Reaction pathway, energy profile

Figure 8 An accurate estimate of the barrier height can be found by adding a sufficient number of intermediate points in the discretized transition pathways. The solid line in the graph represents the energy profile for a reaction path described by 11 intermediate configurations of the system. The dashed line shows a coarse pathway described by only two intermediate configurations. The latter path underestimates the true energy ban ier.

Fig. 2.5. Comparison of energy profile (AG) for pathways to E- and Z-product from the reaction of lithio methyl dimethylphosphonoacetate and acetaldehyde. One molecule of dimethyl ether is coordinated to the lithium ion. Reproduced from J. Org. Chem., 64, 6815 (1999), by permission of the American Chemical Society.

In aqueous solutions, a water molecule can bind to the activated complex to complete the coordination sphere. The complex formed, [MLnHjO], has some stability, so it represents a lower energy than the transition state, [ML ], Therefore, the complex [Ml ThO] is known as an intermediate because it is more stable than the transition state either before the H20 enters or after it leaves. Figure 20.2 shows the energy profile for a substitution reaction that follows an associative pathway. 

PROTON TRANSFER REACTION PATHWAY 4.1 Free Energy Profile... 

Ah initio calculations to map out the gas-phase activation free energy profiles of the reactions of trimethyl phosphate (TMP) (246) with three nucleophiles, HO, MeO and F have been carried out. The calculations revealed, inter alia, a novel activation free-energy pathway for HO attack on TMP in the gas phase in which initial addition at phosphorus is followed by pseudorotation and subsequent elimination with simultaneous intramolecular proton transfer. Ah initio calculations and continuum dielectric methods have been employed to map out the lowest activation free-energy profiles for the alkaline hydrolysis of a five-membered cyclic phosphate, methyl ethylene phosphate (247), its acyclic analogue, trimethyl phosphate (246), and its six-membered ring counterpart, methyl propylene phosphate (248). The rate-limiting step for the three reactions was found to be hydroxyl ion attack at the phosphorus atom of the triester. ... 

The scope of the rearrangement reaction whereby azido-l,2,3-triazolide ion (66) is converted to the (diazomethyl)tetrazolide ion (68) has been studied. Where R = H, substituted phenyl, Me, and C02Me the reaction proceeds at a rate which is largely independent of substituent extensive decomposition is observed where R = COMe, COAr, and CN. PM3 calculations used to explore the energy profile of the reaction pathway indicate that the order of anion stability is (67) < (66) < (68) and that the rearrangement is of the type (66) (67) (68) for which 2 and k i ki. 

To assess the role of electron transfer in the dioxetane decomposition, a comparison of the reaction pathways for the neutral dioxetane and its negatively charged ion is relevant (cf. Figure 1). The energy profiles for the cleavage of a 1,2-dioxetane and its 1,2- dioxetane radical anion as a function of stretching the 0-0 bond, as calculated by the PM3 method. 

FIGURE 8. The energy profile (kcalmol ) for the HMgH + H2CO reaction along a polar pathway. Reprinted from Reference 23, copyright 1982, with permission from Elsevier... 

Figure 18.7 Energy profiles for compact states in the folding reaction. Left The fully unfolded state collapses to a genuine intermediate that rearranges to form products. Right. The compact state C is off the pathway and has to unfold for productive folding to occur.

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