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971.
Topological indices are graph invariants used in computational chemistry to encode molecules. A frequent problem when performing structure-activity studies is that topological indices are inter-correlated. We consider a simple topological index and show asymptotic independence for a random tree model. This continues previous work on the correlation among topological indices. These findings suggest that a size-dependence in a certain class of distance-based topological indices can be eliminated.AMS subject classification: 05C80, 60E10, 92E10 相似文献
972.
Wei Wang Rongliang Wang Fan Wu Boshun Wan 《Reaction Kinetics and Catalysis Letters》2005,85(2):277-282
Summary A Ni-promoted ligand free palladium catalyst system for Suzuki coupling of aryl bromides has been developed in high efficiency under mild reaction conditions. It was obtained in situ by introducing NiCl2 to PdCl2/PVP using a parallel high-throughput screening technique. A wide range of aryl bromides bearing a variety of functional groups was evaluated. 相似文献
973.
Peifeng Su Fuming Ying Wei Wu Prof. Philippe C. Hiberty Prof. Sason Shaik Prof. 《Chemphyschem》2007,8(18):2603-2614
The recently developed (L. Song, W. Wu, Q. Zhang, S. Shaik, J. Phys. Chem. A 2004 , 108, 6017–6024) valence bond method coupled to a polarized continuum model (VBPCM) is applied to the Menshutkin reaction, NH3+CH3Cl→CH3NH3++Cl?, in the gas phase and in aqueous solution. The computed barriers and reaction energies at the level of the breathing orbital VB method (P. C. Hiberty, J. P. Flament, E. Noizet, Chem. Phys. Lett. 1992 , 189, 259), BOVB and VBPCM//BOVB, are comparable to CCSD(T) and CCSD(T)//PCM results and to experimental values in solution. The gas‐phase reaction is endothermic and leads to an ion‐pair complex via a late transition state. By contrast, the reaction in the aqueous phase is exothermic and leads to separate solvated ions as reaction products, via an early transition state. The VB calculations provide also the reactivity parameters needed to apply the valence bond state correlation diagram method, VBSCD (S. Shaik, A. Shurki, Angew. Chem. Int. Ed. 1999 , 38, 586). It is shown that the reactivity parameters along with their semiempirical derivations provide together a satisfactory qualitative and quantitative account of the barriers. 相似文献
974.
Solid Phase Extraction and Flame Atomic Absorption Spectrometric Determination of Trace Amounts of Zinc and Cobalt Ions in Water Samples 总被引:1,自引:0,他引:1
A simple and rapid method for the preconcentration of Co2+ and Zn2+ as their 2-guanidino-benzimidazole chelates is proposed using an octadecyl silica cartridge. The retained analytes were recovered with a mixture of 5mL of 0.01M HNO3 and 5mL of methanol. The metal ions in the effluent were determined by flame atomic absorption spectrometry. The effect of different parameters, such as sample matrix, amount of ligand, type of eluent for elution of ions from cartridge, flow rates of sample solution and eluent, breakthrough volume, and limit of detection, were evaluated. The effects of various cationic and anionic interferences on the percent recovery of Co2+ and Zn2+ were also studied. Quantitative extraction efficiencies were obtained by elution of the cartridge with a minimal amount of solvent. Hence, with a typical preconcentration factor of 50, the limits of detections of the proposed method are 0.26 and 1.62ngmL–1 for Zn2+ and Co2+, respectively. The method was applied to the determination of zinc and cobalt ions in different water samples. 相似文献
975.
Darja?Rudan-Tasic Cveto?KlofutarEmail author 《Monatshefte für Chemie / Chemical Monthly》2004,135(7):773-784
Summary. The freezing temperatures of dilute aqueous solutions of some poly(oxyethylene) glycols (PEG, HO–(CH2CH2O)
n
–H, n varying from 4 to 117) were measured over a solute to solvent mass ratio from 0.0100 to 0.3900. The second and third osmotic virial coefficient (A
22 and A
222) of poly(oxyethylene) glycols in aqueous solution were determined. The molecular weight dependence of the second virial coefficient can be described by a simple relation A
22=2×10–5
M
n
1.86, and the third virial coefficient is A
222=0.038A
22
2. The activity coefficients of the solute were calculated using the Gibbs-Duhem equation as applied by Bjerrum. From the osmotic and activity coefficients the excess Gibbs energies of solution, as well as the respective partial molar functions of solute and solvent and the virial pair interaction coefficients for the excess Gibbs energies were estimated. The second and the third osmotic virial coefficients are correlated with the Mc-Millan-Mayer virial coefficients. 相似文献
976.
Thirty-five chemical solutions, acids, bases, complexing, argentometric, reducing, oxidising, salts and eluent mixtures, were studied continuously over a 7.5-month period. Twenty-seven remained reasonably stable and had acceptable expiration dates, which were equal to or more than 2 months longer than the established average deviation of ±2% relative to the initial value. Exceptions were made for potassium permanganate (0.1 M), perchloric acid (0.1 M) and disodium hydrogen phosphate (10%) solutions, as well as for HPLC eluent mixtures, which showed stability with expiration dates equal to stated values or shorter by 1 month. The eight chemical solutions presenting expiration dating higher than 6 months were hydrochloric acid (0.1 and 1 M), sulphuric acid (0.5 and 1.5 M), ethylenediamine tetraacetic acid (EDTA) (0.1 M), silver nitrate (0.1 M), ammonium thiocyanate (0.1 M) and iodine (0.1 M). The present data is a suitable guideline for the date of several chemical solutions routinely used in the analytical laboratories. 相似文献
977.
Ayfer?Kalkan Zehra A.?BayirEmail author 《Monatshefte für Chemie / Chemical Monthly》2003,134(12):1555-1560
Summary. Unsymmetrical porphyrazines bearing a single peripheral bis(hydroxyethylthio) moiety were synthesised by mixed condensation of bis(2-hydroxyethylthio)maleonitrile and phthalonitrile. Complexation of the thioether groups of metal-free porphyrazine with PdCl2 further lowered the intensity of the Q-band absorption of the porphyrazine core. The new compounds were characterised by elemental analyses, IR, 1H NMR, UV-Vis, and mass spectra. 相似文献
978.
R. ZielińskaE. Mulik A. MichalskaS. Achmatowicz M. Maj-?urawska 《Analytica chimica acta》2002,451(2):243-249
All-solid-state ion-selective electrodes with plastic membrane (poly(vinyl chloride) (PVC), bis(2-ethylhexyl) sebacate (DOS), methyltri-n-tetradecylammonium chloride (MTTACl)), a conducting poly(pyrrole) (PPy) film doped either with chloride ions (PPyCl) or hexacyanoferrate(II) ions (PPyFeCN), and glassy carbon (GC) or screen-printed graphite layer (S-PG) as an inner electric contact were investigated. All the electrodes show close to Nernstian response, but their lifetimes vary. The at least 2-months lifetime of screen-printed electrodes is only achieved for the electrodes containing PPyFeCN (cation-exchanging film). Shorter lifetime of other screen-printed electrodes, i.e. without PPy, or with PPyCl (anion-exchanging film), was attributed to the diffusion of anionic products of the hydrolysis of organic components of the graphite paste used to prepare the electric contact. The properties of miniature, screen-printed electrodes comprising PPyFeCN solid contact, were comparable to those ion-selective electrodes with PPy solid contact (regardless the ion-exchanging characteristic of the polymer) deposited on GC electric contact. 相似文献
979.
Boron-doped diamond (BDD) electrodes were used to investigate the possibility of detecting aniline by linear-sweep cathodic stripping voltammetry. It was found that the dimeric species (p-aminodiphenylamine and benzidine) formed by anodic oxidation of aniline during the accumulation period are involved in electrochemically reversible redox processes and, in acidic media, the shape of the stripping voltammetric response is suitable for aniline detection in the micromolar concentration range. The low background current of conductive diamond is an advantage compared to other electrode materials and allows a detection limit of 1 μM. Weak adsorption properties and the extreme electrochemical stability are additional advantages of BDD and it was found that, even after long-time measurements, the electrode surface can regain its initial activity by an anodic polarization in the potential region of water decomposition. 相似文献
980.
Renata Toczy?owska 《Analytica chimica acta》2005,540(1):167-172
Back-side contact Au and Ag microelectrodes were used as transducers to construct planar all-solid-state electrodes suitable for flow-through analysis. The microsensors were based on plasticized PVC potassium-selective membranes containing ion-electron conducting polymer—polypyrrole doped with di(2-ethylhexyl) sulfosuccinate. The proposed technique allowed simple construction of microsensors in one step, by membrane solution casting directly on the surface of the planar metallic transducers. The performance of the microsensors based on Au and Ag transducers were determined and compared with planar sensors based on internal electrolyte immobilized in polyHEMA. The addition of the polypyrrole to the membrane composition did not influence on the selectivity, reproducibility and long-term stability of the microsensors but improved their standard potential stability in time in comparison with coated-wire type sensors. Moreover, all-solid-state microsensors based on Au transducers exhibited better signal stability than Ag based sensors. 相似文献