Combined translational-rotational jumps in solid (alpha)-CO and CO(,2)

• Main Author: Liu, Shang-Bin
• Format: Others
• Language: English
• Published: W&M ScholarWorks 1985
• Subjects: Condensed Matter Physics
• Online Access: https://scholarworks.wm.edu/etd/1539623757; https://scholarworks.wm.edu/cgi/viewcontent.cgi?article=3635&context=etd

Combined translational-rotational jumps in two orientationally ordered solids, (alpha)-CO and CO(,2), have been measured using C('13) NMR. In the Pa3 structure of these solids, a molecule which jumps to a neighboring (presumably vacant) site will reorient, due to the orientationally ordered structure.;The rates of translation and rotation have been measured independently by using different NMR techniques. The rotations were detected at high field (14.7 MHz) through the modulation of the chemical shift anisotropy; spin echoes and stimulated echoes were used. The translational jumps modulate the dipolar interactions and were studied at low fields (1.256 MHz) with line narrowing and Slichter-Ailion slow motion (T(,1D)) experiments. The rates of translations and rotations agree, indicating that they are two aspects of one combined motion.;The shift anisotropies of (alpha)-phase C('13)O and C('13)O(,2) were found to be 350 (+OR-) 15 ppm and 325 (+OR-) 15ppm, respectively; both values are in good agreement with previous NMR measurements. The jump rates of the combined motion in both materials obey the thermally activated expression: (omega)(,j) = (tau)(,j)('-1) = (omega)(,0) exp(-E(,a)/kT). The activation parameters are E(,a)/k = 2100 K and (omega)(,0) = 2 x 10('18) s('-1) for (alpha)-CO and E(,a)/k = 6600 K and (omega)(,0) = 2 x 10('17) s('-1) for CO(,2). The activation energies of (alpha)-CO and CO(,2) from this study agree by corresponding states analysis with that found previously for the same motion in N(,2)O. All three molecular solids belong to the family of solids composed of small, linear molecules with Pa3 crystal structure. Unusually high frequency prefactors ((omega)(,0)) are seen in all three solids and are not understood. The high prefactors are also shown to appear in other molecular solids such as benzene and ammonia.