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Evidence for orbital-specific electron transfer to oriented haloform molecules.
Beike Jia1, Jonathan Laib, R F M Lobo
1Chemistry Department and Rice Quantum Institute, Rice University, Houston, Texas 77251, USA.
Journal of the American Chemical Society
|November 15, 2002
Summary
Potassium atom collisions with haloforms reveal orientation-dependent reactivity. Fluorine atom (F) end attack becomes dominant at higher energies, unlike chlorine (Cl) and bromine (Br) where halogen end reactivity prevails.
Area of Science:
- Chemical Physics
- Molecular Collisions
- Reaction Dynamics
Background:
- Understanding molecular orientation effects in chemical reactions is crucial for controlling reaction pathways.
- Haloforms (CHX3) present unique steric challenges due to their tetrahedral structure and varying halogen substituents.
Purpose of the Study:
- To investigate the role of molecular orientation in electron transfer reactions between hyperthermal potassium (K) atoms and oriented haloforms (CF3H, CCl3H, CBr3H).
- To determine how collision energy influences the preferred reaction site and product formation.
Main Methods:
- Crossed molecular beams technique employing oriented haloform molecules.
- Detection of coincident K+ and X- (halide) ions to analyze reaction products.
- Varying collision energy near the ion-pair formation threshold (approx. 5.5 eV).
Main Results:
- CF3H exhibits significant orientation dependence, with H-end attack favored near the threshold for F- formation, switching to F-end dominance at higher energies.
- CCl3H and CBr3H show minimal steric asymmetry, with the halogen end being more reactive.
- Electron transfer to the sigma(CH) antibonding orbital in CF3H is observed near threshold, indicated by CF3- ion signals, while this pathway is less active in CCl3H and CBr3H.
Conclusions:
- Molecular orientation strongly influences electron transfer dynamics in CF3H, unlike CCl3H and CBr3H.
- Collision energy plays a critical role in determining the reactive site and the dominant electron transfer pathway (sigma(CH) vs. sigma(CX) orbitals).
- The formation of ionic or molecular products at low energies is likely governed by the proximity of the nascent ions.
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