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Formation and Decay of the Biphenyl Cation Radical
Formation and Decay of the Biphenyl Cation Radical in Aqueous Acidic Solution K. Sehested* Danish Atomic Energy Commission, Research Establishment Riso. DK-4000 Roskilde, Denmark
and E. J. Hart Argonne National Laboratory, Argonne, Illinois 60439 (Received September 26, 1974; Revised Manuscript Received May 5, 1975) Publication costs assisted by the Danish Atomic Energy Cornniission
The biphenyl cation radical forms from the OH adduct of biphenyl in aqueous acidic medium. Its spectrum is similar to the established spectra of the biphenyl cation radical obtained in organic solvents and glasses. The OH adduct was shown to be the precursor by eliminating effects due to H atoms and by the use of OH radical scavengers. I t forms from the OH adduct with a rate constant of 2 X lo6 M-l sec-l and it decays with a first-order rate constant of 5.0 X lo3 sec-l. The hydrated electron forms the anion radical with a rate constant h = 1.2 X 1O1O M-l sec-l by reaction with biphenyl. The OH and H radicals react with the biphenyl with rate constants of 9.0 X lo9 and 5.0 X lo9 M-l sec-l, respectively. The mechanism of cationradical formation is briefly discussed.
Introduction The electronic structure of aromatic anion and cation radicals has been studied extensively by optical absorption spectroscopy and electron spin resonance. The biphenyl cation radical has been investigated in several organic solvents,1-6 in some cases with electron scavengers, and in organic glasses7-12by the pulse radiolysis technique. In these experiments the biphenyl cation radical was created by charge transfer from the solvent cation and some kinetic data have been r e p ~ r t e d . ' - ~ The b absorption spectrum of this cation radical is well established. It has absorption bands with maxima around 380 and 690 nm and a ratio of extinction coefficients of the two bands between 2 and 3.1,2,6This paper describes the absorption spectrum and the kinetics of formation and decay of the biphenyl cation radical formed in aqueous acidic solutions from the OH adduct.
Experimental Section The biphenyl was of BDH p.a. quality. The perchloric acid was reagent grade from G. F. Smith, and the water was triply distilled. The pulse radiolysis setup was described previ0us1y.l~ The irradiations were performed at Riso with 1-ysec pulses with maximum 0.25 A from the 10-MeV electron linear accelerator. The dosimetry was performed with the hexacyOH) = 5.25 and anoferrate(I1) dosimeter14 using g(e,,t(420 nm) 1000 M-l cm-l. The solutions were prepared in 100-cc syringes by bubbling the appropriate gas through the solution containing excess of biphenyl for 30 min. The concentration of the biphenyl was determined spectrophotometrically from its absorption band a t 247 nm in light petroleum.15 This band has an extinction coefficient of 17,000 M-l cm-l and it is the same in tetrahydrofuran and in water to within 5%. In water the extinction coefficient was determined by dissolving 5 mg of biphenyl in 10 ml of methanol and diluting to 1 1. with water. The saturated aqueous biphenyl solution contains 4.4 X 10-6 M biphenyl a t pH 7 and 5.5 X 10-5 M a t pH zero.
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Results and Discussion p H 10-3. The transient absorption spectrum observed in neutral NzO-saturated solution of biphenyl has uv bands a t 305 and 360 nm. The decay characteristics are the same for both of the wavelengths indicating that the observed spectrum is to be attributed to a single transient species. In Arsaturated solutions, however, two additional bands are observed, a uv band at 390 nm and a weaker visible band at 590 nm partly masked by the hydrated electron spectrum. The 305 and 360 nm bands are reduced to about 60% of the bands in NzO solution. Solutions containing oxygen show only the 305- and 360-nm bands (about half of that in N20 solution). In solutions containing 0.1 M tert-butyl alcohol the observed bands are a t 390 and 590 nm with a very weak absorption at 305 nm. Consequently the 305- and 360-nm bands are attributed to the OH adduct of the biphenyl containing a small fraction (0.095) of the H adduct (Figure 1). The corrected extinction coefficients for the two bands are 11,500 and 7,000 A4-l cm-l, respectively, and the rate constant for the formation of the adduct is determined from experiments with different initial concentrations of biphenyl to be h(OH biphenyl) = (9.0 f 1.0) X lo9 M-l sec-l. The decay is second order with a rate constant 2k = (1.6 f 0.2) X lo9 M-l sec-l. The 390- and 590-nm bands in Ar-saturated solutions at p H 9 are attributed to the biphenyl anion radical formed in the reaction of the hydrated electron with biphenyl (Figure 2). The weak absorption at 305 nm in solutions containing tert- butyl alcohol together with a small hidden absorption a t 360 nm is due to a small fraction of the H adduct. The formation of the anion radical as measured from the buildup at the 390-nm peak and the decay of the hydrated elecbitron absorption at 700 nm has a rate constant k(e,,phenyl) = (1.2 f. 0.2) X 1O1O M - l sec-l. The half-life of this species is about 25 ysec at pH 9.8-6.4, although the decay kinetics do not appear to be pure first order. The 590-nm peak can be separated satisfactorily from electron absorption by a kinetic treatment at the highest possible biphenyl concentration where the half-lives of the electron and the anion radical are about 1.5 and 25 ysec, respectively. In solutions containing tert- butyl alcohol the decay of the anion
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The Journal of Physical Chemistry, Vol. 79, No. 16, 1975
K. Sehested and E. J. Hart
1640
4
1I
1'
I
300
"\>gy+,/:
, 500 600 Wavelength (nm)
400
, 1
I
x\
700
800
I
Wavelength ( n r )
Figure 1. Absorption spectrum (0)of the OH adduct of biphenyl obtained in N20-saturated aqueous solution, pH 7, and corrected for the contribution from the H adduct. Absorption spectrum (X) of Php' obtained in Op-saturated aqueous acidic biphenyl solution (1 M HCIO4).
Figure 2. Absorption spectrum (0)of the H adduct of biphenyl obtained in Ar-saturated aqueous solution, pH 3, containing 0.05 M tert-butyl alcohol. Absorption spectrum (X) of Php- obtained in Arsaturated aqueous biphenyl solution, pH 9.8, and corrected for the contribution from the H adduct.
radical is faster (half-life less than 5 wsec in 0.1 M tertbutyl alcohol) probably due to reaction with the alcohol1 and the alcohol radical. The observed spectrum is similar to the spectrum of the biphenyl anion obtained in T H F and 2-propano11y3 except for a small blue shift of the two bands. The reported bands in 2-propanol are at 400 and 620 nm (Figure 1 ref 3) with an extinction ratio of 3.2. The observed bands in aqueous solutions, 390 and 590 nm, have extinction coefficients of 18,500 and 8,000 M-l cm-l, respectively, yielding a ratio of 2.3. In slightly acid solution the reaction of the hydrated electron with hydrogen ions compete efficiently with the relatively slow reaction of the electron with low concentrations of biphenyl. No evidence of a reaction of the biphenylide ion with hydrogen ions to form a hydrogen adduct1 was found. As the pH is lowered to 3 there is no change in the observed spectrum in NnO-saturated solutions. In Ar-saturated solutions, however, the spectral change is very pronounced. Two of the bands, 390 and 590 nm, observed in neutral solution, have completely vanished and the 360-nm band is increased to about the same extinction as the 305nm band. In air- and 02-saturated solutions the 360-nm peak is reduced to about half while the 305-nm peak decreases about 10-15%. In these solutions the H is scavenged by the oxygen (k(H 0 2 ) = 2 X 1010 M-l sec-l)16 forming HOz, which absorbs at 240 nm. The remaining spectrum is that of the OH adduct. In solutions containing 0.05 M tertbutyl alcohol at pH 3 bands a t 305 and 360 nm are observed. 98% of the OH radicals are scavenged by the alcohol and the bands are consequently ascribed to the H adduct of biphenyl (Figure 2). The extinction coefficients are 3200 and 5000 M-l cm-1 and the rate of formation is k = (5.0 k 1.0) X lo9 M-1 sec-1. The decay constant is second order
with 2k = 1.5 X lo9 M-l sec-l, but the kinetics appear to be influenced by the reaction of the H adduct with the tertbutyl alcohol radical. p H below 3. In Ar-saturated solutions pH
