Presentation on theme: "Hydrogen Chemisorption on Polycyclic Aromatic Hydrocarbons via Tunnelling Alexander Parker European Astrobiology Network Association T.P.M. Goumans Mon."— Presentation transcript:
Hydrogen Chemisorption on Polycyclic Aromatic Hydrocarbons via Tunnelling Alexander Parker European Astrobiology Network Association T.P.M. Goumans Mon. Not. R. Astron. Soc. 415, 3129-3134 (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 ~10 -16.9 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.