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121.
π-electron energies and bond orders of benzenoid hydrocarbons with up to five fused hexagons have been considered by the simple
Bond Orbital Resonance Theory (BORT) approach. The corresponding ground states were determined according to four BORT models.
In the first three models a diagonalisation of the Hückel-type Hamiltonian was performed in the bases of Kekulé, of Kekulé
and mono-Claus and of Kekulé and Claus resonance structures, respectively. In the fourth model a simple BORT ansatz was used.
According to this ansatz, the ground state is a linear combination of the positive Kekulé structures, all with equal coefficients.
It was shown that π-electron energies and bond orders obtained by these models correlate much better with the PPP energies
and bond orders than with the Hückel energies and bond orders. This indicates that a simple BORT approach is quite reliable
in predicting the more sophisticated PPP results. Concerning the relative performance of the four BORT models, the best results
were obtained with the BORT ansatz. The performance deteriorates with the expansion of the basis set. This is attributed to
the fact that in these models the improvement of the basis set is not accompanied with the corresponding improvement of the
Hamiltonian. Comparing the BORT-ansatz bond orders with the Pauling bond orders, it was shown that BORT-ansatz bond orders
correlate much better with the PPP bond orders.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
122.
Twenty laboratories from 17 countries around the world participated in two analytical quality control exercises, coded NAT-5
and NAT-6, on determining trace and minor elements in plant bioindicator samples. 1398 laboratory mean values for 51 elements
were submitted for two lichen and two moss materials. The submitted measurement results were evaluated as interlaboratory
comparison (IC) exercises and as proficiency tests (PT) following standard procedures developed by the International Atomic
Energy Agency (IAEA). The evaluations confirmed good performance of the participating laboratories for many elements. Furthermore,
the laboratories performance based on IC criteria improved from the first exercise to the second one. Subsequent NAT-5 evaluation
revealed systematic differences between the analytical values obtained non-destructively or after the total sample dissolution
and the measurements following nitric acid sample dissolution (without the use of hydrofluoric acid) for some elements. The
most critical elements for this kind of discrepancies appeared to be Al, Ca, Cr, Fe, Na, Ni, and Pb. After changing analytical
methodologies in the concerned laboratories, more consistent values for those elements were obtained in NAT-6.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
123.
Syntheses and spectral characteristics of cadmium(II) compounds (CdSeO4, CdSeO3, and Cd(NCSe)2(nia)2) containing selenium in oxidation states (VI), (IV), and (-II) are described. In Cd(NCSe)2(nia)2, nicotinamide (nia) and selenocyanate anions are bonded to Cd atom as N-donor monodentate ligands. Nicotinamide is coordinated
through the ring nitrogen atom. The effects of these selenium compounds as well as Cd(NCS)2(nia)2 on the growth and Cd accumulation in roots and shoots of hydroponically cultivated chamomile plants (cultivar Lutea) were studied. In the applied concentration range (12–60 μmol dm−3) Cd(NCS)2(nia)2 affected neither the length nor the dry mass of roots and shoots. Other compounds applied at 24 μmol dm−3 and 60 μmol dm−3 significantly reduced dry mass of roots and shoots. Selenium oxidation state in the cadmium compounds affected Cd accumulation
in plant organs as well as Cd translocation within the plants, which was reflected in the values of bioaccumulation (BAF)
and translocation factors (S/R). Cd amount accumulated by shoots was lower than that in the roots. The highest BAF values
determined for Cd accumulation in shoots were obtained with CdSeO4. Substitution of S with Se in the Cd(NCX)2(nia)2 (X = Se or S) caused an increase of Cd translocation into the shoots.
Presented at the XVIIIth Slovak Spectroscopic Conference, Spišská Nová Ves, 15–18 October 2006. 相似文献
124.
Matej Babjak 《Tetrahedron》2005,61(9):2471-2479
A total synthesis of (+)-goniothalesdiol, a 3,4-dihydroxy-2,5-disubstituted tetrahydrofuran isolated from Goniothalamus borneensis (Annonaceae), and its 7-epimer is reported using oxycarbonylation methodology for construction of polyhydroxylated substituted heterocycles. Diastereoselectivity of addition of organometallic reagents to 2,3-O-isopropylidene-d-threose derivatives using theoretical calculations based on the semiempirical PM5 was studied. 相似文献
125.
František Kvasnička Jaroslav Dobiáš Kamila Klaudisová-Chudáčková 《Central European Journal of Chemistry》2003,1(1):91-97
A simple, rapid and reproducible capillary isotachophoretic on-line coupled with capillary zone electrophoresis (CITP-CZE)
method for the determination of IMz in food packaging extracts and its residues in apples is described. A good separation
of the IMZ from other sample constituents was achieved within 15 minutes without any sample clean up. Method characteristics
(linearity, accuracy, intra-assay and detection limit) were determined. Less amount of time involved, sufficient sensitivity
and low running cost are the important attributes of CITP-CZE method. 相似文献
126.
Nataša Pejić Slobodan Anić Vesna Kuntić Vladana Vukojević Ljiljana Kolar-Anić 《Mikrochimica acta》2003,143(4):261-267
A kinetic method is described for the microquantitative (microconcentration/microvolume) determination of rutin based on potentiometric monitoring of the concentration perturbations of the Bray-Liebhafsky (BL) oscillatory reaction being in a non-equilibrium stationary state close to a bifurcation point. The experiments are carried out in an open reactor. The response of the matrix system to perturbations by different concentrations of rutin ethanolic solutions is followed by a Pt-electrode. In the concentration range between 7.8×10–8moldm–3 and 9.1×10–6mol dm–3, we found a linear dependence of the maximal potential shift, Em, on the logarithm of the rutin concentrations. The unknown concentrations can be determined from the calibration curve up to an accuracy of ±5%. The detection limit is 3.6×10–8mol dm–3. The amount of required sample can be as small as 10µL. 相似文献
127.
P. Rajec F. Macášek M. Féder P. Misaelides E. Šamajová 《Journal of Radioanalytical and Nuclear Chemistry》1998,229(1-2):49-55
The sorption of caesium and strontium from its aqueous solutions by ten clinoptilolite-and mordenite-containing sedimentary
materials from Slovakia, Bulgaria, Ukraine and Greece was investigated by batch-type procedure and radiotracer techniques.
The concentration of the solutions, that were performed in and without the presence of competing cations (0.005 mol·dm−3 KCl) varied between 1·10−4 and 5·10−2 mol·dm−3. The uptake and distribution coefficient (K
d
) values determined for the materials of the different origin, were correlated with their mineralogical composition and gross
cation exchange capacity (CEC). The identification of the specific uptake sites was attempted on the basis of the sorption
isotherms and the content of exchangeable cations. The experimental results provide information on the suitability of the
individual materials for the treatment of radioactive wastes and their application as backfills in potential nuclear waste
repositories. 相似文献
128.
The kinetics of the dehydration of CsNd(SO4)2 · 4 H2O to CsNd(SO4)2 · H2O and then to CsNd(SO4)2 are studied by isothermal weight change. The reactions are phase-boundary-controlled. Reaction mechanism and activation energy depend on sample weight. 相似文献
129.
The construction of the Z-counting polynomial for edge-weighted graphs is discussed.Dedicated to Professor Haruo Hosoya (Tokyo) on the occasion of his 55th birthday for enriching chemical graph theory with the elegant concept of the Z-counting polynomial. 相似文献
130.
David Bevk Uroš Grošelj Anton Meden Jurij Svete Branko Stanovnik 《Helvetica chimica acta》2007,90(9):1737-1744
A simple and efficient synthesis of four new substituted pyrimidines, compounds 9a – d , from the title compound 3 is described. Conversion of 3 to methyl (E)‐3‐(dimethylamino)‐2‐(6‐methoxy‐2‐phenylpyrimidin‐4‐yl)prop‐2‐enoate ( 4 ), followed by condensation with various dinucleophiles according to the ‘enaminone methodology’, afforded the target compounds 9 in medium‐to‐good yields. 相似文献