S. Tajima et al., Electron transfer reaction from triplet 1,4-dimethoxybenzene to hydronium ion in aqueous solution, J PHYS CH A, 103(31), 1999, pp. 6097-6105
The electron transfer (ET) reaction from the triplet state of 1,4-dimethoxy
benzene ((DMB)-D-3*) to the hydronium ion (H-aq(+)) in aqueous solution at
295 K was studied by means of the laser photolysis method. (DMB)-D-3* was q
uenched by H-aq(+) and D-aq(+) with the quenching rate constants of 7.7 (+/
-0.4) x 10(6) and 2.6 (+/-0.4) x 10(6) M-1 s(-1), respectively. The occurre
nce of the ET reaction was substantiated by formation of a DMB cation radic
al (DMB+.) coinciding with disappearance of (DMB)-D-3*. From kinetic analys
es, the rate constant (k(et)(H)) for the ET reaction from (DMB)-D-3* to H-a
q(+) was determined to be 7.5 (+/-0.4) x 10(6) M-1 s(-1). The observed ET r
ate exhibited a significantly smaller value compared to the diffusion-contr
olled rate in aqueous solution. The free energy changes for the ET reaction
were estimated to be +28 and -175 kJ mol(-1) for the reactions (DMB)-D-3*
+ H-aq(+) --> DMB+. + H and (DMB)-D-3* + H-aq(+) --> DMB+. + 1/2H(2), respe
ctively. The observed small ET rate constant (7.5 (+/-0.4) x 106 M-1 s(-1))
revealed that the ET corresponds to the former reaction. The large isotope
effect (k(et)(H)/k(et)(D) = 3.0) was observed for the ET reaction between
(DMB)-D-3* and H-aq(+) (or D-aq(+)). The fluorescence of DMB was also quenc
hed by H-aq(+) with a quenching rate constant of 6.2 (+/-0.4) x 10(7) M-1 s
(-1). The singlet quenching was similarly ascribable to the ET reaction fro
m (DMB)-D-1* to the hydronium.