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How Isomer and Conformer Structures Impact Dissociation Dynamics of Alkane Radical Cations
Madison K Minvielle1, Mikaela Aftel1, Timothy Hill1
1Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, United States.
The Journal of Physical Chemistry. A
|September 8, 2025
Summary
Alkane radical cations, formed by ionization, readily break C-H bonds. This study reveals isomer-specific dissociation dynamics, influenced by structure and leading to rapid C-C bond cleavage and hydrogen migration.
Area of Science:
- Physical Chemistry
- Chemical Dynamics
- Spectroscopy
Background:
- Ionization of alkanes generates radical cations, activating normally inert C-H bonds.
- This activation is crucial for processes like hydrocarbon cracking.
- Understanding dissociation dynamics is key to controlling these reactions.
Purpose of the Study:
- Investigate the femtosecond dissociation dynamics of hexane radical cation isomers.
- Elucidate the influence of isomeric and conformational structures on dissociation pathways.
- Determine the timescales of hydrogen migration and C-C bond cleavage.
Main Methods:
- Femtosecond time-resolved mass spectrometry.
- Quantum chemical calculations.
- Analysis of fragment ion yields and molecular ion depletion.
Main Results:
- All five hexane isomers showed competing C-C bond cleavage and hydrogen migration within 50-300 fs.
- Branched isomers preferentially dissociated via geometric relaxation to elongated C-C bonds.
- Coherent vibrations were observed in specific branched isomers, causing ion yield oscillations.
- n-Hexane exhibited faster molecular ion depletion due to a strongly coupled excited state.
Conclusions:
- Dissociation dynamics of alkane radical cations are highly sensitive to isomer and conformer structure.
- Hydrogen migration occurs on ultrafast timescales (femtoseconds).
- Geometric relaxation and coherent vibrational motion play significant roles in dissociation pathways.
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