Temperature jump intramolecular proton

Results which have an important bearing on the mechanism of proton removal from intramolecular hydrogen bonds have been obtained quite recently for the second ionisation of phenylazoresorcinols (73). Kinetic measurements were made using the temperature-jump technique (Perlmutter-Hayman and Shinar, 1975 Perlmutter-Hayman et al., 1976). 

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... 

Jnmps of a proton along the hydrogen bond represent another type of dynamics observed in hydrogen-bonded complexes. Mechanistically, this process is simplest for intramolecular hydrogen bonds. The fast enol-enolic equilibrium shown in Scheme 2.2 illustrates an intramolecular proton-jumping system [27]. Here, substituent X dictates the equilibrium constant as well as the rate of proton transfer. It should be noted that such proton jumps can be stopped on the H NMR time scale only at very low temperatures. 

THERMODYNAMICS AND KINETICS OF INTRAMOLECULAR PROTON-TRANSFERS. TEMPERATURE-JUMP STUDIES. 

Bensaude, O., Dreyfus, M., Dodin, G., and Dubois, J. (1977) Intramolecular nondissociative proton transfer in aqueous solutions of tautomeric heterocycles a temperature-jump kinetic study. J. Am. Chem. Soc., 99, 4438-4446. 

Above 200 K changes in line shape as shown in Fig. 26 reveal the onset of molecular dynamics (the simulations (solid lines) are carried out assuming a particular motional model, the moderate jump model, which is discussed below.) The line width has dropped to about 12 G, which is known to result from intramolecular interactions manifested as unresolved proton hyperfine splittings [132]. The dipolar interactions are now averaged out by the molecular dynamics. The remarkable intensity behavior (increasing intensity at higher temperature) is caused by the reversible conversion of DTBN moleeules from the ESR-inactive to the ESR-active state and is discussed elsewhere [133]. 

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