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81.
Eckhoff DA Sutin JD Clegg RM Gratton E Rogozhina EV Braun PV 《The journal of physical chemistry. B》2005,109(42):19786-19797
Studying the properties and stability of silicon nanoparticles (Si-np) in aqueous environments may lead to novel applications in biological systems. In this work, we use absorption and photoluminescence (PL) spectroscopy to characterize ultrasmall Si-np prepared through anodic etching and ultrasonic fractionation of a crystalline Si wafer. Their behavior is studied over time in 2-propanol and during treatments with water, NaOH, HCl, and H(2)O(2). The observed population is divided into two types of material: bright species consisting of well-etched Si-np, approximately 1 nm in diameter, and dark species derived from partially etched or aggregated Si structures. The dark material is seen by its scattering in the 2-propanol and water solutions and is largely removed via precipitation with the NaOH or HCl treatment. The bright material includes three distinct species with their respective emissions in the UV-B, UV-A, and hard-blue regions of the spectrum. The hard-blue PL is shown to have a simple pH dependence with a pK(a) approximately 3, providing an important insight into its chemical origin and signaling for possible application of Si-np as environmental probes. Our results offer some potential for tailoring the PL properties of ultrasmall Si-np through control of their surface chemistry. 相似文献
82.
Janulewicz KA Schnürer M Tümmler J Priebe G Risse E Nickles PV Greenberg B Levin M Pukhov A Mandelbaum P Zigler A 《Optics letters》2005,30(12):1572-1574
Investigations of plasma produced by a boron nitride capillary discharge irradiated with a guided 20-TW Ti: sapphire laser pulse at a peak intensity of 4 x 10(18) W/cm2 are presented. The guided laser radiation in the plasma channel generated He-like ions that, subject to suitable plasma temperature, recombined into Li-like nitrogen ions. Intense radiation at a wavelength of 24.77 nm was observed, indicating possible lasing at the 3d(5/2) - 2p(3/2) transition in Li-like nitrogen. 相似文献
83.
Edoardo Ballico Claudio Fontanari Cristiano Bocci Enrico Carlini 《Rendiconti del Circolo Matematico di Palermo》2004,53(3):429-436
We generalize the classical Terracini’s Lemma to higher order osculating spaces to secant varieties. As an application, we
address with the so-called Horace method the case of thed-Veronese embedding of the projective 3-space.
This research is part of the T.A.S.C.A. project of I.N.d.A.M., supported by P.A.T. (Trento) and M.I.U.R. (Italy). 相似文献
84.
Boiocchi M Del Boca L Gómez DE Fabbrizzi L Licchelli M Monzani E 《Journal of the American Chemical Society》2004,126(50):16507-16514
1,3-bis(4-nitrophenyl)urea (1) interacts through hydrogen bonding with a variety of oxoanions in an MeCN solution to give bright yellow 1:1 complexes, whose stability decreases with the decreasing basicity of the anion (CH3COO- > C6H5COO- > H2PO4- > NO2- > HSO4- > NO3-). The [Bu4N][1.CH3COO] complex salt has been isolated as a crystalline solid and its molecular structure determined, showing the formation of a discrete adduct held together by two N-H...O hydrogen bonds of moderate strength. On the other hand, the F- ion first establishes a hydrogen-bonding interaction with 1 to give the most stable 1:1 complex, and then on addition of a second equivalent, induces urea deprotonation, due to the formation of HF2-. The orange-red deprotonated urea solution uptakes carbon dioxide from air to give the tetrabutylammonium salt of the hydrogencarbonate H-bond complex, [Bu4N][1.HCO3], whose crystal and molecular structures have been determined. 相似文献
85.
Mimmi MC Gullotti M Santagostini L Battaini G Monzani E Pagliarin R Zoppellaro G Casella L 《Dalton transactions (Cambridge, England : 2003)》2004,(14):2192-2201
The dinuclear and trinuclear Cu(II) complexes of an octadentate ligand derived from (S)-1,1'-binaphthyl-2,2'-diamine have been prepared and characterized by UV/Vis, CD, EPR and NMR spectroscopy. The ligand contains two tridentate aminobis(benzimidazole) donor arms connected to a central bidentate diaminobinaphthyl linker, which hosts the chiral unit. In the dinuclear Cu complex the ligation occurs essentially within the tridentate arms of the ligand. The two Cu centers are EPR nonequivalent and noninteracting. The EPR data suggests that one of the Cu ions additionally interacts with one of the tertiary aminonaphthyl donors. In the trinuclear complex the two aminonaphthyl donors bind the third Cu ion. The EPR spectrum of this complex shows the signal for a mononuclear Cu(II) center bound to a tridentate arm, while the remaining two Cu(II) centers are coupled through hydroxo groups. The CD spectrum shows that in the free ligand a severe reduction of the dihedral angle between the naphthyl groups from the strain free range occurs. This conformation is stabilized by ring stacking interactions with the benzimidazole groups. On complex formation this interaction is removed because the benzimidazole groups are involved in metal binding. In the dinuclear Cu complex the conformation of the binaphthyl chromophore probably approaches the strain free range, while in the trinuclear Cu complex a marked flattening of the dihedral angle between the two naphthyl rings occurs. Both complexes are active catalysts in the oxidation of L-/D-Dopa derivatives to quinones. High enantioselectivity is observed in the oxidation of L-/D-Dopa methyl ester catalyzed by the dinuclear Cu complex, which exhibits strong preference for the d enantiomer. The enantioselectivity is largely lost for the trinuclear Cu complex. 相似文献
86.
[structure: see text] The first examples of epoxy-p-quinol and diepoxy-p-quinol calixarene derivatives have been obtained by base-promoted direct addition of O(2)(oxygenation) to calixarene phenol rings. The regio- and stereochemistry of these derivatives was determined by 2D NMR studies, in conjunction with MM3 calculations, and X-ray crystallography. Both the oxygenation and the subsequent carbonyl reduction occur with a preferential attack to the less hindered exo face of the calixarene rings. 相似文献
87.
Enrico Leuzinger 《Proceedings of the American Mathematical Society》2004,132(3):919-927
Let be a noncompact semisimple Lie group and an arbitrary discrete, torsion-free subgroup of . Let be the bottom of the spectrum of the Laplace-Beltrami operator on the locally symmetric space , and let be the exponent of growth of . If has rank , then these quantities are related by a well-known formula due to Elstrodt, Patterson, Sullivan and Corlette. In this note we generalize that relation to the higher rank case by estimating from above and below by quadratic polynomials in . As an application we prove a rigiditiy property of lattices.
88.
89.
Bonardi M Birattari C Groppi F Gini L Ghioni A Mainardi HS Arginelli D Gallorini M Rizzio E Sabbioni E 《Annali di chimica》2003,93(4):363-372
In this paper are presented the production methods for very "high specific activity" radionuclides (HSA-RN) of vanadium, manganese and thallium which have been developed in our laboratories for labelling different chemical forms of these elements present in the echo-systems in ultra-trace amounts, for metallo-toxicological and bio-kinetic studies. Use was made of both cyclotron and thermal nuclear reactor. If the nuclear reaction product has atomic number different from irradiated target, it is possible separating the radioactive nuclide from irradiated target, without addition of isotopic carrier. This kind of radionuclide is named No Carrier Added, NCA, and his specific activity, As is very high and can reach values close to the theoretical Carrier Free one, CF. The experimental determination of specific activity, chemical and radiochemical purities is mandatory for all these kinds of applications. 相似文献
90.
Traversa Enrico Di Vona Maria Luisa Licoccia Silvia Sacerdoti Michele Carotta Maria Cristina Gallana Massimiliano Martinelli Giuliano 《Journal of Sol-Gel Science and Technology》2000,19(1-3):193-196
Pure, 5 at%, and 10 at% Ta- or Nb-doped TiO2 nanosized powders were prepared by the sol-gel method. The powders heated to 400°C have the crystalline anatase structure. While the pure TiO2 powder heated to 850°C has the rutile structure, the addition of Ta and Nb inhibited the anatase-to-rutile phase transformation at this temperature. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired at 650°C and 850°C for 1 h. SEM observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO2 grain size at the nanometric level even at 850°C. Conductance measurements showed that a good gas response is observed only for the nanostructured titania-based films. The CO response of these materials is only slightly affected by humidity. 相似文献