Transition state sensitivity

From Table IV the relative magnitudes of the transition state "sensitivity factor" (M) for the reactions are in the order 2 > 4 >... 

Direct fluorination at saturated carbon is possible with either elemental fluorine or CF30F and a radical inhibitor.57-8 The transformation is an electrophilic fluorination, regiospecific by virtue of the highly polar transition state sensitive to the inductive effects of nearby or remote polar substituents. Substitution occurs almost exclusively at tertiary... 

Calculated transition structures may be very sensitive Lo the level of theory employed. Semi-empirical methods, since they are parametrized for energy miriimnm structures, may be less appropriate for transition state searching than ab initio methods are. Transition structures are norm ally characterized by weak partial" bonds, that is, being broken or formed. In these cases UHF calculations arc necessary, and sometimes even the inclusion of electron correlation effects. 

The sensitivity of the reaction to ortho steric hindrance shows that the transition state is probably angular (894). 

The relatively large p shows that the reaction is very sensitive to substituent effects and implies that there is a relatively large redistribution of charge in the transition state. 

Because a relates the sensitivity to structural changes that the proton-transfer process exhibits to that exhibited by dissociation of the acid, it is frequently assumed that the value of a can be used as an indicator of transition-state structure. The closer a approaches unity, the greater is the degree of proton transfer in the transition state. There are limits to the generality of this interpretaton, however. ... 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

Another line of evidence that bridging is important in the transition state for solvolysis has to do with substituent effects for groups placed at C-4, C-5, C-6, and C-7 on the norbomyl system. The solvolysis rate is most strongly affected by C-6 substituents, and the exo isomer is more sensitive to these substituents than is the endo isomer. This implies that the transition state for solvolysis is especially sensitive to C-6 substituents, as would be ejqiected if the C(l)—C(6) bond participates in solvolysis. ... 

Several lines of evidenee led to the eonclusion that the reaetion is eoneerted. Experimental values of the sensitivity eoeffieients were aHA = —0.54 and 3 uc = 0.63. These values are plotted in Fig. 5-23 to define the position of the transition state, which probably has the following charge distribution, where the net charge is — 1, and bond formation from the nueleophile to carbon is well advanced. 

Usually, geometries of transition states are significantly more sensitive with respect to method than are stmctures of stable species. Since electron correlation effects are of particular importance for these stmctures, the determination of transition states at the Hartree-Fock level should be avoided. It is recommended to compare the stmctural parameters of transition states obtained from different methods (for instance DFT and MP2) in order not to be misled. 

In reactions in which separated ion pairs are involved, e.g., R4N+, K or Na +, and as a borderline case, Li +, the cation does not contribute to the adjustment of the reaction partners in a dense, well-ordered transition state poor selcctivities arc usually the result of these carbanionic carbonyl additions. Further, the high basicity of such carbanionic species may cause decomposition or racemization of sensitive reactions partners. 

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. 

The Brpnsted coefficient a represents the sensitivity of the rate to the acid strength of the catalyst. It is a measure of the degree of proton transfer from catalyst to substrate in the transition state. For nearly all reactions where BH+ contains acidic N-H or O-H groups, a is in the range 0-1. 

We shall now derive a result first obtained24 by more complicated mathematics than the alternative25 given here. The 1992 Nobel Prize in Chemistry was awarded to R. A. Marcus for developing this work. We construct a family of reaction profiles (see Fig. 10-8) for different members of the series. The horizontal axis is now used to show the relative locations of the transition states. The larger AG is, the closer to product the transition states lies, and the larger AG is. By assuming that the sensitivity coefficient... 

Equation 6 would hold for a family of free radical initiators of similiar structure (for example, the frarw-symmetric bisalkyl diazenes) reacting at the same rate (at a half-life of one hour, for example) at different temperatures T. Slope M would measure the sensitivity for that particular family of reactants to changes in the pi-delocalization energies of the radicals being formed (transition state effect) at the particular constant rate of decomposition. Slope N would measure the sensitivity of that family to changes in the steric environment around the central carbon atom (reactant state effect) at the same constant rate of decomposition. 

It is not intended that the equations of this study be used to supplant the much more elegant molecular orbital calculations, both semiempirical and ab initio, and the mechanical modeling studies of radical forming reactions. However, it may be possible to make some hypotheses about differences in mechanisms between reaction families, based on the values of the slopes in Table IV. The slopes could be considered "sensitivity factors" (like rho values) for measuring the relative magnitude of transition state effects (U) and reactant state effects (N) on the rates of the four reactions of this study. 

From Table IV the relative magnitudes of the reactant state "sensitivity factor" (N) are 4>1>2=3= zero. From this analysis the decomposition rates of traiw-phenyl, alkyl diazenes (2) and iert-butyl peresters (3) can be predicted by assuming a dependence only on transition state effects, with no need to incorporate the back strain of the reactants into the equation. 

As long as there are no important steric contributions to the transition-state energies, the elementary rate constant of Eq. (1.22) does not sensitively depend on the detailed shape of the zeolite cavity. Then the dominant contribution is due to the coverage dependent term 9. 

Figure 1.22 schematically summarizes the principle of the preferred transition states without sharing of a common metal atom. Whereas we have earher discussed surface sensitivity as a function of the relative ratio of particle surface edge sites and surface terrace atoms, the discussion given above provides a principle for particle size shape differences. 

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