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

Kütle spektrometresinde, analit molekülü, analit molekülünden bir elektron daha az olan bir moleküler iyona iyonize olur.
Bir elektron kaybı, moleküler iyondaki bazı bağları zayıflatır ve bunlar daha sonra nispeten kararlı, daha küçük türlere yol açan yollardan parçalanır.
Örneğin, pentan moleküler iyonunun parçalanmasını düşünün. Bölünme, tercihen terminal ile bitişik karbon arasında değil, ikinci ve üçüncü karbon arasındaki bağda meydana gelir. İlki nispeten stabil bir karbokatyon verir.
Parçalanma, eşleşmemiş elektronun düzenlenmesinin iki bölünme modu ile sonuçlandığı bir katyon ve bir radikal oluşturur. Kararlı bir karbokatyon oluşturan bölünme, kararlı bir radikal oluşturan bölünmeye tercih edilir.
Parçalanma ayrıca iki bağın bölünmesi yoluyla da gerçekleşir. Örneğin, alkollerin moleküler iyonları radikal bir katyon ve nötr bir molekül verir.
Başka bir parçalanma modeli, analit molekülünde pi bağlarının veya bağ yapmayan elektronların varlığı tarafından yönlendirilen rezonansla stabilize edilmiş karbokatyonların oluşumudur.

kütle spektrometrisi: moleküler parçalanmaya genel bakış

Mass Spectrometry: Molecular Fragmentation Overview

Kütle spektrometresi içinde bir molekülün moleküler iyon halinde iyonlaşması, elektron kaybına bağlı olarak molekülün yapısında kararsızlığa neden olur. Bu sonuçta moleküldeki bazı bağların parçalanmasına veya kopmasına yol açar. Parçalanma, nispeten stabil parçalar elde etmek için ağırlıklı olarak spesifik bağlarda meydana gelir.

Parçalanma modellerinden biri, moleküler iyondaki tek bir bağın bölünmesidir. Bölünme bir radikale ve bir katyona yol açar. Bölünme moleküldeki çeşitli tekli bağlarda meydana gelebilir, ancak bölünme tercihen stabil bir pozitif yüklü karbon iyonu (karbokatyon) ve bir radikal oluşturan pozisyonlarda meydana gelir. Belirli bir bağda, eşleşmemiş elektron her iki parçaya da aktarılarak iki ürün kümesi oluşturulabilir. Elektron transferinin tercih edilen yönelimi, kararlı bir radikal yerine daha kararlı bir karbokatyon oluşturacak şekilde gerçekleşir. Figure 1'de gösterilen pentanın parçalanma modelini düşünün.

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

Figure 1. Pentan moleküler iyonunun parçalanması.

Figure 2'de gösterildiği gibi kütle spektrumlarındaki karbokatyonların bolluğundan, moleküler iyonların bölünmesinin herhangi iki bitişik karbon atomu arasında meydana gelebileceği açıktır. Bununla birlikte, iki iç karbon atomu arasındaki bölünme, terminal ile bitişik karbon arasındaki bölünmeye göre daha fazla tercih edilir. İç karbonların bölünmesi nispeten kararlı bir birincil karbokatyon ve birincil radikal verir. Terminal ile bitişik karbon arasındaki bağın bölünmesi, bir birincil karbokasyon ve bir metil radikali veya bir birincil radikal ve bir metil karbokatyon verir. Bu bölünme meydana gelirse, bu, bir bütil karbokatyona ve bir metil radikaline yol açan bölünme modu aracılığıyla gerçekleşir.

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

Figure 2. Pentanın kütle spektrumu.

Başka bir bölünme türü, çoklu bağların fragmanlar verecek şekilde ayrılmasıdır. Bu tür parçalanma daha küçük bir radikal katyon ve nötr bir molekül üretir. Örneğin alkoller, bir su molekülünü dışarı atarak daha küçük bir radikal katyona parçalanabilir. Sonuç olarak, kütle spektrumları bir M−18 zirvesi sergiliyor. Figure 3, 1-heptanolün şematik parçalanmasını göstermektedir.

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

Figure 3. 1-heptanolün parçalanması.

Heptanolün moleküler kütlesi 116 u'dur ve Figure 4'te yakalanan kütle spektrumunda m/z = 98'deki bir tepe görülebilir. Benzer şekilde sikloalkenler, bir alkeni ve daha küçük bir radikal katyonu dışarı atmak için bir retro-Diels-Alder reaksiyonuna girer.

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

Figure 4. Heptanolün kütle spektrumu.

Üçüncü tip parçalanma, paylaşılmamış elektronlara veya π bağlarına sahip moleküllerde görülür. Yalnız çift elektronlu moleküllerde, yalnız çift taşıyan atomun yanındaki karbon-karbon bağı kırılarak rezonansla stabilize edilmiş bir karbokatyon elde edilir. Figure 5'te örnek olarak eterin parçalanması gösterilmektedir.

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

Figure 5. Eterlerin parçalanması.

Moleküler bir iyondaki π bağları, rezonansla stabilize edilen alilik karbokatyon oluşturmak için molekülün parçalanmasını teşvik eder. Figure 6, bir terminal alkenin parçalanma modelini göstermektedir.

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

Figure 6. Alkenlerin parçalanması.

Kütle Spektrometrisi: Moleküler Parçalanmaya Genel Bakış

Bir molekülün kütle spektrometresi içindeki bir moleküler iyona iyonlaşması, molekülün yapısında bir molekülün kaybı nedeniyle kararsızlığa neden olur.

mass-spectrometry-molecular-fragmentation-overview

Education

JoVE Core

Analytical Chemistry

Analitik Kimya

Unit 1

Ünite 1

Principles of Mass Spectrometry

Kütle Spektrometrisi İlkeleri

KEY TERMS AND CONCEPTS

TEMEL TERİMLER VE KAVRAMLAR

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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key terms and concepts

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

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analytical chemistry

Mass Spectrometry Fragmentation Patterns

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