Unimolecular reaction mechanisms: the role of reactive intermediates

Grützmacher, Hans‐Friedrich

doi:10.1016/0168-1176(92)85087-g

Cited by 33 publications

(7 citation statements)

References 53 publications

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“…This ion was depicted as either a one-bond cleavage product (hh, calculated mass of 150 + mass shift of 2 ) or as a two-bond cleav-age product (ii, calculated mass of 151, including a rearranged hydrogen atom + mass shift of 1). The latter process would be either a McLafferty-type, one-step, site-specific hydrogen rearrangement, or a three-step process initiated by a simple bond cleavage followed by an ion-neutral complex (INC) [14][15][16][17][18][19][20][21] and proton-bound complex (PBC) mediated, sitenon-specific, migration of a hydrogen atom. In any case, the rule of mass shift can predict the correct m/z value for the observed ion.…”

Section: Structural and Mechanistic Implicationsmentioning

confidence: 99%

“…Since the precursor ion is isolated in a mass spectrometer during CID experiments, the hydrogen atoms and/or protons responsible for the mass shift of product ions appear to come from the precursor ion itself. The intervention of the ion-neutral complex [14][15][16][17][18][19][20][21] would then possibly play an important role during fragmentation. A detailed discussion of this topic will be reported elsewhere.…”

Section: Structural and Mechanistic Implicationsmentioning

confidence: 99%

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A rule to account for mass shifts of even-electron fragment ions in fast-atom bombardment mass spectrometry

Nakata

1999

Eur. J. Mass Spectrom.

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A rule to account for mass shifts of even-electron fragment ions in fast-atom bombardment mass spectrometry a Hisao Nakata b 147-24, Arimatsu-ura, Narumi-cho, Midori-ku, Nagoya, 458-0824, Japan A rule is proposed for prediction of correct mass numbers of fragment ions in fast-atom bombardment (FAB) ionization mass spectrometry. The term "mass shift" in the rule is defined as the increment or diminution for the mass of an observed peak against the mass calculated from the corresponding part of the structural formula of a sample molecule. For positively-charged fragment ions, the mass shift is either 0, +1, +2 or even +3, according to the nature of the cleaved bond, and 0,-1 or-2 for negatively-charged fragment ions. The rule is based on the recognition that, with a few exceptions, fragment ions formed under FAB ionization are even-electron species and should have a stable structure which includes either (a) charged carbon atoms stabilized by conjugation with double bonds or with any heteroatom-containing functional groups, or (b) positively-or negatively-charged heteroatoms. The rule is also applicable to product ions observed in collision-induced dissociation.

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Section: Structural and Mechanistic Implicationsmentioning

confidence: 99%

Section: Structural and Mechanistic Implicationsmentioning

confidence: 99%

A rule to account for mass shifts of even-electron fragment ions in fast-atom bombardment mass spectrometry

Nakata

1999

Eur. J. Mass Spectrom.

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“…Both the Zhang and Hows research groups attribute the principal ion at m/z 128 to a quinolinium structure attained by an undescribed mechanism. While it is true that some low molecular weight nitrogenous compounds such as amino acids may form heterocyclic rings under conditions of pyrolysis (Sharma et al 2003) and as a general principle many unimolecular processes can lead to reactive intermediates capable of rearrangements (Gruetzmacher 1992), the simple mechanism described here (Scheme 1) is a superior argument because it accounts for generation of both m/z 128 and 127 by loss of either a hydrogen atom or of an H 2 molecule, respectively. Zhang et al (2008) in contrast felt obliged to replace the N atom in quinoline with a carbon to explain m/z 127 by an inexplicable mechanism.…”

Section: Discussionmentioning

confidence: 99%

Liquid chromatographic–electrospray mass spectrometric determination of 1-methyl-4-phenylpyridine (MPP⁺) in discrete regions of murine brain

Lehner

Johnson

Simkins

et al. 2010

Toxicology Mechanisms and Methods

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1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is widely used as a neurotoxin in several models of Parkinson’s disease in mice. MPTP is metabolized to 1-methyl-4-phenylpyridinium (MPP+), which is a mitochondrial toxicant of central dopamine (DA) neurons. There are species, strain, and age differences in sensitivity to MPTP. Simultaneous measurement of the MPTP active metabolite MPP+ and dopamine (DA) in the brain would be helpful in mechanistic studies of this neurotoxin. The objective of this study was to develop a liquid chromatography–mass spectrometry (LC/MS) method for analysis of MPTP and MPP+ in brain tissue and correlate these in the same sample with changes in DA measured via HPLC coupled with electrochemical detection. Twenty-five C57BL/6J7 8-week old female mice were used in the study. Mice were given a single subcutaneous injection of MPTP (20 mg/kg) and were sacrificed 1, 2, 4, or 8 h later. Zero time control mice received an injection of 0.9% normal saline (10 ml/kg) and were killed 1 h later. Brains were rapidly harvested and quickly frozen, and microdissected brain regions were placed in 0.1 M phosphate-citric acid buffer containing 20% methanol (pH 2.5). A new LC/MS method was successfully developed that utilized selected reaction monitoring (SRM) of MPP+ m/z 170→127, 170→128, and 170→154 fragmentation for quantitation and area ratios (m/z 127)/(m/z 128) and (m/z 154)/(128) for identity confirmation. A similar SRM strategy from m/z 174 was unable to detect any significant levels of MPTP down to 0.4 ppb. According to this method, MPP+ was detected in the nucleus accumbens (NA) and the striatum (ST), with the levels in the NA being 3-times higher than those in the ST. The advantage of this approach is that the tissue buffer used in this procedure allowed concurrent measurement of striatal DA, thus enabling direct correlation between accumulation of tissue MPP+ and depletion of DA concentrations in discrete regions of the brain.

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“…Protonated aryl ketones, ArC(R)=OH + , undergo the same step, generating the respective arenium ion, ArH + -C(R)=O prior to scission into the neutral arene and the corresponding acyl cation. [58][59][60][61][62][63] Unfortunately, little knowledge has been gained about the intermediacy of the tautomeric forms of such "formylarenium" intermediates, since any proton ring walk would inevitably be a hidden process. In fact, bimolecular probe reactions would help in this respect.…”

Section: Proton Transfer Processesmentioning

confidence: 99%

Concomitant Hydride and Proton Transfer: An Essay on Competing and Consecutive Key Reactions Occurring in Gaseous Ion/Neutral Complexes

Kuck

2012

Eur J Mass Spectrom (Chichester)

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The interplay of proton transfer and hydride transfer reactions in alkylbenzenium ions and related protonated di- and oligophenylalkanes is presented and discussed. While intra- and interannular proton exchange has been recognised to be an ubiquitous feature in protonated arenes, hydride abstraction is much less obvious but can become a dominating fragmentation channel in metastable ions of tert-butyl-substituted alkylbenzenium ions and related carbocations. In such cases, proton-induced release of the tert-butyl cation gives rise to ion/neutral complexes as reactive intermediates, for example, [(CH(3))(3)C(+)...arylCH(2)(α)(CH(2))(n)CH(2)(ω)aryl '] with n ≥ 0, and highly regioselective intra-complex hydride transfer occurs from all of the benzylic methylene hydride ion donor groups (α-CH(2) and ω-CH(2)) to the tert-butyl cation acting as a Lewis acid. Substituent effects on the individual contributions to the overall hydride transfer from different donor sites, including ortho-methyl groups, in particular, and the concomitant intra- complex proton transfer from the tert-butyl cation to the neutral diarylalkane constituent corroborate the view of "bisolvated" complexes as the central intermediates, in which the carbenium ion is coordinated to both of the aromatic π-electron systems. The role of cyclisation processes converting the benzylic, [M - H](+) type, ions into the isomeric benzenium, [M + H](+)-type, ions prior to fragmentation is demonstrated for several cases. This overall scenario, consisting of consecutive and/or competing intra-complex hydride abstraction and proton transfer, intraannular proton shifts (H+ ring walk) and interannular proton transfer, hydrogen exchange ("scrambling") processes, and cyclisation and other electrophilic substitution reactions, is of general importance in this field of gas-phase ion chemistry, and more recent examples concerning protonated ethers, benzylpyridinium and benzylammmonium ions are discussed in which these recurring features play central and concerted mechanistic roles as well.

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Unimolecular reaction mechanisms: the role of reactive intermediates

Cited by 33 publications

References 53 publications

A rule to account for mass shifts of even-electron fragment ions in fast-atom bombardment mass spectrometry

A rule to account for mass shifts of even-electron fragment ions in fast-atom bombardment mass spectrometry

Liquid chromatographic–electrospray mass spectrometric determination of 1-methyl-4-phenylpyridine (MPP⁺) in discrete regions of murine brain

Concomitant Hydride and Proton Transfer: An Essay on Competing and Consecutive Key Reactions Occurring in Gaseous Ion/Neutral Complexes

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Unimolecular reaction mechanisms: the role of reactive intermediates

Cited by 33 publications

References 53 publications

A rule to account for mass shifts of even-electron fragment ions in fast-atom bombardment mass spectrometry

A rule to account for mass shifts of even-electron fragment ions in fast-atom bombardment mass spectrometry

Liquid chromatographic–electrospray mass spectrometric determination of 1-methyl-4-phenylpyridine (MPP+) in discrete regions of murine brain

Concomitant Hydride and Proton Transfer: An Essay on Competing and Consecutive Key Reactions Occurring in Gaseous Ion/Neutral Complexes

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Liquid chromatographic–electrospray mass spectrometric determination of 1-methyl-4-phenylpyridine (MPP⁺) in discrete regions of murine brain