IR spectrum of HCF2CF2Br: hindered intramolecular vibrational energy redistribution Original Research Article

The CH stretch band of the anti isomer of HCF2CF2Br shows a Q branch of only 1 cm−1 of width and in the first overtone, it is the syn isomer which has a very narrow Q branch. Obviously, the hot bands starting, e.g., from the thermally populated states of the torsional vibration, practically coincide with the fundamental. Hence, the torsional frequency does not depend on the degree of CH excitation, and therefore neither does the torsional potential, although already a single CH quantum has more energy than the torsional barrier, above which there could be free internal rotation. A barrier height of 1590 cm−1 is deduced from the observed torsional frequency of 76 cm−1. This is too high for tunnelling. We conclude that there is no direct conversion of the energy from the CH vibration to internal rotation, although higher order coupling can probably cause a slow redistribution. Similarly, we conclude that the flow of CH vibrational energy to the CBr stretch and other low frequency modes is hindered, too. We point out the similarity to van der Waals molecules, in which the low-frequency mode also has little coupling to the other vibrations. But the torsional-CH non-coupling is especially intuitive, because it infers an unchanged barrier height and therefore an unchanged tunnelling frequency. We also propose that generally the nondiagonal anharmonicity between high- and low-frequency modes is small (⪡1 cm−1).