Zeolite computed frequencies

This model approximation is known as the cluster approach. The fragment extracted from a zeolite framework is terminated with hydrogen atoms. Such method has been used with success for the estimate of properties such as frequencies of vibration, NMR shielding constants, or reactivity. Another mandatory reason of the use of this method was the too heavy computational cost to simulate larger systems. 

Owing to the complexity of zeolitic systems, most computational studies are still performed with the help of classical models. These methods use a set of potential functions to describe the potential energy surface (PES) in a manydimensional space of geometrical parameters of the system. Although the PES can be tested in terms of observables such as equilibrium atom positions, vibrational frequencies, heats of formation, and other experimental information, the PES itself is not an observable quantity. Because of that, there is no unique representation of the PES, and several coordinate systems and parameteriza-... 

The vibration frequencies were calculated ab initio for a model structure comprising a bare metal cation interacting with the fliran ring. For both the experimental and calculated spectra the interaction of furan with the zeolite caused the out-of-plane vibrational modes to move to higher wave numbers, see Fig. 7.28. This result can be explained if one assumes an interaction, oriented perpendicular to the molecular plane of furan, between the zeolite cation and the 7t electron system of furan, which lead to an increase of the CH out-of-plane bending force constant. However, for the experimental spectra the wavenumbers of the out-of-plane modes then decrease in the series Li > Na > > Cs whereas the computed... 

EXAFS experiments of the Rh CO)2 complex in DAY zeolite [234] provided an excellent opportunity to compare the interatomic distances derived from EXAFS spectra with the calculated structural data. Good agreement between the EXAFS parameters and the optimized structure of the Rh (CO)2 complex on a zeolite four-ring support the hypothesis that the system Rh (CO)2/DAY is correctly modeled by the calculated T4 stmcture (Table 4.4). Also the computed C-0 vibrational frequencies of this system are close to those from IR spectra [234,236,237], with deviations below 12 cm (Table 4.4). 

Nevins N, K Chen and N L Allinger 1996c Molecular Mechamcs (MM4) Vibrational Frequency Calculations for Alkenes and Conjugated Hydrocarbons. Journal of Computational Chemistry 17 730-746 Nicholas J B, A J Hopfmger, F R Trouw and L E Iton 1991 Molecular Modelling of Zeolite Structure. 2. Structure and Dynamics of Silica Sodalite and Silicate Force Field. The Journal of the American Chemical Society 113.4792-4800. 

Whereas Eq.(5.58) serves for the determination of local interactions between cluster models of a zeolite and interacting molecules, analytical expressions are needed for the interaction potential if one wishes to compute vibrational frequencies for purpose of comparison with experiment or if the potentials are to be used in Monte Carlo or molecular-dynamics simulation calculations. Sauer and co-workers developed such analytical potentials for the water-silica interaction system. The method makes use of the molecular electrostatic potential (MEP) maps and the functional form of EPEN/2 (Empirical Potential based on interactions of Electrons and Nuclei). EPEN/2 potential functions consist of a point-charge interaction term and... 

In summary, the direct quantum mechanical simulation of zeolite vibrational spectra is evidently a formidable task and is often severely hampered by limited computational resources. Pure ab initio methods are well-suited if local effects or groups with characteristic vibrational frequencies like Bronsted acidic OH groups are under study. In theoretical studies of vibrational spectra of zeolite frameworks and cations on extra-framework sites, QM calculations are of crucial importance in developing force field parameters which can be used in a subsequent step in MM, MD or NMA calculations. Due to the lack of sufficient exper-... 

A more physical t (= 10" s) value for ferromagnetic particles was obtained by Lazaro et al. from measurements on iron particles embedded in a zeolite matrix, which, because of its microporous structure, allows the isolation of the magnetic grains. From a computational analysis of the experimental data, the authors also obtained information about the particle size distribution. However, the explored frequency range was too narrow ( = 10-10 Hz). 

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