Transition state index

It was concluded that while kinetic isotope effects are much more sensitive than Bronsted exponents to variations in pKa, the use of either quantity as an index of transition state symmetry may be doubtful. 

Table 17 Selectivity relationships substituent" and solvent effects in bromination of arylolefins as indexes of transition state shifts with reactivity.

The relationship between the geometry of the saddle point of index one (SPi-1) and the accessibility to the quantum transition states cannot be proved, but it can be postulated [43,172], To some extent, invariance of the geometry associated with the SPi-1 would entail an invariance of the quantum states responsible for the interconversion. Thus, if a chemical process follows the same mechanism in different solvents, the invariance of the geometry of the SPi-1 to solvent effects would ensure the mechanistic invariance. This idea has been proposed by us based on computational evidence during the study of some enzyme catalyzed reactions [94, 96, 97, 100-102, 173, 174, 181-184],... 

Another example of the use of transition state pKa values has been provided by Pollack (1978). From the rate constants for the decarboxylation of substituted a,a-dimethylbenzoylacetic acids ([37] — [38]) and their anions, he calculated pK for reaction of the acids (Table A6.2). The values vary significantly with the phenyl substituent (p = +1.7), much more so than the p/(a values of the substrate acids (p = +0.2). This difference is consistent with the proton being much closer to the phenyl group in the transition state than in the initial state, and it may even denote a relatively late transition state (Pollack, 1978). However, from the pKa values of the reactant acids (approximately 3.4), the transition states (approximately 4.4), and the enol product (11.8) (Pruszynski et al., 1986), the Leffler index... 

The hydration of simple ketenes (RCH=C=0—> RCH2COOH) also shows relatively constant values of oh w which are quite low (100-1000) (Tidwell, 1990 Allen et al., 1992), implying p/fj = 11 to 12 for the transition state for water attack. Corresponding to this, the Leffler index and the /3nuc are both about 0.25. Whether these low values really indicate an early transition state or arise because water and hydroxide ion react quite differently is not yet clear. However, it appears possible that water attack proceeds through a cyclic mechanism involving two (or more) water molecules (Allen et al., 1992) whereas hydroxide ion probably attacks conventionally as a nucleophile (Tidwell, 1990). Of course, any mechanism for the water reaction which is superior to simple nucleophilic attack will elevate kw and necessarily lead to low kOH/kw ratios. 

Brown (1959) has presented a charge transfer model of the transition state for electrophilic reactions which differs appreciably from that proposed by Fukui and his collaborators and leads to the definition of a new reactivity index termed the Z value . The model is based on a more conventional formulation of the charge transfer mechanism, which avoids the complete transfer of electrons associated with v = 0,1,2 in Fukui s model. There is no dependence on the formation of a pseudo tt orbital in the transition state, nor is hyperconjugation invoked. A wave function for a charge transfer complex is written as a linear combination of a wave function < o describing the unperturbed ground state of the molecule under attack, and a function which differs from (Pq in the replacement... 

The catalytic isomerization of 1-methylnaphthalene and all lation of 2-methylnaphtha-lene with methanol were studied at ambient pressure in a flow-type fixed bed reactor. Acid zeolites with a Spaciousness Index between ca. 2 and 16 were found to be excellent isomerization catalysts which completely suppress the undesired disproportionation into nwhthalene and dimethylnaphthalenes due to transition state shape selectivity. Examples are HZSM-12, H-EU-1 and H-Beta. Optimum catalysts for the shape selective methylation of 2-methylnaphthalene are HZSM-5 and HZSM-li. All experimental finding concerning this reaction can be readily accounted for by conventional product shape selectivity combined with coke selectivation, so there is no need for invoking shape selectivity effects at the external surface or "nest effects", at variance with recent pubhcations from other groups. 

In addition to allowing comparison among experiments carried out in different solvents, the m—Y system serves as an important tool for study of mechanism. Sensitivity to ionizing power, measured by m, is an index of the degree of charge separation at the transition state. The ratio of rates in two solvents of equal Y but different nucleophilicity provides evidence about nucleophilic assistance by a solvent molecule during the ionization. 

The reaction between ammonia and methyl halides has been studied by using ab initio quantum-chemical methods.90 An examination of the stationary points in the reaction potential surface leads to a possible new interpretation of the detailed mechanism of this reaction in different media, hr the gas phase, the product is predicted to be a strongly hydrogen-bonded complex of alkylammonium and halide ions, in contrast to the observed formation of the free ions from reaction hr a polar solvent. Another research group has also studied the reaction between ammonia and methyl chloride.91 A quantitative analysis was made of the changes induced on the potential-energy surface by solvation and static uniform electric fields, with the help of different indexes. The indexes reveal that external perturbations yield transition states which are both electronically and structurally advanced as compared to the transition state in the gas phase. 

The aromaticities of symmetry-allowed and -forbidden transition states for electrocyclic reactions and sigmatropic rearrangements involving two, four, and six r-electrons, and Diels-Alder cycloadditions, have been investigated by ab initio CASSCF calculations and analysis based on an index of deviation from aromaticity. The order of the aromaticity levels was found to correspond to the energy barriers for some of the reactions studied, and also to the allowed or forbidden nature of the transition states.2 The uses of catalytic metal vinylidene complexes in electrocycliza-tion, [l,5]-hydrogen shift reactions, and 2 + 2-cycloadditions, and the mechanisms of these transformations, have been reviewed.3... 

In any case, the analysis given suggests that the value of a deduced from H20-D20 experiments is as useful an index of transition-state structure for the hydrogen ion-catalysed reaction as the value of aA is for the transition state in catalysis by weak acids. 

Finally, there is an extra-thermodynamic assumption, which one can make about two molecules whose reactivity one wishes to compare. The basic idea is not unfamiliar, since it is inherent in the Bronsted linear free-energy relation. The assumption is that the free-energy difference in the transition states is bracketed by reactants and products. The factor provides a numerical index between zero and unity of the... 

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