Temperature dependence proton transfer

Other methods of calculating the O N separation dependent proton transfer rates, such as a Fermi Golden Rule approach (Siebrand et al. 1984), can also be employed. In this approach, two harmonic potential wells (e.g., O-H N and, O H-N) are considered to be coupled by an intermolecular term in the Hamiltonian. Inclusion of the van der Waals modes into this approximation involves integration of the coupling term over the proton and van der Waals mode wavefunctions for all initial and final states populated at a given temperature of the system. Such a procedure requires the reaction exothermicity and a functional form for the variation of the coupling as a function of well separation. In the present study, we employ the barrier penetration approach this approach is calculationally straightforward and leads to a clear qualitative physical picture of the proton transfer process. 

Cyclic voltammetry studies reveal striking differences between complex 13 and the analogous complex [HFe(depp)(dmpm)(CH3CN)f (17) in which the NMe group of 13 has been replaced by a methylene group. At normal scan rates the Fe " couple is reversible for complex 17, but irreversible for 13. Scan rate dependence measurements and potential step experiments indicated that this difference in behavior arises from a rapid transfer of the proton of the Fe hydride to the N atom of the pendant base with a rate constant of 1.1 x 10 s at room temperature. This proton transfer results in an irreversible Fe " " couple at low scan rates. A similar process cannot occur for 17, and the Fe " " couple remains reversible, even at slow scan rates in the presence of an external base. These results indicate that pendant bases in the second coordination sphere can facilitate the coupling of electron and proton transfer reactions. 

The temperature dependence of the rate constant of rapid hydrogen transfer, measured by Mordzinski and Kuhnle [1986], is given in fig. 1. A similar dependence has been found by Grellmann et al. [1989] for one-proton transfer in half of the above molecule (6.16), which does not include the two rightmost rings. 

This prediction is fulfilled by all the values in Table 9 in every case the value of — kT nK increases throughout the range covered by experiment. Having established this fact, the next step is to look at the rate at which the values of J increase with rise of temperature. For most of the substances in Table 9 values of K are available at 20 and at 40°C. Let us then calculate the values of J at these two temperatures and by subtraction find in each case the increment in J over this range of temperature. From what has been said above we know that we expect the increment in J to depend on whether the proton transfer belongs to class... 

First calculations of the optimum distance between the reactants, R, taking into account the dependence of the probability of proton transfer between the unexcited vibrational energy levels on the transfer distance have been performed in Ref. 42 assuming classical character of the reactant motion. Effects of this type were considered also in Ref. 43 in another model. It was shown that R depends on the temperature and this dependence leads to a distortion of the Arrhenius temperature dependence of the transition probability. 

Deuterium isotope effects on 1SN chemical shifts Deuterium isotope effects on 15N chemical shifts AN(D), similarly as for AC(D), can be employed in determination of mole fraction of the proton transferred form Schiff bases.44 A similar correlation between the mole fraction of NFI-form and AN(D) values was found (Figure 2). For the Schiff bases in which proton transfer takes place, the AN(D) values varied from —2 to + 5 ppm and depend on solvent and temperature. The AN(D) values of... 

In recent years, evidence has been found that both mechanisms of proton transfer can occur for certain intramolecularly hydrogen-bonded acids. Also, new kinetic behaviour has been obtained which allows a much more detailed examination of the reaction steps in (22). Kinetic data for the second ionization of substituted phenylazoresorcinols in the presence of hydroxide ions (25) were some of the first to be obtained for an intramolecularly hydrogen-bonded acid. The reciprocal relaxation time (t ) for the approach to equilibrium in a temperature-jump experiment was measured at different hydroxide-ion concentrations. A linear dependence of x on [OH] was obtained of the form of (26) (Eigen and Kruse, 1963 Inskeep et al., 1968 Rose and Stuehr, 1971). However, careful measurements at lower hydroxide-ion concentrations (Perlmutter-Hayman and Shinar, 1975 Perl-mutter-Hayman et al., 1976 Yoshida and Fujimoto, 1977) revealed that the... 

It is difficult to correlate the kyi/k values with the operation of a hydride, proton or H atom transfer. The temperature dependence of the kinetic isotope effect, which is now easier to measure accurately, is more diagnostic of mechanism but has been applied mainly to organic systems. ... 

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