  The chemisorption of atomic hydrogen on the single-walled armchair carbon
nanotube is studied with ab initio calculations. A single H atom is found to be
chemisorbed on both the inside and outside wall of the nanotube. The binding
energy of H adsorption at the exterior of the nanotube is much greater than
that at the interior of the nanotube. For the first time, we predict that two
rows of H atoms chemisorbed on selective sites exterior to the nanotube can
break the nearest-neighbor C-C bond of the nanotube through the concerted
formation of C-H bonds, leading to the unzipping of the nanotube wall. We
provide insights into the underlying electronic structure responsible for the
H-induced unzipping of the nanotube, lending strong support to the recent
experimental observations for the coalescence of single-walled nanotubes in the
presence of atomic hydrogen. Interestingly, H atoms chemisorbed inside the
nanotube do not lead to the breaking of the C-C bonds.
