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 CH3CN/CD3CN in the 5-60 degrees C temperature rang e. By using these two techniques, it has been ascertained that the substitu tion of hydrogen with deuterium weakens the intermolecular interactions so that the local structures of aligned and parallel dipoles are destroyed at a temperature which is far below the boiling point. The behaviour of the tr ansport quantities versus temperature in liquid CD3CN thus gives evidence o f structural-like relaxations at a temperature falling in the range 35-40 d egrees C. The comparison of thermal and orientational relaxation data obtai ned for the two isotopomers is the key for a correct interpretation of the dynamics in the two liquids.