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961.
Giorgi T Grepioni F Manet I Mariani P Masiero S Mezzina E Pieraccini S Saturni L Spada GP Gottarelli G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(9):2143-2152
Lipophilic guanosine derivatives are self-assembled into ribbonlike aggregates, both in the crystal state and in solution. The structure of the ribbons has been characterised by single-crystal X-ray diffraction and, in solution, by NMR spectroscopy and ESI-MS. Two different ribbons with different patterns of hydrogen bonds are present in the solid state and in chloroform solutions. The gel-like phases obtained in hexadecane, toluene and chloroform have been investigated by optical microscopy and small-angle X-ray diffraction: the type of phase observed is related to the molecular structure of the compounds and depends dramatically on the solvent. The structures of the phases are discussed, with the presence of the two different ribbons being taken into account. 相似文献
962.
Roberto Grisorio Cosimo Francesco Nobile Gian Paolo Suranna E.W Meijer 《Tetrahedron letters》2004,45(28):5367-5370
Ni(COD)2 promoted coupling of bromofluorenes functionalised with boronic esters or trimethylsilyl groups proves to be an efficient method for the preparation of reactive bifluorenes, which are key intermediates for the synthesis of bis-substituted oligofluorenes. The synthetic method has been exploited as a key step for the synthesis of a chiral 2,7?-diiodo-quaterfluorene and a 2,7?-bis-amino quaterfluorene. 相似文献
963.
The use of Nafion-coated thin mercury film electrodes (NCTMFE) for determining trace amounts of Eu3+ and Yb3+ ions was examined. Ion-exchange preconcentration of submicromolar levels of the two rare earths was achieved efficiently at a rotating NCTMFE, while the use of “classical” Nafion-coated glassy carbon electrodes is restricted to the determination of Eu3+ alone. Differential-pulse voltammetry was used to quantify the accumulated ions. The influence of variables such as rotation rate, preconcentration time and scan parameters was assessed. A preconcentration -voltammetry-regeneration scheme suitable for multiple analysis with the same modified electrode was developed. Calibration graphs with a linearity range extending up to 2 μM and detection limits of 0.03 and 0.08 μM for Eu3+ and Yb3+, respectively, were obtained. For Yb3+, the detection limit can be lowered to 0.02 μM by electrocatalytic amplification of the signals achieved by operating in the presence of ammonium nitrate as supporting electrolyte. However, in this instance a more restricted linearity range is observed. The effect of competing incorporation caused by the presence in solution of an excess of La3+ is also discussed, together with the competition between Eu3+ and Yb3+ when one of the two is present in large excess over the other. 相似文献
964.
Stefano Evangelisti Gian Luigi Bendazzoli Laura Gagliardi 《International journal of quantum chemistry》1995,55(3):277-280
We performed CAS –CI calculations on Li2 using a set of molecular orbitals (MO ) optimized with a procedure that, in the case of highly symmetric molecules, permits extraction of a small set of MO out of a large set of atomic orbitals (AO ). The dimension of the CAS –CI space was of about 12 million symmetry-adapted determinants. We determined some spectroscopic constants of Li2 with three different atomic basis sets of increasing quality. The values obtained with the largest atomic basis set are very close to the experimental results. © 1995 John Wiley & Sons, Inc. 相似文献
965.
A. De Giacomo M. Dell'Aglio O. De Pascale M. Capitelli 《Spectrochimica Acta Part B: Atomic Spectroscopy》2007
The interaction between ns-laser pulse at 532 nm and water, or heavy water (deuterium dioxide), has been studied by Stimulated Raman Scattering (SRS) and optical emission spectroscopy. Both the photolysis and breakdown processes have been considered. When the photolysis is the main process, structural change in water occurs as a consequence of electron and proton hydration. The rearrangement of the water structure and the subsequent photon absorption by free electrons raising the breakdown threshold occur. Moreover, charge separation in bulk water, under laser induced electromagnetic field, leads to a notable enhancement of the SRS signal. On the other hand, for a high laser pulse energy density, electrons gain energy enough to escape from the hydrating water structure, generating electron impact dominated plasma. 相似文献
966.
Riva S Mendozza M Carrea G Chattopadhyay P Tramontano A 《Applied biochemistry and biotechnology》1998,75(1):33-44
A monoclonal antibody (MAb) was produced against thep-nitrophenylphosphate derivative of 3α,5β-lithocholic acid, a transition-state analog for hydrolysis of a steroidalp-nitrophenylcarbonate. The indicated reaction was catalyzed by this Ab with kinetic constants kcat = 4.0 × 10-2min and Km = 3.3 μM at pH 9.0 and 35°C. The Ab also hydrolyzed the isomericp-nitrophenylcarbonate of 3β,5β-lithocholic acid with kcat = 8.4 × 10-2/min and Km = 1.0 μM. Bovine serum albumin (BSA) was found to catalyze the same reactions with similar turnover rates and Michaelis constants
of 15 and 14 μM, respectively. Although the BSA-catalyzed reaction was only weakly inhibited by the phosphate ester TSA (IC50 ca. 40 μM), the Ab-catalyzed reaction was completely inhibited at less than 1 μM of the TSA. The relative rates and efficiencies
of the MAbcatalyzed and BSA-catalyzed reactions are discussed in the context of the hydrophobic sites and intrinsic reactivity
of the protein surfaces, and the induction of groups on the Ab to enhance the enzymatic function. 相似文献
967.
Sedona F Rizzi GA Agnoli S Llabrés i Xamena FX Papageorgiou A Ostermann D Sambi M Finetti P Schierbaum K Granozzi G 《The journal of physical chemistry. B》2005,109(51):24411-24426
Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate. 相似文献
968.
Casavecchia P Capozza G Segoloni E Leonori F Balucani N Volpi GG 《The journal of physical chemistry. A》2005,109(16):3527-3530
The crossed molecular beam scattering technique with soft electron ionization (EI) is used to disentangle the complex dynamics of the polyatomic O(3P) + C2H4 reaction, which is of great relevance in combustion and atmospheric chemistry. Exploiting the newly developed capability of attaining universal product detection by using soft EI, at a collision energy of 54.0 kJ mol(-1), five different primary products have been identified, which correspond to the five exoergic competing channels leading to CH2CHO(vinoxy) + H, CH3CO(acetyl) + H, CH3(methyl) + HCO(formyl), CH2(methylene) + HCHO(formaldehyde), and CH2CO(ketene) + H2. From laboratory product angular and velocity distributions, center-of-mass product angular and translational energy distributions and the relative branching ratios for each channel have been obtained, affording an unprecedented characterization of this important reaction. 相似文献
969.
Given the four-velocityV describing the motion of a continuous medium, the covariant derivativeg(V) may be uniquely decomposed in terms of the vorticity tensor , the expansion tensorK, and the four-accelerationC. In the present paper we examine the integrability conditions on the fields ,K, andC ensuring the existence of a four-velocity fieldV. 相似文献
970.