Thermal properties of the valence electrons in alkali metal clusters

The finite-temperature density functional approach is applied for the first time to calculate thermal properties of the valence electron system in metal clusters using the spherical jellium model. Both the canonical and the grand canonical formalism are applied and their differences are discussed. We study the temperature dependence of the total free energyF(N) (including a contribution from the ionic jellium background) for spherical neutral clusters containingN atoms. We investigate, in particular, its first and second differences, Δ₁ F =F (N ? 1) ?F (N) and Δ₂ F =F(N + 1) +F(N ? 1) ? 2F(N), and discuss their possible relevance for the understanding of the mass abundance spectra observed in cluster production experiments. We show that the typical enhancement of magic spherical-shell clusters withN=8, 20, 34, 40, 58, 92, 138, 186, 254, 338, 398, 440, 508, 612..., most of which are well established experimentally, is decreasing rather fast with increasing temperatureT and cluster sizeN. We also present electronic entropies and specific heats of spherical neutral clusters. The Koopmans theorem and related approximations for calculating Δ₁ F and Δ₂ F atT > 0 are discussed.     

Thermal Isomerization of Isoborneols and Dehydroisoborneols to New Chiral Building Blocks in Terpenoid Synthesis

The substituted isoborneols 1a – 1g and 5,6‐dehydroisoborneols 6a – 6c , readily prepared in excellent yields from (+)‐camphor and (+)‐5,6‐dehydrocamphor ( 2 ) by aryl, vinyl, or alkyl Grignard addition in the presence of stoichiometric amounts of CeCl₃, were thermally isomerized in a flow reactor system under DGPTI (dynamic gas‐phase thermo‐isomerization) conditions at temperatures between 480 and 630° to give the enantiomerically pure monocyclic carbonyl compounds 7a – 7d, 19a , b, 23 , and 24 . In all cases, product formation proceeded highly regio‐ as well as stereoselectively. The absolute configurations of the new stereogenic centers were determined by ¹H‐NOE measurements. DGPTI of the aryl substrates 1a – 1d is proposed to effect initial cleavage of the weakest single bond in the molecule under formation of a diradical intermediate state followed by intramolecular H‐abstraction to afford the acetophenone derivatives 7a – 7d . This reaction path was further supported by a ²H‐labeling study showing the OH group to be the exclusive H‐source. In contrast, DGPTI of the vinyl substrates 1e and 6b allowed concerted retro‐ene and oxy‐Cope rearrangements. In the case of 5,6‐dehydro‐2‐phenylisoborneol ( 6a ), concomitant diradical and retro‐Diels–Alder reaction pathways could be observed. In addition, a new route to (+)‐trans‐α‐campholanic acid ( 9 ) and (+)‐trans‐α‐dihydrocampholytic acid ( 14 ) is presented by regioselective Baeyer–Villiger oxidation and subsequent hydrolysis of 7c and 7d , respectively.     

Partial agonists and subunit selectivity at NMDA receptors

Subunit-selective ligands for glutamate receptors remains an area of interest as glutamate is the major excitatory neurotransmitter in the brain and involved in a number of diseased states in the central nervous system (CNS). Few subtype-selective ligands are known, especially among the N-methyl-D-aspartic acid (NMDA) receptor class. Development of these ligands seems to be a difficult task because of the conserved region in the binding site of the NMDA receptor subunits. A few scaffolds have been developed showing potential to differentiate between the NMDA receptors.     

Photoinduced reactions of aryl-2H-azirines with carbonyl compounds. Preliminary communication

Irradiation of 3-phenyl-( 4 ), 2-methyl-3-phenyl-( 8 ), and 2,3-diphenyl-2H-azirine ( 10 ), in benzene solution in the presence of aldehydes, yields the corresponding aryl-3-oxazolines. Photochemical reaction of 4 and 10 with carbon dioxide leads to the formation of 4-phenyl- ( 15 ) and 2,4-diphenyl-3-oxazolin-5-one ( 16 ), respectively.     

Zur Photodimerisierung von 3-Phenyl-2H-azirinen. Vorläufige Mitteilung

Irradiation of 3-phenyl-2H-azirine ( 2 ) in benzene solution with a high-pressure mercury lamp yields 4,5-diphenyl-1,3-diazabicyclo[3,1,0]hex-3-ene ( 4 ) and not 3-phenylimino-4-phenyl-1-azabicyclo[2,1,0]pentane ( 1 ), as had been reported previously by others [2]. 2-Methyl-3-phenyl-2H-azirine ( 3 ) yields on irradiation a 2:1 mixture of 2-exo, 6-exo- and 2-exdo, 6-exo-dimethyl-4,5-diphenyl-1,3-diazabicyclo[3,1,0]hex-3-ene (2-exo,6-exo- and 2-endo, 6-exo- 5 ). Irradiation of 2,3-diphenyl-2H-azirine ( 8 ) leads to the formation of 2,4,5-triphenyl-imidazole ( 9 ) and tetra-phenylpyrazine ( 10 ). The suggested reaction path for the generation of 9 and 10 is shown in Scheme 2.     

Eine einfache Allencarbonsäureester-Synthese mittels der Wittig-Reaktion

A Simple Synthesis of Alkyl Allenecarboxylates (Allenic Esters) by the Wittig-Reaction A simple one-pot synthesis of variously substituted α-allenic esters ( 7–23 , cf. Scheme 4) is described. Either of the phosphonium salts 1 and 2 or the phosphoranes 3–6 in methylene chloride (or acetonitrile) when treated with acid chlorides at room temperature in the presence of triethylamine give the corresponding allenic compounds in fairly good yields (cf. Tables 3a and 3b). This procedure shows for the first time, that the Wittig-reaction with acid chlorides as starting material can also be used for the synthesis of 2-H-substituted allenic esters ( 7–13 ; for numbering see Scheme 7). The deuterium labeled compounds 26–31 (cf. Scheme 7) are prepared in the same manner. The course of the reaction involves an in situ generation of ketenes which are trapped by the resonance stabilized phosphonium ylides. The thus formed betains readily decompose, already at room temperature, to the anticipated allenes by the well known Wittig-reaction.     

Modified Nilsson model for large sodium clusters

We propose a modified Nilsson model for spheroidal sodium clusters and investigate the modification of shell structure by deformation for sizes up toN=850. For spherical clusters, our potential is fitted to the single-particle spectra obtained from microscopically selfconsistent Kohn-Sham calculations using the jellium model and the local density approximation. Employing Strutinsky's shell-correction method, the surface energy of the jellium model is renormalized to its experimental value. We find good agreement between our theoretically predicted deformed magic numbers and the experimentally observed ones extracted from recent sodium mass abundance spectra.     

Energy dependence of multiplicity in proton-nucleus collisions and models of multiparticle production

This is a continuation of our earlier investigation (Gurtuet al 1974Phys. Lett. 50 B 391) on multiparticle production in proton-nucleus collisions based on an exposure of emulsion stack to 200 GeV/c beam at the NAL. It is found that the ratioR _em = 〈n _s〉/〈n _ch〉, where 〈n _ch〉 is the charged particle multiplicity in pp-collisions, increases slowly from about 1 at 10 GeV/c to 1·6 at 68 GeV/c and attains a constant value of 1·71 ± 0·04 in the region 200 to 8000 GeV/c. Furthermore,R _em = 1·71 implies an effectiveA-dependence ofR _A =A ^0.18,i.e., a very weak dependence. Predictions ofR _em on various models are discussed and compared with the emulsion data. Data seem to favour models of hadron-nucleon collisions in which production of particles takes place through adouble step mechanism,e.g., diffractive excitation, hydrodynamical and energy flux cascade as opposed to models which envisage instantaneous production.     

有机稀土铕的螯合物的发光和能量传递的研究

本文应用荧光光谱和瞬时时间分辨光谱研究了稀土铕有机螯合物,以及在二苯酮做为三线态敏化剂存在时,对能量的吸收、传递和发光的过程,探讨了有机配位体与稀土离子的能级、三线态敏化剂和配位体分子的性质对发光的影响。     

Measuring the longitudinal NMR relaxation rates of fast relaxing nuclei using a signal eliminating relaxation filter

A new experiment for selective determination of the relaxation rates of fast relaxing NMR signals is presented. The experiment is derived from the conventional inversion recovery experiment by substituting the 180 degrees inversion pulse of this experiment with a signal eliminating relaxation filter (SERF) consisting of three 180 degrees pulses separated by two variable delays, Delta1 and Delta2. The SERF experiment allows a selective suppression of signals with relaxation rates below a given limit while monitoring the relaxation of faster relaxing signals. The experiment was tested on a sample of 20% oxidized plastocyanin from Anabaena variabilis, where the fast exchange of an electron between the reduced (diamagnetic) and the oxidized (paramagnetic) form results in a series of average signals with widely different relaxation rates. To ensure an optimum extraction of information from the experimental data, the relaxation rates were obtained from the SERF experiment by a simultaneous analysis of all the FIDs of the experiment using a fast linear prediction model method developed previously. The reliability of the relaxation rates obtained from the SERF experiment was confirmed by a comparison of the rates with the corresponding rates obtained from a conventional inversion recovery experiment.