Reorientational relaxation in a simple polar liquid: CD3CN. A comparison with light acetonitrile

FTIR and Rayleigh depolarized scattering experiments have been performed on the isotopomeric couple CH3 CN/CD3 CN in the 5-60 °C temperature range. By using these two techniques, it has been ascertained that the substitution of hydrogen with deuterium weakens the intermolecular interactions so that the local structures of aligned antiparallel dipoles are destroyed at a temperature which is far below the boiling point. The behaviour of the transport quantities versus temperature in liquid CD3 CN thus gives evidence of structural-like relaxations at a temperature falling in the range 35-40 °C. The comparison of thermal and orientational relaxation data obtained for the two isotopomers is the key for a correct interpretation of the dynamics in the two liquids.