A pulse radiolysis study of the reactions of hydrogen atoms with dimethyl-, tetramethyl- and hexamethyl-substituted viologens

Abstract

The reactions of ·H, ·OH and e^-_aq with four viologens in aqueous solution were studied by spectrophotometry. With 1,1'-dimethyl-4,4'-bipyridinium dichloride, the main transient species caused by H reaction is the viologen radical cation formed by an electron-transfer reaction. Smaller amounts of a hydrogen-deficient radical are also formed by an abstraction reaction at one of the methyl groups together with a radical formed by addition of ·H to one of the rings. There is no evidence that a protonated radical cation species was formed. A similar pattern of reactivity is seen with 1,1',2,2'-tetra-methyl-4,4'-bipyridinium diperchlorate. However, the electron-transfer reaction is either absent in the case of 1,1',3,3'-tetramethyl-4,4'-bipyridinium diperchlorate or less important in 1,1',2,2',6,6'- hexamethyl-4,4'-bipyridinium dichloride, and the main initial transient is assigned to a hydrogen-deficient radical formed by an abstraction reaction at one of the ring methyl groups. The possible role of the hydrogen-deficient radical in reducing the loss of viologen observed in the photochemical splitting of water is discussed.