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

Why This Was Investigated  H n -PAHs may be intermediates in CO & H 2 formation in the InterStellar Medium (ISM).  A mechanism for H n -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 H n -PAH be formed in the ISM?

PAHs and the ISM  ISM is what exists in space between stars and galaxies.  Dust = PAHs, Fullerenes etc...  Gas = H 2 or another small molecules.  Found with IR deep space spectroscopy. 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.

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

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

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

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 3a 1 models central atoms.  Larger PAHs (>50 C atoms) expected to be comparable to pyrene.

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&3a 1

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 ~ cm 3 s -1 at 40K.  Deuterium atom addition much slower as it tunnels much less efficiently.

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.