Molecular motion in anisotropic medium. II. The proton relaxation study of CH3CN: The general A3 spin system

Abstract

The coupled relaxation of an A₃ spin system (CH₃CN) dissolved in a perdeuterated nematic phase (EBBA‐d23) is studied for the first time. The partially averaged dipolar interaction allows one to observe a triplet pattern on which four different spin perturbations have been performed: a selective soft π pulse on either an external or the central transition, and a nonselective π pulse affecting all three transitions with or without spin tickling on one of the external transitions. The last perturbation provides a measure of the time evolution of the doublet components in the central line of the triplet. The relaxation processes are modeled as intramolecular dipole–dipole interactions plus other mechanisms which are treated collectively as external random magnetic fields (RF). A restricted diffusion model is used which takes account of the molecular orientation of the solute molecule. Cross relaxation terms are very important, and RF mechanisms are not negligible even in a perdeuterated nematic phase. The anisotropy of the rotational diffusion is obtained, and R, the rotational diffusion constant of an axis perpendicular to the axis of symmetry of the rotor, is found to be 25 times smaller than in neat liquid. The diffusion constant about the principal axis R_∥ is not affected significantly by the liquid crystal. The values of R_⊥ and R_∥ give correlation times which allow extreme narrowing approximations, but in accordance with theoretical predictions it is demonstrated experimentally that the zeroth, first, and second order projections of the dipolar spectral densities are not equal due to preferential ordering of CH₃CN in the liquid crystal. Optimal values of the effective ordering parameter S_ZZ obtained from dipolar splittings in the spectrum and from the dipolar relaxation parameters differ from each other. This difference may be rationalized on the basis of models which account for the two different physical phenomena.