  An extensive first-principles study of fully exo-hydrogenated zigzag (n,0)
and armchair (n,n) single wall carbon nanotubes (C$_n$H$_n$), polyhedral
molecules including cubane, dodecahedrane, and C$_{60}$H$_{60}$ points to
crucial differences in the electronic and atomic structures relevant to
hydrogen storage and device applications. C$_n$H$_n$'s are estimated to be
stable up to the radius of a (8,8) nanotube, with binding energies proportional
to 1/R. Attaching a single hydrogen to any nanotube is always exothermic.
Hydrogenation of zigzag nanotubes is found to be more likely than armchair
nanotubes with similar radius. Our findings may have important implications for
selective functionalization and finding a way of separating similar radius
nanotubes from each other.
