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Unimolecular Fragmentation Properties of Thermometer Ions from Chemical Dynamics Simulations
Abdul Malik1, Riccardo Spezia2, William L Hase1
1Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061 United States.
Journal of the American Society for Mass Spectrometry
|November 19, 2020
Summary
Thermometer ions, like benzylpyridinium and benzhydrylpyridinium ions, are crucial for calibrating internal energy in mass spectrometry. Simulations confirm a single fragmentation pathway, ensuring reliable calibration accuracy.
Area of Science:
- Computational chemistry
- Mass spectrometry
- Physical chemistry
Background:
- Thermometer ions are essential for calibrating internal energy in electrospray ionization mass spectrometry.
- Benzylpyridinium and benzhydrylpyridinium ions are commonly used, with the latter offering lower bond dissociation energies.
Purpose of the Study:
- To characterize activation energies of representative thermometer ions using direct dynamics simulations.
- To evaluate the reliability of these ions for internal energy calibration in mass spectrometry.
Main Methods:
- Direct dynamics simulations were performed using M06-2X/6-31G(d), DFTB, and PM6-D3 levels of theory.
- Rate constants were calculated and used to determine Arrhenius activation energies and RRK fitted threshold energies.
- Fragmentation mechanisms and potential competing pathways were analyzed.
Main Results:
- A single C-N bond dissociation mechanism was observed for both para-methylbenzylpyridinium and methyl,methylbenzhydrylpyridinium ions.
- Calculated energies showed good agreement with theoretical bond dissociation energies.
- Tropylium ion formation was observed only at higher energies, indicating no significant competition between pathways.
Conclusions:
- The observed single fragmentation pathway validates the reliability of these ions for internal energy calibration.
- DFTB with a modified-Arrhenius model is suggested for future studies on thermometer ions.
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