Hydrogen Chemisorption on Polycyclic A romatic Hydrocarbons via Tunnelling

1. European Astrobiology Network Association Hydrogen Chemisorption on Polycyclic Aromatic Hydrocarbons via Tunnelling Alexander Parker T.P.M. Goumans Mon. Not. R. Astron. Soc. 415, 3129-3134 (2011)

2. Alexander Parker 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?

3. Alexander Parker PAHs and the ISM Interstellar medium here • 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.

4. Alexander Parker Reaction Investigated • H adsorption - high classical barriers. • Previously shown: • Reaction barrier can be lowered via tunnelling. • Barrier at the periphery is lower.

5. Alexander Parker Methodology of Modelling Used • Harmonic Quantum Transition State Theory (HQTST). • Density Functional Theory basis set choice. • MPWB1K/6-31G*(*)*

6. 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

7. 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”. Tunnelling Paths at 40K Blue = Reactant, Red = Product

8. Alexander Parker 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 3a1models central atoms. • Larger PAHs (>50 C atoms) expected to be comparable to pyrene.

9. Alexander Parker Results • Low K dominated by tunnelling. • Parameters cannot be accurate below 40K. • By 50 K temperature independent rate suggests D may become available. Classical rate vs. HQTST for 1,4&3a1

10. Alexander Parker 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 at40K. • Deuterium atom addition much slower as it tunnels much less efficiently.

11. 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.