Electronic structure of halogen-substituted methyl radicals: Equilibrium geometries and vibrational spectra of CH2Cl and CH2F
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 a 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 (C2v) structure with the calculated pyramidal (Cs) equilibrium geometry.