Name: mass spectrometry fragmentation patterns
Uploaded: 2024-04-04T10:51:33.000Z
Duration: 1 min 20 s
Channel: JoVE

Bei der Massenspektrometrie wird das Analytmolekül zu einem molekularen Ion ionisiert, das ein Elektron weniger als das Analytmolekül hat.
Der Verlust eines Elektrons schwächt einige Bindungen im Molekülion, das anschließend auf Wegen fragmentiert, die zu relativ stabilen, kleineren Spezies führen.
Betrachten Sie zum Beispiel die Fragmentierung des Pentan-Molekülions. Die Spaltung erfolgt bevorzugt an der Bindung zwischen dem zweiten und dritten Kohlenstoff, nicht zwischen dem Terminal und dem benachbarten Kohlenstoff. Ersteres ergibt eine relativ stabile Karbokation.
Bei der Fragmentierung werden ein Kation und ein Radikal erzeugt, wobei die Anordnung des ungepaarten Elektrons zu zwei Spaltungsmodi führt. Die Spaltung, die ein stabiles Carbokation erzeugt, wird der Spaltung vorgezogen, die ein stabiles Radikal erzeugt.
Die Fragmentierung erfolgt auch über die Spaltung zweier Bindungen. Zum Beispiel ergeben molekulare Ionen von Alkoholen ein radikalisches Kation und ein neutrales Molekül.
Ein weiteres Fragmentierungsmuster ist die Bildung von resonanzstabilisierten Karbokationen, die durch das Vorhandensein von Pi-Bindungen oder nichtbindenden Elektronen im Analytmolekül angetrieben werden.

massenspektrometrie: Überblick über die molekulare fragmentierung

Mass Spectrometry: Molecular Fragmentation Overview

Die Ionisierung eines Moleküls zu einem Molekülion im Massenspektrometer führt aufgrund des Verlusts eines Elektrons zu einer Instabilität der Molekülstruktur. Dies führt schließlich zur Fragmentierung oder zum Bruch einiger Bindungen im Molekül. Die Fragmentierung erfolgt vorwiegend an bestimmten Bindungen und führt zu relativ stabilen Fragmenten.

Eine Art Fragmentierungsmuster ist die Spaltung einer Einzelbindung im Molekülion. Die Spaltung führt zu einem Radikal und einem Kation. Die Spaltung kann an verschiedenen Einzelbindungen im Molekül erfolgen, bevorzugt erfolgt die Spaltung jedoch an den Positionen, die ein stabiles Carbokation und ein Radikal erzeugen. An einer bestimmten Bindung kann das ungepaarte Elektron auf eines der beiden Fragmente übertragen werden, wodurch zwei Produktgruppen entstehen. Die bevorzugte Ausrichtung des Elektronentransfers erfolgt so, dass ein stabileres Carbokation statt eines stabilen Radikals entsteht. Betrachten Sie das in Abbildung 1 dargestellte Fragmentierungsmuster von Pentan.

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

Figure 1. Fragmentierung des Pentan-Molekülions.

Aus der Häufigkeit von Carbokationen in Massenspektren, wie in Abbildung 2 dargestellt, geht hervor, dass die Spaltung von Molekülionen zwischen zwei beliebigen benachbarten KohlenSubstanzatomen erfolgen kann. Allerdings ist die Spaltung zwischen zwei inneren KohlenSubstanzatomen bevorzugter als die Spaltung zwischen dem endständigen und seinem benachbarten KohlenSubstanzatom. Die Spaltung der inneren KohlenSubstanzen führt zu einem relativ stabilen primären Carbokation und Primärradikal. Die Spaltung der Bindung zwischen dem endständigen und seinem benachbarten KohlenSubstanz ergibt ein primäres Carbokation und ein Methylradikal oder ein primäres Radikal und ein Methylcarbokation. Wenn diese Spaltung stattfindet, erfolgt sie über den Spaltungsmodus, der zu einem Butylcarbokation und einem Methylradikal führt.

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

Figure 2. Das Massenspektrum von Pentan.

Eine andere Art der Spaltung ist die Dissoziation mehrerer Bindungen zu Fragmenten. Bei dieser Art der Fragmentierung entstehen ein kleineres Radikalkation und ein neutrales Molekül. Beispielsweise können Alkohole durch Abspaltung eines Wassermoleküls in ein kleineres Radikalkation fragmentieren. Folglich zeigen die Massenspektren einen M−18-Peak. Abbildung 3 zeigt die schematische Fragmentierung von 1-Heptanol.

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

Figure 3. Fragmentierung von 1-Heptanol.

Die Molekülmasse von Heptanol beträgt 116 u und ein Peak bei m/z = 98 ist in seinem Massenspektrum in Abbildung 4 sichtbar. Ähnlich verhält es sich mit Cycloalkenen, bei denen durch eine Retro-Diels-Alder-Reaktion ein Alken und ein kleineres Radikalkation freigesetzt werden.

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

Figure 4. Das Massenspektrum von Heptanol.

Die dritte Art der Fragmentierung wird in Molekülen mit ungeteilten Elektronen oder π-Bindungen beobachtet. In den Molekülen mit Einzelpaarelektronen bricht die KohlenSubstanz-KohlenSubstanz-Bindung neben dem einsamen Elektronenpaar tragenden Atom auf und es entsteht ein resonanzstabilisiertes Carbokation. Abbildung 5 zeigt beispielhaft die Fragmentierung von Ether.

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

Figure 5. Fragmentierung von Ethern.

Die π-Bindungen in einem Molekülion fördern die Fragmentierung des Moleküls, um ein durch Resonanz stabilisiertes Allylcarbokation zu erzeugen. Abbildung 6 zeigt das Fragmentierungsmuster eines terminalen Alkens.

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

Figure 6. Fragmentierung von Alkenen.

Massenspektrometrie: Überblick über die molekulare Fragmentierung

Die Ionisation eines Moleküls zu einem molekularen Ion im Massenspektrometer führt zu einer Instabilität in der Struktur des Moleküls aufgrund des Verlusts eines ...

mass-spectrometry-molecular-fragmentation-overview

Education

Lehre

JoVE Core

Analytical Chemistry

Analytische Chemie

Unit 1

Einheit 1

Principles of Mass Spectrometry

Prinzipien der Massenspektrometrie

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

mass spectrometry fragmentation patterns

Mass spectrometry fragmentation patterns show how a molecular ion breaks apart after ionization. When a molecule loses an electron inside the mass spectrometer, its structure becomes unstable. The ion then fragments at bonds that often produce the most stable pieces.

One common pattern is cleavage of a single bond in the molecular ion. This break forms a radical and a cation. The bond can split in more than one way, but the favored path is the one that gives the more stable carbocation rather than the more stable radical.

Pentane shows this idea clearly. Its molecular ion can break between any two adjacent carbon atoms, but cleavage between two inner carbon atoms is preferred over cleavage near the end of the chain. That inner break gives a relatively stable primary carbocation and primary radical. When the terminal bond breaks, the products can include a primary carbocation and a methyl radical, or a primary radical and a methyl carbocation. In the preferred case, the cleavage leads to a butyl carbocation and a methyl radical.

A second pattern is loss of a neutral fragment from the ion. This type of fragmentation gives a smaller radical cation plus a neutral molecule. Alcohols can lose water in this way, so their mass spectra often show an M−18 peak. For example, 1-heptanol has a molecular mass of 116 u, and its spectrum shows a peak at m/z = 98. Cycloalkenes can also follow a retro-Diels–Alder reaction to expel an alkene and leave a smaller radical cation.

A third pattern appears in molecules with lone pairs or π bonds. In compounds with lone pairs, the carbon-carbon bond next to the atom bearing the lone pair can break to form a resonance-stabilized carbocation, as seen with ethers. In molecules with π bonds, fragmentation often creates an allylic carbocation that is stabilized by resonance. A terminal alkene is a common example of this behavior.

Mass Spectrometry Fragmentation Patterns

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Mass Spectrometry Fragmentation Patterns

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