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1982 (1)
1981 (1)
1978 (1)
1Author    Chrysostomos Wesdemiotis, Helmut Schwarz, Friedrich Borchers, Heinz Heimbach, Karsten LevsenRequires cookie*
 Title    Massenspektrometrische Untersuchungen zur Genese von [M—Me] [M-Et] [C7H7]+ und [C7H8]+ aus n-Butyl-und n-Pentylbenzol: Zeitabhängige Nachbargruppeneffekte, Isomerisierungen und kompetitive Wasserstoff-Umlagerungen Mass Spectrometric Investigations on the Formation of [M-Me]+, [M-Et]+, [C7H7] + and [C7H8] + - from w-Butyl and w-Pentyl Benzenes: Time Dependent Neighbouring Group Participation, Isomerization and Competitive Hydrogen Rearrangements  
 Abstract    The details of the elimination of CH3 1 and C2Hs -from the molecular ions of n-butyl and n-pentyl benzenes as well as the formation of C7H7+ and C7Hs + -have been established using a combination of different mass spectrometric techniques. Among these techniques are field ionization kinetics (FIK), collisional activation (CA), unimolecular decomposi-tion of metastable ions (MI), kinetic energy release determinations (Tkin)> appearance potential measurements (AP) as well as high resolution mass spectrometry. The appli-cation of these methods and the investigation of nine 13 carbon and eleven deuterium labelled n-butyl and n-pentyl benzenes clearly demonstrate that 1) the loss of methyl and ethyl from terminal positions of the alkyl chains are accompanied by an interaction with the phenyl ring and 2) the regioselectivity of these processes is time dependent. Contrary to previous conclusions it is shown that hydrogen transfer in the formation of C7H8 + -occurs via five-, six-and seven-membered transition states. The time dependence of the hydrogen exchange reaction preceding the alkene eliminations is discussed in detail. 
  Reference    Z. Naturforsch. 33b, 1150—1164 (1978); eingegangen am 1. März 1978 
  Published    1978 
  Keywords    Hydrocarbons, Hydrogen Rearrangement, Metastable Ions, Collisional Activation, Field Ionization Kinetics 
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 TEI-XML for    default:Reihe_B/33/ZNB-1978-33b-1150.pdf 
 Identifier    ZNB-1978-33b-1150 
 Volume    33 
2Author    Bernd Richter, Helmut SchwarzRequires cookie*
 Title    Zur McLafferty-Umlagerung bei Radikalkationen von Phenylpyridylalkanonen [1] On the McLafferty Rearrangement of Ionized Phenyl Pyridyl Alkanones [1]  
 Abstract    A detailed investigation concerning the genesis of McLafferty rearrangement products from molecular ions of phenyl pyridyl alkanones reveals the following features: 1) The various products are not formed by competitive dissociations of the molecular ion. Most of the relevant fragment ions are generated from the primary McLafferty product of a hydrogen transfer to the ionized carbonyl group (M +> ->m/z 163, ion k). The ion k plays a decisive role for the generation of the abundant product ions at m/z 93 (ion g) and m/z 106 (ion s) both of which are formed by further dissociation of k. Details of the mechanisms for the decomposition of k are obtained by investigating [D]-labelled isotopomers, the analysis of low and high resolution data, the application of MIKE and CA spectra. The problem of keto/enol tautomerism between some ions, relevant in this context, is discussed and it is shown that these isomerization processes are not involved. A detaüed description is given for the syntheses of various [D]-labelled phenyl pyridyl alkanones. 
  Reference    Z. Naturforsch. 36b, 865—877 (1981); eingegangen am 16. Dezember 1980 
  Published    1981 
  Keywords    Mass Spectrometry, Hydrogen Rearrangement, Keto Enol Tautomerism, Metastable Ions, Collisional Activation 
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 TEI-XML for    default:Reihe_B/36/ZNB-1981-36b-0865.pdf 
 Identifier    ZNB-1981-36b-0865 
 Volume    36 
3Author    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