Quantum mechanical stabilization calculations

An essential question in zeolite science is the nature of the chemical bond in zeolitic systems. This paper aims to present current understanding and will focus on the covalent as well as electrostatic interactions in these materials. It will appear that structural differences sensitively depend on the balance between the two kinds of interactions. Especially the use of proper quantum-mechanical cluster calculations and the development of new force flelds based on such calculations has resulted in a signiflcant improved understanding of zeolite stability as weU as the acidity of protonic zeolites that are important for catalytic applications. 

Extensive molecular dynamic simulations of proline-catalysed asymmetric aldol condensation of propionaldehyde in water have revealed that the stereoselectivity can be attributed to differences in transition-state solvation pattems. " The hydrogen bond concept has been applied to design new proline-based organocatalysts. " 4-Hydroxyproline derivatives bearing hydrophobic groups in well-defined orientations have been explored as catalysts in water an advantage of aromatic substituents syn to the carboxylic acid moiety has been attributed to a stabilizing transition-state hydrophobic interaction and this is supported by quantum mechanics (QM) calculations. " Catalysts and solvents were screened for reaction between cyclohexanone andp-nitrobenzaldehyde. 

Devillanova and coworkers have also addressed this issue with some simple thiones and selones. Using spectroscopic analysis and quantum mechanical calculations, they examined the various possible reaction pathways shown in Fig. 5 [72,183]. The geometries and relative stabilities of the charge-transfer and T-shaped hypervalent adducts were compared using DFT cal-... 

In a few instances, quantum mechanical calculations on the stability and reactivity of adsorbates have been combined with Monte Carlo simulations of dynamic or kinetic processes. In one example, both the ordering of NO on Rh(lll) during adsorption and its TPD under UHV conditions were reproduced using a dynamic Monte Carlo model involving lateral interactions derived from DFT calculations and different adsorption... 

Molecular mechanics calculations have become a well established tool in the area of coordination chemistry, including the coordination chemistry of nickel375-379 where they are often applied for the analysis or the prediction of structures,380 the computation of isomer or conformer ratios and metal ion selectivities,381,382 and for simulating spectroscopic properties in combination with AOM calculations or by hybrid quantum mechanics/molecular mechanics (QMMM) methods.383,384 Details of the various approaches, e.g., the incorporation of d-electron stabilization energy... 

In spite of its limitations, molecular mechanics (MM) is a technique that is widely used for the computation of molecular structures and relative stabilities. The advantage of MM over quantum mechanical methods is mainly based on the computational simplicity of empirical force field calculations, leading to a comparatively small computational effort for MM calculations. Therefore, even large... 

The first (exponential) term represents repulsion between electron orbitals on the atoms. The second term can be seen to be opposite in sign to the first and so represents an attraction—the weak van der Waals interaction between the electron orbitals on approaching atoms. The adjustable parameters can sometimes be calculated using quantum mechanics, but in other systems they are derived empirically by comparing the measured physical properties of a crystal, relative permittivity, elastic constants, and so on, with those calculated with varying parameters until the best fit is obtained. Some parameters obtained in this way, relevant to the calculation of the stability of phases in the system SrO-SrTiC>3, are given in Table 2.3. 

Extensive computational calculations have been performed by using molecular mechanics (MM) [79], quantum mechanics (QM) [80], or combined MM/QM methods [81]. As major contributions, these theoretical studies predict the greater stability of the major isomer, explain the higher reactivity of the minor diastereomer, introduce the formation of a dihydrogen adduct as intermediate in the oxidative addition of H2 to the catalyst-substrate complexes, and propose the migratory insertion, instead of the oxidative addition, as a turnover-limiting step. 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

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