| | 1 | Author
| Gernot Frenking, Jochen Schmidt, Helmut Schwarz | Requires cookie* | | | Title
| MNDO-Rechnungen zur unimolekularen CO-Abspaltung aus Isomeren C3H5CO + -Ionen MNDO Calculations on the Unimolecular CO Loss from Isomerie C3HsC0 + Ions  | | | | 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 CHsCH = 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 a-oxo-and /?-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. | | | |
Reference
| Z. Naturforsch. 35b, 1031—1034 (1980); eingegangen am 21. März 1980 | | | |
Published
| 1980 | | | |
Keywords
| Carbocation Chemistry, Quantum Mechanical Calculations, Potential Energy Surfaces | | | |
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| default:Reihe_B/35/ZNB-1980-35b-1031.pdf | | | | Identifier
| ZNB-1980-35b-1031 | | | | Volume
| 35 | |
| 2 | Author
| Gemot Frenking, Jochen Schmidt, Helmut Schwarz | Requires cookie* | | | Title
| Quantenmechanische Berechnungen zu pseudo-einstufigen (C-C)-Spaltungen bei Radikalkationen von Carbonsäuren Quantum Mechanical Calculations on Pseudo-One-Step Cleavages of Carbonic Acid Cation Radicals | | | | Abstract
| Extensive quantum mechanical calculations on the unrestricted MINDO/3 level reveal that for some ionised carbonic acids the energetically most feasible pathway for uni-molecular C-C cleavage cannot be described as being a one-step process. In complete agreement with experimental results the computational data are consistent with an inter-pretation of the reaction as being a two-step process, in which the reaction commences with a hydrogen migration from a CH-bond onto the ionised C = 0 group. The dissociation is completed with a radical induced C-C cleavage thus generating quite stable protonated unsaturated carbonic acids. Whereas the MINDO/3 results clearly show that the two-step process is energetically favoured over the direct C-C cleavage, this computational method fails badly in explaining the experimentally established fact that in the two-step sequence the hydrogen migration is the rate-determining step. This failure is due to the MINDO/3 inherent weakness of overestimating the stability of cyclic ions, e.g. cyclic transition states as they are formed in the course of the hydrogen migration step. Single point ab initio calculations carried out on the 4-31G level substantiate this interpretation in that the transition state for the hydrogen migration is calculated to lie substantially higher than that for the actual C-C dissociation step, but is still much lower than that calculated for a direct simple bond cleavage process of the respective cation radical. | | | |
Reference
| Z. Naturforsch. 37b, 355—360 (1982); eingegangen am 23. Oktober 1981 | | | |
Published
| 1982 | | | |
Keywords
| Potential Energy Surfaces, Carbonic Acids, Hydrogen Rearrangement, C-C Cleavage, Mass Spectrometry | | | |
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| default:Reihe_B/37/ZNB-1982-37b-0355.pdf | | | | Identifier
| ZNB-1982-37b-0355 | | | | Volume
| 37 | |
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