  By means of pseudopotential calculations based on density functional theory
(DFT) we studied the effect of hydrogenation on electronic properties of
armchair single-wall carbon nanotubes. The calculations demonstrate strong
preference for formation of monoatomic H chains along the (5,5) nanotube axis
with the H binding in an infinite H chain reaching the value of 2.58 eV per
atom. Upon formation of chains of H adatoms, initially metallic (5,5) nanotubes
change electronic structure to the semiconducting. The opening of the band gap
of $\sim$0.6 eV is accompanied with antiferromagnetic coupling of
ferromagnetically ordered magnetic moments on C atoms in vicinity of the H
chain. These electronic properties are strikingly similar to those previously
observed in narrow graphene nanoribbons with zigzag edges.
