Nuclear magnetic relaxation measurements on liquid acetonitrile and acetonitrile water mixtures

Precise temperature dependent H, D, C-13 and N-14 relaxation measurements on acetonitrile in liquid acetonitrile and acetonitrile water mixtures are presented. ii and, for the first time, D induced nuclear Overhauser enhancements on C-13 are reported. Isotopic dilution experiments for the acetonitrile deuteron relaxation and relaxation measurements in the two phase region were performed. By a new method, the acetonitrile deuterium quadrupole coupling constant was determined to 162(6) kHz. An isotope effect of about 5% on the acetonitrile correlation time of the reorientation of the symmetry axis is observed. It decreases if the water content is increased. For the effective correlation time of the C-H/D vector the isotope effect is about 25%. This effect was separated into a contribution of 3% for only one molecule which is deuterated and 22% fur the deuteration of the remaining molecules. The anisotropy of the reorientation of acetonitrile was calculated in the limit of rotational diffusion. In neat deuterated ammonia the reorientation about the symmetry axis is about ten times faster than the reorientation of the symmetry axis itself. Due to different activation energies the anisotropy increases with decreasing temperature. The isotope effect is also anisotropic and the rotational anisotropy in protonated acetootirile is about fourteen. The anisotropy is higher in the binary mixtures. Ar the critical composition it is fourteen for the deuterated and twenty for the protonated sample. In the mixture of critical composition simultaneous relaxation measurements on both phases were possible. This allows a determination of the coexistence curve. No anomaly of the rates was observed on approaching the coexistence curve.