Field-dependent relaxation and molecular reorientation of C-60 in chlorobenzene

To advance our understanding of the C-13 spin-lattice relaxation process an d rotational dynamics of C-60 in various environments, we have acquired rel axation data on this molecule in chlorobenzene-d(5) as a function of field strength and at various temperatures. Field-dependent measurements allowed us to separate the contributions arising from the two possible modes for re laxation in this molecule: chemical shift anisotropy (CSA) and spin-rotatio n (SR). The CSA and SR values were used to extract the reorientational time , tau (c), and the angular momentum time, tau (J), which allowed us to inve stigate the rotational dynamics of C-60 in this solvent. We found that the longitudinal relaxation rate is dominated by the chemical shift anisotropy mechanism and that spin-rotation becomes rapidly more important with rising temperature. This is seen especially at 4.7 T, where SR dominates at the m oderate temperature of 333 K. We also compared our field-dependent findings with values derived via the Hubbard relation, and we have found the Hubbar d approach to overestimate the CSA contribution, causing underestimation of the SR effect. Finally, in an attempt at characterizing our experimental c orrelation times, we applied the Stokes-Einstein-Debye (SED), Gierer-Wirtz (GW), and Hynes-Kapral-Weinberg (HKW) models to our system and found their predictions to be inconsistent with our experimental findings. Far better a greement was obtained with the GW and HKW theories when these models were e xpanded to account for the inertial contribution to the overall rotational time. Our investigation also revealed that C-60 reorients in the intermedia te regime rather than at either the "slip" or the "stick" extremes.