MECHANISMS OF MOLECULAR-MOTION OF MO(CO)(6) IN ZEOLITES NAY AND HY FROM C-13-NMR LINESHAPES

The potential of C-13-NMR spectroscopy of stationary samples for the s tudy of mechanisms of anisotropic molecular motion of Mo(CO)(6) adsorb ed in zeolites has been examined. Free diffusional rotation or jump pr ocesses about the various symmetry axes of the octahedral complex caus e motional averaging of C-13 chemical shift anisotropy, allowing for t he characterization of the different motional mechanisms. At 110 K a l arge portion of the Mo(CO)(6) molecules is firmly fixed in zeolites Na Y and HY as indicated by a broad (ca. 410 ppm) chemical shift powder p attern. A narrow C-13-NMR component is observed in both zeolites for m olecules undergoing isotropic motion or rotation about a three-fold ax is. An additional line, which is inverted in shape and reduced in widt h by a factor of -1/2 compared to the rigid case, is only observed for NaY. This line is assigned to molecules undergoing free rotation abou t the four-fold axis, but it may also be due to Mo(CO)(6) jumping abou t a two-fold axis. The changes in the distribution of molecules in dif ferent motional states was explored between 1 10 K and ambient tempera ture. Molecules undergoing rotations or jumps about the four-or two-fo ld axes only exist in NaY at lower temperatures when stationary Mo(CO) (6) molecules are also present. At 180 K and higher temperatures, rota tion about the three-fold axis or isotropic motion are the sole types of molecular motion. (C) 1997 Elsevier Science B.V.