Etude de la Dynamique Orientationnelle des Molécules d'Allène a l'Etat Liquide par Spectrométrie Raman

The orientational dynamics of liquid allene has been studied by means of the analysis of the isotropic and anisotropic profiles of the symmetrical A1 band at 1074 cm−1 and of the anisotropic profile of an E band at 3067 cm−1. These profiles have been studied in the temperature range from 300 to 150 K and for pressures ranging from 100 to 3000 atm. Using the hypothesis of a diffusional rotation, the analysis of the former band allows the determination of the principal component D⊥ of the diffusion tensor, as well as of the associated activation energy and activation volume. The comparison of the observed orientational correlation times with the free‐rotation times and the shape of the correlation functions are not in too bad agreement with this hypothesis. The second band of E symmetry, which corresponds to a vibration whose Coriolis constant is very small, allows the evaluation of the diffusional Constant D∥ associated to the rotation of the molecule about its axis. A strong anisotropy in the rotations of the allene molecule in the liquid state is evidenced: D∥/D⊥ ≃ 7. Furthermore, the rotational motion about the axis seems to digress from a diffusional motion more than the reorientation of the axis itself. Finally, the paper discusses those conditions in which the evaluation of the rotational diffusion constant from the Raman profile of a depolarized band is valid.