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'Unimolecular Isomerization' in keywords
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1979 (1)
1Author    Karsten Levsen, RichardD. Bowen, DudleyH. WilliamsRequires cookie*
 Abstract    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 C3HsCO + ions formed by "OCH3 loss from the molecular ions of 14, 15 and other isomeric precursors. Four distinct C3HsCO+ 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 4 -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 endo-thermic. 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 C3Hs+ 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). Die interessanten chemischen Eigenschaften klei-ner Ringe werden seit vielen Jahren intensiv stu-diert [1]. Besondere Beachtung haben substituierte Cyclopropane (1 und 3) gefunden, für die Hoffmann [2] aufgrund von Molekülorbitalrechnungen das folgende Reaktivitätsmuster vorhergesagt hat: Elektronenliefernde Substituenten B schwächen die C(2)-C<3)-Bindung und sollten bei einer chemischen Reaktion (wie z.B. Hydrogenolj'se) zu C-verzweigten Produkten der allgemeinen Konstitution 2 führen. Für elektronenziehende Substituenten A gilt das Umgekehrte: Sie verstärken die C(2)-C(3>-und labili-sieren die C(i)-C(2)-Bindung mit dem Resultat, daß Produkte mit einer unverzweigten C3-Einheit ent-stehen. Tatsächlich konnte Musso [3] beweisen, daß 
  Reference    Z. Naturforsch. 34b, 488—494 (1979); eingegangen am 27. November 1978 
  Published    1979 
  Keywords    Unimolecular Isomerization, Ion Lifetime, Ion Structure, Collisional Activation, Kinetic Energy Release 
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 TEI-XML for    default:Reihe_B/34/ZNB-1979-34b-0488.pdf 
 Identifier    ZNB-1979-34b-0488 
 Volume    34