Proton asynchronous

On the basis of the obtained results, the concerted outer-sphere mechanism seems to be more favorable than any of the inner-sphere evaluated mechanisms. The transition-state for the outer-sphere mechanism shows a certain degree of asynchronicity the proton was relatively more transferred to the iminic nitrogen atom than the hydride to the iminic carbon atom. 

In the work by Shida et al. [45] a synchronous concerted double proton transfer is found to be the major mode of the reaction, which is confirmed in a recent molecular dynamics calculation by Wolf et al. [46]. In his molecular dynamics calculation the time evolution of the potential energy was additionally taken into account. The double proton transfer has also been investigated in a Car-Parinello ah initio molecular mechanics study by Miura et al. [41]. Quantum fluctuations are shown to cause significant deviations from the minimum energy path. While an asynchronous movement of the two protons close to the equilibrium structure was... 

Stepwise PT and ET reactions occur along the edges of Fig. 17.1, while PCET includes the entire space within the square. The stepwise and PCET mechanisms (including HAT) are clearly distinct. The PCET mechanism is defined by a single transition state in which the proton and electron both transfer in one step, with no intermediate states populated along the reaction coordinate PCET is thus concerted but the electron and proton events can be asynchronous as opposed to their synchronous transfer for a HAT reaction. In a stepwise mechanism, an intermediate is formed and there are two distinct rate constants for the forward reaction and two separate transition states. Stepwise ET/PT or PT/ET can, in principle, be broken down and treated experimentally and theoretically as separate ET and PT events. Like any series of reactions, the rate-limiting rule applies. 

When, however, the rates of electron and proton transfer are comparable and coupled to each other, intermediate slopes are obtained (between -0.5 and -1.0) [76-78]. Accordingly, the measured Marcus slopes of-0.72 and -0.71 support the assignment of a concerted PCET process for the oxidation of the substituted phenols by the Cu metal complexes. Additionally, KIEs of 1.21 to 1.56 are also consistent with an asynchronous PCET process in which there is some proton motion but a large ET component in the transition state. 

Transfer of a proton from f/2-H2 to the ju-thiolates in H-ases is also possible, and calculations support such heterolysis (although it is endothermic by 15 kcal/mol).41 Transfer of a proton to CN is nearly isoenergetic but a high barrier is computed (38 kcal/mol, compared to 17 kcal/mol for transfer to sulfide). The next steps involve movement of protons away from the active site and synchronous or asynchronous electron transfer to the cubane cluster and away from the site via other Fe-S clusters. The electrons in the H-H bond could essentially flow through the Fe-Fe bond and, depending on whether one- or two-electron transfer process takes place, one-electron Fe---Fe bonds (2.9-3.1 A)42 may be present in the intermediates (one-electron transfer steps are shown in Scheme 2). The flexibility of the M-M separation (2.6-3.2A, corresponding to 0, 1, or 2e M-M bonds) could facilitate electron/proton transfer here and in the [NiFe] H-ases. 

Scheme 3.1 Concerted synchronous (CS) and asynchronous (CA) double proton transfer processes.

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