Study of the reaction238U (α, α′f) atE α=480 MeV

The excitation function of the fission probability P _E E _x) for²³⁸U has been measured in the reaction²³⁸U(α, α′ f) at 480 MeV bombarding energy. The reaction mechanism of this reaction is discussed for excitation energies belowB _nf , the threshold for second chance fission, and aboveB _nf up toE _x =37 MeV. In comparing with results from fission induced by photons and by particle transfer reactions the (α, α′f) reaction gives too low values for the fission probabilityP _f at excitation energies well aboveB _nE . The role of the quasi-elastic knock-out process in this reaction is discussed.     

Analysis of thermally induced magnetization dynamics in spin-transfer nano-oscillators

The thermally induced magnetization dynamics in the presence of spin-polarized currents injected into a spin-valve-like structure used as microwave spin-transfer nano-oscillator (STNO) is considered. Magnetization dynamics is described by the stochastic Landau-Lifshitz-Slonczewski (LLS) equation. First, it is shown that, in the presence of thermal fluctuations, the spectrum of the output signal of the STNO exhibits multiple peaks at low and high frequencies. This circumstance is associated with the occurrence of thermally induced transitions between stationary states and magnetization self-oscillations. Then, a theoretical approach based on the separation of time-scales is developed to obtain a stochastic dynamics only in the slow state variable, namely the energy. The stationary distribution of the energy and the aforementioned transition rates are analytically computed and compared with the results of direct integration of the LLS dynamics, showing very good agreement.     

Design and synthesis of novel [60]fullerene derivatives as potential HIV aspartic protease inhibitors

[structure] Two water-soluble fullerene derivatives have been computer-designed and synthesized. They may exhibit interesting anti-HIV activity owing to the presence of two ammonium groups strategically located on the spheroid surface.     

Mutagenicity and DNA damage studies of N-acyloxy-N-alkoxyamides--the role of electrophilic nitrogen

N-Acyloxy-N-alkoxyamides are anomeric amides that are direct-acting mutagens. They have been shown to damage DNA in the major and the minor grooves in a pH and sequence-selective manner. In acidic media, they damage adenines at N3 in the minor groove but above neutral pH, only guanine is damaged at N7 in the major groove. Both the acyloxy leaving group and the alkoxy group at the amide nitrogen are responsible for their electrophilicity and Salmonella mutagenicities in TA 100 and DNA damage data confirm that the mutagens react with DNA in an intact form, rather than by solvolysis to electrophilic nitrenium ions in the cytosol, or in vitro, prior to reacting with DNA. Hydrophobicity plays a role in both mutagenicity and DNA damage.     

Induced fit process in the selective distal binding of imidazoles in zinc(II) porphyrin receptors

The respective affinities of various imidazole derivatives, imidazole (ImH), 2-methylimidazole (2-MeImH), 2-phenylimidazole (2-PhImH), N-methylimidazole (N-MeIm), 2-methylbenzimidazole (2-MeBzImH), and 4,5-dimethylbenzimidazole (4,5-Me(2)BzImH), for two phenanthroline (Phen) strapped zinc(II) porphyrin receptors porphen-Zn 1-Zn and 2-Zn have been studied. The formation of a supplementary H-bond considerably enhances the affinity of the zinc(II)-porphen receptor for imidazoles unsubstituted on the pyrrolic nitrogen (ImH) versus N-substituted imidazoles such as N-MeIm. The ImHs subset porphen-Zn complexes are formed with association constants up to 4 orders of magnitude superior to those measured either for N-MeIm as substrate or TPP-Zn as receptor. Distal or proximal binding of the substrates was determined by (1)H NMR measurements and titration. In two cases, the very high stability of the inclusion complex enabled the use of 2D NMR techniques. Excellent correlation between solution and solid-state structures has been obtained. A total of six X-ray structures are detailed in this article showing that the evolution of the shape of the zinc(II) receptor is mostly dependent on the steric constraints induced by the substitution on the imidazole. Hindered guests also progressively induce considerable mobility restrictions and severe distortions on the receptor, especially in the case of 2-MeBzImH and 2-PhImH.     

The lock‐in phase in the urotropine–sebacic acid system

The 1,10‐decane­dioic acid–1,3,5,7‐tetra­aza­tri­cyclo­[3.3.1.1^3,7]­decane (1/1) system, C₁₀H₁₈O₄·C₆H₁₂N₄, was studied at 215 (2) K. Its analysis provides important information with regard to the long‐standing acid–carboxyl­ate controversy in the urotropine–alkanedioic acid system. In the present structure, all the chain end‐groups display a clear acid character. The asymmetric unit of this commensurate modulated phase contains two mol­ecules of diacid as well as two mol­ecules of urotropine. Furthermore, the chain packing suggests a possible order parameter for the lock‐in transition.     

Possible evidence for a narrow isovectorB=2 resonance below pion production threshold

Some evidence is presented for a narrow peak at 1969±2 MeV (FWHM=9±2 MeV) in the missing mass spectrum of the³He(p, d) reaction, with 3 standard deviations. The nature of this state, the mass of which is below the NN threshold, is discussed in connection with structures found in other experiments.     

Asymmetric Synthesis. 39.(1) Synthesis of 2-(1-Aminoalkyl)piperidines via 2-Cyano-6-phenyl Oxazolopiperidine

The asymmetric synthesis of a series of 2-(1-aminoalkyl) piperidines using (-)-2-cyano-6-phenyloxazolopiperidine 1 is described. LiAlH(4) reduction of 1 followed by hydrogenolysis led to the diamine 3. The same strategy applied to C-2-methylated compound 7 afforded [(2S)-2-methylpiperidin-2-yl]methanamine (9). Addition of lithium derivatives to the cyano group of 1 resulted in the formation of an intermediate imino bicyclic system (11a-c) which could be diastereoselectively reduced to substituted diamino alcohols 13a-c. The addition of an excess of PhLi to 1 in the presence of LiBr furnished disubstituted amine 19, the precursor of diphenyl[(2S)-piperidin-2-yl]methanamine (22).     

Every group is the automorphism group of a rank-3 matroid

Every group is the automorphism group of a rank-3 extension of a rank-3 Dowling geometry.Partially supported by The George Washington University UFF grant.Partially supported by the National Security Agency under grant MDA904-91-H-0030.