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

Abstract

The potential of ¹³C-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 ¹³C 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 13C-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 110 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.