Force constant carbon-halogen bond

In general, the spectral features of vibrations of haloalkanes are affected by the mass of the halogen atom and the force constant of the carbon-halogen bond. If a halogen is bound to the same carbon atom as the hydrogen, the neighboring C-H overtone bands are shifted to a lower wavelength and their intensities are intensified. ... 

FIG. 3.4. Plots of the dependencies between experimental atomic equilibrium charges of halogens in CH3a and (a) difference of electronegativity between halogen and carbon, (b) stretching force constant of the bond aC, and (c) interatomic distance aC (Reproduced from Ref. [81] widi permission. Copyright [1984] American Chemical Society). 

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