= Molecular vibration
{wiki}
<Molecular vibration> changes internal separations and <bond angles>, unlike the fixed internal <molecular geometry> of a <rigid rotor>. Its coupling to <molecular rotation> makes measured <rotational constants> depend on the vibrational <quantum state>.
A <diatomic molecule> has one internal stretching coordinate: its two <atomic nuclei> approach and separate about an equilibrium <bond length>. The <diatomic bond stretch> turns this motion into a particularly useful model. The local <potential energy> curvature supplies the restoring <force>, while the <reduced mass> accounts for both moving partners. Thus the same <chemical bond> can vibrate more slowly after an <isotope> substitution even when its electronic structure is nearly unchanged.
This separation makes <infrared spectroscopy> informative: transition <frequencies> constrain <chemical bond> stiffness, and <isotope scaling of diatomic vibration> tests the mass dependence. The <University of Florida HCl and DCl FTIR experiment> illustrates the measurement. Detectability also requires <vibrational infrared activity>; having vibrational <energy eigenvalues> alone does not guarantee an <infrared> absorption line.
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