Combination translationhyphen;rotation jumps in solid CO2have been measured using13C NMR techniques previously applied to agr;hyphen;CO. In thePa3 structure of CO2, a molecule which jumps to a neighboring (presumably vacant) site will also reorient, due to the orientationally ordered structure. The rates of translation and rotation have been measured independently by using low and high field NMR. The two rates agree, indicating that one combined motion occurs, as expected. The jump rate obeys the thermal activation expression with activation energyE/k=6600 K and frequency prefactor ohgr;0=2times;1017sminus;1. The activation energy in CO2agrees with that found previously for the same motion in N2O and agr;hyphen;CO, after scaling by the latent heats of sublimation. All three molecular solids belong to the family of solids composed of small, linear molecules with thePa3 crystal structure. Unusually high frequency prefactors are seen in all three solids. The shift anisotropy of13CO2has been measured as 325 ppm.
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