Slow intramolecular vibrational redistribution: the latest results for trifluoropropyne, a comparison with the other terminal acetylenes and the mechanism*

We studied the dynamics of intramolecular vibrational redistribution (IVR) from the initially excited mode ν 1 ≈3330 cm −1 (acetylene-type H–C bond) in molecules in the gaseous phase by means of time-resolved anti-Stokes spontaneous Raman scattering. The time constant of this process was estimated as 2.3 ns—this is the slowest IVR time reported so far for the room-temperature gases. We have compared this result with earlier results on the other terminal acetylene molecules, and give an explanation of this low IVR rate. Our suggestion for it follows from an assumption that the most probable doorway state leading to IVR from to the bath of all vibrational–rotational states consists of one quantum of the stretch and two quanta of the bend, and the matter is that the energy defect of Fermi resonance is essentially larger in trifluoropropyne than in other similar molecules. In addition, we have obtained the rate of collision-induced IVR for trifluoropropyne from experiments with various gas pressures and have shown that the observed dynamics is in agreement with a theoretical model assuming strong vibrational–rotational mixing.