Mechanisms of molecular motion of Mo(CO)6 in zeolites NaY and HY from 13C-NMR lineshapes

The potential of C-13-NMR spectroscopy of stationary samples for the study of mechanisms of anisotropic molecular motion of Mo(CO)(6) adsorbed in zeolites has been examined. Free diffusional rotation or jump processes about the various symmetry axes of the octahedral complex cause motional averaging of C-13 chemical shift anisotropy, allowing for the characterization of the different motional mechanisms. At 110 K a large portion of the Mo(CO)(6) molecules is firmly fixed in zeolites NaY and HY as indicated by a broad (ca. 410 ppm) chemical shift powder pattern. A narrow C-13-NMR component is observed in both zeolites for molecules undergoing isotropic motion or rotation about a three-fold axis. An additional line, which is inverted in shape and reduced in width 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 about the four-fold axis, but it may also be due to Mo(CO)(6) jumping about a two-fold axis. The changes in the distribution of molecules in different motional states was explored between 1 10 K and ambient temperature. Molecules undergoing rotations or jumps about the four-or two-fold axes only exist in NaY at lower temperatures when stationary Mo(CO)(6) molecules are also present. At 180 K and higher temperatures, rotation about the three-fold axis or isotropic motion are the sole types of molecular motion. (C) 1997 Elsevier Science B.