MNDO-Rechnungen zur unimolekularen CO-Abspaltung aus Isomeren C3H5CO+ -Ionen / MNDO Calculations on the Unimolecular CO Loss from Isomerie C3H5CO+-Ions

Abstract

Abstract In agreement with experimental results MNDO calculations indicate that unimolecular CO loss from the isomeric ions CH2 = CHCH2-CO+ (1), c-C3H5-CO+ (2), and CH3CH = CH-CO+ (3), gives exclusively allyl cation (4). It is shown that, whereas CO loss from 1 is a continuously endothermic process, the analogous reaction from 2 and 3 follows relatively complicated pathways. For 2, the energetically most feasible reaction involves an isomerization of 2 to 1 via the intermediate α-oxo-and β-oxocyclobutyl cations (8) and (9). This reaction sequence is 10 kcal • mol-1 lower in energy than direct CO loss from 2 coupled with isomerization of the incipient cyclopropyl cation (7) to allyl cation (4). For 3 it is shown that this ion undergoes isomerization to 2 prior to CO loss. The alternative reaction, i.e. direct CO elimination giving rise to 1-propenyl cation (11) or isomerization to 1 were 35 and 43 kcal • mol-1 higher in energy than the process 3 → 2.