Effects of slow motion on deuteron relaxation time anisotropy

When molecular motion occurs on multiple timescales, interpreting deuteron Zeeman relaxation times, T1Z, yields ambiguous results. Simultaneous measurements of the orientation dependence of T1Z and the quadrupole order relaxation time, T1Q, are required to provide a more accurate description. Processes with microsecond correlation times necessitate analysis based on the stochastic Liouville equation. Relaxation of methyl deuterated bisphenol-A polycarbonate is considered as an illustration. Numerous Redfield models are consistent with the T1Z data, but their failure to reproduce the observed T1Q anisotropy is characteristic of slow motion. In complex systems where slow motion is ubiquitous, it is essential that motional studies include measurement of both T1Z and T1Q anisotropies.