Semiempirical methods zeolites

At the time of their inception, cluster calculations of adsorbate-zeolite systems were largely treated by using a semiempirical method. In the mid-... 

We have calculated those values by following the reaction coordinate during alkylation of toluene by zeolite-CHj and have observed that the densities of frontier orbital in the atoms which intervene directly in the reaction have strongly increase, and C(.j (HOMO) = 0.54 q7m-ch3 (lUMO) = 0.89 values have been calculated by means of PM3 and MNDO semiempirical methods. Furthermore, the population of the frontier orbitals show a larger electronic transfer from the HOMO to the LUMO, while PMO calculations show that the effect is larger for the attack in para than in ortho position [35]. 

The next problem to investigate was whether the cationic positions could also be determined in zeolite A and faujasites by semiempirical methods. From this aspect the MNDO method is the best choice, since most of alkali and alkaline earth elements are parameterized within this approximation. 

Models used in the zeolite science can be divided into two categories (i) models that do not explicitly consider any electron in the system (molecular mechanics, interactions described with interatomic potential functions) and (ii) models that explicitly consider part of the electrons in the system (either at semiempirical level or at ab initio level). This text should serve as an introductory overview of quantum chemical approaches (excluding semiempirical methods) and models available for zeolite modeling. It is impossible to review the quantum chemical calculations in zeolite science on pages available here. Only qualitative description of methods will be given, avoiding mathematical equations. More details can be found, e. g., in Refs. [1-5]. 

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CaY zeolite has been found either to act as a reagent to reduce stilbenes to 1,2-diarylethanes or to act as a catalyst and isomerize (Z)-stilbenes to the more stable E-form, depending on the number of Bronsted acid sites present in the zeolite. It has also been suggested that Z-to-E one-way isomerization of (Z)-stilbene through proton addition-elimination and electron-transfer processes occurs in acidic zeolite cavities. The isomerization reaction of (E)- and (Z)-a-phenylcinnamic acid molecules has been studied " at the level of semiempirical quantum chemical methods. The calculations revealed that the (Z)-a-phenylcinnamic acid is slightly more stable than the -isomer. Kinetic studies have been made on the thermal Z-E isomerization of C(40)-carotenoids, and the rotational barriers for Z-E isomerization of different proline analogues have been investigated by dynamic H NMR spectroscopy. The effects of... 

A semiempirical model that has been particularly useful in zeolite studies is known as the electronegativity equalization method (EEM). EEM is based on the following assumptions (1) the electron density can be partitioned into spherical atomic contributions (2) T[p] and Ve [p] in Eq. [6] can be written as a second-order Taylor expansion of the effective charges and (3) each atom i carries an effective charge q,- = Z, - N,, where Z, and N, are the charge on the nucleus and the number of electrons in the atom, respectively. Then the total energy is given ... 

There have been a large number of quantum chemical studies on cluster models representing zeolitic lattice and the molecules adsorbed over them. Semiempirical quantum chemical methods such as CINDO [8], MNDO [2], AMI [4], PM3 [5] have been used to study the structural parameters [9,10] as well as interaction with probe molecules such as H2O, CO, NH3, Pyridine and other organic reactants [11-13]. 

Semiempirical calculations of simple molecules on zeolite model clusters were carried out by Beran, Lochmann et al., Kustov et al. and Tasi et al. [64-69]. One of these procedures, the Self Consistent Charge (SCC)-Xa method has been successfully applied also to zeolites [70-76], because of its higher reliability in comparison to other methods in the determination of atomic charges within a reasonable computation time. A combination of semiempirical MO calculations and vibrational analysis supplies detailed information on zeolitic systems when consistently combined with experimental data [77, 78]. 

Very often a specific semiempirical MO method was adopted to represent Hqu and a specific MM method was used for //mm and the coupling term / qm-mm introduced additional empirical parameters to allow the combined energy to reproduce experiments. Thus they are specific to the MO method and the MM method adopted at the initial definition of the method. The empirical valence bond approach has also been combined with MM for more complex systems. There have been other approaches attempting to incorporate different procedures which show the importance of combining these methods and the difficulty in doing so. Numerous QM/MM studies have been made in the past, including various versions, some applications, " the treatment of charges, application to zeolites and biochemistry, molecular dynamics, and solvent effects. These variations of the so-called QM/MM methods are treated in separate articles. 

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