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81.
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Claudio Albanese 《Communications in Mathematical Physics》1996,178(3):527-540
Summary We estimate the accuracy of the adiabatic approximation in predicting the time evolution of local observables for an XY quantum magnet with a slowly variable external magnetic field. The system evolves according to the natural Hamiltonian dynamics and the spectral gap produced by the magnetic field is assumed to be large with respect to the term inducing quantum fluctutions. The proof is based on a finite order truncation of a time dependent cluster expansion in inverse powers of the time scale . In the analytic case, we show that the accuracy of this truncated expansion is of order
for any >1. If the time dependent perturbation is suddenly switched on at time zero and switched off at time , the accuracy of the adiabatic approximation is proven to be of orderO(
–1. 相似文献
84.
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87.
Giorgio Abbiati Alessandro Cirrincione de CarvalhoElisabetta Rossi 《Tetrahedron》2003,59(37):7397-7402
The cycloaddition reactions of 1-p-tolyl and 1-benzyl- 2,4-diphenyl-1,3-diazabuta-1,3-dienes with a variety of aryl and alkyl isocyanate and isothiocyanate are described. The reaction mechanism is also discussed. 相似文献
88.
A time- and cost-saving method for the congener-specific analysis of polychlorinated biphenyls (PCBs) in human serum has been developed and validated. After two fast extraction and clean-up steps, analyses were performed using gas chromatography coupled with mass spectrometry with single ion monitoring (GC/SIM-MS), either in electron impact (EI) or electron-capture negative ionization (ECNI) mode. For the determination of dioxin-like congeners, an improvement in EI-MS sensitivity is desirable and use of NI is thus preferred. The procedure was validated for 12 dioxin-like congeners by analyzing spiked samples on three different days and using (13)C(12)-labelled analogues as internal standards. When using an NCI source, the limit of quantification was assessed at 0.01 microg/L, except for PCBs #77 and #81, which cannot be reliably detected below 0.05 microg/L. For the lower chlorinated non-dioxin-like congeners, NI offers less selectivity because of limited fragmentation. Electron impact ionization and electron-capture negative ionization mode can therefore be considered to be complementary for the determination of PCB congeners in the general population. 相似文献
89.
This communication reports the first intercalation into VOPO4·2H2O performed through a solid-state reaction at room temperature. Ethyleneurea, EU (2-imidazolidone) was intercalated into the oxovanadium phosphate by grinding in a mortar stoichiometric amounts of both reagents at room temperature for 30 min. C, H and N elemental analyses of the obtained intercalation compound gave the formula VOPO4·(EU)·H2O. The profile of the absorption bands due to the VOPO4 lattice vibration around 1000 cm−1 was retained after reaction, providing evidence that an intercalation compound and not merely a salt of V-P-O species was produced. For α-VOPO4·2H2O, the interlayer distance is 0.75 nm, whereas for the EU inter-calation compound it is 0.95 nm. The obtained SEM micrographs show that the microstructure of VOPO4·2H2O is changed after reaction, with a total exfoliation of the oxovanadium matrix. 相似文献
90.
Claudio Tavagnacco Gabriele Balducci Giacomo Costa Karl Tschler Wolfgang von Philipsborn 《Helvetica chimica acta》1990,73(5):1469-1480
In four series of strictly related organocobalt complexes, derived from cobaloximes by replacement of the O…H…O with O…-BF2…O and/or (CH2)3 groups, the trends of 59Co-NMR shielding and electrochemical data are discussed. A largely parallel behaviour of the plots of E1/2(I) values for the first Co(III)/Co(II) electron transfer vs. the 59Co chemical shifts reflects the similar sensitivity of the two parameters to a change in electron affinity of the central metal ion due to a variation of the organic group R. E1/2(II) values for the second Co(II)/CO(I) electron transfer are less sensitive to the change of R, but the trend of the plot vs. δ(59Co) is still parallel in the four series. Consistent deviations from a roughly linear dependence of E1/2(I) on pKa of the hydrocarbon acid corresponding to R, on Taft constant s?* and on 59Co shielding are noticed for the isopropyl derivatives and attributed to a steric effect. This was confirmed in a series of R? Co(DMG)pyridine complexes in which 59Co shielding decreases steadily with increasing steric parameter Es (Taft) of the alkyl group. There is experimental evidence from X-ray data that δ(59Co) decreases with an increase of the Co? C bond length, illustrating steric hindrance in alkyl coordination to be responsible for the decreased shielding of the 59Co nucleus. The relative displacements of the graphic displays for the different series reflect the effect of changes in electron affinity of the redox center, due to the equatorial ligand, which, in turn, is caused by variations in the electron-withdrawing power due to the introduction of the BF2 group and by the change from ?2 to ?1 valence of the (CH2)3-capped ligands. 相似文献