Raman study of the conformational instability of a ferrocene molecule at high pressure: Influence of a crystal field

The Raman spectra of ferrocene crystals were measured at pressures up to 20 GPa, and an abnormally large bandwidth of intermolecular phonons at ambient pressure was found. With an increase in the pressure, the bandwidth increased to a maximum at similar to 2 GPa and then decreased to a minimum at similar to 4 GPa, which was equal to the pressure-independent bandwidth of intramolecular phonons. The unusual behavior of the bandwidth was related to the instability of a ferrocene molecule caused by jumps between its D-5d and D-5h conformations. A decrease in the time of jumps between the conformations to the period of crystal lattice vibrations led to a loss of coherence and broadening of intermolecular phonon bands. The energy barrier between the conformations was determined to be similar to 17.6 meV/molecule under ambient conditions and 80 meV/molecule at 4.9 GPa. An increase in the barrier with pressure was due to the enhancement of the crystal field, which resulted in the inhibition of the jumps and the stabilization of the molecule in the D-5d conformation.