Pulse radiolysis of 3-pyridine methanol and 3-pyridine carboxaldehyde in aqueous solutions

摘要

Reactions of e_(aq)~-, H-atom and OH radicals with 3-pyridine methanol (3-PM) and 3-pyridine carboxaldehyde (3-PCA) have been studied at various pHs using pulse radiolysis technique, e_(aq)~- was found to be highly reactive with both 3-PM and 3-PCA (k approx. 10~(10) dm~3 mol~1 s~(-1)). Semi-reduced species formed in both cases were strongly reducing in nature. Ln the case of 3-PM, electron addition leads to the formation of pyridinyl radicals whereas in the case of 3-PCA, PyCHOH type radicals are formed. At pH 6.8, H-atom reaction with 3-PCA also gives semi-reduced species (PyCHOH), whereas at pH 1, H-atoms add to the ring. (CH_3C_2~·COH radicals were found to transfer electron to 3-PCA at all the pH values tested and by making use of changes in the absorption spectra, pK_a values of the semi-reduced species were determined to be 4.5 and 10.6. OH radicals were found to undergo addition reaction with 3-PCA, whereas in the case of 3-PM they reacted by H-abstraction as well as addition reaction. By following the yield of methylviologen radical cation formed by electron transfer reaction, it was estimated that approx. 50% of OH radicals react with 3-PM by H-atom abstraction at pH 6.8, giving reducing radicals, whereas at pH 3.2, where 3-PM is in the protonated form, the same is only about 10%. At pH 13, O~(-·) radical anions were found to react exclusively by H-atom abstraction. Reaction of SO_4~(-·) radicals with 3-PCA was found to give a species identical to the one formed by one electron reduction of nicotinic acid at acidic pH values.