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

En spectrométrie de masse, la molécule d’analyte est ionisée en un ion moléculaire avec un électron de moins que la molécule d’analyte.
La perte d’un électron affaiblit certaines liaisons dans l’ion moléculaire, qui se fragmente ensuite par des voies qui mènent à des espèces relativement stables et plus petites.
Prenons l’exemple de la fragmentation de l’ion moléculaire pentane. Le clivage se produit préférentiellement au niveau de la liaison entre le deuxième et le troisième carbone, et non entre le carbone terminal et son carbone adjacent. Le premier donne un carbocation relativement stable.
La fragmentation génère un cation et un radical, où l’arrangement de l’électron non apparié entraîne deux modes de clivage. Le clivage qui produit un carbocation stable est préféré à celui qui génère un radical stable.
La fragmentation se produit également via le clivage de deux liaisons. Par exemple, les ions moléculaires des alcools produisent un cation radical et une molécule neutre.
Un autre modèle de fragmentation est la formation de carbocations stabilisées par résonance, entraînées par la présence de liaisons pi ou d’électrons non liants dans la molécule d’analyte.

spectrométrie de masse : aperçu de la fragmentation moléculaire

Mass Spectrometry: Molecular Fragmentation Overview

L'ionisation d'une molécule en un ion moléculaire à l'intérieur du spectromètre de masse provoque une instabilité de la structure de la molécule en raison de la perte d'un électron. Cela conduit finalement à la fragmentation ou à la rupture de certaines liaisons dans la molécule. La fragmentation se produit principalement au niveau de liaisons spécifiques pour produire des fragments relativement stables.

Un type de modèle de fragmentation est le clivage d’une simple liaison dans l’ion moléculaire. La coupure conduit à un radical et un cation. Le clivage peut se produire au niveau de diverses liaisons simples dans la molécule, mais il se produit préférentiellement aux positions qui génèrent un carbocation stable et un radical. Au niveau d’une liaison spécifique, l’électron non apparié peut être transféré vers l’un ou l’autre fragment, créant ainsi deux ensembles de produits. L'orientation préférée du transfert d'électrons se produit de manière à générer un carbocation plus stable plutôt qu'un radical stable. Considérez le modèle de fragmentation du pentane illustré à la figure 1.

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

Figure 1. Fragmentation de l'ion moléculaire pentane.

D’après l’abondance des carbocations dans les spectres de masse, comme le montre la figure 2, il est clair que le clivage des ions moléculaires peut se produire entre deux atomes de carbone adjacents. Cependant, le clivage entre deux atomes de carbone internes est plus préféré que le clivage entre le carbone terminal et son carbone adjacent. Le clivage des carbones internes donne un carbocation primaire et un radical primaire relativement stables. Le clivage de la liaison entre le carbone terminal et son carbone adjacent donne un carbocation primaire et un radical méthyle ou un radical primaire et un carbocation méthyle. Si cette coupure se produit, c'est via le mode de coupure qui conduit à un carbocation butyle et un radical méthyle.

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

Figure 2. Le spectre de masse du pentane.

Un autre type de clivage est la dissociation de multiples liaisons pour produire des fragments. Ce type de fragmentation produit un cation radical plus petit et une molécule neutre. Par exemple, les alcools peuvent se fragmenter en un cation radical plus petit en expulsant une molécule d’eau. Par conséquent, les spectres de masse présentent un pic M−18. La figure 3 présente la fragmentation schématique du 1-heptanol.

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

Figure 3. Fragmentation du 1-heptanol.

La masse moléculaire de l'heptanol est de 116 u et un pic à m/z = 98 est visible dans son spectre de masse capturé sur la figure 4. De même, les cycloalcènes subissent une réaction rétro-Diels-Alder pour expulser un alcène et un cation radical plus petit.

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

Figure 4. Le spectre de masse de l'heptanol.

Le troisième type de fragmentation est observé dans les molécules avec des électrons non partagés ou des liaisons π. Dans les molécules contenant des électrons en doublet isolé, la liaison carbone-carbone située à côté de l'atome porteur du doublet isolé se brise pour produire un carbocation stabilisé par résonance. La figure 5 représente la fragmentation de l'éther à titre d'exemple.

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

Figure 5. Fragmentation des éthers.

Les liaisons π dans un ion moléculaire favorisent la fragmentation de la molécule pour créer un carbocation allylique stabilisé par résonance. La figure 6 montre le modèle de fragmentation d'un alcène terminal.

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

Figure 6. Fragmentation des alcènes.

Spectrométrie de masse : aperçu de la fragmentation moléculaire

L'ionisation d'une molécule en un ion moléculaire à l'intérieur du spectromètre de masse provoque une instabilité de la structure de la molécule en raison de la perte d'un ...

mass-spectrometry-molecular-fragmentation-overview

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Enseignement

JoVE Core

Analytical Chemistry

Chimie analytique

Unit 1

Unité 1

Principles of Mass Spectrometry

Principes de la spectrométrie de masse

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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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