Physisorption and diffusion of hydrogen atoms on graphite from correlated calculations on the H-coronene model system

Correlated, counterpoise corrected wave function calculations on the hydrogen-coronene system are used to investigate the energy landscape and the dynamic behavior of hydrogen atoms physisorbed on graphite. The adopted MP2 correlation level, employing the aug-cc-pVDZ basis set augmented with bond functions, has been selected after extensive investigation on the smaller hydrogen-benzene system. The computed physisorption energy (39.7 meV) is in excellent agreement with the existing experimental value of (39.2 +/- 0.5) meV for a graphite single layer (Ghio, E.; Mattera, L.; Salvo, C.; Tommasini, F.; Valbusa, U. J. Chem. Phys. 1980, 73, 557) and makes one confident of the computed barriers to diffusion. A simple, analytical expression of the corrugated potential energy surface fitted to the calculated energy values is then used in 3D quantum dynamical calculations of the tunneling contribution to the diffusion coefficient. Results show that hydrogen atoms physisorbed on graphite are highly mobile on the surface even at T = 0 K. This suggests that hydrogen formation in cold, interstellar clouds can indeed occur down to very low temperatures through recombination of hydrogen atoms previously physisorbed on the surface of dust grains.