Quantum mechanical energy levels are computed for the hydrogen molecule and its homonuclear isotopes confined within carbon nanotubes of various sizes and structures using three different interaction potentials. Two translational and two rotational degrees of freedom are treated explicitly. We study the dependence on the interaction potential and the size of the nanotube of several features, including zero-pressure quantum sieving selectivities, ortho-para energy splittings, and wave function characteristics. We show that large quantum sieving selectivities, as well as large deviations from gas phase ortho-para splittings, occur only under the condition of extreme two-dimensional confinement, when the characteristic length of the hydrogen-carbon interaction potential is nearly equal to the radius of the nanotube.