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

Na espectrometria de massa, a molécula do analito é ionizada em um íon molecular com um elétron a menos do que a molécula do analito.
A perda de um elétron enfraquece algumas ligações no íon molecular, que posteriormente se fragmenta por meio de vias que levam a espécies menores e relativamente estáveis.
Por exemplo, considere a fragmentação do íon molecular pentano. A clivagem ocorre preferencialmente na ligação entre o segundo e o terceiro carbono, não entre o terminal e seu carbono adjacente. O primeiro produz um carbocátion relativamente estável.
A fragmentação gera um cátion e um radical, onde o arranjo do elétron desemparelhado resulta em dois modos de clivagem. A clivagem que produz um carbocátion estável é preferível àquela que gera um radical estável.
A fragmentação também ocorre por meio da clivagem de duas ligações. Por exemplo, íons moleculares de álcoois produzem um cátion radical e uma molécula neutra.
Outro padrão de fragmentação é a formação de carbocátions estabilizadas por ressonância, impulsionadas pela presença de ligações pi ou elétrons não ligantes na molécula do analito.

espectrometria de massa: visão geral da fragmentação molecular

Mass Spectrometry: Molecular Fragmentation Overview

A ionização de uma molécula em um íon molecular dentro do espectrômetro de massa causa instabilidade na estrutura da molécula devido à perda de um elétron. Isso eventualmente leva à fragmentação ou quebra de algumas ligações na molécula. A fragmentação ocorre predominantemente em ligações específicas para produzir fragmentos relativamente estáveis.

Um tipo de padrão de fragmentação é a clivagem de uma ligação simples no íon molecular. A clivagem leva a um radical e um cátion. A clivagem pode ocorrer em várias ligações simples na molécula, mas preferencialmente ocorre nas posições que geram um carbocátion estável e um radical. Em uma ligação específica, o elétron desemparelhado pode ser transferido para qualquer um dos fragmentos, criando dois conjuntos de produtos. A orientação preferida da transferência de elétrons ocorre de forma a gerar um carbocátion mais estável em vez de um radical estável. Considere o padrão de fragmentação do pentano mostrado na Figura 1.

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

Figure 1. Fragmentação do íon molecular pentano.

A partir da abundância de carbocátions nos espectros de massa, conforme representado na Figura 2, fica claro que a clivagem de íons moleculares pode ocorrer entre quaisquer dois átomos de carbono adjacentes. Contudo, a clivagem entre dois átomos de carbono internos é mais preferida do que a clivagem entre o terminal e o seu carbono adjacente. A clivagem dos carbonos internos produz um carbocátion primário e um radical primário relativamente estáveis. A clivagem da ligação entre o terminal e seu carbono adjacente produz um carbocátion primário e um radical metila ou um radical primário e um carbocátion metila. Se ocorrer essa clivagem, ela será através do modo de clivagem que leva a um carbocátion butila e a um radical metila.

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

Figure 2. O espectro de massa do pentano.

Outro tipo de clivagem é a dissociação de múltiplas ligações para produzir fragmentos. Esse tipo de fragmentação produz um radical catiônico menor e uma molécula neutra. Por exemplo, os álcoois podem fragmentar-se em um num radical catiônico menor ao expelir uma molécula de água. Como resultado, os espectros de massa exibem um pico M-18. A Figura 3 apresenta a fragmentação esquemática do 1-heptanol.

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

Figure 3. Fragmentação de 1-heptanol.

A massa molecular do heptanol é 116 u, e um pico em m/z = 98 é visível em seu espectro de massa capturado na Figura 4. Da mesma forma, os cicloalcenos sofrem uma reação retro-Diels-Alder para expelir um alceno e um radical catiônico menor.

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

Figure 4. O espectro de massa do heptanol.

O terceiro tipo de fragmentação é observado em moléculas com elétrons não compartilhados ou ligações π. Nas moléculas com pares de elétrons solitários, a ligação carbono-carbono próxima ao átomo que contém o par solitário se rompe para produzir um carbocátion estabilizado por ressonância. A Figura 5 mostra a fragmentação do éter como exemplo.

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

Figure 5. Fragmentação de éteres.

As ligações π em um íon molecular promovem a fragmentação da molécula para criar carbocátion alílico estabilizado por ressonância. A Figura 6 mostra o padrão de fragmentação de um alceno terminal.

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

Figure 6. Fragmentação de alcenos.

Espectrometria de Massa: Visão Geral da Fragmentação Molecular

A ionização de uma molécula em um íon molecular dentro do espectrômetro de massa causa instabilidade na estrutura da molécula devido à perda de um ...

mass-spectrometry-molecular-fragmentation-overview

Education

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidade 1

Principles of Mass Spectrometry

Princípios da Espectrometria de Massas

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

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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