The mass to use in the expression for the vibrational frequency of a diatomic molecule is the effective mass µ = m_Am_B/(m_A + m_B), where m_A and m_B are the masses of the individual atoms. The following data on the infrared absorption wavenumbers (in cm⁻¹) of molecules are taken from Spectra of diatomic molecules, G. Herzberg, van Nostrand (1950):

Calculate the force constants of the bonds and arrange them in order of increasing stiffness.

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H³5 Cl 2990 H81 Br 2650 HI 2310 CO 2170 NO 1904