AB-INITIO MOLECULAR-ORBITAL CLUSTER STUDIES OF THE ZEOLITE ZSM-5 .1. PROTON AFFINITIES

In this study we carried out ab initio molecular orbital calculations using the STO-3G, 3-21G, and 6-31G basis sets on aluminosilicate clus ters representing ZSM-5 containing up to 46 tetrahedrally coordinated tetravalent atoms and including (SiO)n rings. The proton affinity and OH stretching frequency of the central bridging oxygen site (O(24) sit e) are examined as a function of cluster size and shape. The proton af finity is found to depend significantly (5-15 kcal/mol) on the shape o f the cluster. Calculation of the electrostatic potential in the regio n of the central bridging oxygen site for the clusters indicates that this dependence is largely due to electrostatic effects. The effects o f geometry relaxation are studied with a constant-volume relaxation me thod in which a part of the cluster is optimized with the remaining be ing fixed in its crystal structure. Results indicate that relaxation i ncreases the proton affinity by about 10-15 kcal/mol over the value fr om the rigid experimental structure. The final calculated proton affin ity for the O(24) site in ZSM-5, including correction for zero-point e ffects, O:Si ratio, and higher level theory, is consistent with experi ment. The OH stretching vibrational frequency is in good agreement wit h the experimental value.