Activation energy for proton exchange

Solomon and Bloombergen (1921) estimated that the activation energy for proton exchange between ion and HF is 1,940 db250... 

I. Solomon and N. Bloombcrgen. J. Chem. Phys. 25, 261-6 (1956). NMR activation energy for proton exchange between HF and HjO" in aqueous solution. 

As a matter of fact, proton exchange of imidazolium ion in aqueous solutions is significantly faster than acid dissociation. Since the activation energies for proton exchange and acid dissociation are equal, it is probable that the two processes have the same ratedetermining step, formation of an ionized intermediate [23]. 

Contributions to (T )e include effects of and rs. A plot of log [(T2)eTctl-— C/tJ vs. 1/T gives 8.4 kcal/mole for the activation energy of proton exchange between water and the hydration sphere of the manganous ion. For more recent data on exchange involving Mn(H20)6 H, the interested reader is directed to the O17 NMR studies of Connick and Swift (16a). 

Fackler and coworkers (18, 19) have used the methyl proton resonances to determine activation energies for the exchange of methyldi-phenylphosphine coordinated to the bis-complexes of platinum (II) with ligands such as p-dithiocumate (18), 3,4,5-trimethoxydithiobenzoate (18), XXXI, and o-benzylxanthate (19). 

Qualitative studies of the proton exchange in pure ethanol were reported by Schneider and Reeves (1958). The simultaneous shift to high field and collapse of the —OH triplet was demonstrated as the temperature is raised. Connor and Loewenstein (1961) measured the activation energies for proton transfer reactions in ammonium and methyl ammonium ion solutions. Proton transfer reactions of the type... 

Rate constants were measured at different temperatures and from the Eyring plots activation energies of proton exchanges were obtained as follows 7.9 kcal/mole for A-BU, 12.7 for A-U, 13.6 for A-TH and 15.3 for A-DH. The order is parallel to that of pk valves of the imino protons of uracil derivatives. The more acidic protons exchange more easily. 

The appearance of N resonances of amino or amide groups is quite sensitive to the exchange rate of attached protons with those of the solvent water. This rate is pH dependent. The activation energy for the exchange in glutamine was measured as 75.2 kJ/mole at pH 4.8 in lysine values of 61.6 kJ/mole were measured at pH 1.0 (Blomberg et al, 1976). 

Nuclear Overhauser enhancement spectroscopy ( H- H NOESY and NOE) experiments show only the TTC isomer in equilibrium with the TTT isomer for 6,8-dinitro-BIPS [36,55] and 6-nitro,8-bromo-BIPS. The TTC form dominates the equilibrium. Spectral broadening for several proton resonances in the spectra of 6,8-dinitro-BIPS and 6-nitro,8-bromo-BIPS indicate a rapid exchange between these two isomeric forms. The activation energy for this isomerisation is reported to be 43.6 kJ mol and the energy difference between the the TTC and TTT forms is 4.6 kJ mol [55]. 

Occasionally rearrangements from more stable to less stable carbocations occur, but only if (1) the energy difference between them is not too large or (2) the carbocation that rearranges has no other possible rapid reactions open to it.9 For example, in superacid medium, in the temperature range 0-40°C, the proton nmr spectrum of isopropyl cation indicates that the two types of protons are exchanging rapidly. The activation energy for the process was found to be 16 kcal mole-1. In addition to other processes, the equilibrium shown in Equation 6.7 apparently occurs.10 In the superacid medium, no Lewis base is available... 

In the case of benzene, a number of epoxides are known, e.g., mono-, di-, and triepoxides. All undergo interesting electrocyclic reactions. Benzene epoxide itself exists as a valence tautomer of 86 and 96.8 The enthalpy difference between the two forms is 1.7 kcal/mol. The activation energies for forward and reverse reactions are 9.1 and 7.2 kcal/mol, respectively. At room temperature the rate of exchange is so fast that the NMR spectrum is the average of chemical shifts that are due to protons represented by structures 86 and 96. 

Both kinetic and thermodynamic factors are important in determining protic character. Although the equilibrium concentration of solvated protons in a protic solvent such as water or ethanol may be very small, there is a low activation energy for dissociation or exchange of protons, and these labile protons can rapidly react with any species having an appreciable proton affinity. Aprotic solvents usually have a lower equilibrium concentration of solvated protons (perhaps by a factor of as much as 1010) and the activation energy for... 

Lin, J., et al. (1998). Fractionation factors and activation energies for exchange of the low-barrier hydrogen bonding proton in peptidyl trifluoromethyl ketone complexes of chymotrypsin. Proc. Natl. Acad. Sci. 95 (25), 14664-14668... 

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