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81.
O. I. Arillo-Flores M. F. Ruiz-López M. I. Bernal-Uruchurtu 《Theoretical chemistry accounts》2007,118(2):425-435
We discuss the failure of commonly used AM1 and PM3 semiempirical methods to correctly describe acid dissociation. We focus
our analysis on HCl because of its physicochemical importance and its relevance in atmospheric chemistry. The structure of
non-dissociated and dissociated HCl – (H2O)
n
clusters is accounted for. The very bad results obtained with PM3 (and also with AM1) are related to large errors in gas-phase
proton affinity of water and gas-phase acidity of HCl. Indeed, estimation of pKa values shows that neither AM1 nor PM3 are able to predict HCl dissociation in liquid water since HCl is found to be a weaker
acid than H3O+. We have proposed in previous works a modified PM3 approach (PM3-MAIS) adapted to intermolecular calculations. It is derived
from PM3 by reparameterization of the core–core functions using ab initio data. Since parameters for H–Cl and O–Cl core–core
interactions were not yet available, we have carried out the corresponding optimization. Application of the PM3-MAIS method
to HCl dissociation in HCl–(H2O)
n
clusters leads to a huge improvement over PM3 results. Though the method predicts a slightly overestimated HCl acidity in
water environment, the overall agreement with ab initio calculations is very satisfying and justifies efforts to develop new
semiempirical methods. 相似文献
82.
Solid-phase spectrophotometric microdetermination of iron with ascorbic acid and ferrozine 总被引:1,自引:0,他引:1
M. L. Fernández-de Córdova A. Ruíz-Medina A. Molina-Díaz 《Analytical and bioanalytical chemistry》1997,357(1):44-49
A very sensitive and selective method for the determination of trace amounts of iron has been developed, based on the reduction
of Fe(III) to Fe(II) by ascorbic acid, followed by chromogenic chelation of Fe(II) with ferrozine. The complex Fe(II)-ferrozine
is easily sorbed on a dextran-type anion-exchange gel packed in a 1 mm cell, and the absorbance of the gel is measured directly
at 569 and 800 nm. The extended linear range of the determination is 0.5–10 ng ml-1 of iron (apparent molar absorptivity=4.4×107 l mol-1 cm-1) and the precision (RSD) 1.3% for a concentration of 5 ng ml-1 of iron (n=10). The detection limit for a sample volume of 1000 ml, using 0.040 g of anion-exchanger, corresponds to 0.12 ng ml-1. The method has been successfully applied to the determination of iron in natural and waste waters, wine, soil extract and
previously digested vegetal tissues, drugs and human hair.
Received: 20 November 1995/Revised: 23 January 1996/Accepted: 26 January 1996 相似文献
83.
C.B. Cosmovici E. DAnna A. DInnocenzo G. Leggieri A. Perrone R. Dirscherl 《Chemical physics letters》1977,47(2):241-244
Dissociation energy (Do = 4.29 ± 0.05 eV), threshold energy (E = 0.83 ± 0.05 eV) and forward scattering at a collision-al energy Ec = 1.1 cV have been found by crossing beam of Yb atoms with a supersonic seeded beam of He + O2 at collisional energies up to Ec = 1.4 eV. 相似文献
84.
The behaviour of iron in the Bayer process has been investigated by neutron activation analysis and Ge(Li) spectrometry, using
bauxite samples of various origin. The amount of iron impurities in alumina was found to be independent of the origin of the
bauxites. Alumina is contaminated by iron in the process of decomposition of aluminate liquors; regardless whether iron is
present in ‘dissolved’ form or as a floating impurity, it will pass into the alumina almost completely. 相似文献
85.
L. L. Stachó 《Annali di Matematica Pura ed Applicata》1981,128(1):207-225
Summary Banach M-lattices are studied from the view point whether all the biholomorphic automorphisms of their unit balls admit fixed points when continuously extended to the closure of the unit ball. A characterization of compact topologieal F-spaces is found in terms of the fixed points of the elements of AutB(C()) which enables to establish some particular properties also of the topological automorphisms of compact F-spaces. Finally it is shown that if the M- lattice E admits a predual then each member of Aut B(E) has fixed point if and only if E is isometrically isomorphic with some l-space. 相似文献
86.
In this paper, by capturing the atomic information and reflecting the behaviour governed by the nonlinear potential function,
an analytical molecular mechanics approach is proposed. A constitutive relation for single-walled carbon nanotubes (SWCNT’s)
is established to describe the nonlinear stress-strain curve of SWCNT’s and to predict both the elastic properties and breaking
strain of SWCNT’s during tensile deformation. An analysis based on the virtual internal bond (VIB) model proposed by P. Zhang
et al. is also presented for comparison. The results indicate that the proposed molecular mechanics approach is indeed an
acceptable analytical method for analyzing the mechanical behavior of SWCNT’s.
The project supported by the National Natural Science Foundation of China (10121202, 90305015 and 10328203), the Key Grant
Project of Chinese Ministry of Education (0306) and the Research Grants Council of the Hong Kong Special Administrative Region,
China (HKU 7195/04E). 相似文献
87.
Xueyu Qiu Yadong Han Xiuli Zhuang Xuesi Chen Yuesheng Li Xiabin Jing 《Journal of nanoparticle research》2007,9(5):901-908
Nano-hydroxyapatite (HA)/poly(l-lactide) (PLLA) composite microspheres with relatively uniform size distribution were prepared by a solid-in-oil-in-water
(s/o/w) emusion solvent evaporation method. The encapsulation of the HA nanopaticles in microshperes was significantly improved
by grafting PLLA on the surface of the HA nanoparticles (p-HA) during emulsion process. This procedure gave a possibility
to obtain p-HA/PLLA composite microspheres with uniform morphology and the encapsulated p-HA nanoparticle loading reached
up to 40 wt% (33 wt% of pure HA) in the p-HA/PLLA composite microspheres. The microstructure of composite microspheres from
core-shell to single phase changed with the variation of p-HA to PLLA ratios. p-HA/PLLA composite microspheres with the diameter
range of 2–3 μm were obtained. The entrapment efficiency of p-HA in microspheres could high up to 90 wt% and that of HA was
only 13 wt%. Surface and bulk characterizations of the composite microspheres were performed by measurements such as wide
angle X-ray diffraction (WAXD), thermal gravimetric analysis (TGA), environmental scanning electron microscope (ESEM) and
transmission electron microscopy (TEM). 相似文献
88.
Youness Ouahid María C. Zaccaro Gloria Zulpa Mónica Storni Ana M. Stella Juan C. Bossio 《International journal of environmental analytical chemistry》2013,93(6):525-536
Microcystis is one of the most common bloom-forming cyanobacteria genera in diverse ecosystems. More than 80% of its strains are toxic, mainly due to their ability to produce metabolites known as microcystins (MC). Here we report on a M. aeruginosa bloom that appeared in the summer of 2001 at a site of the Río de la Plata, within the city of Buenos Aires. The symptoms in mice indicated that the bloom was hepatotoxic. LC-PDA analysis revealed a similar high concentration (0.9–1?mg?g–1?d w) of just one MC in the bloom biomass during the 3-month study period. During this period most of the MC (ca. 98 %) was found intracellularly, and the content remained almost the same. The molecular mass of the bloom MC was 1036?Da, as deduced from LC-ESI-MS data. Fragment ion analysis by LC-IT-MS-MS allowed identifying 6 out of the 7 amino acids, as well as their position in the molecule. The molecular mass of the unidentified amino acid residue was 155?Da. According to the data obtained, the MC under study was MC-XR, X representing the unidentified amino acid. This is the first report both on the characterisation of MCs in an urban site of the Río de la Plata waters, and on an Argentinean bloom exhibiting only one MC variant. 相似文献
89.
Mario Gómez José Quincoces Klaus Peseke Manfred Michalik Helmut Reinke 《Journal of carbohydrate chemistry》2013,32(7):851-865
ABSTRACT 1,6-Anhydro-2-(dicyanomethylene)-2,3-dideoxy-4-S-ethyl-4-thio-β-D-erythro-hexopyranose (1) reacted with tosyl azide or sulfur and triethylamine to furnish the 5-aza-10, 11-dioxatricyclo[6.2.1.02,6]undeca-2(6),3-diene-3-carbonitrile 2 and the 10,11-dioxa-5-thiatricyclo[6.2.1.02,6]undeca-2(6),3-diene-3-carbonitrile 3, respectively. The reactions of 1 with arylisothiocyanates furnished the 11,12-dioxa-5-thiatricyclo[7.2.1.02,7]dodeca-2(7),3-diene-3-carbonitriles 4 and 5. 3 underwent cyclization with triethyl orthoformate and ammonia or hydrazine hydrate to afford the 5,7-diaza-14,15-dioxa-9-thiatetracyclo[10.2.1.02,10.03.8]pentadecatetra(tri)enes 7 and 8, respectively. 相似文献
90.