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?

Alexander Parker

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.

Alexander Parker

Methodology of Modelling Used Harmonic Quantum Transition State Theory

(HQTST). Density Functional Theory basis set choice.• MPWB1K/6-31G*(*)*

Alexander Parker

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

Alexander Parker

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.

Alexander Parker

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.

Alexander Parker