On the structure of the C2H4O2 neutrals (acetic acid versus 1,1-dihydroxyethene) generated from ionized n-hexanoic acid and n-butyl acetate in the gas phase. Implications for the mechanism of the McLafferty rearrangement

Baar, Ben L. M. van; Terlouw, Johan K.; Akkök, Semiha; Zummack, Waltraud; Schwarz, Helmut

doi:10.1016/0168-1176(87)80014-9

Cited by 29 publications

(7 citation statements)

References 12 publications

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“…This latter spectrum is very similar to the one from the [c4H,] + * ions from n-butyl acetate cations demonstrating that, unexpectedly, this rearrangement leads to [2-C4Hs]+' ions at high energies as well as at low energies. 24 On the other hand, sodium neutralization of four [C,H4J+' isomers gave very similar +NR+ and +NR-spectra. Bowers2, had earlier shown that only [m/z 27/[m/z 261 varied in the CAD spectra of [C4H4]+' isomers.…”

Section: Minimization Of Competing Isomerization Reactionsmentioning

confidence: 77%

Isomeric characterization via ion neutralization and dissociation. Experimental variables

McLafferty

Wesdemiotis

1989

Org. Mass Spectrom.

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Major deficiencies of mass spectrometry for characterizing isomeric molecules, and of collisionally activated dissociation for characterizing isomeric ions, can be alleviated by complementary information from new techniques of neutralization-reionization (NR) mass spectrometry. Mass data can be obtained from most fragments of the original species, irrespective of their abdity to retain the charge; dissociation of fast neutrals prepared from isomeric ions can involve novel reaction pathways and can minimize competing isomerization reactions; isomeric neutrals undergoing similar dissociations can be differentiated by forming them with different internal energies; reionization of the neutral products to negative as well as positive ions can provide increased selectivity; and structural information on the resulting ions can be derived using MS/MS/MS. Dissociation by novel non-isomerization pathways can also be effected by a second addition (or subtraction) of an electron to produce an unstable ion of opposite charge. Special techniques can yield neutralized products in favorable dissociative states by collisional activation, by using neutralization targets of selected ionization energy, or through Franck-Condon factors. Optimum excitation of the neutral is important, as this should be high enough to minimize rearrangement, to maximize the differences in the dissociation pathways of isomers, and to minimize the further dissociation of the characteristic primary products of the neutral. NR experiments can, thus, also provide information on the energy surfaces for unimolecular dissociations of neutrals that are difticult to study by conventional techniques. Dissociations of the neutrals can be differentiated from those occurring after reionization by separate collisional activation of the neutrals, by changing the ionization energy of the neutralization agent, or by reionization to ions of opposite charge. INTRODUCTIONCharacterization of molecular isomers by electronionization (EI) mass spectrometry,' or of ionic isomers by collisionally activated dissociation (CAD)2 has been successfully applied to a wide variety of important problems. However, in a number of cases EI or CAD spectra do not provide sufficiently extensive or different information to distinguish all possible isomers. For example, if the isomeric ions are separated by an isomerization barrier that is low relative to the respective individual dissociation thresholds, equilibration of the isomers can occur before fragmentation, leading to EI or CAD spectra in which most, or all, peaks exhibit very similar abundances. , Fig. 1); although ABCD" and ABDC" ions of sufficiently low internal energy will retain their isomeric identity indefinitely, increasing this energy to their dissociation thresholds will make their rates of isomerization much faster than those of dissociation. Ion-molecule reactions' offer one method of t Dedicated to Professor Allan Maccoll on the occasion of his 75th$ Authors to whom correspondence should be addressed. characterizing such io...

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Section: Minimization Of Competing Isomerization Reactionsmentioning

confidence: 77%

Isomeric characterization via ion neutralization and dissociation. Experimental variables

McLafferty

Wesdemiotis

1989

Org. Mass Spectrom.

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“…However, the detailed mechanism is still under debate between a concerted and a stepwise mechanism via formation of an intermediate distonic ion [6][7][8] by g-hydrogen shift and subsequent homolytic C-C bond cleavage to yield uncoordinated free products. The stepwise mechanism is strongly favored, [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] mainly based on arguments concerning the lifetime of intermediates 12 and the competitive H/D exchange in some isotopically labelled ketones prior to the C-C bond cleavage, depending on the stability of the intermediate distonic ion with branched and straight aliphatic chains 13 or the product stability. 14 An extension of the last step assumes the formation of an ion/molecule complex as intermediate for the products but also for intra-cage reactions like hydrogen scrambling or a further hydrogen transfer.…”

Section: Introductionmentioning

confidence: 99%

Electron-induced ionization of undeuterated and deuterated benzoic acid isopropyl esters and nicotinic acid isopropyl esters: Some implications for the mechanism of the McLafferty rearrangement

Opitz

Hashmi

Miehlich

et al. 2019

Eur J Mass Spectrom (Chichester)

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Electron ionization mass spectra, ionization, and appearance energies and bond energies (as dissociation energies) are reported for benzoic acid-1-methyl-ethyl ester (BAIPE), benzoic acid-1-deutero-1-methyl-ethyl ester (BAIPED1), benzoic acid-2,2,2-trideutero-1-trideuteromethyl-ethyl ester (BAIPED6) as well as nicotinic acid-1-methyl-ethyl ester (NAIPE), nicotinic acid-1-deutero-1-methyl-ethyl ester (NAIPED1), and nicotinic acid-2,2,2-trideutero-1-trideuteromethyl-ethyl ester (NAIPED6). Ionization energies of 9.39 eV for BAIPE, 9.40 eV for BAIPED1, 9.26 eV for BAIPED6 as well as 9.70 eV for NAIPE, 9.79 eV for NAIPED1, and 9.65 eV for NAIPED6 were determined. A gas-phase formation enthalpy of [Formula: see text] = (−4.10 ± 0.1) eV for BAIPE is calculated as well as [Formula: see text] = (−3.35 ± 0.1) eV for NAIPE. Molecular ions show two main fragmentation pathways. The first is a classical McLafferty rearrangement, characterized by the transfer of one γ-hydrogen atom from the isopropyl ester chain leading to the ions of the corresponding acid and neutral propene. The second is the double hydrogen transfer from the ester chain leading to the formation of the protonated acid and a C3H5√ allyl radical. For BAIPE, both hydrogen atoms originate from the methyl groups of the aliphatic chain with a probability of ≥98%, whereas the C-1-hydrogen is transferred with a probability of ≤2%. For NAIPE, both hydrogen atoms originate from the methyl groups of the aliphatic chain with a probability of 90%. Experimental proton affinities of PA = (8.75 ± 0.2) eV for benzoic acid and PA = (8.43 ± 0.2) eV for nicotinic acid are derived. For the protonation of the carbonyl group, B3LYP DFT calculations yielded PA = 8.66 eV for benzoic acid and PA = 8.41 eV for nicotinic acid. The overall fragmentation mechanism is explained with the initial formation of a 1,5-distonic ion by transfer of the first hydrogen. For the transfer of the second hydrogen, an intermediate ion/neutral complex is formulated.

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“… The study of the major MS fragmentations of these carbonyl derivatives, however, has not received due attention. For example, although the McLafferty rearrangement of carbonyl compounds has been the focus of extensive and continuous research , studies on oximes and silyl oxime ethers have been limited. Surprisingly, to date, only one article has reported the McLafferty rearrangement for ketoximes.…”

Section: Introductionmentioning

confidence: 99%

Fragmentation of oxime and silyl oxime ether odd‐electron positive ions by the McLafferty rearrangement: new insights on structural factors that promote α,β fragmentation

et al. 2012

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The McLafferty rearrangement is an extensively studied fragmentation reaction for the odd-electron positive ions from a diverse range of functional groups and molecules. Here, we present experimental and theoretical results of 12 model compounds that were synthesized and investigated by GC-TOF MS and density functional theory calculations. These compounds consisted of three main groups: carbonyls, oximes and silyl oxime ethers. In all electron ionization mass spectra, the fragment ions that could be attributed to the occurrence of a McLafferty rearrangement were observed. For t-butyldimethylsilyl oxime ethers with oxygen in a β-position, the McLafferty rearrangement was accompanied by loss of the t-butyl radical. The various mass spectra showed that the McLafferty rearrangement is relatively enhanced compared with other primary fragmentation reactions by the following factors: oxime versus carbonyl, oxygen versus methylene at the β-position and ketone versus aldehyde. Calculations predict that the stepwise mechanism is favored over the concerted mechanism for all but one compound. For carbonyl compounds, C–C bond breaking was the rate-determining step. However, for both the oximes and t-butyldimethylsilyl oxime ethers with oxygen at the β-position, the hydrogen transfer step was rate limiting, whereas with a CH2 group at the β-position, the C–C bond breaking was again rate determining. n-Propoxy-acetaldehyde, bearing an oxygen atom at the β-position, is the only case that was predicted to proceed through a concerted mechanism. The synthesized oximes exist as both the (E)- and (Z)-isomers, and these were separable by GC. In the mass spectra of the two isomers, fragment ions that were generated by the McLafferty rearrangement were observed. Finally, fragment ions corresponding to the McLafferty reverse charge rearrangement were observed for all compounds at varying relative ion intensities compared with the conventional McLafferty rearrangement.

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On the structure of the C2H4O2 neutrals (acetic acid versus 1,1-dihydroxyethene) generated from ionized n-hexanoic acid and n-butyl acetate in the gas phase. Implications for the mechanism of the McLafferty rearrangement

Cited by 29 publications

References 12 publications

Isomeric characterization via ion neutralization and dissociation. Experimental variables

Isomeric characterization via ion neutralization and dissociation. Experimental variables

Electron-induced ionization of undeuterated and deuterated benzoic acid isopropyl esters and nicotinic acid isopropyl esters: Some implications for the mechanism of the McLafferty rearrangement

Fragmentation of oxime and silyl oxime ether odd‐electron positive ions by the McLafferty rearrangement: new insights on structural factors that promote α,β fragmentation

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