Transition state, charges

For another method of determining transition state charge, see Williams, A. Acc. Chem. Res., 1984, 17, 425. 

Equations (37)—(39), where the non-additivity of multiple substituent effects is described by a cross-term, express correctly the rate data for bromination and other reactions of polysubstituted substrates. The question arises, therefore has the interaction constant, q, any physicochemical meaning in terms of mechanism and transition state charge To reply to this question, selectivity relationships (42) that relate the p-variation to the reactivity change and not to any substituent constant, have been considered (Ruasse et al., 1984). 

In conclusion, for the SN2 reactions of primary alkyl derivatives, the author is of the view that the Bronsted parameter a does not constitute measure of transition state charge development, and is unlikely to represent a meaningful measure of transition state geometry. As we shall see in the following section, however, for reactions described by three or more configurations, the Bronsted parameter or Hammett p parameter may provide a relative measure of transition state charge development (Pross, 1984). 

Reactions (109) and (110) as well as nitroalkane deprotonation are all examples of central substitution so that for these reactions a does not measure the earliness or lateness of the transition state. End substitution, such as in the nucleophile or leaving group of SN2 reactions, in which the formal charge does vary between reactants and products generates -values which may be utilized as a relative measure of transition state charge this, provided that the reaction is composed of three configurations. Such applications were discussed in Section 3, p. 152. 

As in the case for alkene additions, if the SOMO of the radical is relatively high in energy, such as is the case for alkyl radicals, the principal interaction with the abstractable X-H bond will be with its unoccupied a MO (one-electron-two-orbital type), and such a radical would be considered nucleophilic. If the SOMO is relatively low in energy, such as is the case for perfluoroalkyl radicals, the principal interaction with the abstractable X-H bond will be with its occupied a MO (three-electron-two-orbital type), and the radical is considered electrophilic. Either way, a good match-up in polarities in an H-atom transition state will give rise to beneficial transition state charge-transfer interaction [130,136,137]. 

In contrast to the heteroatom in five-membered heterocycles (Chapter 6), a pyridine nitrogen atom should be able to transmit electronic effects in a manner similar to a CH group in benzene. Difficulty attends assessment of such transmission abilities by comparison of the reactivities of pyridines and benzenes because the Hammett p factor obtained will be a measure of both the transmission ability (64JCS627) and the transition-state charge. Because pyridine is less reactive, the transition state for its reactions should be nearer to the Wheland intermediate than the corresponding transition states for reactions of the corresponding benzene... 

Uyeda, C. Jacobsen, E. N. Transition-state charge stabilization through multiple non-covalent interactions in the guanidinium-catalyzed enantioselective claisen rearrangement, 7 Am. Chem. Soc. 2011,133, 5062-5075. 

Analogous to the case of the inner-shell reorganization, energy conservation requires that the transition-state charges for the solvent reorganization be equal. 

The substituent effect data indicates that the charge polarity in the vinyl ether transition states is less than in the corresponding ester elimination transition states. The activation energies (corrected) naturally parallel the rate coefficient variations and show a systematic decrease of 2.3 to 2.4 kcal.mole for successive methyl substitutions for hydrogen at the a-carbon. Corrected for gauche destabilizations of the ether ground states, this indicates a transition state charge stabilization by methyl of about 1.6 kcal per CH3. Ester stabilizations have previously been estimated to be about 3.3 kcal per CH3. 

Value of the i nuclear coordinate (i = 1, N) in the initial and final system, at the transition state Charge consumed (coulombs)... 

In this transition state, charge is dispersed. The transition state is less polar than the starting material... 

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