Res. Chem. Intermed.
, Vol. 28, No. 1, pp. 29 –39 (2002)
Nature of transient species formed during pulse radiolysis
of 4-mercaptopyridine in aqueous solutions: Formation
of a dimer radical species by one-electron reduction
, G. R. DEY and D. B. NAIK
Applied Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
Received 25 July 2001; accepted 29 September 2001
Abstract—Reactions of one-electron reducing as well as oxidizing radicals with 4-mercaptopyridine
(4-MPy) were studied in aqueous solutions at different pH values. One-electron oxidizing radicals
such as N
, react with 4-MPy by electron transfer reaction at pH 11 to give 4-pyridylthiyl
radical. The reduction potential for the couple 4-PyS
was estimated to be 0.93V
. NHE by
equilibrium reaction with I
couple. At pH 6.8, where the compound is predominantly present
in the thione form, the transient species formed is a cation radical. OH radicals react with 4-MPy by
addition to the pyridine ring at pH 6.8 and 11. At pH 0, OH radicals as well as one-electron oxidants
radicals react with 4-MPy to produce the protonated form of 4-pyridylthiyl radical.
At pH 6.8 and 11, e
reaction with 4-MPy gave an initial adducts which reacted with the parent
molecule to give dimer radicals. Acetone ketyl radicals were unable to reduce 4-MPy at neutral
pH. Reducing radicals like H-atoms and acetone ketyl radicals reacted with 4-MPy at acidic pH by
H-abstraction reaction to give the same species as produced by oxidizing radicals.
: 4-mercaptopyridine; pulse radiolysis; dimer radical species.
In the case of aliphatic thiols (RSH), OH radical reaction leads to the formation
of thiyl radicals (RS
) which do not have any absorption in the visible region.
But thiyl radicals can react with the parent thiolate ion to give an inter-molecular
three electron bonded species (RS SR)
which has strong absorption in the visible
region (»420 nm) . In the case of aromatic thiols, however, OH radical reaction
leads to the formation of aryl thiyl radicals, which are stabilized by the presence
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