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71.
Kostko O Wrigge G Cheshnovsky O von Issendorff B 《The Journal of chemical physics》2005,123(22):221102
We present photoelectron spectroscopy studies on Zn(n) (-) in the size range of n=3-117. We show that zinc clusters exhibit a distinct transition in their electronic structure as a function of size. At small sizes (up to n=18) the clusters follow the Bloch-Wilson picture of the development of a metal from closed-shell atoms, exhibiting a gradual decrease of the gap between the fully occupied s band and the empty p band. For large sizes (n approximately or > 32) the band overlap allows the valence electrons to fully delocalize. This leads to an almost perfect free-electron density of states, as is demonstrated by discussing the spectra in the light of standard free-electron models and by comparison to the results obtained on sodium clusters. 相似文献
72.
Dodds ED Kennish JM Von Hippel FA Bernhardt R Hines ME 《Analytical and bioanalytical chemistry》2004,379(5-6):881-887
The perchlorate anion (ClO
4
–
) is an anthropogenic contaminant of increasing concern in water supplies, and has been shown to disrupt thyroid activity. Most perchlorate analyses are currently carried out by ion chromatography (IC) with suppressed conductivity detection (SCD). While this procedure has been demonstrated to provide acceptable performance for analysis of water samples, the determination of perchlorate in high-conductivity aqueous extracts of plant or animal material is not readily accomplished by IC-SCD unless lengthy cleanup protocols are applied. With the addition of electrospray ionization mass spectrometry (ESI-MS) to IC, it was hypothesized that the interference imposed by various ionic species could be significantly reduced without the need for purification; however, the analysis of perchlorate in relatively unpurified extracts of biologically derived homogenates by IC-ESI-MS has not previously been described in the literature. The research presented here represents a comparison of the capabilities of IC-SCD and IC-ESI-MS to detect perchlorate in reagent water and in crude extracts of perchlorate-exposed fish (threespine stickleback, Gasterosteus aculeatus). ESI-MS was found to compare favorably to SCD for the detection of perchlorate in deionized water, and to exceed SCD performance in perchlorate analysis of fish-derived extracts. 相似文献
73.
A high-performance liquid chromatographic assay for the determination of isoniazid, acetylisoniazid, acetylhydrazine and diacetylhydrazine (plasma and urine) was developed. The m-chlorobenzoyl derivatives of isoniazid, acetylhydrazine and the internal standard propionylhydrazine were prepared, separated on a RP-18 column and detected at 220 nm. Acetylisoniazid, diacetylhydrazine and the internal standard dipropionylhydrazine were converted to isoniazid, acetylhydrazine, and propionylhydrazine by acidic hydrolysis and subsequently derivatized with m-fluorobenzoyl chloride, separated on a RP-18 column and detected at 220 nm. The lower limits of detection in plasma are acetylhydrazine 0.5 nmol/ml, isoniazid 1.0 nmol/ml, diacetylhydrazine 1.0 nmol/ml and acetylisoniazid 2.0 nmol/ml, and in urine, acetylhydrazine 10 nmol/ml, isoniazid 15 nmol/ml, diacetylhydrazine 20 nmol/ml and acetylisoniazid 40 nmol/ml. This method is sensitive, reproducible, accurate and precise; therefore, it is well suited for detailed pharmacokinetic studies. 相似文献
74.
Helmut Quast Josef Christ Yvonne Görlach Wolfgang von der Saal 《Tetrahedron letters》1982,23(36):3653-3656
The 2,6-dicyano-1,5-dimethylsemibullvalene (2d) synthesized via the bicyclo[3.3.0]octanedione 7 exists as a classical ground state which is lower in energy by less than 5 kcal/mole than the homoaromatic transition of the Cope rearrangement of 2d. 相似文献
75.
76.
Zusammenfassung Es wurden die Dipolmomente der, -dichlorierten Kohlenwasserstoffe in Lösung mit Hexan über den ganzen Konzentrationsbereich bei 25 °C bestimmt. Untersucht wurden die Verbindungen dieser Reihe vom 1,2-Dichloräthan bis zum 1,10-Dichlordekan.Die Dipolmomente dieser Verbindungen ließen sich gut deuten, wenn ein Gemisch starrer Rotationsisomerer angenommen wurde. Dabei zeigten sich auch noch die durch eine Kohlenstoffkette von 10 C-Atomen getrennten Partialmomente deutlich in ihrer Richtung gegenseitig abhängig.Die Konzentrationsabhängigkeit der Dipolmomente wurde an Hand der durch dieKirkwood-Fröhlichsche Theorie gegebeneng-Faktoren diskutiert. Es ergab sich, daß dieg-Faktoren des 1,2-Dichloräthans weitgehend auf eine DK-abhängige Konformationsänderung von der unpolaren trans- in die polare gauche-Konformation zurückzuführen sind. Dies scheint eine grundsätzliche Eigenschaft von Verbindungen zu sein, die Partialmomente in 1,2-Stellung haben. Dieg-Faktoren der Verbindungen, deren Partialmomente weiter voneinander entfernt sind, wie bei den höheren Gliedern der Reihe, wurden nahezu ausschließlich auf zwischenmolekulare Ausrichtung der Partialmomente zurückgeführt.Ein Vergleich mit deng-Faktoren des Hexylchlorids und des 2,2-Dichlorpropans zeigte, daß bis zu einer Konzentration von 1 Mol/l in erster Linie dimere bzw. kettenartige antiparallele bzw. parallele Assoziate der Partialmomente zu erwarten sind. Nach höheren Konzentrationen weichen dieg-Faktoren von diesem Verhalten nach größeren Werten hin ab. Daraus wurde auf parallele Ausrichtung in höherer Koordinationszahl geschlossen.Für die Unterstützung dieser Arbeit sprechen wir der Deutschen Forschungsgemeinschaft unseren verbindlichsten Dank aus.Der Firma Bayer, Leverkusen, danken wir für die Hilfe bei Beschaffung einiger der Substanzen. 相似文献
77.
The Metropolis Monte Carlo method has been added to the program FANTOM for energy refinement of polypeptides and proteins using a Newton–Raphson minimizer in torsion angle space. With this extension, different strategies for global minimization of the semiempirical energy function ECEPP/2 by various temperature schedules and restriction of conformational space were tested for locating local minimum conformations with low energy of the pentapeptide Met-enkephalin. In total, 1881 conformations below ?10 kcal/mol were found. These conformations could be represented by 77 nonidentical conformations which were analysed for their pattern of hydrogen bonds, types of tight turn, pairwise root-mean-square-deviation (rmsd), Zimmermann codes and side chain conformations. All low energy conformations below ?10.4 kcal/mol show strong similarity to the global minimum conformation in the backbone structure. 相似文献
78.
Jockusch RA Kroemer RT Talbot FO Snoek LC Carçabal P Simons JP Havenith M Bakker JM Compagnon I Meijer G von Helden G 《Journal of the American Chemical Society》2004,126(18):5709-5714
The beta(1-->4) glycosidic linkage found in lactose is a prevalent structural motif in many carbohydrates and glycoconjugates. Using UV and IR ion-dip spectroscopies to probe benzyl lactoside isolated in the gas phase, we find that the disaccharide unit adopts only a single, rigid structure. Its fully resolved infrared ion-dip spectrum is in excellent agreement with that of the global minimum structure computed ab initio. This has glycosidic torsion angles of phi(H) (H1-C1-O-C4') approximately 180 degrees and psi(H) (C1-O-C4'-H4') approximately 0 degrees which correspond to a rotation of approximately 150 degrees about the glycosidic bond compared to the accepted solution-phase conformation. We discuss the biological implications of this discovery and the generality of the strategies employed in making it. 相似文献
79.
In picosecond spectroscopy transient absorption methods are utilized to measure ground-state repopulation kinetics. Since linearly polarized light pulses from mode-locked lasers are used, the transient absorption characteristics in solutions are governed by the rotational diffusion of the absorbing molecules, too. The theory for isotropic diffusion is given and compared to a measurement on the rhodamine 6G molecule dissolved in solvents of different viscosity using a novel pulse spectrophotometer. 相似文献
80.
Micha? K. Cyrański Paul von Ragué Schleyer Tadeusz M. KrygowskiHaijun Jiao Georg Hohlneicher 《Tetrahedron》2003,59(10):1657-1665
The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of CC or CX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G∗∗. 相似文献