Energetics of the transition state

Figure 10. The calculated geometries (distances in A and angles in deg) and energetics of the transition states and products of the reactions B3 (Cl) + H2, B13 (C4) + H2 and B21 (C7) + H2i the addition of the third hydrogen molecule to the complex Al. For clarity, the ancillary PH3 and NH2 ligands are omitted in the illustration. All these results are obtained upon constraining the Zr-P bond distances to 2.80 A.

Such studies, when well designed, provide for a more fundamental understanding of catalysis. Catalytic concepts and models are thereby developed and "catalysts by design" come within the realm of reality. This area of catalyst characterization is exciting, complex and requires researchers of many talents. This is where the issue of true fundamental interest, the structure and associated energetics of the transition state should be identified as a major challenge. 

Another area of interest has centered on the role that such a-agostic interactions may play in influencing the energetics of the transition state for olefin insertions. This is of consequence since such an interaction can have a profound influence on the tacticity of the growing alkyl chain. Initial studies by Grubbs et al. focused on the... 

The various methods of the quantum chemical calculations and the evaluation of rate parameters of the various elementary steps on the potential energy surface are discussed in detail. The evaluation required the knowledge of the energetics of the transition states and the intermediates, and their entropy, the degeneracy of their reaction coordinates,... 

Table 6.6 The resultant geometries and energetics of the transition states for the reaction CCI2F2 + Fsurf CCIF3 + a surf- The transition state energy barrier (TS Energy) is relative to the reactants...

This is a question of reaction prediction. In fact, this is a deterministic system. If we knew the rules of chemistry completely, and understood chemical reactivity fully, we should be able to answer this question and to predict the outcome of a reaction. Thus, we might use quantum mechanical calculations for exploring the structure and energetics of various transition states in order to find out which reaction pathway is followed. This requires calculations of quite a high degree of sophistication. In addition, modeling the influence of solvents on... 

The next level seeks a molecular description, and kinetics again makes a contribution. As will be seen in Chapter 5, the experimental kinetics provides information on both the energetics of the reaction (i.e., the height of the energy barrier on the reaction path) and the atomic composition of the transition state. Any proposed mechanism must therefore be consistent with the kinetic evidence. 

Intramolecular C/H insertion by copper-catalyzed decomposition of a-diazoketones provides a convenient cyclization procedure which is limited, however, to diazo compounds which allow energetically favorable realization of the transition state leading to the cyclized product. 

Scheme 13 may look unfavorable on the face of it, but in fact the second two reactions are thermally allowed 10- and 14-electron electrocyclic reactions, respectively. The aromatic character of the transition states for these reactions is the major reason why the benzidine rearrangement is so fast in the first place.261 The second bimolecular reaction is faster than the first rearrangement (bi-molecular kinetics were not observed) it is downhill energetically because the reaction products are all aromatic, and formation of three molecules from two overcomes the entropy factor involved in orienting the two species for reaction. 

Equation (5.86c), written as a strict equality, may also be taken to define the NRT transition state as an alternative to (and slightly different from) the usual definitions based on energetic, saddle-point-curvature, or density-of-states criteria. Note that this NRT alternative definition can be employed for non-IRC choices of reaction coordinate, and remains valid even in the case of barrierless processes (such as many ion-molecule or radical-recombination reactions) for which the reaction profile does not exhibit an energy maximum as in Fig. 5.52. The NRT definition is practically identical to the usual saddle-point definition of the transition state in the present examples. 

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