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Mass Spectrometry: Molecular Fragmentation Overview01:20

Mass Spectrometry: Molecular Fragmentation Overview

6.2K
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.
One type of fragmentation pattern is the cleavage of a single bond in the molecular ion. The cleavage leads to a radical and a cation. The cleavage can occur at...
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Mass Spectrometry: Alcohol Fragmentation01:03

Mass Spectrometry: Alcohol Fragmentation

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Alcohols (R-OH) ionize to lose one non-bonded electron from the oxygen atom, forming molecular ions. Due to their tendency to fragment rapidly, the intensity of the molecular ion peak in the mass spectrum is weak or sometimes absent. The fragmentation patterns for alcohols occur in two ways, i.e. ⍺-cleavage and dehydration. During ⍺-cleavage, the bond at the ⍺-position adjacent to the hydroxyl group cleaves to give a resonance-stabilized cation and a radical. However, intramolecular...
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Mass Spectrometry: Long-Chain Alkane Fragmentation01:18

Mass Spectrometry: Long-Chain Alkane Fragmentation

2.7K
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...
2.7K
Mass Spectrometry: Alkene Fragmentation00:59

Mass Spectrometry: Alkene Fragmentation

3.9K
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...
3.9K
Mass Spectrometry: Branched Alkane Fragmentation01:29

Mass Spectrometry: Branched Alkane Fragmentation

1.9K
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.
1.9K
The Molecular Nature of Internal Energy01:27

The Molecular Nature of Internal Energy

45
The internal energy of a molecule is determined by its degrees of freedom, including translational, rotational, and vibrational motions. In addition to these kinetic activities, the energy of molecules is also shaped by electronic energy, intermolecular forces, and the rest-mass energy of electrons and nuclei. These factors collectively influence the energy state of the molecules. The equipartition theorem of classical mechanics provides insight into this energy distribution. It posits that the...
45

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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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Molecular energies from an incremental fragmentation method.

Oinam Romesh Meitei1, Andreas Heßelmann1

  • 1Lehrstuhl für Theoretische Chemie, Universität Erlangen-Nürnberg, Egerlandstr. 3, D-91058 Erlangen, Germany.

The Journal of Chemical Physics
|March 3, 2016
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Summary

This study refines the molecular fragmentation method for accurate energy calculations in various molecular systems. The enhanced approach combines accurate quantum chemistry with cost-effective density functional theory for reliable results.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Molecular Modeling

Background:

  • The Collins and Deev molecular fragmentation method offers a way to calculate molecular energies.
  • Accurate energy calculations are crucial for understanding molecular behavior and interactions.

Purpose of the Study:

  • To test and enhance the accuracy of the molecular fragmentation method using an incremental scheme.
  • To evaluate the performance of the method across different molecular systems and decomposition types.

Main Methods:

  • Utilizing an incremental scheme with accurate quantum chemistry (random-phase approximation and extensions) for low-level fragmentation energies.
  • Employing density functional theory (DFT) methods for higher-level incremental corrections.
  • Applying the fragmentation method with DFT-Symmetry-Adapted Perturbation Theory (DFT-SAPT) for interaction energy analysis.

Main Results:

  • The complete incremental fragmentation method accurately reproduces supermolecule results.
  • High accuracy is achieved regardless of molecular type, size, or decomposition strategy.
  • The method successfully breaks down nonbonding energy contributions into individual terms.

Conclusions:

  • The enhanced incremental fragmentation method provides a robust and accurate approach for molecular energy calculations.
  • The method demonstrates broad applicability and reliability for diverse molecular systems.
  • Potential issues with capping hydrogen atoms were identified and addressed with proposed solutions.