Hydrogen Chemisorption on Polycyclic A romatic Hydrocarbons via Tunnelling
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Transcript of Hydrogen Chemisorption on Polycyclic A romatic Hydrocarbons via Tunnelling
Hydrogen Chemisorption on Polycyclic Aromatic Hydrocarbons via Tunnelling
Alexander Parker
European Astrobiology Network Association
T.P.M. GoumansMon. Not. R. Astron. Soc. 415, 3129-3134 (2011)
Why This Was Investigated
Hn-PAHs may be intermediates in CO & H2 formation in the InterStellar Medium (ISM).
A mechanism for Hn-PAH formation will add weight to all the mechanistic theories e.g. dimer-mediated reaction (Cuppen & Hornekaer 2008) or by direct H atom abstraction (Sha et al. 2002).
Could Hn-PAH be formed in the ISM?
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PAHs and the ISM ISM is what exists
in space between stars and galaxies.
Dust = PAHs, Fullerenes etc...
Gas = H2 or another small molecules.
Found with IR deep space spectroscopy.
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Interstellar medium here
Reaction Investigated H adsorption - high classical
barriers.
Previously shown: • Reaction barrier can be lowered via
tunnelling.• Barrier at the periphery is lower.
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Methodology of Modelling Used Harmonic Quantum Transition State Theory
(HQTST). Density Functional Theory basis set choice.• MPWB1K/6-31G*(*)*
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Results Vibrational adiabatic barrier calculated.
Shows enhanced activity of edge caused by increased flexibility of rehybridised Carbon atom.
Calculated barriers give:• High K = fast rate• Low K(40) = negligible rate
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Results Core C atoms
affected by 0.15Å “puckering”.
Edge C has little participation.
Less favourable paths become allowed at lower K, “corner-cutting”.
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Tunnelling Paths at 40KBlue = Reactant, Red = Product
Results Barrier for H Tunnelling at 2 is greater than
at 1 or 4. 1 and 4 are model sites for larger PAHs
edges whilst 3a1 models central atoms.
Larger PAHs (>50 C atoms) expected to be comparable to pyrene.
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Results Low K dominated
by tunnelling. Parameters cannot
be accurate below 40K.
By 50 K temperature independent rate suggests D may become available.
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Classical rate vs. HQTST for 1,4&3a1
Summary Quantum tunnelling makes rate of H-PAHs
formation non-negligible despite sizable classical barriers.
Edges always preferable and makes H-PAH formation possible in ISM at rate ~10-16.9 cm3 s-1 at 40K.
Deuterium atom addition much slower as it tunnels much less efficiently.
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Future How does tunnelling compete with other
pathways? e.g. H atom addition to PAH cations followed by charge neutralisation.
Full reaction network scheme assessment.
Currently underway by Goumans.
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