Proton transfer synchronous

There is an intermediate mechanism between these extremes. This is a general acid catalysis in which the proton transfer and the C—O bond rupture occur as a concerted process. The concerted process need not be perfectly synchronous that is, proton transfer might be more complete at the transition state than C—O rupture, or vice versa. These ideas are represented in a three-dimensional energy diagram in Fig. 8.1. 

The small solvent isotope effect shows clearly that a proton transfer is not part of the rate-determining step of the hydrolysis. Christensen (1966, 1967) favors an SN2-type mechanism (164) for the hydrolysis, with nucleophilic attack of water on a sulfonyl group synchronous with the departure of ArSO. The alternative formulation (165), however, where a pentacovalent inter-... 

A condition arising when reaction parameters for different bond-breaking or bond-making processes have developed synchronously as the transition state is approached. Imbalance is common in elimination and addition reactions as well as in proton transfer reactions. See also Synchronous Synchronization... 

Because of the disorder present in ice, an array of water molecules such as that in Eq. 9-94 wouldn t revert to its exact original form in step b. However, active sites of enzymes are highly structured and proton transfers may occur with precision. For example, a synchronous shift of protons in an array of carboxylic acid, imidazolium, and phosphate groups can be envisioned readily (Eq. 9-96). The net effect of the process is to transfer a proton from one end of the chain to the other (as in Eq. 9-94) with facile tauto-... 

The conclusions reached by Costentin and SavOant are in fact quite consistent with our own. The main difference is that, according to these authors, the notion of an imbalanced transition state should be placed within the context of charge localization-delocalization heavy-atom intramolecular reorganization rather than of synchronization (or lack thereof) between charge delocalization and proton transfer. ... 

The evidence for perfect synchronization between bond cleavage, bond formation and positive charge delocalization was obtained for the proton transfer from hydronium ion to substituted a-methoxystyrenes ArC(OMe)=CH264. 

Figure 6.20. (a) Projection of a three-dimensional PES K(p,p2,p3) for two-proton transfer in formic acid dimer onto the (p, p,) and (p, p3) planes. In contrast with points A and B, in points C and D the potential along the p3 coordinate is a double well resulting in bifurcation of the reaction path [from Shida et al., 1991b]. (b) The contour lines correspond to equilibrium value of p3 and potential (6.37) when V(Q) = V0(Q4 - 2Q2), V0 = 21 kcal/ mol, C = 5.()9V0, A = 5.351/, Qn = 0.5. When Q > Qc, two-dimensional tunneling trajectories exist in the shaded region between curves 1 and 2. Curve 3 corresponds to synchronous transfer. 

We studied, at the B3LYP/6-31+G theoretical level, four monomers and 12 NH-pyrazole cyclamers, C-unsubstituted or bearing fluoro, chloro and bromo substituents at positions 3 and 5 [100], Two mechanisms of proton transfer, stepwise and synchronous, were calculated for dimers, trimers, and tetramers. The set of values of energies and geometries thus obtained provide useful insights about the dynamics of NH-pyrazoles in the solid state. It has been shown that pyrazole cyclamers exist not only in condensed phases but in the gas phase as well [101], thus our gas-phase calculations will provide information about the solid state. 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

Another possible two-electron mechanism involves the direct transport of two electrons from a mononuclear transition metal complex to a substrate (S). Such a transport alters sharply the electrostatic states of the systems and obviously requires a substantial rearrangement of the nuclear configuration of ligands and polar solvent molecules. For instance, the estimation of the synchronization factor (asyn) for an octahedral complex, with Eq. 2.44 shows a very low value of asyn = 10 7to 10 8 and, therefore, a very low rate of reaction. The probability of two-electron processes, however, increases sharply if they take place in the coordination sphere of a transition metal, where the reverse compensating electronic shift from the substrate to metal occurs. Involvement of bi- and, especially, polynuclear transition metal complexes and clusters and synchronous proton transfer in the redox processes may essentially decrease the environment reorganization, and, therefore, provide a high rate for the two- electron reactions. 

Figure 12.10. Potential-energy functions of the S0 state, the locally excited 1 hit states of guanine and cytosine, the lowest1 rnr state, and the tt-jt charge-transfer state of the WC conformer (a), the conformer B (b), and the conformer C (c) of the CG dimer. The PE functions have been calculated along the linear-synchronous-transit proton-transfer reaction path from the S0 minimum to the biradical minimum. Insets show the potential-energy function of the locally excited 1mr state of guanine calculated along the minimum-energy path for stretching of the NH bond...

Senchenya et al. (96) have treated the adsorption of ethanol on a structural hydroxyl group (Fig. 14) using a CTP scheme and the CNDO/BW method. The separation of a molecule and cluster with respect to the z axis was optimized, the optimal values being r = 1.19 A and R = 1.28 A The adsorption energy was 23.2 kcal/mol, which was close to the experimental value (97). Note that this was essentially the two-point adsorption involving both acid and base sites. This case is quite similar to the above propylene adsorption (90). There is also no definite trend toward proton transfer from the hydroxyl group of a zeolite to the alcohol molecule. The carbocation state is also predicted to be activated. This, in turn, increases relative efficiency of the synchronous mechanism (with the same recommendation for its experimental examination). The estimation (96) of the energetics of the intermediate structures of the synchronous mechanism showed that such a mechanism is quite realistic. 

Another example of tautomerism which involves a double-proton transfer is tautomerization of porphyrins. Mechanisms for the double-proton transfer of porphine [18], the parent compound of porphyrins, in solution have been discussed for a long time. A point of special interest is whether the double-proton transfer occurs in a synchronous or stepwise manner. 

Although Bronsted proton transfer reactions appear to belong to a unique category not described by Scheme 14, they are examples of polar-group transfer reactions and are not different in principle from nucleophilic displacement reactions. Deprotonation by hydroxide ion can be regarded as the shift of an electron from HO to the Bronsted acid synchronously with the transfer of a hydrogen atom from the Bronsted acid to the incipient HO- radical, with the reaction driven by covalent bond formation between the HO- radical and the H- atom to form water (equation 161). 

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