Mandelstam-Brillouin spectra and hyperacoustic velocities dispersion of trideuteroacetonitrile in the liquid state

Brillouin scattering spectra, as a function of temperature and transferred wave vector, were measured on deuterated acetonitrile. Its collective mode dynamics was compared with that of the light molecule, previously studied in this laboratory [P. Sassi, G. Paliani, R.S. Cataliotti, J. Chem. Phys. 108 (1998) 10197]. The results obtained in this study demonstrate that the hydrodynamic behaviour of the two isotopomers is quite similar. Small differences are observable at temperatures higher than 40 degrees C. These differences regarding shear viscosity, adiabatic compressibility, sound absorption coefficient and thermal relaxation times, are discussed in terms of a local-structure relaxation in such molecular liquids. It concerns the local organization of small clusters of antiparallely aligned dipoles. (C) 2000 Elsevier Science B.V. All rights reserved.