Molecular rotation in the presence of intramolecular vibrational energy redistribution

At high energy, the vibrational dynamics of a polyatomic molecule are quali tatively different from the separable normal-mode dynamics that characteriz e the low energy region of the spectrum. Once the total rovibrational state density exceeds 10-100 slates cm(-1), the effects of intramolecular vibrat ional energy redistribution (IVR) are readily observed in the frequency-dom ain spectrum. In an energy region where IVR occurs, the time scale for the flow of vibrational energy is comparable to the time scale for molecular ro tation. The jostling of nuclear positions caused by the IVR dynamics leads to a time-dependent moment of inertia for the molecular rotation. The time- dependent modulation of the moment of inertia, in turn, affects the appeara nce of the rotational spectrum of the molecule. These effects can be descri bed by the motional narrowing formalism first developed for nuclear magneti c resonance spectroscopy. We present a basic description of the rotational problem for the case where the molecule has a single energetically accessib le nuclear geometry and the case where the total energy of the molecule is above the barrier to isomerization. In the latter case, the microcanonical isomerization rate can be obtained from the overall line shape of the rotat ional spectrum. An example of using rotational spectroscopy to measure the isomerization rate of 4-chlorobut-1-yne at 3330 cm(-1) is presented.