| | 3 | Author
| Helmut Schwarz, Chrysostomos Wesdemiotis, Thomas Weiske, Klaus-Peter Zeller, Roland Müller | Requires cookie* | | | Title
| H. Schwarz et al. * Methyl-Eliminierung aus dem metastabilen Homoadamantan-Radikalkation 219 Methyl Loss from Metastable Homoadamantane Cation Radical  | | | | Abstract
| It is demonstrated that methyl loss from ionized homoadamantane (1) yields exclusively the 1-adamantyl cation (4); there is no experimental evidence for the formation of the secondary adamantyl cation (5). From both model calculations and the investigation of [4-13 C]-homoadamantane (la) and l(13 C-methyl)adamantane (2a) it is concluded, that 24% of the metastable homoadamantane cation radicals dissociate after one isomerization (1 2); the remaining 76% are able to undergo at least a second (degenerate) isomerization cycle (2->l->2) prior to methyl loss. 78% of metastable 1-methyl-adamantane cation radical, obtained upon direct ionization of the corresponding neutral hydrocarbon, dissociate directly, whereas the loss of methyl from the remaining 22% is preceded by an isomerization 2 ->1 ->2. | | | |
Reference
| Z. Naturforsch. 35b, 207—211 (1980); eingegangen am 23. Oktober 1979 | | | |
Published
| 1980 | | | |
Keywords
| Ion Structures, Carbocation Rearrangements, Hydrocarbon Chemistry, Mass Spectrometry, Collisional Activation | | | |
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| default:Reihe_B/35/ZNB-1980-35b-0207.pdf | | | | Identifier
| ZNB-1980-35b-0207 | | | | Volume
| 35 | |
| 4 | Author
| Karsten Levsen, RichardD. Bowen, DudleyH. Williams | Requires cookie* | | | Title
| | | | | 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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| default:Reihe_B/34/ZNB-1979-34b-0488.pdf | | | | Identifier
| ZNB-1979-34b-0488 | | | | Volume
| 34 | |
| 5 | Author
| Bernhard Ciommer, Helmut Schwarz | Requires cookie* | | | Title
| [1.3] -Hydroxylwanderung bei Radikalkationen von Allen-und Acetylenalkoholen [1] [l,3]-Hydroxyl Migration in Cation Radicals of Allene and Acetylene Alcohols | | | | Abstract
| Contrary to previous conclusions it is shown that the molecular ions of 1, 2, and 3 do not interconvert prior to dissociation. Investigation of labelled compounds establishes a new typ of intramolecular [1,3]-hydroxyl migration to allenic and acetylenic carbons. It is shown that both 1 and 2 undergo irreversible rearrangement to an a,^-unsaturated ketone 4. Methyl loss from the intermediate 4, which accounts for 90% of the total methyl elimination, is preceded by partial hydrogen exchange processes between the three methyl groups of 4. The actual dissociation can be described as an a-cleavage to the carbonyl function. The unusual formation of CH3CO 4 " is shown to occur exclusively from 1. However, | | | |
Reference
| Z. Naturforsch. 34b, 1307—1319 (1979); eingegangen am 15. Mai 1979 | | | |
Published
| 1979 | | | |
Keywords
| Rearrangement of Allenes, Hydroxyl Migration, Exchange Processes, Ion Structures, Gas Phase Chemistry | | | |
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| default:Reihe_B/34/ZNB-1979-34b-1307.pdf | | | | Identifier
| ZNB-1979-34b-1307 | | | | Volume
| 34 | |
| 6 | Author
| Wilfried Franke, Helmut Schwarz, Chrysostomos Wesdemiotis | Requires cookie* | | | Title
| Elf stabile C5H9+-Kationen in der Gasphase. Zur dissoziativen Ionisierung von 31 isomeren C6H9Br-Verbindungen Eleven Stable CsHo 4 " Cations in the Gas Phase. On the Dissociative Ionization of 31 Isomeric CsHgBr Compounds | | | | Abstract
| Collisional activation mass spectrometry (CA) reveals the existence of 11 stable CsH9+ cations in the gas phase, e. g. the substituted allyl cations a, b, c, d,and e, the sub-stituted vinyl cations f, g, h, and i, the methyl cyclobutyl cation j and the cyclopentyl cation k, respectively. The ethyl substituted allyl cation a is formed via dissociative ionization of the isomeric precursors 1, 3, 4, 5, 18, 19, 20, 22, and 28 by means of various mechanistic processes, whereas the 1,3-dimethylallyl cation b is generated from both 2 (by allylic cleavage) and in part from the stereoisomeric cyclopropan derivatives 25, 26 and 27. 6"l + -gives a mixture of the vinyl cations 1 and g. From 13 and 14 the main product generated is the 1,2-dimethylallyl cation d, which is formed directly from 11 and also by quite complicated processes from 13, 14 and to a certain extent from 25, 26 and 27. The dissociative ionization of 9, 15, 16, 21, 24 and (in part) 23 give rise to the formation of the substituted vinyl cation h. Decomposition of 23"!+ • results not only in formation of h but generates also the 1,1-dimethylallyl cation e. From 291+-and 30~l +-both the methylcyclo-butyl cation j and cyclopentyl cation k are produced, whereas the isomeric precursor 28 gives mainly the substituted allyl cation a and a second, as yet, unidentified CsH^ cation. In general, it can be stated that the gas phase chemistry of cation radicals of substituted cyclopropanes is characterised by multistep-reactions, commencing with spontaneous ring opening. The so formed intermediates undergo various rearrangements (including hydrogen and alkyl shifts) prior to expulsion of Br\ Direct elimination of Br' from intact cyclopropan-like structures, followed by ring opening of the intermediate cyclopropyl cation, cannot compete with the above-mentioned multistep-sequences. | | | |
Reference
| Z. Naturforsch. 36b, 1315—1323 (1981); eingegangen am 2. Juni 1981 | | | |
Published
| 1981 | | | |
Keywords
| Isolated Carbocations, Ion Structures, Gas Phase Ion Chemistry, Mass Spectrometry, Collisional Activation | | | |
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| default:Reihe_B/36/ZNB-1981-36b-1315.pdf | | | | Identifier
| ZNB-1981-36b-1315 | | | | Volume
| 36 | |
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