Modeling proton mobility in acidic zeolite clusters. I. Convergence of transition state parameters from quantum chemistry

We have applied electronic structure methods to the calculation of transition state parameters for the O(1)--> O(4) proton transfer in H-Y zeolite. We arrive at a set of recommendations for calculating these transition state parameters accurately and efficiently. Density functional theory using the B3LYP functional and basis sets of triple-zeta quality in the valence space, and including polarization functions on all atoms, is the most efficient method for converging structures and vibrational frequencies. For converging classical barrier heights, we find it necessary to augment MP2 barrier heights calculated using large basis sets with MP4 energies obtained in more limited basis sets. We obtain an O(1)--> O(4) barrier height of 86.1 kJ mol(-1), and find the curvature of the barrier at the transition state to be 1570 cm(-1). Including long range effects from the work of Sauer [ACS Symp. Ser. 721, 358 (1999)] results in a higher barrier, which we estimate to be 97.1 kJ mol(-1). We attribute the fact that our barriers are significantly larger than those reported in the experimental literature to the neglect of tunneling in the interpretation of experimental data. (C) 2000 American Institute of Physics. [S0021-9606(00)70815-4].