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

En espectrometría de masas, la molécula de analito se ioniza en un ion molecular con un electrón menos que la molécula de analito.
La pérdida de un electrón debilita algunos enlaces en el ion molecular, que posteriormente se fragmenta a través de vías que conducen a especies relativamente estables y más pequeñas.
Por ejemplo, considere la fragmentación del ion molecular pentano. La escisión se produce preferentemente en el enlace entre el segundo y el tercer carbono, no entre el terminal y su carbono adyacente. El primero produce un carbocatión relativamente estable.
La fragmentación genera un catión y un radical, donde la disposición del electrón desapareado da como resultado dos modos de escisión. Se prefiere la escisión que produce un carbocatión estable a la que genera un radical estable.
La fragmentación también se produce a través de la escisión de dos enlaces. Por ejemplo, los iones moleculares de los alcoholes producen un catión radical y una molécula neutra.
Otro patrón de fragmentación es la formación de carbocationes estabilizados por resonancia, impulsados por la presencia de enlaces pi o electrones no enlazantes en la molécula de analito.

espectrometría de masas: descripción general de la fragmentación molecular

Mass Spectrometry: Molecular Fragmentation Overview

La ionización de una molécula en un ion molecular dentro del espectrómetro de masas provoca inestabilidad en la estructura de la molécula debido a la pérdida de un electrón. Esto eventualmente conduce a la fragmentación o ruptura de algunos enlaces en la molécula. La fragmentación ocurre predominantemente en enlaces específicos para producir fragmentos relativamente estables.

Un tipo de patrón de fragmentación es la escisión de un enlace simple en el ion molecular. La escisión da lugar a un radical y un catión. La escisión puede ocurrir en varios enlaces simples de la molécula, pero ocurre preferentemente en las posiciones que generan un carbocatión estable y un radical. En un enlace específico, el electrón desapareado puede transferirse a cualquier fragmento, creando dos conjuntos de productos. La orientación preferida de la transferencia de electrones se produce de tal manera que se genera un carbocatión más estable que un radical estable. Considere el patrón de fragmentación del pentano que se muestra en la Figure 1.

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

Figure 1. Fragmentación del ion molecular pentano.

A partir de la abundancia de carbocationes en los espectros de masas, como se muestra en la Figure 2, está claro que la escisión de iones moleculares puede ocurrir entre dos átomos de carbono adyacentes cualesquiera. Sin embargo, la escisión entre dos átomos de carbono internos es más preferida que la escisión entre el carbono terminal y su carbono adyacente. La escisión de los carbonos internos produce un carbocatión primario y un radical primario relativamente estables. La ruptura del enlace entre el carbono terminal y su adyacente produce un carbocatión primario y un radical metilo o un radical primario y un carbocatión metilo. Si se produce esta escisión, es a través del modo de escisión que conduce a un carbocatión butilo y un radical metilo.

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

Figure 2. El espectro de masas del pentano.

Otro tipo de escisión es la disociación de múltiples enlaces para producir fragmentos. Este tipo de fragmentación produce un catión radical más pequeño y una molécula neutra. Por ejemplo, los alcoholes pueden fragmentarse en un catión radical más pequeño al expulsar una molécula de agua. En consecuencia, los espectros de masas exhiben un pico M-18. La Figure 3 presenta la fragmentación esquemática del 1-heptanol.

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

Figure 3. Fragmentación de 1-heptanol.

La masa molecular del heptanol es 116 u, y un pico en m/z = 98 es visible en su espectro de masas capturado en la Figure 4. De manera similar, los cicloalquenos experimentan una reacción retro-Diels-Alder para expulsar un alqueno y un catión radical más pequeño.

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

Figure 4. El espectro de masas del heptanol.

El tercer tipo de fragmentación se observa en moléculas con electrones no compartidos o enlaces π. En las moléculas con pares de electrones libres, el enlace carbono-carbono próximo al átomo que lleva el par solitario se rompe para producir un carbocatión estabilizado por resonancia. La Figure 5 muestra la fragmentación del éter como ejemplo.

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

Figure 5. Fragmentación de éteres.

Los enlaces π en un ion molecular promueven la fragmentación de la molécula para crear un carbocatión alílico estabilizado por resonancia. La Figure 6 muestra el patrón de fragmentación de un alqueno terminal.

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

Figure 6. Fragmentación de alquenos.

Espectrometría de masas: Descripción general de la fragmentación molecular

La ionización de una molécula en un ion molecular dentro del espectrómetro de masas provoca inestabilidad en la estructura de la molécula debido a la pérdida de un ...

mass-spectrometry-molecular-fragmentation-overview

Education

Educación

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidad 1

Principles of Mass Spectrometry

Principios de la espectrometría de masas

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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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