排序方式: 共有204条查询结果,搜索用时 15 毫秒
41.
Zino Boisdenghien Stijn Fias Fabiana Da Pieve Frank De Proft Paul Geerlings 《Molecular physics》2015,113(13-14):1890-1898
We investigate the local polarisability or polarisability density using both a conceptual density functional theory approach based on the linear response function and time-dependent density functional theory. Using a zero frequency in the latter, we can immediately compare both approaches. Using an analytical expression for the linear response kernel, we are able to systematically analyse α(r) throughout the periodic table. An extension to molecules is also made with a study of the CO molecule retrieving the connection between local softness and local polarisability. 相似文献
42.
Benoit Louis Dr. M. Maciel Pereira Prof. Fabiana M. Santos Pierre M. Esteves Prof. Jean Sommer Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(2):573-576
The heterogeneous acid‐catalyzed activation step of alkanes leading to the reaction intermediates (carbocationic or alkoxy species) was up to now the matter of a longstanding controversy. Gas chromatography and online mass spectroscopy measurements show that H2 and methane are formed over H‐zeolites, whereas HD and CH3D are formed over D‐zeolites as the primary products in the reaction with isobutane. These results indicate that σ‐bond protolysis by strong acid sites is the first step for hydrocarbon activation on these catalysts at mild temperatures (473 K), in analogy to the activation path occurring in liquid superacid media. 相似文献
43.
Tormena CF Yoshinaga F Doi TR Rittner R 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2006,63(3):511-517
Conformational preferences and orbital interactions of methyl chloroacetate (1), methyl bromoacetate (2) and methyl iodo-acetate (3) were analyzed using experimental infra-red data, theoretical calculations and NBO analyses. The conformational equilibria of compounds 1-3 can be represented by their cis and gauche rotamers. The gauche form of 1 is stable in the vapour phase and in a non-polar solvent, but the cis is predominant in a polar solvent. For 2 the gauche form is more stable than the cis, in both the vapour and liquid phases, but for compound 3 only the gauche form was observed both in vapour phase as in solution. These conformational preferences were attributed to the orbital interaction between two antibonding orbitals pi(C=O)(*)-->sigma(C-X)(*). This unexpected interaction was possibly due to the high (0.2) electron density on pi(C=O)(*), which results from the interaction between ether oxygen lone pair and pi(C=O)(*). 相似文献
44.
We obtain existence results for some strongly nonlinear Cauchy problems posed in
and having merely locally integrable data. The equations we deal with have as principal part a bounded, coercive and pseudomonotone
operator of Leray-Lions type acting on
, they contain absorbing zero order terms and possibly include first order terms with natural growth. For any p > 1 and under
optimal growth conditions on the zero order terms, we derive suitable local a-priori estimates and consequent global existence
results. 相似文献
45.
Danielle Goveia Fabiana Aparecida Lobo Peter Burba Leonardo Fernandes Fraceto Newton Luiz Dias Filho André Henrique Rosa 《Analytical and bioanalytical chemistry》2010,397(2):851-860
This paper deals with the development and optimization of an analytical procedure using ultrafiltration and a flow-injection system, and its application in in-situ experiments to characterize the lability and availability of metal species in humic-rich hydrocolloids. The on-line system consists of a tangential flow ultrafiltration device equipped with a 3-kDa filtration membrane. The concentration of free ions in the filtrate was determined by atomic-absorption spectrometry, assuming that metals not complexed by aquatic humic substances (AHS) were separated from the complexed species (M–AHS) retained by the membrane. For optimization, exchange experiments using Cu(II) solutions and AHS solutions doped with the metal ions Ni(II), Mn(II), Fe(III), Cd(II), and Zn(II) were carried out to characterize the stability of the metal–AHS complexes. The new procedure was then applied in-situ at a tributary of the Ribeira do Iguape river (Iguape, São Paulo State, Brazil) and evaluated using the ions Fe(III) and Mn(II), which are considered to be essential constituents of aquatic systems. From the exchange between metal–natural organic matter (M–NOM) and the Cu(II) ions it was concluded that Cu(II) concentrations >485 μg L?1 were necessary to obtain maximum exchange of the complexes Mn–NOM and Fe–NOM, corresponding to 100% Mn and 8% Fe. Moreover, the new analytical procedure is simple and opens up new perspectives for understanding the complexation, transport, stability, and lability of metal species in humic-rich aquatic environments. 相似文献
46.
47.
Isabeau?Birindelli Giulio?Galise Fabiana?LeoniEmail author Filomena?Pacella 《Calculus of Variations and Partial Differential Equations》2018,57(6):158
We study the asymptotic behavior of positive solutions of fully nonlinear elliptic equations in a ball, as the exponent of the power nonlinearity approaches a critical value. We show that solutions concentrate and blow up at the center of the ball, while a suitable associated energy remains invariant. 相似文献
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49.
Adina Arvinte Federica Valentini Antonio Radoi Fabiana Arduini E. Tamburri Lucian Rotariu Giuseppe Palleschi Camelia Bala 《Electroanalysis》2007,19(14):1455-1459
In this work, the capability of carbon nanofibers to be used for the design of catalytic electrochemical biosensors is demonstrated. The direct electrochemistry of NADH was studied at a glassy carbon electrode modified using carbon nanofibers. A decrease of the oxidation potential of NADH by more than 300 mV is observed in the case of the assembled carbon nanofiber‐glassy carbon electrode comparing with a bare glassy carbon electrode. The carbon nanofiber‐modified electrode exhibited a wide linear response range of 3×10?5 to 2.1×10?3 mol L?1 with a correlation coefficient of 0.997 for the detection of NADH, a high specific sensitivity of 3637.65 (μA/M cm2), a low detection of limit (LOD=3σ) of 11 μM, and a fast response time (3 s). These results have confirmed the fact that the carbon nanofibers represent a promising material to assemble electrochemical sensors and biosensors. 相似文献
50.
Oliva FY Avalle LB Cámara OR De Pauli CP 《Journal of colloid and interface science》2003,261(2):299-311
The adsorption of human serum albumin (HSA) onto colloidal TiO2 (P25 Degussa) particles was studied in NaCl electrolyte at different solution pH and ionic strength. The HSA-TiO2 interactions were studied using adsorption isotherms and the electrokinetic properties of HSA-covered TiO2 particles were monitored by electrophoretic mobility measurements. The adsorption behavior shows a remarkable dependence of the maximum coverage degree on pH and was almost independent of the ionic strength. Other characteristic features such as maximum adsorption values at the protein isoelectric point (IEP approximately 4.7) and low-affinity isotherms that showed surface saturation even under unfavorable electrostatic conditions (at pH values far away from the HSA IEP and TiO2 PZC) were observed. Structural and electrostatic effects can explain the diminution of HSA adsorption under these conditions, assuming that protein molecules behave as soft particles. Adsorption reactions are discussed, taking into account acid-base functional groups of the protein and the surface oxide in different pH ranges, considering various types of interactions. 相似文献