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1.
2.
Albert J. R. Heck Nico M. M. Nibbering 《Journal of the American Society for Mass Spectrometry》1995,6(1):11-18
The gas phase chemistry of C3H6Br+ cations generated via low energy electron impact on various dibromopropanes has been studied by using Fourier transform ion cyclotron resonance mass spectrometry. Neutral substrate molecules that have been selected to probe the bimolecular reactivity of the C3H6Br+ isomers are ammonia, methylamine, trimethylamine, cis-butene, and 2, 3-dimethyl-2-butene. At least three different isomers are characterized on the basis of their different reactivity toward the various substrate molecules. It is suggested that these isomers have (a) the 2-bromo-2-propyl cation structure, (b) the propylenebromomum ion structure, and (c) the cyclic four-membered trimethylenebromonium ion structure. The 2-bromo-2-propyl cations react predominantely via proton transfer. This reaction is hampered for the propylenebromonium ions, which react mainly as electrophiles or bromanyl cation donors. Cyclic trimethylenebromoruum ions react predominantly via adduct formation, even under low pressure conditions, which implies that tturd body collisions are not the only stabilization mechanism. 相似文献
3.
Susuma Tajima Jan van der Greef Nico M. M. Nibbering 《Journal of mass spectrometry : JMS》1978,13(10):551-555
On the basis of field ionization kinetic and deuterium labelling experiments, it is shown that the molecular ions of isobutyl alcohol generate [CH5O]+ ions at 10?11 s via a 1,4-shift of a hydrogen atom from one of the methyl groups to the oxygen atom, followed by a 1,2-elimination of protonated methanol with a hydrogen atom of the other methyl group. At times > 10?11 s two distinct interchange processes between hydrogen atoms appear to compete with this reaction, as shown from field ionization kinetic experiments and metastable decompositions. Ion cyclotron resonance experiments on the long-lived [CH5O]+ ions further demonstrate that they are protonated methanol ions. Arguments are put forward that the ions, generated by a specific 1,3-elimination of a molecule of water from metastable decomposing molecular ions, have an isobutene structure. 相似文献
4.
Mass spectra of 1-phenylethanol-1 and its analogues, specifically deuterated in the aliphatic chain, suggest that the [M? CH3]+ ion is represented partly by an α-hydroxybenzyl fragment. Moreover, the molecular ion loses successively—after scrambling of all hydrogen atoms, except those of CH3? a hydrogen atom and C6H6, generation the CH3CO+ ion. Diffuse peaks, found in the spectra of of 2-phenylethanol-1 and its analogues, specifically deuterated in the aliphatic chain and in the phenyl ring, show that the molecular ion loses C2H4O, possibly via a four-center mechanism, after an exchange of aromatic and hydroxylic hydrogens. Mass spectra of 1-phenylpropanol-2 and its analogues, specifically, deuterated in the aliphatic chain, demonstrate that in the molecular ion exclusively the hydroxyl hydrogen atom is transferred to one of the ortho-positions of the phenyl ring via a McLafferty rearrangement, generating the [M ? C2H4O]+ ion. Furtherore, an eight-membered ring structure is proposed for the [M ? CH3]+ ion to explain the loss of H2O and C2H2O from this ion after an extensive scrambling of hydrogen atoms. 相似文献
5.
6.
C. E. M. Heeremans R. A. M. van der Hoeven W. M. A. Niessen J. van der Greef N. M. M. Nibbering 《Journal of mass spectrometry : JMS》1991,26(5):519-527
The application of a high potential at the repeller electrode, positioned opposite to the sampling cone in order to increase the sampling efficiency, can induce fragmentation in thermospray mass Spectrometry. Until now, this fragmentation has been attributed to collision-induced dissociation. As a result of studies on the changes in the reagent gas composition in the thermospray buffer ionization mode as a function of the repeller potential in the positive-ion mode, it appears that three different processes are occurring. At low repeller potentials, the thermospray mass spectra of the eluent are determined by the proton affinities and the concentrations of the various solvent constituents, and the stabilities of the formed cluster ions under the ion source conditions. With an increase in the repeller potential, collision-induced dissociation of the background ions starts to occur. When the kinetic energy of the ions and cluster ions becomes high enough, endothermic proton transfer and solvent-switching reaction pathways are opened. For the relatively volatile analytes studied, e.g. aniline, acetophenone, benzaldehyde and benzoic acid, similar effects are observed. 相似文献
7.
H J van Manen R H Fokkens N M Nibbering F C van Veggel D N Reinhoudt 《The Journal of organic chemistry》2001,66(13):4643-4650
The noncovalent synthesis of "layer-block" metallodendrimers containing hydrophobic shells of covalent dendritic wedges at the periphery is described. Starting from first- and second-generation Fréchet wedges having phosphines at their focal point, convergent dendritic growth yields third- and fourth-generation metallodendrimers in which the coordination of nitriles, pyridines, and phosphines to SCS Pd(II) pincers is used as the assembly motif. In this convergent growth, the number of terminal hydrophobic phosphine wedges increases with generation. The solubility of the dendritic structures in apolar organic solvents such as chloroform and dichloromethane increases accordingly, in contrast to previously reported metallodendrimers. All dendritic structures were characterized by (1)H and (31)P NMR spectroscopy, elemental analysis, and MALDI-TOF mass spectrometry. 相似文献
8.
Rini M Pines D Magnes BZ Pines E Nibbering ET 《The Journal of chemical physics》2004,121(19):9593-9610
We investigate one of the fundamental reactions in solutions, the neutralization of an acid by a base. We use a photoacid, 8-hydroxy-1,3,6-trisulfonate-pyrene (HPTS; pyranine), which upon photoexcitation reacts with acetate under transfer of a deuteron (solvent: deuterated water). We analyze in detail the resulting bimodal reaction dynamics between the photoacid and the base, the first report on which was recently published. We have ascribed the bimodal proton-transfer dynamics to contributions from preformed hydrogen bonding complexes and from initially uncomplexed acid and base. We report on the observation of an additional (6 ps)(-1) contribution to the reaction rate constant. As before, we analyze the slower part of the reaction within the framework of the diffusion model and the fastest part by a static, sub-150 fs reaction rate. Adding the second static term considerably improves the overall modeling of the experimental results. It also allows to connect experimentally the diffusion controlled bimolecular reaction models as defined by Eigen-Weller and by Collins-Kimball. Our findings are in agreement with a three-stage mechanism for liquid phase intermolecular proton transfer: mutual diffusion of acid and base to form a "loose" encounter complex, followed by reorganization of the solvent shells and by "tightening" of the acid-base encounter complex. These rearrangements last a few picoseconds and enable a prompt proton transfer along the reaction coordinate, which occurs faster than our time resolution of 150 fs. Alternative models for the explanation of the slower "on-contact" reaction time of the loose encounter complex in terms of proton transmission through a von Grotthuss mechanism are also discussed. 相似文献
9.
Timmerman P Jolliffe KA Calama MC Weidmann JL Prins LJ Cardullo F Snellink-Ruël BH Fokkens RH Nibbering NM Shinkai S Reinhoudt DN 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(22):4104-4115
Herein we describe our results on the characterization of a wide variety of different hydrogen-bonded assemblies by means of a novel matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique with Ag+ labeling. The labeling technique with Ag+ ions is extremely mild and provides a nondestructive way to generate charged assemblies that can be detected by mass spectrometry. Up to now more than 25 different single (1(3).2(3)), double (3(3).2(6)), and tetrarosettes (4(3).2(12)) have been successfully characterized by the use of this method. The success of the method entirely depends on the presence of a suitable binding site for the Ag+ ion. A variety of functionalities has been identified that provide strong binding sites for Ag+, either acting in a cooperative way (pi-arene and pi-alkene donor functionalities) or individually (cyano and crown ether functionalities). The method works well for assemblies with molecular weights between 2,000 and 8,000 Da, and most likely far beyond this limit. 相似文献
10.
The bimolecular hydrogen-deuterium exchange behavior of protonated alkyl dipeptides in the gas phase
Erez H. Gur Leo J. de Koning Nico M. M. Nibbering 《Journal of the American Society for Mass Spectrometry》1995,6(6):466-477
As part of an ongoing characterization of the intrinsic chemical properties of peptides, thermal hydrogen-deuterium exchange has been studied for a series of fast-atom-bombardment-generated protonated alkyldipeptides and related model compounds in the reaction with D2O, CH3OD, and ND3 in a Fourier transform ion cyclotron resonance mass spectrometer. Despite the very large basicity difference between the dipeptides and the D2O and CH3OD exchange reagents, efficient exchange of all active hydrogen atoms occurs. From the kinetic data it appears that exchange of the amino, amide, and hydroxyl hydrogens proceeds with different efficiencies, which implies that the proton in thermal protonated dipeptides is immobile. The selectivity of the exchange at the different basic sites is governed by the nature of both the dipeptide and the exchange reagent. The results indicate that reversible proton transfer in the reaction complexes, which effectuates the deuterium incorporation, is assisted by formation of multiple hydrogen bonds between the reagents. Exchange is considered to proceed via the intermediacy of different competing intermediate complexes, each of which specifically leads to deuterium incorporation at different basic sites. The relative stabilization of the competing intermediate complexes can be related to the relative efficiencies of deuterium incorporation at different basic sites in the dipeptide. For all protonated dipeptides studied, the exchange in the reaction with ND3 proceeds with unit efficiency, whereas all active hydrogen atoms are exchanged equally efficiently. Evidently specific multiple hydrogen bond formations are far less important in the reversible proton transfers with the relatively basic ammonia, which allows effective randomization of all active hydrogen atoms in the reaction complexes. 相似文献