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.