Quantum mechanical simulations bond activation

Quantum-chemical cluster models, 34 131-202 computer programs, 34 134 methods, 34 135-138 for chemisorption, 34 135 the local approach, 34 132 molecular orbital methods, 34 135 for surface structures, 34 135 valence bond method, 34 135 Quantum chemistry, heat of chemisorption determination, 37 151-154 Quantum conversion, in chloroplasts, 14 1 Quantum mechanical simulations bond activation, 42 2, 84—107 Quasi-elastic neutron scattering benzene... 

The previous two sections of this review deal with classical simulation methods. A description of the activation of adsorbates by acidic sites, together with any bond breaking or bond formation that may take place, is the realm of quantum mechanical (QM) simulations. These types of calculations are particularly well-suited to zeolite-adsorbate systems when the cluster approximation is used. The active acidic site in the zeolite is modeled by a molecular cluster, formed by cutting out a small portion of... 

Early quantum mechanical calculations treating chemisorption employed a valence bond-type method and a few atoms to simulate the surface. Sherman and Eyring (54) used a 4-atom model to simulate H2 dissociation on charcoal. The calculations were unable to duplicate the low activation energy observed experimentally for this reaction. Later calculations by Sherman and Eyring (55) showed that H2 dissociation on Ni2 has a low activation energy, suggesting that quantum mechanics is a useful tool for such studies. 

Through ab initio quantum mechanical/molecular dynamics methods, Perakyla and Kollman [386] have presented a detailed model of the mechanism of aspartylglucosaminidase-catalysed cleavage of an amide bond. The QM model consisting of the model substrate, N-terminal amino acid, and oxyanion hole was considered large enough to include all the important interactions present at the active site between the enzyme and the scissile amide bond. The QM calculations have been performed at the MP2/6-31G //HF/6-31G level and MD simulations for all the intermediates and transition states of the reaction were carried out. 

In view of Sharma et al., [29] the classical simulations address the FIR activity of the translational modes by means of force fields, where each water molecule is polarized according to a local field so that the induced polarization is intramolecular. On the contrary, in the above-cited work it was shown in terms of the first-principle quantum approach that for a tetrahedral coordination of oxygen atoms the liquid is regarded as an assembly of electrons and nuclei and that quantum mechanical evidence is that the FIR spectra depend on concerted tetrahedral fluctuations of the H bonds. In this assembly the polarization effects depend on the environment in a strong intemolecular way. ... 

Direct dynamics calculations of the type just described, with all degrees of freedom included, are very expensive if the local quadratic approximations to the potential energy surface are obtained from an ab initio computation. In applications we have used a hybrid parameterized quantum-mechanical/force-field method, designed to simulate the CASSCF potential for ground and covalent excited states. A force field is used to describe the inert molecular a-framework, and a parameterized Heisenberg Hamiltonian is used to represent the CASSCF active orbitals in a valence bond space. Applications include azulene and benzene excited state decay dynamics. 

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