Transition state from DFT calculations

The following results stem from DFT calculations on model rhodium complexes. Based on these model system transition states, force-field calculations were carried out in order to incorporate steric effects. The global energy minima of the new systems were searched for by MD simulations (cf. Section 3.1.2.1). This QMMM method with frozen reaction centers (FRC) is related to approaches recently discussed in the literature [29] cf. [23] for more details. 

Cluster DFT calculations were also used to identify a transition state for the formation of ethylene oxide. In this transition state, the Ag-0 bonds are elongated relative to the oxametallacycle. The product of this reaction is gaseous EO. The activation energy for this step had previously been determined by Linic and Barteau experimentally.62 The predicted activation barrier from DFT calculations, 16 kcal/mol, is in very good agreement with the experimental result of 17 kcal/mol. 

Enthalpies of transition state structures are determined with the above composite ab initio and DFT methods. Enthalpies of transition states structures are calculated as the difference between the calculated value of the TS structure and the value of the stable radical adduct(s) (adjacent product and reactant where both are a single species) at the corresponding levels of calculation. The computational methods for enthalpies are G3MP2B3 and G3 (whenever possible), and DFT. Zero-point energies (ZPVEs) and thermal corrections to 298.15 K are from DFT. Table 

There can be no cross calculations between the methods, meaning one cannot, for example, take the difference of energies of a minimum calculated by HF and a transition state from a DFT method to obtain an activation energy. Doing so would produce bizarre results. As with the choice of basis sets, one needs to make a decision depending on the merits and appropriateness of the methods on the particular system in consideration. 

The molecular interpretation of major topics in catalytic kinetics will be highlighted based on insights on the properties of transition-state intermediates as deduced from computational chemical density functional theory (DFT) calculations. 

A Gst is the difference in free energy due to steric constants in reactant and transition state, k is the rate constant of the nonsterically constrained reaction. The contribution of the steric component to the transition-state energy cannot be deduced accurately from DFT calculations because van der Waals energies are poorly computed. Force field methods have to be used to properly account for such interactions. 

This reaction is an interesting test of the modern approach to chemical reactivity. DFT calculations have been used to construct a potential energy surface for this reaction, which is reproduced in Fig. 3(c).45 One can see that the transition state for this reaction occurs at an N-N separation of 1.85X. Furthermore the energy release from the transition state to products is very large (250 kJ mol-1). 

The previous analysis shown that the initial values of most of the kinetic parameters obtained from DFT calculations provide a good description of the reaction kinetics data collected over a wide range of conditions. The principal difference between the values of the final kinetic parameters used in the model and the initial values obtained from DFT calculations is that the fitted enthalpy changes for the formation of C2Ha transition states involved in cleavage of the C-C bond are lower than the initial values predicted from DFT calculations. This difference may be explained by the structure sensitivity of the system and/or by the inherent error of the DFT calculations. 

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