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

W spektrometrii mas cząsteczka analitu zostaje zjonizowana do jonu molekularnego z jednym elektronem mniej niż cząsteczka analitu.
Utrata elektronu osłabia niektóre wiązania w jonie molekularnym, który następnie ulega fragmentacji przez szlaki prowadzące do stosunkowo stabilnych, mniejszych gatunków.
Rozważmy na przykład fragmentację jonu molekularnego pentanu. Rozszczepienie zachodzi preferencyjnie w wiązaniu między drugim a trzecim węglem, a nie między końcem a sąsiednim węglem. Ten pierwszy daje stosunkowo stabilny karbokation.
Fragmentacja generuje kation i rodnik, w których ułożenie niesparowanego elektronu powoduje dwa tryby rozszczepienia. Rozszczepienie, które wytwarza stabilny karbokation, jest preferowane niż to, które generuje stabilny rodnik.
Fragmentacja następuje również poprzez rozszczepienie dwóch wiązań. Na przykład jony molekularne alkoholi dają rodnik kationowy i obojętną cząsteczkę.
Innym wzorcem fragmentacji jest tworzenie się karbokationów stabilizowanych rezonansem, napędzanych obecnością wiązań pi lub elektronów niewiążących w cząsteczce analitu.

spektrometria mas: przegląd fragmentacji molekularnej

Mass Spectrometry: Molecular Fragmentation Overview

Jonizacja cząsteczki w jon molekularny wewnątrz spektrometru mas powoduje niestabilność struktury cząsteczki w wyniku utraty elektronu. To ostatecznie prowadzi do fragmentacji lub zerwania niektórych wiązań w cząsteczce. Fragmentacja zachodzi głównie przy specyficznych wiązaniach, dając stosunkowo trwałe fragmenty.

Jednym z typów wzorców fragmentacji jest rozerwanie pojedynczego wiązania w jonie cząsteczkowym. Rozszczepienie prowadzi do rodnika i kationu. Rozszczepienie może nastąpić przy różnych pojedynczych wiązaniach w cząsteczce, ale rozszczepienie preferencyjnie zachodzi w pozycjach, które generują stabilny karbokation i rodnik. Przy specyficznym wiązaniu niesparowany elektron może zostać przeniesiony na którykolwiek fragment, tworząc dwa zestawy produktów. Preferowana orientacja przenoszenia elektronów zachodzi w taki sposób, aby wygenerować bardziej stabilny karbokation niż stabilny rodnik. Rozważmy wzór fragmentacji pentanu pokazany na rysunku 1.

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

Figure 1. Fragmentacja jonu cząsteczkowego pentanu.

Z obfitości karbokationów w widmach masowych, jak pokazano na rysunku 2, jasno wynika, że ​​rozszczepienie jonów molekularnych może nastąpić pomiędzy dowolnymi dwoma sąsiednimi atomami węgla. Jednakże rozszczepienie pomiędzy dwoma wewnętrznymi atomami węgla jest bardziej korzystne niż rozszczepienie pomiędzy terminalem a sąsiadującym z nim węglem. Rozszczepienie węgli wewnętrznych daje stosunkowo stabilny pierwotny karbokation i pierwszorzędowy rodnik. Rozszczepienie wiązania między terminalem a sąsiadującym z nim węglem daje pierwszorzędowy karbokation i rodnik metylowy lub pierwszorzędowy rodnik i karbokation metylu. Jeśli nastąpi takie rozszczepienie, to poprzez tryb rozszczepienia prowadzi się do karbokationu butylu i rodnika metylowego.

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

Figure 2. Widmo masowe pentanu.

Innym rodzajem rozszczepienia jest dysocjacja wiązań wielokrotnych w celu uzyskania fragmentów. Ten typ fragmentacji wytwarza mniejszy rodnikowy kation i obojętną cząsteczkę. Na przykład alkohole mogą ulegać fragmentacji na kation o mniejszym rodniku poprzez wydalenie cząsteczki wody. W konsekwencji widma masowe wykazują pik M-18. Na rycinie 3 przedstawiono schematyczną fragmentację 1-heptanolu.

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

Figure 3. Fragmentacja 1-heptanolu.

Masa cząsteczkowa heptanolu wynosi 116 u, a pik przy m/z = 98 jest widoczny w jego widmie masowym przedstawionym na rysunku 4. Podobnie cykloalkeny przechodzą reakcję retro-Dielsa-Aldera, mającą na celu usunięcie alkenu i kationu o mniejszym rodniku.

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

Figure 4. Widmo masowe heptanolu.

Trzeci rodzaj fragmentacji obserwuje się w cząsteczkach z niewspółdzielonymi elektronami lub wiązaniami π. W cząsteczkach z wolną parą elektronów wiązanie węgiel-węgiel obok atomu niosącego samotną parę pęka, tworząc karbokation stabilizowany rezonansem. Figure 5 przedstawia jako przykład fragmentację eteru.

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

Figure 5. Fragmentacja eterów.

Wiązania π w jonie molekularnym sprzyjają fragmentacji cząsteczki, tworząc karbokation allilowy stabilizowany rezonansem. Figure 6 przedstawia wzór fragmentacji terminalnego alkenu.

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

Figure 6. Fragmentacja alkenów.

Spektrometria mas: Przegląd fragmentacji molekularnej

Jonizacja cząsteczki do jonu molekularnego wewnątrz spektrometru mas powoduje niestabilność struktury cząsteczki z powodu utraty ...

mass-spectrometry-molecular-fragmentation-overview

Education

JoVE Core

Analytical Chemistry

Chemia analityczna

Unit 1

Jednostka 1

Principles of Mass Spectrometry

Zasady spektrometrii mas

KEY TERMS AND CONCEPTS

KLUCZOWE TERMINY I POJĘCIA

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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high-resolution mass spectrometry (hrms)

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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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