CARBONYL C-13 SHIELDING TENSORS AND HEATS OF ADSORPTION OF ACETONE ADSORBED IN SILICALITE AND THE 1 1 STOICHIOMETRIC COMPLEX IN H-ZSM-5/

The principal components of the carbonyl carbon chemical shift tensor of the hydrogen-bonded 1:1 stoichiometric acetone-H-ZSM-5 adsorption c omplex have been determined from an analysis of C-13 NMR spectra of st atic and magic angle sample spinning powder samples at 78 and 130 K, r espectively. In a similar manner the principal elements of this tensor have been determined for physisorbed acetone in silicalite and the pu re solid in order to separate changes due to hydrogen bonding of the a cetone molecule in the zeolite complex from confinement effects define d as interactions of the adsorbed molecule with the siliceous cavity. The energetics associated with such changes have also been measured us ing microcalorimetry. The differential heats of adsorption of acetone adsorbed in H-ZSM-5 and silicalite over a wide range of surface covera ge are reported. The results are compared with ab-initio calculations of the reaction of acetone with model zeolite structures to form a sto ichiometric hydrogen-bonded cluster-molecule complex. At the Hartree-F ock level the agreement with respect to both energetics and isotropic shifts is good but only fair for the shielding anisotropy. The magnitu des of the chemical shifts due simply to confinement of the acetone mo lecule are of the same order of magnitude as those associated with hyd rogen bonding.