Reactions of hydrated electrons with pyridinium salts in aqueous solutions

Rate coefficients for the reactions of the hydrated electron (eaq−) with pyridinium salts in aqueous solutions have been determined using pulse radiolysis techniques. The rate coefficients for pyridine, 1-hydropyridinium chloride, and 1-hydropyridinium nitrate were observed to be 1.4 × 1010, 4.5 × 1010, and 5.3 × 1010 M−1 s−1, respectively. The eaq− was found to primarily attack the pyridine ring, the proton coordinated to the nitrogen atom, and the nitrate counterion, but not the chloride. Results for the corresponding dimer structures of 4,4′-dipyridyl, 1,1′-dihydro-4,4′-bipyridinium dichloride, and 1,1′-dihydro-4,4′-bipyridinium dinitrate had similar trends for eaq− attack sites. The rate coefficients for pyridinium salts were lower when the pyridinium nitrogen atom is coordinated to a methyl group rather than to a proton. This reduction is probably due to the increase in electron density of the pyridine ring due to the electron-donating methyl group. Pyridinium salts are not major contributors to the production of molecular hydrogen in the radiolysis of aqueous solutions and actually decrease molecular hydrogen yield due to scavenging reactions of the eaq−. The yield of molecular hydrogen decreases from 0.45 to ∼0.2 molecule/(100 eV) over the scavenging capacity range for the eaq− of 105−109 s−1. Absorption spectra of the transient species produced by the reactions of pyridinium salts with OH radical and H atom formed in water radiolysis were observed, and rate coefficients for these reactions were determined.