Volume 203, number 2,3 CHEMICAL PHYSICS LETTERS 19 February 1993 Spin-rotation relaxation in CbO J.H. Walton, A.K. Kamasa-Quashie, J.M. Joers and Terry Gullion Department of Chemistry, Florida State University, Tallahassee, FL 32306-3006, USA Received 9 November 1992 Recent solid-state NMR studies have shown that the nuclear spin-lattice relaxation time of CsO is dominated by the chemical shift anisotropy relaxation mechanism. Those studies were performed at high magnetic field strengths and at temperatures not exceeding 320 K. We find that the spin-rotation relaxation mechanism contributes significantly to the total relaxation rate at low magnetic field strengths. This relaxation mechanism also becomes significantlyat high temperatures for high magnetic field strengths. We find that the average value of the spin-rotation coupling constant is equal to 41 Hz. 1. Introduction Solid, pristine Cso is a plastic crystal with several interesting properties with respect to molecular mo- tion. X-ray and calorimetric studies have shown that there is a solid-solid phase transition at 249 K [ l- 31. Recent solid-state NMR studies and X-ray ex- periments provide the following picture [ 1,4-71. Below the phase transition the CbO molecules undergo a thermally activated jump process among the sym- metry equivalent orientations. Since there are a fi- nite number of spatially equivalent sites (60), this phase has been described as the ratchet phase [ 71. Above the phase transition the C6,, molecules reo- rient through a thermally activated isotropic diffu- sional process. This region has been labeled as the rotator (or rotor) phase [ 71. The orientational cor- relation time tc in the rotator phase, as determined by NMR, is in the picosecond regime. These results are consistent with the behavior of other plastic, ro- tor crystals [ 81. The activation energy and 7, were determined in the NMR experiments by the temperature depen- dence of the nuclear spin-lattice relaxation times T1, In this rare-spin system, T, is expected to be dom- inated by the chemical shift anisotropy (CSA) re- laxation mechanism [ 93. This was found to be the case by Tycko et al. [ 61 and Johnson et al. [ 71. However, in our preliminary high-pressure NMR studies of CeO we noticed that our room temperature, 1 atm T, value was shorter than the value obtained by extrapolation of the earlier high-field T, data [ 6 I. Since our magnetic field strength is lower than that used by the earlier investigators, experiments were performed to determine whether an additional re- laxation mechanism is present which may not be ev- ident at high magnetic field strengths. The results of spin-lattice relaxation experiments in the rotator phase for temperatures between 293 and 513 K at magnetic field strength of 3.55 and 6.34 T and for magnetic field strengths between 0.03 and 11.75 T at 293 K are reported. 2. Experimental The CGO sample was prepared by extracting a CbO/ CT0 mixture from soot (purchased from Texas Ful- lerenes) by using toluene followed by separation of the fullerene mixture by liquid chromatography us- ing a 5% toluene and 95% hexane carrier stream and a neutral alumina stationary phase [ 101. Since the sample was found to be contaminated with silicone grease (the original soot contained this contami- nant), the following purification procedure was per- formed. First, the sample was placed in an 8 inch long glass tube and connected to a vacuum line. The sam- ple was maintained at 460 K under vacuum for 2 days. Then the glass tube, while still under vacuum, was sealed. Second, the sealed tube was placed in a 0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. AU rights reserved. 237