J . Am. Chem. SOC. 1991, 113, 6435-6441 6435 for isozyme 111, very strong anion binding is reported.3 Isozyme 111 has a much more positive potential in the active site region that would enhance anion binding. The model is so simple that it is easy to suggest that water or other nearby ionic residues or couples such as the Thr-Glu will alter details. It does show the need for the more complete treatment exists and that all of the studies of CA have not properly considered the problem of the ionicity of the metal active site. Abstract: A comparison of force field calculations on extended systems and ab initio quantum chemical calculations on ring structures reveals that the relative energy content of neutral-framework silicas and aluminophosphates is determined by that of the smallest substructures. Energy-minimized structures containing only four, five, and six rings have the same energy content to within 10 kJ/mol. Hypothetical three-ring-containing structures have considerably higher energy content, which may well inhibit their synthesis in the pure SiOz modification. The results accord with sparse experimental information. Using the same combination of techniques, we demonstrate that substitution of aluminum in silica causes an appreciable, albeit local, distortion of the lattice. The relaxation energy for such a substitution amounts to 100-200 kJ/mol, depending on the way charge compensation is accomplished.
We have calculated the adsorption energy of NH3 and NHZ in a zeolite crystal and on four different dusters cut out from the crystal. The adsorption energies were calculated for two different geometries of the chabazite: a geometry determined with x-ray diffraction and a shell model optimized geometry. The effect of the geometry optimization of the crystal is large. In the crystal for which the geometry has been optimized the adsorbates are less stable.For NH, the difference between the two geometries is 29 kJ mol-' (adsorption energies -101 and -72 W mol-'), and for NHZ the difference between the two geomevies is 137 ! d mol-' (-15 and 122 kJ mol-'). The adsorption energy of NHI is a local process. The effect of the Madelung potential is relatively small: for the two geometries we found -6 and +5 kJ mol-'. The adsorption of NHf is more affected by the Madelung potential -53 and t 2 3 kJ mol-' for the two geometries. Clusters thal have bonds saturated with hydrogen atoms close to the adsorbate do not give a proper descnption of the adsorption process: this type of boundary error can be as large as 60 W mol-',
Rigid ion and shell model calculations of the infrared and Raman active modes of cy-quartz, aluminum-free sodalite, and dealuminated faujasite are presented. Results of the rigid ion calculations show poor agreement with experimental data but the agreement significantly improved when using shell model calculations. A group theoretical analysis of the spectra enables an analysis in terms of dominantly stretching, bending, or torsional modes. A rather good agreement of the frequencies in the bending and torsional mode regions is found. One concludes from comparing with the stretching modes that the short range potential for the Si-0 stretch frequency is too weak. The improvement of the shell model versus the rigid ion model results can be ascribed to screening of the long range electrostatic interactions by polarization of the 02-anions. Calculated frequency splittings between transversal and longitudinal optical modes indicate that the use of partial ionic charges, in combination with an improved short range potential, might result in a better simulation of the infrared and Raman spectra.
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