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11.
Seymour Meyerson 《Journal of mass spectrometry : JMS》1989,24(8):653-662
A series of deuterium-labeled n-octadecyl benzoates serves to extend the literature using lower homologues for study of the mass-spectral reactions characteristic of alkyl esters. The benzoic acid radical cation (a) arises largely via hydrogen migration from C(2), as expected for the conventional mechanism passing through a six-membered quasicyclic intermediate; the presence of contributions from other processes is simply noted in passing. The octadecene radical cation (b) and protonated benzoic acid (c) arise by paths seemingly closely related to each other but differing from the path dominant for a. The data are rationalized in terms of a reaction sequence passing through two ion-neutral complex intermediates. The first, (d), consists of octadecyl carbenium ion and benzoyloxy radical; the second, (e), consists of octadecene radical cation and benzoic acid. The ionized partner in d appears to undergo none to several hydride migrations, but most prominently one; that in e may dissociate promptly to yield b or may persist, undergoing multiple hydride migrations to effect essentially complete scrambling of hydrogens, before dissociating to yield c. The argument for this mechanistic sequence to form b and c rests basically on its ability to account plausibly for the experimental data, and literature precedents cited for specific aspects of it furnish additional support. The low critical energies and small kinetic energy releases observed in this work are in accord with the characteristics listed by McAdoo for decompositions mediated by ion-neutral complexes, and the selection of an octadecyl, as opposed to a smaller alkyl, ester predisposes the molecule to favor such low-energy processes. 相似文献
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Sudipto Guha Adam Meyerson Kamesh Munagala 《Journal of Algorithms in Cognition, Informatics and Logic》2003,48(2):429-440
We consider a generalization of the classical facility location problem, where we require the solution to be fault-tolerant. In this generalization, every demand point j must be served by rj facilities instead of just one. The facilities other than the closest one are “backup” facilities for that demand, and any such facility will be used only if all closer facilities (or the links to them) fail. Hence, for any demand point, we can assign nonincreasing weights to the routing costs to farther facilities. The cost of assignment for demand j is the weighted linear combination of the assignment costs to its rj closest open facilities. We wish to minimize the sum of the cost of opening the facilities and the assignment cost of each demand j. We obtain a factor 4 approximation to this problem through the application of various rounding techniques to the linear relaxation of an integer program formulation. We further improve the approximation ratio to 3.16 using randomization and to 2.41 using greedy local-search type techniques. 相似文献
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Competing primary reactions by which 1-tetralone loses C2H4 and CO give rise to a doublet at mass 118 in the high-resolution mass spectrum corresponding to 96·4% C8H6O+ and 3·6% C9H10+, respectively. Study of the remainder of the spectrum however, suggests that these intensities constitute a poor measure of the relative importance of the two reaction paths. The C9H10+ ion evidently degrades more readily than C8H6O+, since an estimated 26% of the ions arising in these paths involve primary loss of CO. This estimate is supported by voltage dependence of the intensity distribution between the members of the doublet. As ionizing voltage is decreased, the intensity distribution is constant to about 20 volts, but between 20 and 10 volts the value for C9H10+ rises smoothly from about 3.5% to about 25%. 相似文献
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Seymour Meyerson Catherine Fenselau J. L. Young W. R. Landis Edward Selke L. C. Leitch 《Journal of mass spectrometry : JMS》1970,3(6):689-707
The mass spectra of hexanal, heptanal and nonanal variously labeled with deuterium confirm γ-hydrogen migration and β cleavage as the mechanism leading to [C2H4O]+˙ and [M ? C2H4O]+˙, although the data on the latter are complicated by contributions from other, related paths. In addition, they show that three other major primary decomposition products, [M ? C2H4]+˙, [M ? H2O]+˙ and [C3H5O]+, all arise in large part by processes involving γ-hydrogen migration to the oxygen atom. The ethylene lost to yield the first of these products consists of the α and β methylene groups. The loss of ethylene most likely occurs by way of a cyclobutanol intermediate, which, via alternative reaction paths, may well contribute to the yields of the other two products as well. These findings further extend the range of parallelism between photochemical and electron-impact-induced reactions. 相似文献
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The dominant primary reactions of propionitrile under electron-impact effect loss of HCN and loss of H·. Deuterium labeling shows that the hydrogen atom lost as HCN comes chiefly from C-2, but that lost as a free atom comes chiefly from C-3. Both reactions are probably preceded by a 1,2 hydrogen migration to yield an allylic-like molecular ion, \documentclass{article}\pagestyle{empty}\begin{document}$ \left[{{\rm CH}_{\rm 3}{\rm C}^+ {\rm HCH = N}^{\rm .} \leftrightarrow {\rm CH}_{\rm 3} \mathop {\rm C}\limits^{\rm .} {\rm HCH = N}} \right]^+ $\end{document}. 相似文献
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The mass spectrum of nitromethane points to rupture of the CH3? NO2 bond as the dominant primary reaction, as also observed in pyrolysis, photolysis and radiolysis. Isomerization of the molecular ion to the nitrite configuration seems to contribute little in the mass spectrum of nitromethane, in contrast to those of nitrobenzene and other nitroarenes. The nitrite ion is probably the immediate precursor of [NO]+ at its appearance potential, but most of the [NO]+ yield seems to stem from secondary decomposition of excited [NO2]+. 相似文献
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Carla B. Theissling Nico M. M. Nibbering Seymour Meyerson 《Journal of mass spectrometry : JMS》1976,11(8):838-843
Evidence has been reported that primary loss of H and of HCN from the molecular ions of propionitrile, isobutyronitrile and butyronitrile in the mass spectrometer is preferentially preceded by hydrogen migration from C-2 to C-1. Ion cyclotron double resonance spectra of proton (or deuteron-) transfer products derived from propionitrile-2-d2 and -3-d3 and a series of bases provide evidence that such migration occurs also in long-lived propionitrile molecular ions. 相似文献
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Seymour Meyerson 《Journal of mass spectrometry : JMS》1970,3(1):119-125
Mass spectra of isotope-labeled triptycenes, triphenylmethanes and diphenylmethanes rule out the bulk of postulated decomposition mechanisms and fragment-ion structures. The formation of [M ? H]+ and [M ? 2H]2+ from triptycene, of [M ? H]+, [M ? CH3]+ and [M ? CH4]+ from triphenylmethane, and of [M ? H]2+ and [M ? 2H]2+-as well as the previously reported [M ? H]+ and [M ? CH3]+-from diphenylmethane all seem to be preceded or accompanied by complete loss of position identity of the α and ring hydrogens in the original molecules. A statistical preference for loss of α hydrogens is found in the process leading to [M ? 2H]+ and [M ? H]2+ from triptycene, as in the formation of [M ? H]2+ from toluene. 相似文献
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