Bernhard Ciommer

Mechanism of keto Å enol tautomerism of ionized vinyl alcohol versus acetaldehyde and their dissociation to C2H3O+ and H·. An ab initio molecular orbital study

Abstract In complete accord with experiment, ab initio molecular orbital calculations provide a detailed description of the potential energy surface for some isomers of the C2H4O+· ions. In particular, it is predicted that the ionized hydroxy(methyl)carbene, H3CCOH+·, is a stable C2H4O+· isomer and serves as the key intermediate in the isomerization/dissociation processes of the cation radical of gaseous vinyl alcohol. A comparison between the results of semi-empiric (MINDO/3 and MNDO) and ab initio calculations at various levels of theory suggests that (i) MINDO/3 fails to describe properly the central features of the C2H4O+· energy surface, (ii) MNDO gives results which are qualitatively similar to those obtained by the more elaborate ab initio procedures and (iii) inclusion of the effects of correlation and zero-point energies, as well as the use of large basis sets, are essential for obtaining a reliable insight into the gas phase chemistry of these and related cation radicals.

[CH3COH]˙+, the central intermediate in the isomerization–dissociation reactions of ionized vinyl alcohol

In complete agreement with experimental results ab initio molecular orbital calculations predict that ionized hydroxy(methyl)carbene, [CH3COH]+, does not only exist as a stable C2H4O˙+ isomer but also serves as the key intermediate in the isomerization–dissociation processes of the cation radical of gaseous vinyl alcohol.

Dissoziative ionisierung von 2-trimethylsilyl-1-phenoxyethan. Nachweis des nicht-klassischen ethylen-trimethylsilanium-ions in der gasphase

Abstract The mass spectrometric investigation of specifically deuterium and 13C labelled 2-trimethylsilyl-l-phenoxyethanes proves that the dissociative ionization of β-silyl-substituted ethane derivatives (loss of PhO⊎; p-CH3C6H4O⊎; and C4H⊎9 from PhOCH2CH2SiMe3, p-MeC6H4OCH2CH2SiMe3 and CH3CH2CH(CH3)CH2-CH2SiMe3, respectively) yields the non-classical bridge ethylene trimethylsilanium ion and not the open-chain isomer. Other stable C5H13Si+⊎ ions, characterised by collisional activation mass spectrometry, are the dimethyl n-propyl silicenium ion and the l-trimethylsilyl ethyl cation, both generated from the molecular ions of CH3CH2CH2Si(Cl)Me2 and CH3CH(Cl)SiMe3 via unimolecular loss of Cl⊎.

Zur intermediären Existenz von Ethylenfluoronium-Ionen in der Gasphase/ On the Intermediary Existence of Gaseous Ethylen Fluoronium Ions

Abstract The long-sought ethylen fluoronium ion (2) is generated as an intermediate in the dissociative ionization of 1-fluoro-2-(p-methyl)phenoxy ethane (4). However, prior to collisionally induced dissociation 2 undergoes ring-opening, associated with hydrogen migration, to form 1-fluoroethyl cation (3). Other decomposition pathways of the molecular ions of 4 involve (i) direct formation of 3 via a combination of C-O-cleavage (loss of ArO -) and [1,2] hydrogen migration (18%) and (ii) complete positional loss of the a-and β-methylene hydrogen atoms (34%). The remaining 48% of the molecular ions of 4 dissociate via anchimeric assistance of the fluorine in the elimination of ArO·, thus giving rise to the formation of 2.

Silyl-Assistierte Etherspaltung bei Radikalkationen von Hydroxylaminderivaten [1] / Silyl-Assisted Ether Cleavage in Radical Cations of Hydroxylamine Derivatives [1]

Abstract It is demonstrad by means of various mass spectrometric techniques that the dissociative ionization of N.N-bis-silylated hydroxylamine derivatives (5a) and (5b), giving rise to the cleavage of the (O-C) bond, is anchimerically assisted by migration of the trimethylsilyl group to the ether oxygen, thus generating ion (7).