Dynamics of Proton Transfer from Radical Cations. Addition−Elimination or Direct Proton Transfer?

Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway

A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)nCO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar.+(CH2)nCO2H) or radical zwitterions (Ar.+(CH2)nCO2-) depending on pH, and pKa values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with (-)OH (k-OH)) have been obtained. These values are similar to those obtained previously for the (-)OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11.

Proton transfer reactions of methylanthracene radical cations with nitrogen-centered bases under non-steady-state conditions. a search for the possible effect of reactant impurities upon the kinetic data

[reaction: see text] At a recent conference, the issue was raised that our earlier non-steady-state kinetic studies(1)(-)(3) may be in error because of the effects of possible impurities in the N-centered bases employed. This prompted our reinvestigation of these systems with that possibility in mind. It was pointed out that in a series of reactions, if a reactive impurity affects the kinetics in one reaction, then it would exert a consistent effect on the kinetics of all of the reactions in the series. Experimental data for three different series of reactions, proton transfer reactions of 2,6-dimethylpyridine, 2,6-diethylpyridine, and 2,6-diphenylpyridine with four different methylanthracene radical cations, were concurrently examined for possible reactant impurity problems. The analysis confirms that the simulated data for the simple second-order mechanism in the presence of a reactive impurity do not fit experimental data for the proton transfer reactions. The hypothesis that deviations from simple second-order kinetics are caused by the presence of reactive impurities is not valid. It was concluded that the presence of reactive impurities was not a significant problem in the studies of proton transfer reactions of methylanthracene radical cations with pyridine bases.

Proton transfer reactions of methylanthracene radical cations with pyridine bases under non-steady-state conditions. Real kinetic isotope effect evidence for extensive tunneling

The kinetics of the proton transfer reactions between the 9-methyl-10-phenylanthracene radical cation (MPA(+)(.)) with 2,6-lutidine were studied in acetonitrile-Bu(4)NBF(4) (0.1 M) using derivative cyclic voltammetry. Comparisons of extent of reaction-time profiles with theoretical data for both the simple single-step proton transfer and a mechanism involving the formation of a donor-acceptor complex prior to unimolecular proton transfer were made. The experimental extent of reaction-time profiles deviated significantly from those simulated for the single-step mechanism, while excellent fits of experimental to theoretical data, in the pre-steady-state period, for the complex mechanism were observed. In this time period, the apparent deuterium kinetic isotope effects (KIE(app)) were observed to vary significantly with the extent of reaction as predicted by the complex mechanism. Resolution of the apparent rate constants into the microscopic rate constants for the complex mechanism resulted in a real kinetic isotope effect (KIE(real)) equal to 82 at 291 K. Arrhenius activation parameters (252-312 K) for the reactions of MPA(+)(*) with 2,6-lutidine in acetonitrile-Bu(4)NBF(4) (0.1 M) revealed E(a)(D) - E(a)(H) equal to 2.89 kcal/mol and A(D)/A(H) equal to 2.09. In this temperature range, KIE(real) varied from 46 at the highest temperature to 134 at the lowest. The large KIE(real), along with the Arrhenius parameters, are indicative of extensive tunneling for the proton transfer steps.

Detection of short-lived electrogenerated species by Raman microspectrometry

Raman microprobe spectrometry has been applied to the characterization of unstable species generated electrochemically at a microelectrode (radius in the 10 microm range). The ability of the spectroelectrochemical method to detect short-lived intermediates is directly related to its capability to probe small volumes. Raman microprobe spectrometry is appropriate for electrochemical applications because it allows the analysis of approximately 1 microm3 of solution. In spectroelectrochemical experiments, such a volume corresponds to a reaction layer of 1 microm thickness. Potentially, this technique can allow the observation of species with lifetimes of the order of 1 ms. To enhance the capabilities of this spectroscopic technique, we utilized it in combination with steady-state voltammetry at a microelectrode, to increase the concentration of unstable intermediates near the electrode surface. To determine the detection limit of this combined technique, we varied the base concentration as a means for varying the lifetime the radical cation electrogenerated from 9,10-dichloroanthracene. Well-resolved resonance Raman spectra were obtained for this radical cation when the lifetime was > or = 0.1 ms. This short time resolution achieved with micro-Raman spectroelectrochemistry makes this technique a powerful tool for the characterization of short-lived intermediates that are generated electrochemically in solution.