- A study of the gas-phase reactivity of neutral alkoxy radicals by mass spectrometry: α-Cleavages and Barton-type hydrogen migrations
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The reactivity of neutral alkoxy radicals in the absence of any interfering intermolecular interactions is investigated by means of the recently introduced method of neutral and ion decomposition difference (NIDD) spectra. These are obtained from quantitative analysis of the corresponding neutralization - reionization (NR) and charge reversal (CR) mass spectra. The following trends emerge: alkoxy radicals with short (C1-C3) or branched alkyl chains give rise to α-cleavage products, whereas longer-chained alkoxy radicals undergo 1,5-hydrogen migrations from carbon to oxygen, that is, Barton-type chemistry. This facile rearrangement has been studied in detail for n-pentoxy radicals by isotopic labeling experiments and computation at the Becke 3LYP/6-31G* level of theory. Further, the NIDD spectra of 3-methylpentoxy radicals permit for the first time the identification of the diastereoselectivity of the gas-phase hydrogen migrations. The results from the NIDD method are compared to those from earlier studies in the condensed phase. This new mass spectrometric approach is suggested as a tool for the examination of intramolecular reactions of free alkoxy radicals which can usefully complement theoretical studies.
- Hornung, Georg,Schalley, Christoph A.,Dieterle, Martin,Schroeder, Detlef,Schwarz, Helmut
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p. 1866 - 1883
(2007/10/03)
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- Noncompeting Metastable Losses of Methyl and Ethylene from Gaseous Butanoic Acid Ions due to Isomerization Prior to Methyl Loss
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Metastable C4H8O+. ions obtained from butanoic acid and ethyl butanoate undergo considerable γ-hydrogen exchange prior to losing ethylene, but little exchange prior to losing methyl.Therefore the two fragmentations are not directly competing, contrary to the general assumption that all reactions of an ion in the gas phase are competitive.It is concluded that metastable butanoic acid ions which lose methyl isomerize essentially irreversibly to CH3CH2C.HC(OH)2+ and/or CH3CH(C.H2)C(OH)2+ before the γ-methyl becomes exchanged.This accounts for the difference between γ-hydrogen exchange prior to the loss of methyl and ethylene without invoking isolated electronic states, as previously proposed.Butanoic acid ions generated by the McLafferty rearrangement of butanoate esters have a much weaker metastable loss of ethylene than directly ionized butanoic acid.Collisional activation experiments demonstrate that this results from more of the butanoic acid ions derived from ethyl butanoate than from butanoic acid isomerizing prior to collision.Variation in internal energy probably causes this difference in degree of isomerization with the source of the ion.
- McAdoo, David J.,Hudson, Charles E.
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p. 7710 - 7713
(2007/10/02)
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