Enhanced instability of extreme motion states in propyne: Lifetimes of overtone versus isoenergetic combination states

In contrast to the theoretical and experimental evidence for small polyatomic molecules, we demonstrate that in the low overtone region of propyne (νCH=2–4) pure overtone states are more perturbed by intramolecular vibrational energy redistribution (IVR) than some nearly isoenergetic combination states. We argue that for larger molecules, this is the expected behavior when the combination states have their energy delocalized over modes that interact only weakly by direct low order anharmonic resonances.