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

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, H₃CCOH^+·, 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.