Combined translation-rotation jumps in solid carbon dioxide

Abstract

Combination translation-rotation jumps in solid CO2 have been measured using 13C NMR techniques previously applied to α-CO. In the Pa3 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 energy E/k=6600 K and frequency prefactor ω0=2×1017 s−1. The activation energy in CO2 agrees with that found previously for the same motion in N2O and α-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 the Pa3 crystal structure. Unusually high frequency prefactors are seen in all three solids. The shift anisotropy of 13CO2 has been measured as 325 ppm.