Nature of the transition state

Sequences such as the above allow the formulation of rate laws but do not reveal molecular details such as the nature of the transition states involved. Molecular orbital analyses can help, as in Ref. 270 it is expected, for example, that increased strength of the metal—CO bond means decreased C=0 bond strength, which should facilitate process XVIII-55. The complexity of the situation is indicated in Fig. XVIII-24, however, which shows catalytic activity to go through a maximum with increasing heat of chemisorption of CO. Temperature-programmed reaction studies show the presence of more than one kind of site [99,1(K),283], and ESDIAD data show both the location and the orientation of adsorbed CO (on Pt) to vary with coverage [284]. 

The nature of the transition state in elimination reactions is of great importance, since it controls the regiochemistry of p elimination in compounds in which the double bond can be introduced in one of several positions. These effects are discussed in the next section. 

Various ab initio and scmi-cmpirical molecular orbital calculations have been carried out on the reaction of radicals with simple alkenes with the aim of defining the nature of the transition state (Section 1.2.7).2I>,j , 6 These calculations all predict an unsymmetrical transition state for radical addition (i.e. Figure 1.1) though they differ in other aspects. Most calculations also indicate a degree of charge development in the transition state. 

After the completion of this manuscript a paper concerning conformational analyses of 1,1, 3,3 -tetra-r< H-alkylmctallocene of iron and ruthenium including 6 based on thorough NMR spectroscopic measurements (line-shape analysis) has appeared in which the nature of the transition states has conclusively been discussed in detail [164]. 

The nature of the transition state in bromodesilylation is problematical, since the reaction appears to take place in the non-polar solvents benzene and carbon tetrachloride with inversion of configuration at silicon, and, therefore, cannot proceed through a 4-centre intermediate (LVII) as this would lead to retention of configuration746,747. The results are, however, consistent with a six-centre transition state (LVIII), which could follow from the high kinetic order in bromine... 

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

Because of the nature of the transition state in the pericyclic mechanism, optically active substrates with a chiral carbon at C-3 or C-4 transfer the chirality to the product, making this an enantioselective synthesis (see p. 1451 for an example in the mechanistically similar Claisen rearrangement). ... 

Note that if the ratio of partition functions in Eq. (189) leaves a nonvanishing temperature dependence, it generates the additional terms ksTin a and factors e in hv. Consequently, the prefactor hv strongly depends on the nature of the transition state. 

The maximum rate reported at very high acidity accords with protonation of the ester °. To summarise, the ester meehanism has gained general acceptance, although the substituent effects have yet to be explained wholly satisfactorily and the exact nature of the transition state, i.e. whether it is of considerable or only slight carbonyl character, remains a contentious issue ... 

Usually we talk about reactions in solution, but recently techniques have been developed to follow reactions that occur in a vacuum when a stream of reactant A and a stream of reactant B cross each other in a defined direction, as with molecular beams. From the direction in which the products are ejected and their energies, much fundamental information can be deduced about the details of the molecular processes. Lasers, which emit light-energy in a highly focused beam, are sometimes used to put energy into one of the reactants in a defined way. Such a technique reveals less about the nature of the transition state than about what is called the dynamics of the process—how molecules collide so as to react, and how the products carry away the energy of the overall reaction. The development and application of such techniques were recognized by a Nobel Prize in 1986 to Dudley Herschbach, Yuan Lee, and John Polanyi. 

All employed methods to include solvent effects corroborate the nature of the transition state for water exchange via an interchange mechanism and show very satisfactorily the mode... 

An indication of the nature of the transition state in aromatic substitution is provided by the existence of some extrathermodynamic relationships among rate and acid-base equilibrium constants. Thus a simple linear relationship exists between the logarithms of the relative rates of halogenation of the methylbenzenes and the logarithms of the relative basicities of the hydrocarbons toward HF-BFS (or-complex equilibrium).288 270 A similar relationship with the basicities toward HC1 ( -complex equilibrium) is much less precise. The jr-complex is therefore a poorer model for the substitution transition state than is the [Pg.150]

By use of well-established standard potentials, the reported values for K and kg, and the principle of detailed balancing, one can calculate that the reverse of reaction (10) has a rate constant (k g) of 2x103M-1s-1. Normal ligand substitution reactions at Fe2+ are much faster than this, which raises questions regarding the nature of the transition state for this reaction. 

The presence of it- and cyclic-complexes on the catalyst surface and the anionic nature of the transition state suggest the following mechanism where tt- and a-allyl anions are intermediate species on the reaction path (Scheme I). 

A suitable model for the oxygen carrier protein hemerythrin is [Fe2(Et-HPTB)(OBz)](BF4)2 (Et-HPTB = AWAT,iV -tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-l,3-diaminopropane, OBz = benzoate). It can mimic the formation of a binuclear peroxo iron complex in the natural system (101). The measured value of -12.8 cm3 mol1 for the activation volume of the oxidation reaction together with the negative value of the activation entropy confirm the highly structured nature of the transition state. 

Intramolecular general acid catalysis in reactions of salicylic acid derivatives 196 Why are EM s for general acid-base catalysed reactions so low 198 EM and the nature of the transition state 200 The formation of small rings 205... 

If the overall reaction rate is controlled by step three (k3) (i.e. if that is the rate limiting step), then the observed isotope effect is close to the intrinsic value. On the other hand, if the rate of chemical conversion (step three) is about the same or faster than processes described by ks and k2, partitioning factors will be large, and the observed isotope effects will be smaller or much smaller than the intrinsic isotope effect. The usual goal of isotope studies on enzymatic reactions is to unravel the kinetic scheme and deduce the intrinsic kinetic isotope effect in order to elucidate the nature of the transition state corresponding to the chemical conversion at the active site of an enzyme. Methods of achieving this goal will be discussed later in this chapter. 

F. Hollfelder, D. Herschlag, The Nature of the Transition State for Enzyme-Catalyzed Phosphoryl Transfer. Hydrolysis of O-Aryl Phosphorothioates by Alkaline Phosphatase , Biochemistry 1995, 34, 12255-12264. 

Evidence Concerning the Nature of the Transition State Derived from Cycloaddition of a Chiral N-Methylenium Amide... 

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