Gisbert Depke

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.

On the gas phase chemistry of ionized glycine and its enol. A combined experimental and ab initio molecular orbital study

Abstract A combined experimental and ab initio molecular orbital study reveals the following features for the gas phase chemistry of ionized glycine (7) and its enol form (8). The enol form of the all-planar conformer 8.2 is found to be substantially more stable than its ketocounterpart 7.0 by ca. 22 kcal mole−1 (6-31G//ST0-3G and 6-31G//3-21G), in line with many other keto/enol tautomers of cation radicals. The barriers for interconverting 7 and 8 via various possible isomerization reactions were determined to be as high as 80–90 kcal mole−1, thus preventing facile interconversion. In fact, there is no experimental evidence for any mutual isomerization of 7 and 8. The preferred pathway for dissociating 7 corresponds to a CC cleavage process, whereas 8 undergoes loss of water. H2O loss from 8 is preceded by a complete exchange of the hydrogens of the amino and hydroxyl groups followed by H2O loss which is predicted to result in the formation of the as yet unknown cation radical of aminoketene (15). Water loss is subject to a kinetic isotope effect of kH/kD = 2.4. Some comparisons are made between the gas phase chemistry of 7 8 and its analogous acetic acid (1) and their enol form 2, which provides some insight into the effects of the NH2 function.

[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.

Specific reciprocal methyl/hydrogen transfer reactions and hydrogen exchange processes between aryl rings in crowded ketone cation radicals☆

Abstract In distinct contrast to metastable triarylethenol cation radicals, the corresponding isomeric ketones not only show site-specific reciprocal methyl/hydrogen migration but, in addition, some of them undergo hydrogen exchange reactions between the various aryl rings prior to dissociation. A brief discussion is presented for H 2 O loss from ionized 1,2,2-trimesitylethanol and the gas-phase chemistry of the resulting trimesitylethene cation radical.