The formation of [CH5O]+ and ions from the molecular ions of isobutyl alcohol

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.     

Mass spectometry of aralkyl compounds with a functional group—VI1: Mass spectra of 1-phenylethyanol-1, 2-phenylethanol-1 and 1-phenylpropanol-2

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.     

Mechanisms of repeller-induced effects in thermospray liquid chromatography/mass spectrometry

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.     

Convergent synthesis of noncovalent metallodendrimers containing hydrophobic dendrons at the periphery.

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.     

Bimodal proton transfer in acid-base reactions in water

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.     

Ag+ labeling: a convenient new tool for the characterization of hydrogen-bonded supramolecular assemblies by MALDI-TOF mass spectrometry

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.     

A 15N labelling,FD and FAB study of ammonia loss from protonated arginine molecules

The positive ion field desorption (FD) spectrum of arginine taken at the best anode temperature only contains a peak due to [M+H]⁺ ions. At higher emitter temperatures a considerable amount of fragmentation is induced and the [M+H NH₃]⁺ ions become most abundant. Specific ¹⁵N labelling reveals that the eliminated ammonia molecule, exclusively, contains one of the terminal nitrogen atoms of the guanidyl group. This also applies to the ammonia loss from metastably decomposing [M+H]⁺ ions. The positive ion fast atom bombardment (FAB) spectrum shows more fragmentation than the FD spectrum. In contrast with the FD results, the [M+H]⁺ ions generated upon FAB with ion lifetimes <10⁻⁶ s eliminate both ammonia containing one of the terminal nitrogen atoms of the guanidyl group and ammonia containing the α-amino group in the ratio of 1.35, as found by ¹⁵N labelling. The metastably decomposing [M+H]⁺ ions, however, eliminate only the former ammonia molecule. In the negative ion FD and FAB spectra no other peak than that corresponding to the [M H]⁻ ion is observed. Some attention has been paid to the thermal degradation of arginine on the basis of a few Curie-point pyrolysis experiments.