Vibrationally adiabatic ground-state (VAG) potential curves for the CH3 + H-2 --> CH4 + H (I) and CD3 + H-2 --> CD3H + H (II) reactions have been calculated at the QCISD(T, full)/cc-pVTZ// MP2(full)/cc-pVTZ level of theory. It has been found that both the barrier height and barrier width of the VAG potential curve for reaction II are slightly smaller than those for reaction I. Thermal rate constants for reactions I and II have been calculated with the obtained VAG potentials using variational transition state theory combined with the multidimensional semiclassical tunneling method. Rate constants for reaction II have been predicted significantly larger than those for reaction I especially at low temperatures. This computational result qualitatively explains the experimental observation of Momose et al. [J. Chem. Phys. 108, 7334 (1998)] that reaction I does not occur but reaction II occurs at 5 K in solid parahydrogen.