MNDO-rechnungen zum mechanismus der H₂O-abspaltung aus protoniertem cyclohexanon in der gasphase

The potential energy surfaces for some reactive C₆H₁₁O⁺ ions generated under chemical ionisation conditions in a mass spectrometer are explored by semiempirical quantum mechanical calculations (MNDO). It is shown that unimolecular H₂O loss from protonated cyclohexanone (4) proceeds via three competitive pathways, e.g. direct [1.3] and [1,4]-elimination from cyclohexanone-type intermediates and a process involving ring-contracted cations. The computational results do not only account for the experimentally observed fact that all methylene groups of cyclohexanone contribute to H₂O loss: moreover, they explain why only a minor fraction of protonated cyclohexanone undergoes unimolecular H₂O loss and also account for the fact that the non-decomposing [MHH₂O]⁺ ions generated from protonated cyclohexanediols rearrange to protonated cyclopentane carbaldehyd9e (3). The experimental result that non-decomposing 3 and 4 do not interconvert is also reflected by the MNDO calculations.