130,964 research outputs found
Unidirectional triple and double hydrogen rearrangement reactions in the radical cations of gamma-arylalkanols
No full textKuck D, Filges U. Unidirectional triple and double hydrogen rearrangement reactions in the radical cations of gamma-arylalkanols. Organic Mass Spectrometry. 1988;23(9):643-653.A novel fragmentation reaction accompanied by the unidirectional migration of three hydrogen atoms has been found in the radical cations of -arylpropanols with electron-donating substituents in the para position. This triple hydrogen (3H) rearrangement reaction is the dominant fragmentation channel of the long-lived molecular ions of trans-2-(4-dimethylaminobenzyl)-l-indanol, 2, but it occurs also in simpler -arylpropanol ions. Deuterium labelling of 2 reveals that the three hydrogen atoms originate with extraordinarily high specificity from the C(l), C(2) and O positions of the alcohol moiety. Cis- and 3-substituted isomers do not undergo this reaction. Along with the 3H rearrangement reaction a unidirectional double hydrogen (2H) rearrangement reaction takes place independently and with less specificity in the trans-2-(4-X-benzyl)-l-indanol ions 1+· and 2+·. No hydrogen exchange occurs during the 3H and 2H rearrangement reactions. Mechanistic alternatives of these unusual fragmentation reactions are discussed; the experimental evidence strongly favours pathways via several intermediate ion-neutral complexes
Remote fragmentations of protonated aromatic carbonyl compounds via internal reactions in intermediary ion-neutral complexes
No full textThielking G, Filges U, Grützmacher H-F. Remote fragmentations of protonated aromatic carbonyl compounds via internal reactions in intermediary ion-neutral complexes. Journal of the American Society for Mass Spectrometry. 1992;3(4):417-426.Protonated aromatic aldehydes and methyl ketones 1a-10a, carrying initially the proton at the carbonyl group, are prepared by electron impact-induced loss of a methyl radical from 1-arylethanols and 2-aryl-2-propanols, respectively. The aryl moiety of the ions corresponds to a benzene group, a naphthalene group, a phenanthrene group, a biphenyl group, and a terphenyl group, respectively, each substituted by a CH3OCH2 side-chain as remote from the acyl substituent as possible. The characteristic reactions of the metastable ions, studied by mass-analyzed ion kinetic energy spectrometry, are the elimination of methanol, the formation of CH3OCH2+ ions, and the elimination of an ester RCOOCH3 (R = H and CH3). The mechanisms of these fragmentations were studied by using D-labeled derivatives. Confirming earlier results, it is shown that the ester elimination, at least from the protonated aryl methyl ketones, has to proceed by an intermediate [acyl cation/arylmethyl methyl ether]-complex. The relative abundances of the elimination of methanol and of the ester decrease and increase, respectively, with the size of the aromatic system. Clearly, the fragmentation via intermediate ion-neutral complexes is favored for the larger ions. Furthermore, the acyl cation of these complexes can move unrestricted over quite large molecular distances to react with the remote CH3OCH2-side-chain, contrasting the restricted migration of a proton by 1,2-shifts ("ring walk") in these systems
Internal reactions of ion/molecule complexes from isomeric protonated formyl-and acetyl-naphthalenes
No full textFilges U, Grützmacher H-F. Internal reactions of ion/molecule complexes from isomeric protonated formyl-and acetyl-naphthalenes. International Journal of Mass Spectrometry and Ion Processes. 1988;83(1-2):111-133
Fragmentations of protonated acetophenones via intermediate ion-molecule complexes
No full textFilges U, Grützmacher H-F. Fragmentations of protonated acetophenones via intermediate ion-molecule complexes. Organic Mass Spectrometry. 1987;22(7):444-450
Proton migration in naphthalenium ions via [sigma] and [pi] complexes
No full textFilges U, Grützmacher H-F. Proton migration in naphthalenium ions via [sigma] and [pi] complexes. International Journal of Mass Spectrometry and Ion Processes. 1988;83(1-2):93-109
Fragmentations of protonated benzaldehydes via intermediate ion/molecule complexes
No full textFilges U, Grützmacher H-F. Fragmentations of protonated benzaldehydes via intermediate ion/molecule complexes. Organic Mass Spectrometry. 1986;21(10):673-680
Interannular proton exchange and fragmentation of carbonyl-protonated benzophenones
No full textSun J, Grützmacher H-F. Interannular proton exchange and fragmentation of carbonyl-protonated benzophenones. Organic Mass Spectrometry. 1991;26(12):1045-1051.Benzophenones a initially protonated at the carbonyl group Were prepared by electron-impact induced dissocation of 1,1-diphenylpropanols (compounds 1-5). These protonated ketones decompose in the ion source and the second field-free region of a reversed geometry mass spectrometer by proton migration to one of the phenyl groups and subsequent elimination of benzene. In the case of derivatives substituted by methoxy groups and trifluoromethyl groups, respectively, the proton migrates predominantly to the more bask benzene ring, resulting in the elimination of anisole in the former case and of benzene in the latter case. A study of protonated benzophenones labelled at the phenyl ring and at the carbonyl group shows that only a few interannular H/D exchange steps precede the fragmentation. This is observed not only for metastable ions in the magnetic sector instrument but also for ions of long lifetimes investigated by Fournier-transform-ion cyclotron resonance (FT-ICR) spectrometry. This is in contrast to the arene elimination from protonated 1,omega-diphenylalkanes and related polyphenylalkanes which fragment by complete positional exchange of all hydrogen atoms at the aromatic rings. The special behaviour of protonated benzophenones is attributed to a low barrier for the decomposition of a chemically activated arenium ion b, which arises from the initial proton transfer. Once b is formed, it decomposes quickly without much interannular proton exchange
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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