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Radical Formation: Abstraction00:47

Radical Formation: Abstraction

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The electron of an atom can be abstracted from a compound by a relatively unstable radical to generate a new radical of relatively greater stability. For example, an initiator which forms radicals by homolysis can abstract a suitable species like a hydrogen atom or a halogen atom from a compound to generate a new radical. This ability of radicals to propagate by abstraction is a crucial feature of radical chain reactions.
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A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another polar molecule, such as water (H2O), hydrogen fluoride (HF), or ammonia (NH3). The huge electronegativity difference between the H atom (2.1) and the atom to which it is bonded (4.0 for an F atom, 3.5 for an O atom, or 3.0 for an N atom), combined with the very small size of an H atom...
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Hydrogen Bonds00:26

Hydrogen Bonds

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Hydrogen bonds are weak attractions between atoms that have formed other chemical bonds. One of these atoms is electronegative, like oxygen, and has a partial negative charge. The other is a hydrogen atom that has bonded with another electronegative atom and has a partial positive charge.
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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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Chemical Ionization (CI) Mass Spectrometry01:21

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The molecular ion peak of a molecule in the mass spectrum provides vital information for molecular identification. However, conventional electron impact ionization can lead to the rapid dissociation of some molecular ions before they reach the detector. A milder ionization method is required to increase the lifetime of such ionized analyte molecules. Chemical ionization (CI) is a gas-phase protonation reaction useful for mass-analyzing analyte molecules that are easily protonated to yield the...
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Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride01:26

Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

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Radical substitution reactions can be used to remove functional groups from molecules. The hydrogenolysis of alkyl halides is one such reaction, where the weak Sn–H bond in tributyltin hydride reacts with alkyl halides to form alkanes. Here, the reagent Bu3SnH yields tributyltin halide as a byproduct.
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Second hydrogen atom abstraction by molecular ions.

Svetlana Tsizin1, Boaz Seemann1, Tal Alon1

  • 1School of Chemistry, Tel Aviv University, Tel Aviv, 69978, Israel.

Journal of Mass Spectrometry : JMS
|July 8, 2017
PubMed
Summary

A new phenomenon shows molecular ions gaining two hydrogen atoms, forming [M+2H]+ ions, impacting elemental formula determination from mass spectra. This affects mass spectrometry analysis, particularly for protonated molecules.

Keywords:
cluster chemical ionizationcold EIelemental formula generationhydrogen atom abstractionprotonated molecules

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Area of Science:

  • Analytical Chemistry
  • Physical Chemistry
  • Spectroscopy

Background:

  • Mass spectrometry is crucial for identifying compounds and determining elemental formulas.
  • Protonated molecules are common in mass spectrometry, but their interpretation can be complex.
  • Isotope abundance analysis is typically used to support elemental formula assignments.

Purpose of the Study:

  • To report the novel observation of molecular ions acquiring two hydrogen atoms.
  • To investigate the mechanism and prevalence of this hydrogen atom addition phenomenon.
  • To assess the implications of this finding on elemental formula determination using mass spectrometry.

Main Methods:

  • Gas Chromatography-Mass Spectrometry (GC-MS) with supersonic molecular beam (SMB) and methanol.
  • Electron Ionization (EI) Liquid Chromatography-Mass Spectrometry (LC-MS) with SMB and methanol/deuterated methanol.
  • Analysis of molecular ion adducts and isotope patterns for pentaerythritol and trinitrotoluene (TNT).

Main Results:

  • Observed formation of [M+2H]+ ions for pentaerythritol and TNT, with abundances exceeding natural C13 isotope levels.
  • Demonstrated hydrogen atom abstraction with various compounds, including butylglycolate, dioctylphthalate, Vitamin K3, phenazine, and RDX.
  • Confirmed deuterium incorporation using deuterated methanol (CD3OD), forming [M+2D]+ ions in TNT.

Conclusions:

  • The addition of two hydrogen atoms to molecular ions is a significant phenomenon affecting mass spectra.
  • Accurate mass measurements for elemental formula generation are reliable, but isotope abundance analysis may be less dependable due to this effect.
  • GC-MS with cold EI may offer superior elemental formula determination for compounds exhibiting abundant molecular ions compared to standard LC-MS.