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Mass Spectrometry: Branched Alkane Fragmentation01:29

Mass Spectrometry: Branched Alkane Fragmentation

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This lesson delves into the mass spectrometry of branched alkane fragmentation. Branched alkanes possess secondary or tertiary carbon atoms, which generate relatively stable carbocations if the cleavage occurs at the branching point. The high stability of carbocations drives the instant fragmentation of branched alkanes. Accordingly, the branched alkane's molecular ion peak is very weak or invisible in the mass spectra, especially in comparison to a linear alkane.
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The molecular ions of linear alkanes prefer to fragment at the carbon-carbon bond away from the end of the chain since the cleavage of an inner bond creates a stable carbocation and a stable radical. Consequently, the mass signals of linear alkanes feature intense peaks in the middle of the mass-to-charge ratio plot with weaker peaks on either end. The fragmentation of each carbon-carbon bond with the release of a methyl group in each splitting leads to prominent peaks in the mass spectra...
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Introduction to Electrophilic Addition Reactions of Alkenes02:24

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The double bond in a simple, unconjugated alkene is a region of high electron density that can act as a weak base or a nucleophile. The filled π orbital (HOMO) of the double bond can interact with the empty LUMO of an electrophile. A bonding interaction occurs when the electrophile attacks between the two carbons; the electrophile then accepts a pair of electrons from the π bond and undergoes addition across the double bond, yielding a single product.
Addition and elimination...
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Mass Spectrometry: Alkene Fragmentation00:59

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Alkenes lose one electron from the unsaturated π bond upon ionization and form stable molecular ions. Further fragmentation of alkenes occurs through three different reaction pathways. The most prominent fragmentation is the cleavage at the allylic position. The resultant allylic carbocation is resonance stabilized. In the mass spectra of terminal alkenes, this fragment appears at a mass-to-charge ratio of 41. In the internal alkenes, where there are two choices of allylic cleavage, the...
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Mass Spectrometry: Alkyne Fragmentation00:53

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The fragmentation of alkynes preferentially occurs at the carbon–carbon bond between the α and β carbon of the alkyne bond to generate a 3-propynyl cation (or propargyl cation). In terminal alkynes, there is the only type of fragmentation that yields the 3-propynyl cation. The unsubstituted 3-propynyl cation exhibits a peak at a mass-to-charge ratio of 39. In internal alkynes, the 3-propynyl cation is substituted. For example, 2-pentyne fragments into methyl-substituted 3-propynyl cation,...
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Mass Spectrometry: Molecular Fragmentation Overview01:20

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The ionization of a molecule into a molecular ion inside the mass spectrometer causes instability in the molecule's structure due to the loss of an electron. This eventually leads to the fragmentation or breaking of some bonds in the molecule. The fragmentation occurs predominantly at specific bonds to yield relatively stable fragments.
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Positron Binding and Annihilation in Alkane Molecules.

A R Swann1, G F Gribakin1

  • 1School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, United Kingdom.

Physical Review Letters
|October 2, 2019
PubMed
Summary
This summary is machine-generated.

This study introduces a model-potential approach to investigate positron interactions with alkanes. Positron binding energies and annihilation rates were calculated, predicting a second bound state for larger n-alkanes.

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Area of Science:

  • Computational physics
  • Molecular science
  • Positron interactions

Background:

  • Understanding positron interactions with molecules is crucial for various fields.
  • Previous studies have explored positron binding with simple molecules.

Purpose of the Study:

  • To develop and apply a model-potential approach for studying positron interactions with alkane molecules.
  • To calculate binding energies and annihilation rates for positron bound states with alkanes, including rings and isomers.

Main Methods:

  • Employed a model-potential approach to simulate positron-molecule interactions.
  • Calculated binding energies and annihilation rates for positron bound states.
  • Investigated a range of alkane molecules, encompassing rings and isomers.

Main Results:

  • Calculated binding energies show good agreement with experimental data.
  • Predicted the existence of a second bound state for n-alkanes (CnH2n+2) with n≥12, consistent with experimental findings.
  • Demonstrated a linear scaling of annihilation rate with the square root of binding energy for the ground positron bound state.

Conclusions:

  • The model-potential approach is effective for studying positron interactions with alkanes.
  • The study confirms and predicts specific positron binding states in alkanes.
  • Annihilation rates are directly correlated with binding energy in these systems.