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1.
We have studied the dissociation of the trifluoromethane molecule, CHF3, into negative ionic fragments at the C 1s and F 1s edges. The measurements were performed by detecting coincidences between negative and positive ions. We observed five different negative ions: F?, H?, C?, CF?, and F2?. Their production was confirmed by the analysis of triple coincidence events (negative‐ion/positive‐ion/positive‐ion or NIPIPI coincidences) that were recorded with cleaner signals than those of the negative‐ion/positive‐ion coincidences. The intensities of the most intense NIPIPI coincidence channels were recorded as a function of photon energy across the C 1s and F 1s excitations and ionization thresholds. We also observed dissociation channels involving the formation of one negative ion and three positive ions. Our results demonstrate that negative‐ion/positive‐ion coincidence spectroscopy is a very sensitive method to observe anions, which at inner‐shell edges are up to three orders of magnitude less probable dissociation products than cations.  相似文献   

2.
The electron capture negative ion (ECNI) mass spectra of two complex mixtures of polychlorinated decanes (PCDe) and polychlorinated dodecanes (PCDo) are presented. The number of isomers in these mixtures is still high but is drastically reduced in comparison to technical products of chlorinated paraffins (CP), due to their fixed chain length. As a result, the mass spectra are simplified and less complex. Different modes of negative ion formation were observed in the spectra of the PCDe and PCDo. [M+Cl] adduct ions were the most abundant ions in the spectra of lower chlorinated molecules. Higher chlorinated isomers formed prominently [M-Cl] and [M-HCl] fragments besides [M+Cl]. Possible consequences for the determination of chlorinated paraffins by ECNI-MS that result from the variation in ion formation are addressed.  相似文献   

3.
Inclusion complex formation between cyclodextrin and autoinducer of gram negative bacteria in aqueous solution was investigated by 1D 1H-NMR and ROESY spectra. An inhibitioneffect was observed on autoinducer activities of quorum sensing in Pseudomonas aeruginosa by adding cyclodextrins to the bacterial culture medium.  相似文献   

4.
The electron capture negative ion (ECNI) mass spectra of two complex mixtures of polychlorinated decanes (PCDe) and polychlorinated dodecanes (PCDo) are presented. The number of isomers in these mixtures is still high but is drastically reduced in comparison to technical products of chlorinated paraffins (CP), due to their fixed chain length. As a result, the mass spectra are simplified and less complex. Different modes of negative ion formation were observed in the spectra of the PCDe and PCDo. [M+Cl] adduct ions were the most abundant ions in the spectra of lower chlorinated molecules. Higher chlorinated isomers formed prominently [M-Cl] and [M-HCl] fragments besides [M+Cl]. Possible consequences for the determination of chlorinated paraffins by ECNI-MS that result from the variation in ion formation are addressed. Received: 18 August 1997 / Revised: 26 January 1998 / Accepted: 31 January 1998  相似文献   

5.
We have employed electron transmission spectroscopy to detect sharp variations in the total scattering cross section of electron from formaldehyde resulting from temporary negative ion formation. A progression of peaks is observed which we identify with vibrational levels of the H2CO?(2B1) ground state. The adiabatic electron affinity of H2CO is found to be ?0.65 ± 0.05 eV.  相似文献   

6.
The ionization of tetraphenylporphyrin (TPP) in a laser-desorbed metal ion plasma is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Competitive reaction pathways observed to generate abundant molecular ion species include electron detachment, cation attachment, charge exchange, metallation, and transmetallation in the positive ion mode and electron capture, metallation, and transmetallation in the negative ion mode. In general, cation attachment reactions dominate positive ion spectra below the laser irradiance threshold for plasma ignition, although the metallation product from [TPP]+ reaction with the metal atom, M, is observed. Negative ion products are not observed in the FT-ICR spectrum when a plasma is not formed. Under plasma ignition conditions, positive ion spectra include [TPP]+ formed by charge exchange with M+, which is also present in the spectrum. Negative ion spectra are dominated by [TPP]?; which is formed by attachment to thermal electrons generated in the plasma. Metallation reactions involving TPP and the metal substrate are examined. Positive ion metallation products are observed both in the absence of a plasma through reaction of [TPP]+ with M and by a second pathway under plasma ignition conditions through reaction of TPP with M+. In negative ion mode, metallation is only observed under plasma ignition conditions through reaction of [TPP]? with M. Observation of metallated products is found to be consistent with formation of stable metal oxidation states in the metallated porphyrin.  相似文献   

7.
《Electroanalysis》2017,29(11):2437-2443
The voltammetry of bisulfite at a film formed with myoglobin was studied in aqueous solutions. A broad wave was observed for the reduction of bisulfite. Using controlled potential electrolysis, the reduction at potentials positive of the FeII/FeI wave formed dithionite exclusively. As the potential approached the region for the FeII/FeI reduction, bisulfite was reduced primarily to HS. Even at the negative potentials, some dithionite was still formed, which could then be electrochemically reduced to thiosulfate. Analysis of the formation of HS, dithionite and thiosulfate during the electrolysis was consistent with the parallel formation of HS and dithionite, the latter of which was reduced to thiosulfate. Thiosulfate was verified by chemical analysis of the products from controlled potential electrolysis of the solution, and dithionite was observed spectroscopically using spectroelectro−chemistry.  相似文献   

8.
Relative signs of 13C? 13C coupling constants between ring carbons and carbonyl carbons in dimethyl phthalate and phthalic anhydride, 13C-labelled in both carbonyl groups, have been determined. The two-and three-bond coupling constants are shown to be positive, whereas four-bond coupling constants are negative. Substituent effects on carbon-carbon couplings due to a carboxyl group have been determined and effects of 5-membered ring formation in phthalic anhydride have been observed.  相似文献   

9.
Negative ion formation in CF2Cl2, CF3Cl and CFCl3 under low-energy electron impact has been investigated using a trochoidal monochromat The ions observed are F?, Cl?, FCl?, Cl2?, CFCl2? from CF2Cl2; F?, Cl?, FCl?, CF2Cl Quoting available thermochemical data, it can be shown that most of the observed negative ions arise from dissociative attachment processes. Appearance The extremely high yield of Cl? in CFCl3, which is observed at ε = 0.0 eV, will be discussed with regard to the lifetime of this molecule i  相似文献   

10.
Negative-ion/molecule reactions in systems containing methane as the major gas (∼ 4 torr), with NO2 and compounds A at mtorr pressures were studied in a pulsed electron, high pressure mass spectrometer. The compounds A were substituted nitrobenzenes and quinones. All these A compounds have positive electron affinities. Three types of reactions were observed and examined. (1) Electron transfer: A + NO2 = A + NO2. The exothermic electron transfer reactions proceeded with ADO collision rates for exothermicities from 30 to ∼ 10 kcal mol−1. Lower exothermicities led to low collision yields. (2) Adduct formation: NO2 + A = NO2·A. The equilibria for adduct formation were determined. Stable adducts are formed when A has hydrogens with partial protic character. The stability of the adducts NO2·A increased with increase in the electron affinity of A, when A was a substituted nitrobenzene. Substituents that increase the electron affinity of nitrobenzene are electron-withdrawing groups which also increase the protic character of the hydrogens involved in bonding in NO2·A. (3) Some of the compounds A were converted to phenoxy negative ions on reaction with NO2. For example, para-dinitrobenzene leads to formation of the para-nitrophenoxide negative ion. The oxy-negative-ion-forming reaction can be isomer specific. The utility of reaction types (1)–(3) is examined from the standpoint of negative ion chemical ionization where the reagent gas is NO2 in methane and the reagent ion is NO2.  相似文献   

11.
A new method is presented for elemental and molecular analysis of halogen-containing samples by glow discharge time-of-flight mass spectrometry, consisting of detection of negative ions from a pulsed RF glow discharge in argon. Analyte signals are mainly extracted from the afterglow regime of the discharge, where the cross section for electron attachment increases. The formation of negative ions from sputtering of metals and metal oxides is compared with that for positive ions. It is shown that the negative ion signals of F? and TaO2F? are enhanced relative to positive ion signals and can be used to study the distribution of a tantalum fluoride layer within the anodized tantala layer. Further, comparison is made with data obtained using glow-discharge optical emission spectroscopy, where elemental fluorine can only be detected using a neon plasma. The ionization mechanisms responsible for the formation of negative ions in glow discharge time-of-flight mass spectrometry are briefly discussed.  相似文献   

12.
Stereospecific adduct ion formation has been observed in the chemical ionization mass spectra (positive and negative) of certain E- and Z-1,2,3-triaryl-2-propen-1-ones. The Z isomers are found to give higher relative abundances of adduct ions than the E isomers. This has been interpreted in terms of the differences in the proton affinities of the isomers originating from their different degrees of enone resonance. Halide ion (CI? and Br?) attachment spectra of these compounds also show stereochemical differences in the relative abundances of [M]?˙ and [M+halide]? ions, though the effect is not as pronounced as in the case of the positive ion spectra.  相似文献   

13.
196Hg and202Hg isotopes have been measured by neutron activation analysis in samples of twelve stone meteorites. Hg is extracted from an irradiated sample by stepwise heating. The mercury concentrations vary from 0.07 to 33 ppm. While most of the samples give196Hg/202Hg ratios similar to terrestrial value within error limits, in some cases large anomalies are observed. A number of control experiments have been devised, that show the absence of experimental artifacts, during sample preparation, neutron irradiation, chemical separation and counting stages. Several anomalous and normal Hg distillate have been re-irradiated as Hg-diethyl-dithio-carbamate complex to eliminate the influence of neutron self shielding and interfering reactions from matrix elements. The isotopic ratio patterns persist in the distillates too proving that any artifacts during meteorite irradiation and measurement are essentially absent. Both positive and negative anomalies are observed; however, the negative anomalies are much more frequent and abundant. In an extreme case of fine grained magnetic particles of Ambapur Nagla the196Hg is apparently absent in the Hg released at 100 °C. A 2196Hg/202Hg value is only 6% relative to the monitor. This experiment shows the robustness of neutron activation analysis and suggest some constrains on the formation history of stone meteorites.  相似文献   

14.
A novel ditopic receptor possessing two complexation sites and bearing 1,3-alternate conformation based on thiacalix[4]arene was prepared. The binding behaviors with alkali metals and silver ion have been examined by 1H NMR titration experiment. Although the formation of the heterogeneous dinuclear complexes was not observed, the exclusive formation of mononuclear complexes of 1,3-alternate-3 with metal cations is of particular interest in negative allosteric effect in the thiacalix[4]arene family. These findings demonstrate that preorganization, subtle conformational changes and affinity have a pronounced effect on the complexation process between the two different arms placed at the two edges of the thiacalix[4]arene cavity.  相似文献   

15.
Role of added chloride ions on the shift of reaction pathway of oxidation of aromatic ketones (acetophenone, desoxybenzoin) by dichloroisocyanuric acid (DCICA) was studied in aqueous acetic acid—perchloric acid medium. Participation of enolic and protonated forms of ketones in the rate determining steps is manifested from zero and first orders with respect to the oxidant in absence and presence of added chloride ions, respectively. Positive and negative effects of acid and dielectric constant on the reaction rate were observed. The observations deduce plausible mechanisms involving (i) rate-determining formation of enol from the conjugate acid of the ketone (SH+) in the absence of added chloride ions and (ii) rapid formation of molecular chlorine species from HOCl (hydrolytic species of DCICA) in the presence of added chloride ions, which then interacts with SH+ in a rate-determining step prior to the rapid steps of product formation. The order of Arrhenius parameters substantiate the proposed plausible mechanisms based on order of reactants both in presence and absence of added chloride ions.  相似文献   

16.
A new model has been developed to account for adduct formation on multiply charged peptides observed in negative ion electrospray mass spectrometry. To obtain a stable adduct, the model necessitates an approximate matching of apparent gas-phase basicity (GBapp) of a given proton bearing site on the peptide with the gas-phase basicity (GB) of the anion attaching at that site. Evidence supporting the model is derived from the fact that for [Glu] Fibrinopeptide B, higher GB anions dominated in adducts observed at higher negative charge states, whereas lower GB anions appeared predominately in lower charge state adducts. Singly charged adducts were only observed for lower GB anions: HSO4, I, CF3COO. Ions that have medium GBs (NO3, Br, H2PO4) only form adducts having −2 charge states, whereas Cl (higher GB) can form adducts having −3 charge states. The model portends that (1) carboxylate groups are much more basic than available amino groups; (2) apparent GBs of the various carboxylate groups on peptides do not vary substantially from one another; and (3) apparent GBs of the individual carboxylate and amino sites do not behave independently. This model was developed for negative ion attachment but an analogous mechanism is also proposed for the positive ion mode wherein (1) binding of a neutral at an amino site polarizes this amino group, but hardly affects apparent GBs of other sites; (2) proton addition (charge state augmentation) at one site can decrease the instrinsic GBs of other potential protonation sites and lower their apparent GBs.  相似文献   

17.
Dissociative electron attachment spectroscopy is employed to study formation of stable negative ion fragments in the fluoroethylenes. It is found that in each case the energy of the first resonance leading to F? and other stable negative ion fragments coincides with the energy of the temporary Π anions. Tetrafluoroethylene gives fragments by cleavage of the double bond.  相似文献   

18.
The relative spatial distribution and motion with respect to Gd3+ of the 1H and 19F nuclei in the acetate ion and its fluorine derivatives are studied in D2O solutions through the paramagnetic relaxation rate enhancements (PREs) of these nuclei. We derive general theoretical expressions of the longitudinal PRE in terms of the analytical concentrations of metal and ligands, formation constants of the complexes, metal–nucleus distances, and coordination lifetimes of the ligands. The observed formation constants of the 1 metal: 1 ligand complexes markedly decrease with increasing number of fluorine atoms, the electronegativity of which reduces the negative partial charge of the coordinating COO? group. The coordination lifetimes are very short at the scale of the relaxation times of the protons of metal bound acetate, that is, shorter than about 10 μs. The average distance of the acetate protons from Gd3+ is in fair agreement with independent crystallographic determination. The release of free Gd3+ from the very stable Gddtpa (dtpa=diethylene‐triaminepentaacetate) complex caused by the competition of Zn2+ for dtpa, is evidenced by an increase of the PREs with Zn2+ concentration. The observed PRE increase is consistent with the known equilibrium constants governing the speciation involving Gd3+, Zn2+, and dtpa. The present case study illustrates a method which easily yields experimental tunable properties suitable to test the ongoing theories of lanthanide Ln3+ complexation in solution.  相似文献   

19.
The oxidative and reductive electrochemistry of the three isomeric nitroanilines has been studied in neutral (0.1 mol L−1 KClO4) and acidic (0.1 mol L−1 HClO4) aqueous electrolyte solutions by cyclic voltammetry and surface enhanced Raman spectroscopy (SERS). The cyclic voltammograms recorded for o- and p-nitroanilines with a gold electrode in acidic solution, scanning toward negative potentials, revealed formation of phenylenediamine not observed in neutral solution. Similar behavior of nitroanilines and phenylenediamines was observed on gold and platinum electrodes. An oxygen–gold adsorbate stretching mode was detected between 400 and 430 cm−1 in the SER-spectra of the three isomeric nitroanilines in both electrolyte solutions at positive electrode potentials, implying perpendicular adsorption via the nitro group.  相似文献   

20.
Positive and negative ion fast atom bombardment (FAB) mass spectra of some monosubstituted nitroaromatic isomers are reported. Generally ions carresponding to [M + H]+ and M+ are observed in the positive ion FAB spectra; ions such as [M ? H] ? and M?˙ are observed in the negative ion FAB spectra. The use of FAB mass spectra to distinguish the isomers is discussed. Comparisons of FAB, chemical ionization and electron impact mass spectra of the same isomers (wherever possible) are reported. The structural information obtained in the negative ion FAB spectra complement those obtained in the positive ion FAB spectra.  相似文献   

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