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1.
The mechanism of the collision-induced fragmentation of peracetylated methyl-α-D-glucopyranoside was investigated using deuterium-labelled acetates and sequential mass spectrometry. Loss of the substituent at C(1), the anomeric carbon, yields an ion of m/z 331, [C14H19O9]+. This ion further dissociates via two pathways, the first including m/z 271, [C12H15O7]+, 169, [C8H9O4]+ and 109, [C6H5O2]+, and the second including m/z 211, [C10H11O5]+, 169, [C8H9O4]+ and 127 [C6H7O3]+. The first path proceeds via loss of acetate at C(3), followed by a single-step concerted loss of acetates from C(2) and C(4), and ending with loss of acetate from C(6). The second path proceeds predominantly via loss of acetates from C(3) and C(4), elimination of ketene from the C(2)-acetate and finally loss of ketene from the acetate at C(6). This path is also characterized by an ill-defined series of parallel decomposition reactions involving acetates from other sites on the molecule. At low collision energy, and in the absence of collision gas (unimolecular reaction conditions), the former pathway predominates; m/z 331 dissociates via loss of acetate at C(3), followed by a single-step concerted loss of acetates from C(2) and C(4).  相似文献   

2.
The unimolecular fragmentation reactions of 28 protonated nitroarenes, occurring on the metastable ion time-scale, are reported. In addition, the collision-induced fragmentation of the same species have been studied at 10 eV and at 50 eV collision energy. When an OH, COOH or NH2 substituent is ortho to the nitro function, the dominant fragmentation involves loss of H2O, for both unimolecular and collision-induced reactions. When there is an electron-releasing substituent ortho or para to the litro group, loss of OH is the dominant fragmentation reaction both on the metastable ion time-scale and for ions activated by collision. When the electron-releasing substituent is meta to the nitro group, loss of NO2 is the dominant low-energy unimolecular fragmentation reaction while loss of HNO2 is the most important fragmentation for ions activated by 50 eV collisions. Elimination of NO from [MH]+ occurs to a significant extent in the unimolecular fragmentation of protonated nitrobenzene and those protenated nitrobenzenes containing electron- attracting substituents. In the collision-induced dissociation of these species loss of HNO2 occurs at the expense of loss of NO. The results are consistent with protonation predominantly at the nitro group. The results are discussed in terms of the use of neutral loss scans in tandem mass spectrometry to monitor complex mixtures for nitroarenes.  相似文献   

3.
Photoion-photoion coincidence spectra of benzene and benzene-d6 photoionized by He(II) light and synchrotron radiation show the existence of six major and eight minor charge-separation reactions of the [C6H6]2+ ion. Three main groups of ion pairs are related to [C3H3]+ + [C3H3]+, [C2H3]+ + [C4H3]+ and [CH3]+ + [C5H3]+, with appearance energies of 32.2 ± 0.5 eV, 31.3 ± 0.5 eV and 28.4 ± 0.3 eV. The kinetic energy release is the same for all pairs within a group, irrespective of hydrogen number, but differs from group to group. Results are interpreted in terms of fast, direct charge separation of [C6H6]2+, and subsequent hydrogen loss by the singly charged fragments.  相似文献   

4.
Electrospray ionization and tandem mass spectrometry experiments have been used to study the fragmentation and electron-ion interactions of doubly charged zwitterionic clusters, [M(15) + 2H](2+) (where M = Glycine Betaine (GB), (CH(3))(3)N(+)CH(2)CO(2)(-), and Dimethylsulfonioacetate (DMSA), (CH(3))(2)S(+)CH(2)CO(2)(-)) which are close to the stability limit, i.e., the Coulomb repulsion of the charge within the cluster competes with attractive forces. The intercluster chemistry was studied using collision-induced dissociation (CID) and electron-induced dissociation (EID) in which the energy of the electrons has been varied from >0 to 30 eV. Experimental results suggest that the zwitterionic binding energy in the clusters follow the order GB > DMSA, which is consistent with theoretical calculations that highlight that the lower dipole moment of DMSA leads to a binding energy of DMSA that is 0.86 times smaller than that for GB. Multiply protonated clusters of both GB and DMSA dissociate through Coulomb explosion, which is in competition with neutral evaporation for DMSA. Electronic excitation of the cluster under EID conditions at higher electron energies >12 eV can lead to new intercluster reactions associated with bond cleavages where differences between the sulfur and nitrogen betaines are minor.  相似文献   

5.
6.
In this mass spectrometry (MS) study of doubly charged porphyrin salts, fast atom bombardment (FAB) and matrix-assisted laser desorption/ionization (MALDI) MS techniques are utilized to examine several unique ionic species. The predominant transformation of preformed doubly charged ions in the desorption/ionization mechanism of FAB and MALDI is the result of deprotonation reactions to form singly charged ions of the type [M(2+) - H(+)](+) and of one-electron reductions to form radical cations [M(2+) + e(-)](+.). The dependence of this phenomenon and the formation of a number of additional ionic species on the different matrices and the FAB-matrix additive benzoquinone is examined. The significant analogous behavior of doubly charged porphyrins in FAB- and MALDI-MS leads to the conclusion that one-electron reductions are of distinct relevance in the desorption/ionization mechanism of MALDI.  相似文献   

7.
8.
Doubly charged molecular anions M2? of fullerenes are formed in the gas phase under chemical ionization conditions with isobutane as the reagent gas. The efficiency of double electron attachment increases with increasing size of the fullerenes: the C ion is the most abundant doubly charged anion in the negative-ion CI mass spectrum, although the concentration of C70 was about 12% in the fullerene mixture examined. Under low-energy collision-induced dissociation conditions an electron is ejected from the doubly charged C ion resulting in the singly charged molecular anion C˙. This process appears to be the first report of the ejection of an electron (electron stripping) from a doubly charged anion in the gas phase: .  相似文献   

9.
The cis-doubly N-confused porphyrin, H2N2CP, containing two adjacent confused pyrrole rings has been investigated from the point of view of its acid-base and electrochemical behavior in dichloromethane. This novel porphyrin isomer can form two metal-carbon bonds in the central core, stabilizing metal ions in unusually high oxidation states. Furthermore, the two outside N-pyrrole atoms remain available for acid-base and specific solvent interactions. Protonation of the pyrrole N atoms proceeds according to two successive steps, while only a single deprotonation step has been observed in the presence of bases. Similarly, in the case of the silver and copper complexes the protonation and deprotonation of the outer pyrrole rings have been detected, confirming the structure of the metalated species as M(III)-HN2CP. The electrochemical reduction of the metal ions (III/II redox process) and oxidation of the macrocycle ring have been detected respectively at -0.9 and 1.4 V based on spectroelectrochemical measurements in conjunction with the acid/base equilibrium studies. Additional waves observed around -0.5 and 1.3 V have been assigned to redox processes involving water molecules associated with the doubly N-confused porphyrins.  相似文献   

10.
11.
The mass spectra of biological molecules, whose molecular mass exceeds 10 kDa, invariably contain multiply charged ions. For example, a survey scan of a small protein will produce singly, doubly and triply protonated molecules, the intensity of the doubly charged species often being greater than that of the singly charged entity. Although the spectra resulting from doubly charged peptides have not previously been studied, collisional activation of such doubly charged species may result in significant additional information pertaining to molecular structure. The techniques employed to study ions originating from multiply charged species were linked scanning of constant B/E and tandem mass spectrometry, namely low collision energy spectra acquired on a BEQQ hybrid instrument. The methodology was applied to model compounds whose tandem mass spectrometry characteristics are well known, e.g. gramicidin S and angiotensin I. The results for the product ions of the [M + 2H]2+ species of the models were obtained which highlight the methodology required for high-mass materials.  相似文献   

12.
Fragmentation of doubly charged argon clusters is reported. Neutral argon clusters are excited with monochromatized synchrotron radiation in the energy regime of the argonL 3/L 2 absorption edges (240–260 eV) leading predominantly to cluster dication formation. All charged particles are detected in a photoelectron-photoion-photoion-concidence (PEPIPICO) experiment. Symmetric and asymmetric charge separation reactions (Coulomb explosion) are identified for clusters below the critical size of stable dication formation. The peak shapes of the coincidence signals are investigated as a function of neutral cluster size. Characteristic changes in peak shape are observed which are used to derive fragmentation mechanisms involving sequential evaporation of neutrals before and after charge separation. The spectra indicate in accordance with low kinetic energy releases occurring in charge separation of large dissociative cluster dications (Ar n 2+ , withn>50) that due to large charge separation distances the momenta of both singly charged fragments are not any more directed into opposite direction, as it is typical for Coulomb explosion. The results are compared to collision induced fragmentation of mass selected argon cluster dications as well as photon stimulated desorption spectra of condensed argon.  相似文献   

13.
The fragmentation of CH2?CHCH2OCH3 cation-radicals has been investigated by means of 2H- and 13C-labelling experiments and by analysis of collision-induced dissociation spectra. Metastable C4H8O species decompose via one of three main channels which involve loss of (a) a hydrogen atom, (b) a methyl radical or (c) a formaldehyde molecule. Extensive, but not complete, exchange of the hydrogen and deuterium atoms in specifically labelled C4H8-nDnO analogues precedes each of the three fragmentation pathways. The role of distonic ions in the rearrangement steps which bring about hydrogen exchange is discussed. The influence of isotope effects on the relative rates of the major reactions and the associated kinetic energy releases is examined. Only loss of a hydrogen atom is subject to a substantial isotope effect. Elimination of a methyl radical releases a large amount of kinetic energy, as is shown by the broad and dish-topped appearance of the corresponding metastable peak (T1/2 ≈ 42 kJ mol?1). The carbon atom of the original methoxy group is specifically expelled in this process. Both the large T1/2 value and the unusual site selectivity are atypical of methyl and other alkyl radical losses from ionized alkenyl methyl ethers. The carbon atom of the methoxy group also participates specifically in formaldehyde elimination, but the two hydrogen atoms are not always selected from the three contained in the initial methoxy group. The implications of these labelling results for the synchronicity of concert of formaldehyde loss, which can be formu lated as a pericyclic process, is analysed.  相似文献   

14.
The dissociation patterns of doubly charged noble metal clusters (M) n ++ to two singly charged clusters, (M) m Emphasis>1/+ and (M) m Emphasis>2/+ have been investigated using a double focusing mass spectrometer. They are compared with the dissociation patterns from singly charged clusters. The dissociation probabilities to (M) 3 + and (M) 9 + were large and the odd-even alternations were observed in both patterns.  相似文献   

15.
The microsolvation of the suberate dianion, -O2C(CH2)6CO2-, with two separate charge centers was studied by photoelectron spectroscopy and molecular dynamics simulation one solvent molecule at a time for up to 20 waters. It is shown that the two negative charges are solvated in the linear suberate alternately. As the solvent number increases, the negative charges are screened and a conformation change occurs at 16 waters, where the cooperative hydrogen bonding of water is large enough to overcome the Coulomb repulsion and pull the two negative charges closer through a water bridge. This conformation change, revealed both from the experiment and from the simulation, is a manifestation of the hydrophilic and hydrophobic forces at the molecular level.  相似文献   

16.
Anabuki S  Tokuji S  Aratani N  Osuka A 《Organic letters》2012,14(11):2778-2781
Direct meso-alkynylation of β,β'-dipyridylporphyrin with various alkynyllithium reagents has been achieved, in which the β,β'-dipyridyl groups play an important role in facilitating the nucleophilic addition of the reagents through double coordination. This method enabled the synthesis of a meso-ethynylene-bridged diporphyrin.  相似文献   

17.
The blackbody infrared radiation induced dissociation of methyl benzoate (C8H8O2(+*)) radical cation was investigated by using a Fourier transfer ion cyclotron resonance mass spectrometer equipped with a resistively heated (wire temperatures of 400-1070 K) wire ion guide. We observed product ion branching ratios that are strongly dependent upon wire temperature. At low temperatures (670-890 K) the major product ion C7H8 (+*) (m/z 92), which is formed by loss of CO2, and at higher temperatures (above 900 K), loss of methoxy radical ((*)OCH3) competes with loss of CO2. The energies of the various reactant ions and transition states for product ion formation were estimated by using density functional theory molecular orbital calculations, and a proposed mechanism for the dissociation chemistry of C8H8O2 (+*) involving a multistep rearrangement reaction is tested using the Master Equation formalism.  相似文献   

18.
Summary The potential energy of interacting atomic ions A++B+ often shows a shallow local minimum separated by a broad potential barrier from the dissociation products at much lower energy. Early interpretations of dication potential shapes were based on the similarity of the electronic structure between isoelectronic neutral and ionic species and led to a picture of a chemical bond superimposed on a repulsive Coulomb potential. More recently, barriers in dication potentials have commonly been interpreted as avoided curve crossings involving covalent and ionic structures. In this paper, we demonstrate that the former model is the appropriate one except in cases with very small asymptotic ionic/covalent energy splittings. By deriving dication wavefunctions from their neutral isoelectronic counterparts, we obtain upper bound dication potential curves which show all the characteristic features. By further modeling induction effects, we arrive at an almost quantitative fit of accurateab initio dication potentials. The chemical bond plus electrostatic repulsion interpretation of dication interactions also explains why the accurate calculation of potential curves appears to be much more demanding for dications than for isoelectronic neutrals.Dedicated in the honor of Prof. Klaus Ruedenberg  相似文献   

19.
20.
Evidence is presented for the existence of (O2+)2MF62?(M=Ni,Mn) salts. These salts are marginally stable up to about 10°C and are characterized by an OO stretching frequency of about 1805 cm?1.  相似文献   

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