Framework isolated model cluster

The first problem originates from the structural complexity of zeohte frameworks which normally contain several hundred atoms per unit cell. This makes studies of the vibrational behavior of the lattice and the search for modes characteristic of special structural units even more difficult. In this case, a usual approximation is to cut out an isolated model cluster from the framework and treat it hke a molecule. In comparison with quantum mechanics, in NCA it is not necessary to saturate the dangfing bonds of the cluster by terminal pseudoatoms (vide infra). In a first attempt, based on such an assumed decoupHng of modes from the surrounding framework, Blackwell [7] predicted vibrational frequencies... 

It is important to note that the isolated ZOH cluster is not, for many purposes, a physically meaningful simulation of the zeolite framework cavity. As indicated earlier the calculation, of protonic charge, also considered as a possible measure of "acidity", does not lead to sensible values either for the isolated ZOH cluster or the adsorption complex ZOH...B. The natural development of the subject of proton transfer was in the direction of more realistic models of the Bronsted site. In Table 2 below, a list of the principal systems treated by ab initio methods are presented which in many cases include more realistic models. 

Another important zeolite-catalyzed chemical reaction is the decomposition of NO. Cu-exchanged zeolites, especially Cu-ZSM-5, have been shown to catalyze the decomposition of NO in the presence of hydrocarbons and excess oxygen. The increasing awareness of the detrimental effects of automobile exhaust has prompted several theoretical studies on the active site and reaction mechanism. ° Cu-ZSM-5 was described using an empirical force field and energy minimization to locate the copper ions in ZSM-5. Isolated copper atoms and copper clusters were found in the micropores, mostly associated with framework aluminium species. A cluster of two copper ions bridged via an OH species not part of the zeolite framework ( extra-framework ) was proposed as the active site. Quantum mechanical cluster calculations were carried out to study the elementary steps in the NO decomposition. A single T-site model was used to represent the zeolite framework. 

The hydrolysis of formamide has been also studied at the presence of (H20)2 and H30 -H20 clusters at the MP2/6-31G(d,p)//3-21G level [84]. The results of calculations for the isolated clusters and ones surrounded by the dielectric continuum in the framework of SCRF model can be found in [85,86]. The protonation was found again as the most favorable way to decompose the molecule. In the case of the nonacidic catalyzed reaction, there was also found... 

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