Extremely slow intramolecular vibrational redistribution: Direct observation by time-resolved Raman spectroscopy in trifluoropropyne

We have studied the dynamics of intramolecular vibrational redistribution (IVR) from the initially excited mode $\nu_1\approx3330$ cm$^{-1}$ (acetyleneṭype H–C bond) in H–C$\equiv$C–CF$_3$ molecules in the gaseous phase by means of anti-Stokes spontaneous Raman scattering. The time constant of this process is estimated as 2.3 ns – this is the slowest IVR time reported so far for the room-temperature gases. It is suggested that so long IVR time with respect to the other propyne derivatives can be explained by a larger defect, in this case, of the Fermi resonance of $\nu_1$ with $\nu_2+2\nu_7$ – the most probable doorway state leading to IVR from $\nu_1$ to the bath of all vibrational–rotational states with the close energies. In addition, it is shown that the observed dynamics is in agreement with a theoretical model assuming strong vibrational–rotational mixing.