokieli P. Hanke W. Hofmann W. Isenbeck E.E. Kluge V. Korbel D. Linglin A. Minten A. Norton A. Putzer R. Sosnowski D. Wegener 《Physics letters. [Part B]》1976,65(4):394-396
The reaction pp→ppπ+π? at √s=31GeV was studied at the Split Field Magnet Facility of the CERN-ISR. Selecting events with two leading protons of x>0.9 and a rapidity gap Δy>2 between the protons and both pions a sample of 720 events is obtained representing a cross section of 25±10μb. The mass distribution of the dipion system, x and pT distributions for the protons are presented and analyzed for the presence of double pomeron exchange. 相似文献
35.
36.
The polarization of neutrons produced in the reaction12C(d, n 0)13N was measured. Deuterons from the Karlsruhe isochronous cyclotron were used to induce this reaction at 51,5 MeV laboratory energy. The degree of polarization was determined by using the special features of Mott-Schwinger scattering. With an uranium scatterer analysing efficiencies of up to 0.92 can be obtained at very small angles (0.23°). The analysing efficiency can be calculated if the differential cross section at 0° and the total cross section is known. These quantities were experimentally determined. The differential cross section for 49.4 MeV neutrons, scattered by uranium, was measured between 0.88° and 2.10°. By an extrapolation the value 43.4±2.6 b/sr was found for the nuclear differential cross section at zero degree. A total cross section ofσ t=4.80±0.22 b was obtained. The neutron polarization was measured at a reaction angle of 24.5° and the result isP=?0.45±0.07. This value is fairly above the semiclassical 1/3 limit and can be only explained, if spin orbit forces are taken into account. For (d, n) reactions this is the first neutron-polarization measurement above an energy of 20 MeV. 相似文献
37.
The hyperfine structure of the lowest1P1 state of25Mg,43Ca,87Sr,135Ba and137Ba have been measured by the level-crossing and anticrossing technique. The magnetic dipole and electric quadrupole coupling constants determined by these measurements are25Mg(3s3p1P1):A=? 7.7(5) MHz; 16 MHz>B>0 MHz,43Ca(4s4p1P1):A=? 15.3(4) MHz; ¦B¦<12 MHz,87Sr (5s5p1P1:A=? 3.4(4) MHz;B=39(4) MHz,135Ba(6s6p1P1):A=? 97.5(1.0) MHz;B=31(9)MHz,137Ba(6s6p1P1):A=?109.2(1.2) MHz;B=51(12)MHz. The results have been compared with the predictions of the Breit-Wills theory of the two-electron hyperfine structure using the experimental data on the3P states. Large discrepancies have been observed which are due to different radial wave functions of thes andp electron in the triplet and singlet system. This effect has been taken into account by fitting the data with the aid of two additional parameters. That this procedure is justified is shown by an analysis of the fine structure splitting, the life times, and the isotopic shifts in thesp configurations of group II elements. 相似文献
38.
Ewald G Nörtershäuser W Dax A Götte S Kirchner R Kluge HJ Kühl T Sanchez R Wojtaszek A Bushaw BA Drake GW Yan ZC Zimmermann C 《Physical review letters》2004,93(11):113002
The 2s-->3s transition of (6,7,8,9)Li was studied by high-resolution laser spectroscopy using two-photon Doppler-free excitation and resonance-ionization detection. Hyperfine structure splittings and isotope shifts were determined with precision at the 100 kHz level. Combined with recent theoretical work, the changes in the nuclear-charge radii of (8,9)Li were determined. These are now the lightest short-lived isotopes for which the charge radii have been measured. It is found that the charge radii monotonically decrease with increasing neutron number from 6Li to 9Li. 相似文献
39.
Van Roosbroeck J Guénaut C Audi G Beck D Blaum K Bollen G Cederkall J Delahaye P De Maesschalck A De Witte H Fedorov D Fedoseyev VN Franchoo S Fynbo HO Górska M Herfurth F Heyde K Huyse M Kellerbauer A Kluge HJ Köster U Kruglov K Lunney D Mishin VI Mueller WF Nagy S Schwarz S Schweikhard L Smirnova NA Van de Vel K Van Duppen P Van Dyck A Walters WB Weissman L Yazidjian C 《Physical review letters》2004,92(11):112501
Using resonant laser ionization, beta-decay studies, and for the first time mass measurements, three beta-decaying states have been unambiguously identified in 70Cu. A mass excess of -62 976.1(1.6) keV and a half-life of 44.5(2) s for the (6-) ground state have been determined. The level energies of the (3-) isomer at 101.1(3) keV with T(1/2)=33(2) s and the 1+ isomer at 242.4(3) keV with T(1/2)=6.6(2) s are confirmed by high-precision mass measurements. The low-lying levels of 70Cu populated in the decay of 70Ni and in transfer reactions compare well with large-scale shell-model calculations, and the wave functions appear to be dominated by one proton-one neutron configurations outside the closed Z=28 shell and N=40 subshell. This does not apply to the 1+ state at 1980 keV which exhibits a particular feeding and deexcitation pattern not reproduced by the shell-model calculations. 相似文献
40.
J. Dilling F. Herfurth A. Kellerbauer G. Audi D. Beck G. Bollen H. -J. Kluge R. B. Moore C. Scheidenberger S. Schwarz G. Sikler 《The European Physical Journal A - Hadrons and Nuclei》2004,22(2):163-171
The masses of the noble-gas Xe isotopes with
have been directly measured for the first time. The experiments were carried out with the ISOLTRAP triple trap spectrometer at the on-line mass separator ISOLDE/CERN. A mass resolving power of the Penning trap spectrometer of
of close to a million was chosen resulting in an accuracy of
keV for all investigated isotopes. Conflicts with existing, indirectly obtained, mass data by several standard deviations were found and are discussed. An atomic mass evaluation has been performed and the results are compared to information from laser spectroscopy experiments and to recent calculations employing an interacting boson model.PACS:
07.75. + h Mass spectrometers - 21.10.Dr Binding energies and masses - 27.60. + j
- 32.10.Bi Atomic masses, mass spectra, abundances, and isotopes 相似文献