Systematik der Seltenerdmetall-Blei-Legierungen

Zusammenfassung Das systematische Verhalten (sowie dessen Fehlen) der Seltenerdmetall-Blei-Legierungen wird untersucht. Es wurden 12 verschiedene Verbindungsstöchiometrien beobachtet. Fast alle dreiwertigen Lanthanidenelemente bilden die PhasenR ₅Pb₃,R ₅Pb₄ undRPb₃. Von den anderen möglichen Stöchiometrien kommen, mit Ausnahme vonRPb₂, nur von einigen Lanthaniden Verbindungen vor. Die bekannten kristallographischen Daten werden durchbesprochen und die Existenz und Nichtexistenz einiger Phasen auf Grund kristallchemischer Theorien diskutiert. Das Schmelzverhalten derR-reichen Eutektika und der PhasenR ₅Pb₃,RPb₂ undRPb₃ wird dargestellt. Die bekannten Informationen über diese Legierungen werden auf der Basis der relativen Volumskontraktionen und des 4f-Beitrags zur Bindung überprüft und die Ergebnisse zu Voraussagen über die relativen Stabilitäten der VerbindungenR ₅Pb₃,RPb₂ undRPb₃ verwertet.

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Systematics of the rare earth-lead alloys

The systematic behavior (and also the lack of it) in the rare earth-lead alloys is examined. Twelve different compound stoichiometries have been observed. Nearly all of the trivalent lanthanide elements form theR ₅Pb₃,R ₅Pb₄, andRPb₃ phases. For the other stoichiometries, except for possiblyRPb₂, compounds exist for only a limited number of the lanthanide elements. The known crystallographic data are reviewed, and the existence and nonexistence of some of the phases are discussed on the basis of crystal chemistry theories. The melting behaviors of theR-rich eutectic and theR ₅Pb₃,RPb₂ andRPb₃ phases are presented. The known information on these alloys is examined on the basis of the relative volume contractions and the 4f contribution to the bonding, and the results are used to predict the relative stabilities of theR ₅Pb₃,RPb₂, andRPb₃ compounds.

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Mit 8 Abbildungen

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Herrn Prof. Dr.H. Nowotny gewidmet.

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Die Arbeiten wurden im Ames-Laboratorium der US-Atomenergie-kommission durchgeführt (Beitrag Nr. 2972).     

Transition into a magnetic state without magnetic scattering in a rare earth system: CeAl3

Susceptibility measurements on CeAl₃ down to 0.5 K show both the absence of magnetic ordering and a reduced effective moment such as found in dilute Kondo alloys. The resistance decreases a factor of 30 below the maximum at T = 37 K and the magneto resistance which is negative above 0.5 K apparently becomes positive for T < 0.5 K.     

Ground-state interactions of spermine-substituted naphthalimides with mononucleotides

Water soluble spermine, spermine-naphthalimide, and pyridinium-substituted 1,8-naphthalimide derivatives have been synthesized as nucleotide-specific binding agents. Both mono- and bifunctionalized spermine compounds were studied. The photophysical properties of each compound were studied by using time-resolved and steady-state fluorescence and absorption spectroscopies. The fluorescence decay of the mononaphthalimides was adequately fit to a single exponential decay, and in all cases, the lifetime (2.4 ns) was independent of the imide substitutent. In the case of the bisnaphthalimide, emission from both the monomer and ground-state dimer forms was observed. The fluorescence quantum yield of the monomer (0.03) was significantly smaller than that of the mononaphthalimides (0.27). The dimer emission was red-shifted relative to that of the monomer. The singlet-state lifetime of the dimer was found to be 20 ns. In all cases, only absorption from the triplet excited state was observed, indicating no evidence of a naphthalimide radical anion from dimer excitation. The ground-state interactions of the naphthalimides with four nucleotides were investigated. Nucleotide selectivity was evaluated by determining their individual binding constants (Keq). The association constants were measured by using absorption, time-resolved fluorescence, and combined time-resolved and steady-state fluorescence. The equilibrium binding constant was largest for association of the spermine-substituted mononaphthalimide with adenosine monophosphate (Keq=550 M-1) or guanosine monophosphate (Keq=440 M-1). The dimer form of the disubstituted spermine also showed binding constants in excess of 200 M-1 with the purine nucleotides. The association constant for binding of the pyridinium-substituted naphthalimide showed little dependence on the structure of the nucleotide.     

Spectroscopy and reactivity of Kekulé hydrocarbons with very small singlet-triplet gaps.

Two Kekulé hydrocarbons, 2,2-dimethyl-2H-benzo[cd]fluoranthene (1) and its benzannellated analogue 2,2-dimethyl-2H-dibenzo[cd,k]fluoranthene (2), were generated photochemically from two different photoprecursors each and investigated spectroscopically in cryogenic matrices by UV-vis, fluorescence, and EPR and in solution using ns flash photolysis and chemical trapping experiments. Hydrocarbon 1 is a ground-state singlet species, whereas compound 2 has a triplet ground state, the first such neutral Kekulé hydrocarbon. This difference, which is supported by density functional calculations, has profound influence on the spectroscopy and reactivity of the two compounds. Using the results of the spectroscopic measurements, trapping experiments, and density functional calculations, the singlet-triplet gap for 1 is estimated to be 2.3-2.8 kcal mol(-1), with the singlet the ground state, and 0.8-1.3 kcal mol(-1) for 2, in favor of the triplet.     

Surface-induced aggregation of beta amyloid peptide by co-substituted alkanethiol monolayers supported on gold

The primary pathological characteristic of Alzheimer's disease is the presence in the brain of self-assembled beta amyloid (Abeta) protein fibrils, consisting of 35-43 amino acid residues. The toxicity of the aggregated protein structures has previously been proposed to be related to the interaction of Abeta fibrils with neuronal membranes (phospholipid bilayers). Here, surfaces consisting of self-assembled alkanethiol monolayers with different end groups--supported on Au--are used to test the effect of surface chemistry on the structure and morphology of aggregates formed from an active fragment (Abeta10-35) of the Abeta peptide. The influence of monolayer nature (end group) on the aggregation of Abeta10-35 was examined using reflection-absorption infrared spectroscopy (RAIRS) and scanning force microscopy (SFM). Evaluation of the SFM and RAIRS data reveals the presence of Abeta10-35 protein on the various monolayer surfaces, with the surface protein possessing predominantly beta-sheet and random-coil conformations. Time-dependent studies of the extent of Abeta10-35 aggregation and deposition on the various surfaces and the effect of the monolayers on seeding of Abeta10-35 aggregates in solution are also discussed.     

Mode softening and the elastic constants of CeSn3

The transverse and longitudinal acoustic velocities have been measured in the [100] and [111] directions of CeSn₃ as a function of temperature for 4.2 ≤ T ≤ 290 K. The elastic constants c₄₄ and $\text{1}\text{2}\text{(c}{}_{11}\text{?c}{}_{12}\text{)}$ increase smoothly with decreasing temperature and become temperature independent below 10 K but the bulk modulus has a 2% decrease at 135 K which is indicative of an electronically driven mode softening.     

On the lattice dynamics of hcp hafnium

We measured the phonon dispersion curves of hcp Hf at 295, 800, and 1300 K. We find that (unlike the other modes) the zone center [001]LO mode softens appreciably as the temperature decreases and at room temperature this branch exhibits a dip at the zone center. This anomalous behavior is similar to that observed in hcp Zr, Ti, and Tc and seems to be characteristic of the hcp superconducting elements.     

Stereoselective synthesis of a potent thrombin inhibitor by a novel P2-P3 lactone ring opening

The concise synthesis of a potent thrombin inhibitor was accomplished by a mild lactone aminolysis between an orthogonally protected bis-benzylic amine and a diastereomerically pure lactone. The lactone was synthesized by the condensation of l-proline methyl ester with an enantiomerically pure hydroxy acid, which in turn was synthesized by a highly stereoselective (>500:1 er) and productive (100,000:1, S/C) enzymatic reduction of an alpha-ketoester. In addition, a second route to the enantiomerically pure lactone was accomplished by a diastereoselective ketoamide reduction.     

The aromaticity of pyracylene: an experimental and computational study of the energetics of the hydrogenation of acenaphthylene and pyracylene

In this work, the aromaticity of pyracylene (2) was investigated from an energetic point of view. The standard enthalpy of hydrogenation of acenaphthylene (1) to acenaphthene (3) at 298.15 K was determined to be minus sign(114.5 +/- 4.2) kJ x mol(-1) in toluene solution and minus sign(107.9 +/- 4.2) kJ x mol(-1) in the gas phase, by combining results of combustion and reaction-solution calorimetry. A direct calorimetric measurement of the standard enthalpy of hydrogenation of pyracylene (2) to pyracene (4) in toluene at 298.15 K gave -(249.9 plus minus 4.6) kJ x mol(-1). The corresponding enthalpy of hydrogenation in the gas phase, computed from the Delta(f)H(o)m(cr) and DeltaH(o)m(sub) values obtained in this work for 2 and 4, was -(236.0 +/- 7.0) kJ x mol(-1). Molecular mechanics calculations (MM3) led to Delta(hyd)H(o)m(1,g) = -110.9 kJ x mol(-1) and Delta(hyd)H(o)m(2,g) = -249.3 kJ x mol(-1) at 298.15 K. Density functional theory calculations [B3LYP/6-311+G(3d,2p)//B3LYP/6-31G(d)] provided Delta(hyd)H(o)m(2,g) = -(244.6 +/- 8.9) kJ x mol(-1) at 298.15 K. The results are put in perspective with discussions concerning the "aromaticity" of pyracylene. It is concluded that, on energetic grounds, pyracylene is a borderline case in terms of aromaticity/antiaromaticity character.