Bibliothek

Notizen: The modification of a double-focusing mass spectrometer of BE geometry (VG-Analytical ZAB-2F) to permit the field ionization of fast atoms in high Rydberg states is described. Field ionization was achieved by means of a pair of closely spaced, very fine metal meshes with a (kV) potential difference between them. High Rydberg noble gas atoms were generated from their ions by electron transfer from noble gas targets. Also described is a method, involving a field ionization observation, for measuring the net kinetic energy loss associated with the collision-induced neutralization-reionization of polyatomic ions.

Notizen: Methods are described for the unequivocal identification of the acetyl, [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document} =O] (a), 1-hydroxyvinyl, [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OH] (b), and oxiranyl, (d), cations. They involve the careful examination of metastable peak intensities and shapes and collision induced processes at very low, high and intermediate collision gas pressures. It will be shown that each [C2H3O]+ ion produces a unique metastable peak for the fragmentation [C2H3O]+ → [CH3]++CO, each appropriately relating to different [C2H3O]+ structures. [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}=O] ions do not interconvert with any of the other [C2H3O]+ ions prior to loss of CO, but deuterium and 13C labelling experiments established that [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OH] (b) rearranges via a 1,2-H shift into energy-rich leading to the loss of positional identity of the carbon atoms in ions (b). Fragmentation of b to [CH3]++CO has a high activation energy, c. 400 kJ mol-1. On the other hand, , generated at its threshold from a suitable precursor molecule, does not rearrange into [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OH], but undergoes a slow isomerization into [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}=O] via [CH2\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}HO]. Interpretation of results rests in part upon recent ab initio calculations.The methods described in this paper permit the identification of reactions that have hitherto lain unsuspected: for example, many of the ionized molecules of type CH3COR examined in this work produce [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OH] ions in addition to [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}=O] showing that some enolization takes place prior to fragmentation. Furthermore, ionized ethanol generates a, b and d ions. We have also applied the methods for identification of daughter ions in systems of current interest. The loss of OH· from [CH3COOD]+· generates only [CH2=\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}—OD]. Elimination of CH3· from the enol of acetone radical cation most probably generates only [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}=O] ions, confirming the earlier proposal for non-ergodic behaviour of this system. We stress, however, that until all stable isomeric species (such as [CH3—\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^{\rm + } $\end{document}=C:]) have been experimentally identified, the hypothesis of incompletely randomized energy should be used with reserve.

Notizen: An inexpensive data acquisition system has been developed for mass-analyzed kinetic energy experiments which involve scanning the electrostatic analyzer of a reversed geometry mass spectrometer. The various hardware and software design features are described. Results for data obtained with a commercially available VG ZAB-2F mass spectrometer are presented and discussed.

Notizen: The ion retardation method (whereby an ion beam is prevented from entering a collision gas cell by means of a voltage applied thereto) for permitting the examination of the neutral products of unimolecular ion fragmentations has been extended to include observations of neutral products generated by collisions before the gas cell and their related phenomenology. Observations obtained using an ion beam deflection electrode, an alternative method of preventing the ion beam from entering the collision cell, are also reported. When low collision gas pressures are employed (〈2×10-7 Torr He), this latter method provides collisionally induced dissociative ionization (CIDI) mass spectra of unimolecularly generated neutral fragmentation products, free of complications arising from events induced by collisions occurring outside the collision cell. The CIDI mass spectra of CH3·, C2H4, CH3ĊO, CH3OH and C2H6 generated from positive ion precursors and CH3·, CH3O· and C6H5NO2 generated by electron loss from negative ions are described.

Notizen: A method is described for the investigation of the structure of neutral products from the unimolecular (metastable) dissociative ionizations of mass selected ions, by means of the collisionally induced dissociative ionization of the neutral species themselves. The neutral species, with kilovolt translational energies, enter a positively charged collision cell situated in the second field free region of a standard ZAB-2F mass spectrometer. Dissociative ionization of the neutrals results therein from their collisions with He target gas. The resulting ions are analysed by means of the electric sector and the relative ion abundances are shown to be structure characteristic. For such experiments the neutral flux should be c. ≥ 0.5% of the selected precursor ion flux; the collision gas pressure must be insufficient to cause significant precursor ion fragmentation in the field free region preceding the collision cell. It was shown that HNC is generated in the fragmentation of aniline molecular ions, whereas HCN is the neutral product in the dissociative ionizations of pyridine, benzonitrile and benzyl cyanide. The neutral radical [C, H3, O·] formed together with [CH3CO]+ from ionized methyl acetate has the structure ·CH2OH, but that from the analogous fragmentation of the methyl propanoate molecular ion has the structure CH3O·. Allyl radicals were shown to be generated from [(CH3)2CHCH2OH]+· together with [CH3OH2]+ ions.

Notizen: Pure [CH2CHCH2]+ and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{CH}}_{\rm{3}} \mathop {\rm{C}}\limits^{\rm{ + }} = {\rm{CH}}_{\rm{2}} $\end{document} ions are generated only in metastable fragmentations of [CH2=CHCH2X]+·, X=Cl, Br, I, and [CH3CX=CH2]+·, X=Br, I, respectively. For ion source generated [C3H5]+ ions there is some structural interconversion. The structure characteristic feature of their collisional activation mass spectra is the ratio m/z 27 ([C2H3]+): m/z 26 ([C2H2]+·). For \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{CH}}_{\rm{3}} \mathop {\rm{C}}\limits^{\rm{ + }} = {\rm{CH}}_{\rm{2}} $\end{document} the ratio is only weakly dependent upon the translational energy of the ion. For [CH2CHCH2]+, the ratio rises sharply as translational energy is reduced, from 0.9 at 8 kV to c. 3 at 1 kV. [CH2CHCH2]+ ions generated by charge reversal of [CH2CHCH2]- show higher ratios, resulting from their lower average internal energy content. It must therefore be emphasized that [C3H5]+ ion structure assignments should only be made using reference data which apply to specific experimental conditions. [C3H5]+ daughter ion structures for a number of well-known fragmentations have been established. The heat of formation of the 2-propenyl cation was measured to be 969±5 kJ mol-1. Labelling experiments show that at low internal energies, allyl cations do not undergo atom randomization in c. 1-2 μs; high internal energy ions of longer lifetime (c. 8 μs) show complete atom randomization. H· atom loss from [13CH3CH=CH2]+· has been shown to generate [13CH2CHCH2]+ and \documentclass{article}\pagestyle{empty}\begin{document}$ {}^{{\rm{13}}}{\rm{CH}}_{\rm{2}} \mathop {\rm{C}}\limits^{\rm{ + }} - {\rm{CH}}_{\rm{3}} $\end{document} without any skeletal rearrangement.