Second-order perturbation theory corrections to effective Fermi resonance coupling constants

Second-order perturbation theory corrections to effective Fermi resonance coupling constants have been derived analytically within the framework of the traditional rectilinear normal coordinate approach to the analysis of anharmonic molecular vibrations. These corrections have been evaluated for a range of methyl halide molecules (including deuterated isotopomers) and are found to rationalise qualitatively discrepancies between the empirical effective Fermi resonance parameters and theoretical normal coordinate cubic force constants.