Pyridinium ions, radiolysis

The basicity of pyridinyl radicals is of interest since some pyridinyl radicals react with one another in an acid catalyzed reaction (sect. 4.4). The absorption spectra of radicals generated from the pyridinium ion by pulse radiolysis in aqueous solution at different pH values allow the evaluation of the basicity of the radicals 97,98) I jjg Qf protonated form of l-methyl-3-carbamidopyridinyI radical, (CONHj) ), is 1.43 the protonated radical has alsorption maxima at 3(X) nm and 440 nm, at somewhat longer wavelengths than those for the unprotonated radical at 280 nm and 420 nm. The nicotinamide adenine dinucleotide radical (NAD ) has a pK, of 0 or less, with a shift in absorption maximum due to protonation being observed only in 1.5 M HCIO4. 

The l-alkyI-2-, 3- and 4-carbamidopyridinyl radicals may be generated in water from the corresponding pyridinium ions by pulse radiolysis or radiolysis. The rate constant for the disappearance of 3 are pH-independent and close to diffusion controlled (Eq. 27). The rate constants for 4 are pH-dependent. Completely protonat 4 reacts with itself at rates somewhat less than diffusion controlled the rate constants decrease linearly with increasing pH (slope ca. 1) (Eq. 28). Rates in the range pH 8-11 could be followed using a pyridinyl ester. A minimum rate was found near pH 9.2 at higher pH values, the hydrolysis of the pyridinyl ester to the carboxylate controlled the rate (Eq. 29), since the pyridinyl carboxylate would react with itself in a diffusion controlled process (Eq. 30). 

The l-ethyl-4-carbomethoxypyridinyl radical disproportionation products, aside from the pyridinium ion, probably included ethylamine, and a compound thought to be an ester dialdehyde (Eq. 31). The pH changes noted after radiolysis of buffer-free isopropyl alcohol-water solutions of the pyridinium ions revealed that the initial dihydropyridine 4 H) formed from 4 hydrolyzed and consumed another proton, presumably because of ethylamine. 

We have demonstrated a particularly clear and simple technique for evaluating the reactions of pyridinyl radicals in water through measurement of the pH changes resulting from radiolysis of pyridinium ions in unbuffered aqueous solutions containing isopropyl alcohol ( ). Radiolysis generates one proton for each pyridinyl radical, as set forth in Eq. 9. ( ). 

---