Proton transfer transition state

The magnitude of the effect for the elimination reactions indicates a high degree of bond formation between the base and the proton in the transition state. Proton transfer is extensive in the ammonium compound and the proton is just more than half transferred to the base for the sulphonium salt. These... 

M. Nagaoka, Y. Okuno, and T. Yamabe,/. Am. Chem. Soc., 113,769 (1991). The Chemical Reaction Molecular Dynamics Method and the Dynamic Transition State Proton Transfer Reaction in the Formamidine and Water Solvent System. 

Nagaoka M, Okuno Y, Yamabe T (1991) The chemical reaction molecular dynamics method and the dynamic transition state proton transfer reaction in formamidine and water solvent system. J Am Chem Soc 113(3) 769-778... 

One may consider the relaxation process to proceed in a similar manner to other reactions in electronic excited states (proton transfer, formation of exciplexes), and it may be described as a reaction between two discrete species initial and relaxed.1-7 90 1 In this case two processes proceeding simultaneously should be considered fluorescence emission with the rate constant kF= l/xF, and transition into the relaxed state with the rate constant kR=l/xR (Figure 2.5). The spectrum of the unrelaxed form can be recorded from solid solutions using steady-state methods, but it may be also observed in the presence of the relaxed form if time-resolved spectra are recorded at very short times. The spectrum of the relaxed form can be recorded using steady-state methods in liquid media (where the relaxation is complete) or using time-resolved methods at very long observation times, even as the relaxation proceeds. 

Excited-state proton transfer relates to a class of molecules with one or more ionizable proton, whose proton-transfer efficiency is different in the ground and excited states. The works of Forster [2-4] and Weller [5-7] laid the foundation for this area on which much of the subsequent work was based. Forster s work led to the understanding of the thermodynamics of ESPT. He constructed a thermodynamic cycle (Forster cycle) which, under certain acceptable approximations, provides the excited-state proton-transfer equilibrium constant (pK f,) from the corresponding ground-state value (pKa) and electronic transition energies of the acid (protonated) and base (deprotonated) forms of the ESPT molecule ... 

The most common type of biocatalytic reactions is proton transfer (115). Nearly, every enzymatic reaction involves one or more proton-coupled steps. Transition-state proton bridging and intramolecular proton transfer (general acid-base catalysis) are important strategies to accelerate substrate conversion processes. Moreover, proton transfer also plays a fundamental role in bioenergetics (116). 

The systems listed in the lower part of Table 1 have in common a proton transfer potential containing two wells, separated by an energy barrier Et. The H-bond length in the minimum, or equilibrium geometry, is reported as Rgq. The half transfer of the proton to the top of the barrier, the transition state to transfer, causes the H-bond to contract to a distance listed as Rts-This contraction is indeed a common observation, observed in a host of systems in addition to those reported in Table 1, some much larger [13]. 

Figure 3.20 Excited-state proton transfer through water bridges in pyrroloquino-line 7 calculated at the TD-B3LYP/cc-pVDZ level [59, 68]. (a) Structure of the 1 1 solute-HjO complex In the normal form (N), in the transition state (TS), as well...

The solid-state proton transfer and keto - enol tautomeric transformations were accomplished by heating or by mechanochemical treatment (Scheme 13.10). The thermal transition was monitored by in situ time-resolved PXRD and IR methods. The in situ monitoring revealed that the keto-enol transition proceeded through the formation of metastable enol polymorph, which finally converted to the thermodynamically stable form of enol polymorph. 

Fig. 46. Scheme of optical transitions, explaining the dual fluorescence resulting from proton transfer in excited electronic state. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

The catalytic triad consists of the side chains of Asp, His, and Ser close to each other. The Ser residue is reactive and forms a covalent bond with the substrate, thereby providing a specific pathway for the reaction. His has a dual role first, it accepts a proton from Ser to facilitate formation of the covalent bond and, second, it stabilizes the negatively charged transition state. The proton is subsequently transferred to the N atom of the leaving group. Mutations of either of these two residues decrease the catalytic rate by a factor of 10 because they abolish the specific reaction pathway. Asp, by stabilizing the positive charge of His, contributes a rate enhancement of 10. ... 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

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. ... 

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