Ringöffnung CO+- und COOCH3+·-substituierter Cyclopropane und CO-Eliminierung aus isomeren C3H5CO+-Ionen / Ring Opening of CO+ and COOCH3+· Substituted Cyclopropanes and CO Loss from Isomeric C3H5CO+ Ions

The non-decomposing molecular ions of methyl cyclopropanecarboxylate (14) are found to rearrange to ionised methyl but-3-enoate (15). For ions with sufficient internal energy to decompose, this isomerization is in competition with · OCH3 loss, via direct cleavage of the ester group. Collisional activation spectroscopy may be used to distinguish between the C3H5CO+ ions formed by · OCH3 loss from the molecular ions of 14, 15 and other isomeric precursors. Four distinct C3H5CO+ species (18-21) can be identified in this way; these C3H5CO+ ions may themselves decompose, via CO elimination. Consideration of the metastable peak shape for CO loss, in conjunction with collisional activation spectroscopy on the resulting C3H5+ -ions, leads to two main conclusions, (i) Two C3H5+ ions (22 and 27) exist in potential energy wells. The very narrow metastable peaks for CO loss from 19 and 21 (leading to 22 and 27, respectively) show that these processes are continuously endothermic. In contrast, CO loss from either 18 or 20 gives rise to much broader metastable peaks. This suggests that rate-determining rearrangement of the incipient C3H5+ cations, to a more stable isomer, occurs prior to decomposition, (ii) Elimination of CO from the [M- · OCH3]+ fragment of 14 gives rise to a composite metastable peak, thus indicating the occurrence of two competing channels for dissociation. These channels are assigned to CO loss from 18 (larger kinetic energy release) and CO loss from 19 (smaller kinetic energy release).