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

Nella spettrometria di massa, la molecola dell'analita viene ionizzata in uno ione molecolare con un elettrone in meno rispetto alla molecola dell'analita.
La perdita di un elettrone indebolisce alcuni legami nello ione molecolare, che successivamente si frammenta attraverso percorsi che portano a specie relativamente stabili e più piccole.
Ad esempio, si consideri la frammentazione dello ione molecolare pentano. La scissione avviene preferenzialmente al legame tra il secondo e il terzo carbonio, non tra il terminale e il suo carbonio adiacente. Il primo produce un carbocatione relativamente stabile.
La frammentazione genera un catione e un radicale, dove la disposizione dell'elettrone spaiato si traduce in due modalità di scissione. La scissione che produce un carbocatione stabile è preferita a quella che genera un radicale stabile.
La frammentazione avviene anche attraverso la scissione di due legami. Ad esempio, gli ioni molecolari degli alcoli producono un catione radicale e una molecola neutra.
Un altro modello di frammentazione è la formazione di carbocationi stabilizzati per risonanza, guidati dalla presenza di legami pi greco o elettroni di non legame nella molecola dell'analita.

spettrometria di massa: panoramica sulla frammentazione molecolare

Mass Spectrometry: Molecular Fragmentation Overview

La ionizzazione di una molecola in uno ione molecolare all'interno dello spettrometro di massa provoca instabilità nella struttura della molecola a causa della perdita di un elettrone. Ciò alla fine porta alla frammentazione o alla rottura di alcuni legami nella molecola. La frammentazione avviene prevalentemente in corrispondenza di legami specifici per produrre frammenti relativamente stabili.

Un tipo di modello di frammentazione è la scissione di un singolo legame nello ione molecolare. La scissione porta ad un radicale e ad un catione. La scissione può avvenire in vari legami singoli nella molecola, ma avviene preferenzialmente nelle posizioni che generano un carbocatione stabile e un radicale. In corrispondenza di un legame specifico, l'elettrone spaiato può essere trasferito su uno dei due frammenti, creando due serie di prodotti. L'orientamento preferito del trasferimento elettronico avviene in modo tale da generare un carbocatione più stabile piuttosto che un radicale stabile. Considera il modello di frammentazione del pentano mostrato nella Figure 1.

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

Figure 1. Frammentazione dello ione molecolare pentano.

Dall'abbondanza di carbocationi negli spettri di massa, come illustrato nella Figure 2, è chiaro che la scissione degli ioni molecolari può avvenire tra due atomi di carbonio adiacenti. Tuttavia, la scissione tra due atomi di carbonio interni è maggiormente preferita rispetto alla scissione tra il terminale e il suo carbonio adiacente. La scissione dei carboni interni produce un carbocatione primario e un radicale primario relativamente stabili. La scissione del legame tra il terminale e il suo carbonio adiacente produce un carbocatione primario e un radicale metilico oppure un radicale primario e un carbocatione metilico. Se si verifica questa scissione, è attraverso la modalità di scissione che porta ad un carbocatione butilico e un radicale metilico.

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

Figure 2. Lo spettro di massa del pentano.

Un altro tipo di scissione è la dissociazione di più legami per produrre frammenti. Questo tipo di frammentazione produce un catione radicale più piccolo e una molecola neutra. Ad esempio, gli alcoli possono frammentarsi in un catione radicale più piccolo espellendo una molecola d'acqua. Di conseguenza, gli spettri di massa mostrano un picco M−18. La Figure 3 presenta la frammentazione schematica dell'1-eptanolo.

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

Figure 3. Frammentazione dell'1-eptanolo.

La massa molecolare dell'eptanolo è 116 u e un picco a m/z = 98 è visibile nel suo spettro di massa catturato nella Figure 4. Allo stesso modo, i cicloalcheni subiscono una reazione retro-Diels-Alder per espellere un alchene e un catione radicale più piccolo.

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

Figure 4. Lo spettro di massa dell'eptanolo.

Il terzo tipo di frammentazione si osserva nelle molecole con elettroni o legami π non condivisi. Nelle molecole con elettroni a coppia solitaria, il legame carbonio-carbonio accanto all'atomo che porta la coppia solitaria si rompe per produrre un carbocatione stabilizzato per risonanza. La Figure 5 illustra come esempio la frammentazione dell'etere.

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

Figure 5. Frammentazione degli eteri.

I legami π in uno ione molecolare promuovono la frammentazione della molecola per creare un carbocatione allilico stabilizzato dalla risonanza. La Figure 6 mostra lo schema di frammentazione di un alchene terminale.

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

Figure 6. Frammentazione degli alcheni.

Spettrometria di massa: panoramica sulla frammentazione molecolare

La ionizzazione di una molecola in uno ione molecolare all'interno dello spettrometro di massa provoca instabilità nella struttura della molecola a causa della perdita di un ...

mass-spectrometry-molecular-fragmentation-overview

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JoVE Core

Analytical Chemistry

Chimica Analitica

Unit 1

Unità 1

Principles of Mass Spectrometry

Principi della spettrometria di massa

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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: molecular fragmentation overview

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...

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key terms and concepts

Learn how molecular ions fragment in mass spectrometry, including single-bond cleavage, water loss from alcohols, and resonance-stabilized products.

Mass Spectrometry Fragmentation Patterns | JoVE K12

analytical chemistry

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Mass Spectrometry Fragmentation Patterns

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