Anharmonic corrections for the out-of-plane (OPLA) vibrational modes of CH2Cl, CH2F, and CH3 radicals have been calculated. For these radicals, it is possible to describe the OPLA motion within a simple one-dimensional model based on the adiabatic separation of the (slowest) OPLA mode from all other vibrations. The effective potentials have been calculated by CCSD(T) and DFT/B3LYP methods with 6-311(+,+)G(3df,2pd) basis sets. It is found that halogen substitution increases the anharmonicities dramatically, i.e., from 19% in CH3 up to about 100% in CH2Cl and CH2F. The resulting frequencies of the fundamental OPLA transition are in good agreement with the experimental values. In the case of CH2F, the large anharmonicity in the OPLA mode results in a wave function delocalized over the two minima of the double well potential. This reconciles the experimentally determined planar (C-2nu) structure with the calculated pyramidal (C-s) equilibrium geometry.