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91.
The cationization of poly(ethylene glycol)s, PEG 4000 and PEG 6000, under matrix-assisted laser desorption/ionization conditions was studied by using different concentration ratios of the sodium ion, as the reference ion, and another alkali metal ion (Li(+), K(+), Rb(+), Cs(+)). A linear correlation was found between the intensity ratio of the sodiated PEGs and PEGs cationized with alkali metal ions versus the initial concentration ratio of sodium and alkali metal ions. The slopes of these straight lines are proposed as a novel selectivity ratio for the ionization process. The intensity distribution of the cationized PEGs was also investigated. It was found that the cationized oligomers follow Poisson statistics. The M(n) and M(w) values were also evaluated. An explanation for the observed effects is given. 相似文献
92.
Kaupp M Patrakov A Reviakine R Malkina OL 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(9):2773-2782
The characteristic dependence of J(31P,31P) spin-spin coupling constants of alkali metal tetraphosphane-1,4-diides on structure and composition has been analyzed by density functional methods. The computations confirm that the structure of the contact ion pairs is conserved in solution. Calculations on model systems M2P4H4, on naked P4H4(2-) anions, and on models including point charges, show that the role of the cations is mainly structural and to a smaller extent electrostatic. Three of the four J(P,P) coupling constants depend characteristically on the conformation of the anion, which in turn is determined by the substituents R and by cation-anion interactions. Several couplings exhibit a large through-space component and are thus strongly dependent on the relative orientation of nonbonding electron pairs on the phosphorus atoms involved. This is shown by visualization of coupling pathways using the recently introduced coupling energy density (CED), in combination with the electron localization function (ELF). 相似文献
93.
Katalin Mészáros Szécsényi I. Esztelecki G. Pokol 《Journal of Thermal Analysis and Calorimetry》2007,89(3):829-833
In order to increase the nutrition value of bread, one of the most commonly used foodstuff all over the world, different additives
are used in bread processing. In this paper we describe the thermal changes in bread and that of with 0.5% crude soybean lecithin
additive. Their thermal stability has been investigated by TG, DSC and EGD methods. The thermal changes were also followed
of soy products, lecithin and lysine, ingredients used as bread additives in order to check if they may suffer any thermal
degradation during the baking process. The data obtained can be of use only for qualitative conclusions. According to the
obtained data at the usual bread baking temperature only the additives in crust may partly decompose while in the crumb, at
lower temperatures the additives, due to baking, are not damaged. The thermal methods give a possibility for rapid estimation
of processes induced by heat effects in additives during the baking, and they are suitable to detect the changes during the
bread-making procedure. However, they are neither suitable to provide any quantitative data on these changes nor facts affecting
the nutrition value and of the bread. 相似文献
94.
Kolárová L Holcapek M Jambor R Dostál L Nádvorník M Růzicka A 《Journal of mass spectrometry : JMS》2004,39(6):621-629
Electrospray ionization (ESI) mass spectrometry was applied to the structural analysis of 23 2,6-[bis(alkyloxy)methyl]phenyltin(IV) derivatives. The mass spectra were measured in both polarity modes and multistage tandem mass spectrometric (MS(n)) measurements were performed on the ion trap analyser for positively charged tin-containing ions. The sum of complementary ions observed in the positive-ion mode (i.e. [M-R(3)](+) ion) and in the negative-ion mode (i.e. [R(3)](-) ion) permits molecular mass determination in spite of the fact that the molecular adducts were often missing even in the first-order mass spectra. The subsequent fragmentation of [M-R(3)](+) ions studied by MS(n) and the correlation of observed fragment ions with the expected structures of synthesized organotin(IV) compounds allowed us to understand the fragmentation behaviour and the mechanism of the ion formation for studied compounds. The typical neutral losses are alkenes, alcohols and aldehydes. The fragmentation pattern of one selected compound was supported by MS(n) measurements of an isotopically labelled analogue to confirm unusual ion-molecule reactions of some fragment ions with water in the ion trap. 相似文献
95.
Tat'yana T Kucherenko Roman Gutsul Vladimir M Kisel Vladimir A Kovtunenko 《Tetrahedron》2004,60(1):211-217
Previously unknown 3-arylamino-1,2-dihydro-1-isoquinolones were obtained by condensation of 2-cyanomethylbenzoic acid with arylamines. Isonicotinoylation of the compounds was shown to proceed at the carbon atom in the 4-position to give 3-arylamino-4-isonicotinoyl-1,2-dihydro-1-isoquinolones which were quaternized with alkylating agents and formed the corresponding pyridinium salts. Deprotonation of the latter induced intramolecular conjugated addition with the pyrrole ring closure and formation of spiro compounds. The structure of the products was confirmed by NMR, IR and UV spectroscopy and by synthesis of the model compound, 3-(4-tolyl)-2,3,4,5-tetrahydro-1H-pyrrolo[2,3-c]isoquinoline-1,5-dione. 相似文献
96.
Kuśtrowski P Chmielarz L Dziembaj R Cool P Vansant EF 《The journal of physical chemistry. A》2005,109(2):330-336
The novel mesoporous templated silicas (MCM-48, SBA-15, MCF, and MSU) were used as supports for transition metal (Cu, Cr, or Fe) oxides. The catalysts were synthesized using the incipient wetness impregnation, and characterized by low-temperature N2 sorption, DRIFT, photoacoustic IR spectroscopy, UV-vis diffuse reflectance spectroscopy, and temperature-programmed desorption of ammonia. It was shown that the preparation method used results in different distributions and dimensions of the transition metal oxide clusters on the inert support surface. The prepared catalysts were tested in the reaction of oxidative dehydrogenation of ethylbenzene in the presence of nitrous oxide. The iron-containing catalysts showed the highest catalytic activity. The presence of isolated Fe3+ was found to be the most important factor influencing the ethylbenzene conversion. The undesirable effect of the increase in selectivity toward CO2 was observed for the samples with the highest concentrations of acidic surface sites. 相似文献
97.
Boulatov R Collman JP Shiryaeva IM Sunderland CJ 《Journal of the American Chemical Society》2002,124(40):11923-11935
Catalytic reduction of O(2) and H(2)O(2) by new synthetic analogues of the heme/Cu site in cytochrome c and ubiquinol oxidases has been studied in aqueous buffers. Among the synthetic porphyrins yet reported, those employed in this study most faithfully mimic the immediate coordination environment of the Fe/Cu core. Under physiologically relevant conditions, these biomimetic catalysts reproduce key aspects of the O(2) and H(2)O(2) chemistry of the enzyme. When deposited on an electrode surface, they catalyze the selective reduction of O(2) to H(2)O at potentials comparable to the midpoint potential of cytochrome c. The pH dependence of the half-wave potentials and other data are consistent with O-O bond activation at these centers proceeding via a slow generation of a formally ferric-hydroperoxo intermediate, followed by its rapid reduction to the level of water. This kinetics is analogous to that proposed for the O-O reduction step at the heme/Cu site. It minimizes the steady-state concentration of the catalytic intermediate whose decomposition would release free H(2)O(2). The maximum catalytic rate constants of O(2) reduction by the ferrous catalyst and of H(2)O(2) reduction by both ferric and ferrous catalysts are comparable to those reported for cytochrome oxidase. The oxidized catalyst also displays catalase activity. Comparison of the catalytic properties of the biomimetic complexes in the FeCu and Cu-free forms indicates that, in the regime of rapid electron flux, Cu does not significantly affect the turnover frequency or the stability of the catalysts, but it suppresses superoxide-releasing autoxidation of an O(2)-catalyst adduct. The distal Cu also accelerates O(2) binding and minimizes O-O bond homolysis in the reduction of H(2)O(2). 相似文献
98.
In this work, a simple, fast and reproducible method is presented for the determination of fibre/liquid-phase and fibre/gas-phase
partition coefficients of five chlorinated ethenes on a poly-(dimethylsiloxane), PDMS-coated, solid-phase microextraction
fibre, by employing a headspace HS-SPME coupled with gas chromatography. The partition coefficients were estimated by a numerical
method using a Level-I fugacity method coupled with parameter-estimation software. Dimensionless partition coefficients between
SPME fibre and liquid as well as gas phases were obtained at temperatures of 10 °C, 25 °C and 30 °C. The partition coefficients
of the fibre and the gas phase, K
fg, increase with decreasing temperature by a factor of ≈2 to 6, and they are directly proportional to the linear slope of the
regression line. The same tendency is observed for the partition coefficient of the fibre and liquid phase, K
fw, in a factor ≈1.2 to 2.0. The sorption enthalpy is higher in the gas phase; therefore, the sorption onto the fibre is favoured
at lower temperatures. The correlation of the log K
ow versus log K
fw and log K
oa versus log K
fg shows a linear relationship with the number of chlorine atoms in the C = C molecule. Long-term experiments resulted in sorption
to Teflon surfaces and possible losses in 43 mL vials, not observed in 250 mL Boston bottles. 相似文献
99.
Summary Gas chromatographic chiral separation of several chiral 2-and 3-alkanols and diols was studied both in their free hydroxyl
and in their trimethylsilyl ether forms. First, the derivatization procedure was verified through the identification of the
trimethylsilyl ethers formed on the basis of their mass spectra and optimized to obtain quantitative reaction. The optimized
procedure was applied to the trimethylsilylation of racemic mixtures of various hydroxyl compounds. The silylation was found
to be highly effective in the improvement of the separation of the individual enantiomers. The major advantages of the derivatization
process can be summarized as: (i) excellent baseline separation of the enantiomers of the silyl ethers was achieved in contrast
to the parent OH-containing compounds, (ii) the sensitivity of detection highly increased, (iii) the separations do not show
any significant concentration dependence and finally (iv) the analysis time needed decreased significantly.
Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997 相似文献
100.
Polfer NC Haselmann KF Zubarev RA Langridge-Smith PR 《Rapid communications in mass spectrometry : RCM》2002,16(10):936-943
Electron capture dissociation (ECD) of polypeptides has been demonstrated using a commercially available 3 Tesla Fourier transform ion cyclotron resonance (FTICR) instrument. A conventional rhenium filament, designed for high-energy electron impact ionisation, was used to effect ECD of substance P, bee venom melittin and bovine insulin, oxidised B chain. A retarding field analysis of the effective electron kinetic energy distribution entering the ICR cell suggests that one of the most important parameters governing ECD for this particular instrument is the need to employ low trapping plate voltages. This is shown to maximise the abundance of low-energy electrons. The demonstration of ECD at this relatively low magnetic field strength could offer the prospect of more routine ECD analysis for the wider research community, given the reduced cost of such magnets and (at least theoretically) the greater ease of electron/ion cloud overlap at lower field. 相似文献