MOLECULAR MOTIONS AND C-13 CHEMICAL-SHIFT ANISOTROPY OF ACETONE ADSORBED IN H-ZSM-5 ZEOLITE

The adsorption of acetone on H-ZSM-5 at 295 K as a function of surface coverage was studied by observing line shape changes of the C-13 carbonyl carbon shielding tensor. For all coverages below one acetone molecule per Bronsted acid site, the spectrum corresponds to a powder pattern characteristic of a chemical shift anisotropy for static molecules, similar to that of the neat solid. Above one molecule per acid site the powder pattern collapses to a narrow line as a result of rapid random reorientations. The change from a static adsorbent-molecule complex to rapid reorientations is sharp and suggestive of a well-defined acid site.