The mass to use in the expression for the vibrational frequency of a diatomic molecule is the effective mass μ = mAmB/(mA + mB), where mA and mB are the masses of the individual atoms. The following data on the infrared absorption wavenumbers (wavenumbers in cm-1) of molecules are taken from Spectra of diatomic molecules, G. Herzberg, van Nostrand (1950):
H35Cl H81Br HI CO NO
2990 2650 2310 2170 1904
Calculate the force constants of the bonds and arrange them in order of increasing stiffness.