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991.
Summary We investigate the asymptotic behaviour of
n
(n),n where
n
(x) denotes the Laguerre polynomial of degreen. Our results give a partial answer to the conjecture
n
(n)>1 forn>6, made in 1984 by van Iseghem. We also show the connection between this conjecture and the continued fraction approximants of
.Work sponsored by the Consiglio Nazionale delle Ricerche and by the Ministero dell'Università e della Ricerca Scientifica e Tecnologica of Italy 相似文献
992.
Roberto Martínez Lucia Valle Marcos Gonzalez Claudia Romano 《Journal of heterocyclic chemistry》1997,34(3):1043-1045
Reaction of 4(7)-aminobenzimidazole with ethyl 2-alkylmalonates gave the novel 4,5,6,7-tetrahydro-5-alkyl-imidazo[1,5,4-ef][1,5]benzodiazepine-4,6-diones, and the 2-alkyl-4(7)-(2′-ethoxycarbonyl)acetamido-benzimidazoles. The structure of all the products was corroborated by ir, mass spectrometry and 1H-nmr. 相似文献
993.
Attanasi OA De Crescentini L Favi G Filippone P Giorgi G Mantellini F Santeusanio S 《The Journal of organic chemistry》2003,68(5):1947-1953
Alpha-substituted hydrazones obtained from 1,2-diaza-1,3-butadienes and methylenic or methinic activated substrates gave rise to a wide range of cyclic compounds. In particular, in the presence of thionyl chloride as solvent-reagent, they were transformed into 1,2,3-thiadiazoles,(1) with selenium oxychloride in new 4-substituted 2,3-dihydro-1,2,3-selenadiazoles, while with selenium dioxide, they were transformed into 4-substituted 1,2,3-selenadiazoles. We have also examined the nucleophilic behavior of 1,2,3-thiadiazole 4a in the reaction with 1,2-diaza-1,3-butadienes that produced, under basic conditions, 4-hydrazono-1-(1,2,3-thiadiazolyl)pentane derivatives. This event represents an interesting example of stereoselective synthesis because it leads exclusively to the formation of the RR/SS racemic mixture. These latter compounds, treated with thionyl chloride, gave the corresponding 1,3-di-1,2,3-thiadiazolylpropane derivatives, while with sodium methoxide they afforded 1,2,3-thiadiazolyl-2-oxo-2,3-dihydro-1H-pyrrole systems. 相似文献
994.
Lucia Flamigni Etienne Baranoff Jean-Paul Collin Jean-Pierre Sauvage Barbara Ventura 《Chemphyschem》2007,8(13):1943-1949
The effect of photon flux on the yield and lifetime of charge separation over the extreme components of a D-Ir-A triad, where D is a triphenyl amine electron donor, A is a naphthalene bis(imide) electron acceptor and Ir is an Ir(III) bis(terpyridine) complex, has been investigated. In usual laboratory conditions, with nanosecond and picosecond laser pulses in the 4-8 mJ range, biphotonic processes take place. Biphotonic products and their evolution can introduce complications in reaction mechanisms and their interpretation but can also drastically reduce the yield and the lifetime of the charge-separated state. In the present case, after discussion of several possible mechanisms, the process detrimental to charge separation is ascribed to absorption of a photon by the photogenerated charge-separated state D(+)-Ir-A(-). 相似文献
995.
K. P. S. Bhaskara Rao Paolo De Lucia 《Rendiconti del Circolo Matematico di Palermo》1983,32(2):188-198
In una nota del 1976 C. Constantinescu [2], fa osservare che per una misura μ, definita in un δ-anelloR, a valori in un gruppo topologico e non verificante la countable chain condition, i μ-atomi non sono identificati, come nel caso classico, da particolari elementi diR ma sono parti diR godenti di opportu vista è ripreso in [3] da A. B. d’Andrea e P. de Lucia per lo studio degli atomi di una funzione finitamente addittiva in un anello. Nella presente nota si fa vedere che (n. 2), dando un’opportuna definizione di insieme μ-atomico, un μ-atomo può sempre essere individuato da un elemento diR e si stabiliscono poi (n. 3) delle relazioni fra le funzioni fortemente continue e le funzioni prive di atomi. Nel n. 4, infine, sono dimostrate alcune proprietà delle funzioni a due valori. 相似文献
996.
Rassu G Auzzas L Zambrano V Burreddu P Pinna L Battistini L Zanardi F Casiraghi G 《The Journal of organic chemistry》2004,69(5):1625-1628
Efficient, total syntheses of novel 2-deoxy-2-amino-5a-carba-beta-L-mannopyranuronic acid (1) and 2-deoxy-2-amino-5a-carba-beta-L-mannopyranose (2), a positional stereoisomer of validamine, have been achieved in 28% and 24% overall yields and in 12 steps and 13 steps, respectively, from 2-[(tert-butyldimethylsilyl)oxy]furan (3) and (2S)-2,3-O-isopropylideneglyceraldehyde N-benzyl imine (4) via two highly diastereoselective Mukaiyama aldol-related chemical maneuvers. The strategy, which furnishes the targeted carbasugars in enantiopure forms, allows for complete control of the configuration at all five contiguous stereocenters of the targets by utilizing the sole element of chirality present in the aldimine progenitor 4. 相似文献
997.
The first convenient synthesis of 2-S-cysteinyl-3,4-dihydroxyphenylalanine (2-S-cysteinyldopa) in 30% overall yield is reported, which capitalizes on the anomalous regiochemistry of the oxidative coupling of 3,4-dihydroxycinnamic acid derivatives with thiol compounds, leading to 2-S rather than the usual 5-S conjugates. 相似文献
998.
The technique of Raman microscopy has been used to identify and characterise the pigments used in red shards of medieval and earlier items of pottery which have been found in various archaeological sites in the South of Italy. The research has led to the identification, on the basis of their characteristic Raman/resonance Raman spectra, of the red pigments as iron(III) oxide (e.g. Indian Red, Red Ochre or Venetian Red) and the yellow pigments as hydrated iron(III) oxyhydroxide (e.g. Yellow Ochre and Mars Yellow). X-ray powder diffraction experiments confirm the conclusions drawn above. 相似文献
999.
Iengo E Zangrando E Alessio E Chambron JC Heitz V Flamigni L Sauvage JP 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(23):5879-5887
The bis-porphyrin system ZnP(2), in which two zinc porphyrins are connected by a phenanthroline linker in an oblique fashion, acts as a bifunctional receptor towards the complexation of free-base meso-5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cis DPyP). In solution, NMR spectroscopy evidenced quantitative formation of the tris-porphyrin macrocyclic assembly ZnP(2)(4'-cis DPyP), in which the two fragments are held together by two axial 4'-N(pyridyl)-Zn interactions. The remarkable stability of the edifice (an association constant of about 6x10(8) M(-1) was determined by UV/Vis absorption and emission titration experiments in toluene) is due to the almost perfect geometrical match between the two interacting units. The macrocycle was crystallized and studied by X-ray diffraction, which confirmed the excellent complementarity of the two components. Photoinduced energy transfer from the singlet excited state of the zinc porphyrin chromophores to the free-base porphyrin occurs with an efficiency of 98 % (k(en)=2x10(10) s(-1) in toluene, ambient temperature) with a mechanism consistent with a dipole-dipole process with a low orientation factor. 相似文献
1000.
Misbahul Ain Khan Maria Lucia De Brito Morley 《Journal of heterocyclic chemistry》1978,15(8):1399-1401
The diazotization of 3-aminocoumarin followed by its reduction gave the cournarin-3-yl-hydrazine which, without isolation, reacted with various carbonyl compounds in a Fisher's indohzation reaction to give derivatives (Ib-Ih) of the yet unreported system 3H,4H[1]benzopyrano[3,4-b]pyrrol-4-one. 相似文献