Study of low frequency motions in C6H6M (CO)3 with M = Cr,Mo, W by Raman and quasielastic neutron scattering

We have investigated the low frequency region of h₆ and d₆ polycrystalline samples of C₆H₆M(CO)₃ by Raman and IR spectroscopies in the 5–300 K temperature range.A complete assignment of low frequency modes is reported. The torsional mode τ′_z is observed at 89 cm^?1 at 5 K. The intramolecular potential associated with the torsional motion has been found egal to 4.44 and 4.10 kcal.mole^?1 at 5 and 120 K respectively. The intermolecular librational potential barrier (16.1 Kcal.mole^?1) was deduced from the R′_z librational frequency (40 cm^?1). The temperature dependance of torsional mode suggests that a large amplitude motion occurs as soon as 150 K. Quasielastic neutron scattering experiments have shown that jumps of C₆H₆ ring take place and that the corresponding activation energy is equal to 3.9 Kcal.mole^?1 (between 260 and 330 K) in agreement with the above potential barrier. The E.I.S.F. seem to favor a 2π/₆ jump regime with a correlation time of 4.10^?11 s at 300 K.