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1987 (1)
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1Author    Gernot Frenking, Jochen Schmidt, Helmut SchwarzRequires 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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 TEI-XML for    default:Reihe_B/35/ZNB-1980-35b-1031.pdf 
 Identifier    ZNB-1980-35b-1031 
 Volume    35 
2Author    Gemot Frenking, Jochen Schmidt, Helmut SchwarzRequires 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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 TEI-XML for    default:Reihe_B/37/ZNB-1982-37b-0355.pdf 
 Identifier    ZNB-1982-37b-0355 
 Volume    37 
3Author    Martin BohlRequires cookie*
 Title    Molecular Mechanics Investigation on Side-Chain Conformations of a 17 a-Ethyl-17 ß-hydroxy Steroid with Regard to Receptor Binding  
 Abstract    A kadem ie der W issenschaften der D D R , Z entralinstitut für M ikrobiologie und experimentelle T herapie, B eutenbergstraße Energetically favourable conform ations for sim ultaneous intram olecular rotations of both the 17 a ethyl side chain and the 17 ß hydroxyl group of a model steroid are calculated by MM2 m olecular mechanics. In accordance with recent IR and N M R interpretations, the 17a substituent is found to preferably adopt conform ations which may sterically hinder the formation of hydrogen bonds betw een the steroidal 17ß oxygen atom and the receptor protein. Furtherm ore, the 17a ethyl substitution is com puted to influence the D-ring conform ation and to alter the location of the 17ß oxygen function by 28 pm in space. 
  Reference    Z. Naturforsch. 42c, 221 (1987); received N ovem ber 4 1986 
  Published    1987 
  Keywords    Steroid Conform ations, Potential Energy Surface, M olecular Mechanics Calculations, R eceptor Binding, 17a-Ethyl-estradiol 
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 TEI-XML for    default:Reihe_C/42/ZNC-1987-42c-0221.pdf 
 Identifier    ZNC-1987-42c-0221 
 Volume    42