To a first approximation, the stationary perturbation theory could be employed to evaluate the cation effect on the vibrational coupling by treating the cation-water (proton donor) interaction as a perturbation on the quantum mechanical harmonic oscillator of the 'R-mode'. The main contribution of the interaction energy of the cation with the R-mode comes from the cationwater interaction. This interaction is treated as the electrostatic attraction between a point charge (cation) and a dipole (H-OH in the water molecule), which decays with an inverse square law. The perturbation through the cation-water interaction would lead to the deviation of ⟨𝑅 -𝑅 m ⟩ from zero. Assuming that the cation-water interaction strength in the R-mode exhibits a linear relation with ∆𝐺 /0& with a proportionality coefficient 𝛾 (0 < 𝛾 < 1), 𝛿𝑅 can be derived as: