  Density functional theory modelling of the reduction of realistic
nanographene molecules (C42H18, C48H18 and C60H24) by molecular hydrogen
evidences for the presence of limits in the hydrogenation process. These limits
caused the contentions between three-fold symmetry of polycyclic aromatic
hydrocarbon molecules and two-fold symmetry of adsorbed hydrogen pairs.
Increase of the binding energy between nanographenes during reduction is also
discussed as possible cause of the experimentally observed limited
hydrogenation of studied nanographenes.
