Rates proton transfer

After some straightforward manipulations of A3.8.22. the PI-QTST estimate of the proton transfer rate constant can be shown to be given by 48... 

Mavri, J., Berendsen, H.J.C. Calculation of the proton transfer rate using density matrix evolution and molecular dynamics simulations Inclusion of the proton excited states. J. Phys. Chem. 99 (1995) 12711-12717. 

Van der Spoel,D., Berendsen, H.J.C. Determination of proton transfer rate constants using ab initio, molecular dynamics and density matrix evolution calculations. Pacific Symposium on Biocomputing, World Scientific, Singapore (1996) 1-14. 

In this section, we switch gears slightly to address another contemporary topic, solvation dynamics coupled into the ESPT reaction. One relevant, important issue of current interest is the ESPT coupled excited-state charge transfer (ESCT) reaction. Seminal theoretical approaches applied by Hynes and coworkers revealed the key features, with descriptions of dynamics and electronic structures of non-adiabatic [119, 120] and adiabatic [121-123] proton transfer reactions. The most recent theoretical advancement has incorporated both solvent reorganization and proton tunneling and made the framework similar to electron transfer reaction, [119-126] such that the proton transfer rate kpt can be categorized into two regimes (a) For nonadiabatic limit [120] ... 

Finally, in many cases the acidity equilibria cannot be measured but the rate of proton transfer or transmetallation can be measured to give an ionic or ion pair kinetic acidity. Studies using the rates of proton transfer have included the use of isotopes such as tritium and deuterium5,6. The rate is then used to calculate the Brpnsted slope, a, by plotting the logarithm of the proton transfer rate against the pK, as determined by the equilibrium acidity, for a series of compounds. From this plot, the approximate pKa of an unknown compound can be determined by comparison of the same type of compounds. 

It is traditional to treat H 0+ as an exceptional acid with unusual proton-transfer rate properties which preclude it from being included in such a Br nsted plot. In this case H O ... 

There is very little information on proton transfer rates involving M-H bonds against which to check the above prediction. Walker, Kresge, Ford, and Pearson (WKFP) have recently reported... 

All of our measurements of proton transfer rates in acetonitrile have been done by various NMR techniques. One of the simplest has been used to measure the rates of transfer from cis-Os(CO) 2 to Et N and back again (21). Whereas a mixture of... 

DR. STEPHEN NEUMANN (Eastman Kodak Co.) In your observation of the proton transfer rates between the metal base and the amine base, do you have any feeling for whether the slower transfer is unique to the metal bases or whether it has to do more with the bulk of the overall base participating in the proton transfer For instance, would a sterically hindered amine show rates similar to the metal bases. 

DR. NORTON The answer is, as yet, no. I think the experiment to do would be to take a weak base with all three of those hydrides as acids and look at the line broadening. By varying the concentration of the weak base over an enormous range, one should be able to obtain the proton transfer rate constants of all three of those hydrides to the same weak base. That is the next experiment on our list. 

The radical anion pathway (e-c-P-d-p Scheme 2) requires a rate-determining protonation after cyclization, i.e., a slow protonation of a hard oxyanion. However, such proton transfer rates are usually diffusion controlled, so this seems unlikely [32,33], On the other hand, the carbanion closure (e-P-d-c-p) portrayed in Scheme 4 requires a very reasonable suggestion that the ratedetermining step corresponds to protonation of the soft, weakly basic radical anion 42, prior to cyclization [32-35] this is the preferred mechanism. One must use caution, however, realizing that these conclusions are drawn for the particular set of substrates which were examined. In some cases, radical anion cyclization remains a viable option. 

Physically, dv appears as a broadening of the spectral lines. It has been found that the study of NMR band shapes provides a powerful tool for the measurement of proton-transfer rates through the determination of state lifetimes, related to St. Nuclei such as H and have been studied. This allows the simultaneous determination of different rates of proton exchange. NMR methods are particularly valuable for degenerate equilibria, for which AWJ. and AVj are useless. 

Composition dependence of proton transfer rate and nature of the proton acceptor... 

Proton transfer rates. Consider the reversible reaction of a proton acceptor B with acid H-A (Eq. 9-97). Eigen pointed out that the reaction will be fastest if the two reactants form a hydrogen-bonded complex (Eq. 9-97, step a).138 The hydrogen bonding shortens the distance from the proton to B and allows for very rapid transfer of the proton from A to B within the... 

I Table 4.3 Proton-transfer rates involving imidazole (pKa = 6.95)"... 

The acid-base reaction involves the lattice O2. For a given hydrocarbon, the proton transfer rate is enhanced by an increased basicity of O2-. Indeed,... 

Intramolecular proton transfer rates in acetylacetone have been calculated.148... 

Fig. 10 Empirical relationship between the logarithm of the proton transfer rate constants (Table 1) and the corresponding free energies of reaction ArG°. Triangles (y) k /(M 1 s 1). Filled circles ( ) k /(M 1 s-1). Empty circles (O) kjf/s-1.

The ESPT of naphthylammonium [190] and phenanthrylammonium [191] ions in their 18-crown-6 ether complexes in MeOH-H20 (9 1) solvent shows that the excited-state proton-transfer rate decreases markedly on complexing. The back-protonation rate in the excited state is negligibly small compared with those of the other decay processes, which essentially means that there is no excited-state protropic equilibrium in the crown complexes. The one-way proton-transfer reaction is elucidated by the presence of the excited neutral amine-crown complex (RNH2-crown) produced by deprotonation of (RN+H3-crown). There is a large steric effect on protonation to the amino group of the excited neutral complex. 

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