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3. RADIATION CHEMISTRY OF AQUEOUS SOLUTIONS

Hilbert Christensen

3. 1 solutions and the benzyl radical

The gamma radiolysis study of aqueous toluene solutions has been concluded and published [ 1 ]. The investigation indicated the formation of benzyl radicals in irradiated acid solutions. As previous pulse ra- diolysis studies of the system by Dorfman et al [2] and Sauer and Ward [3] did not reveal any formation of benzyl radicals the forma- tion of this radical in our system was probably indirect, most likeiy by water elimination of the primarily formed cyclohexadienyl radical. In order to examine this possibility the system was studied by the pulse radiolysis technique in co-operation with K. Sehested at Ris^ in Den- mark. As the benzyl radical is absorbing light in the same wavelength region as the cyclohexadienyl-type radical, the benzyl radical was first produced and studied in aqueous solutions without interference from the cyclohexadienyl radical by irradiation of various benzyl com- pounds in argon saturated aqueous solutions. The following benzyl com- pounds have been irradiated in this way and were all shown to give the benzyl radical spectrum: chloride, bromide, acetate, formate and thiocyanate. Fig. 3. 1 shows the spectrum in solutions of benzyl acet- ate. Aqueous toluene solutions at various pH values were also studied by the pulse radiolysis technique using small pulses of only 600 rads. Figs. 3. 2 and 3. 3 show the spectra obtained at various times after the pulse in acid (pH I) and in unbuffered solutions, respectively. The increasing contribution of the benzyl radical to the absorption spec- trum with time is clearly seen at pH 1. This shows that the benzyl radical is formed in acid toluene solutions and indicates a water elim- ination reaction.

3. 2 solutions

The pulse radiolysis study of aqueous aniline solutions has been continued with examinations of acid aerated or oxygen-saturated so- lutions. The only difference between the spectrum in argon and oxygen- saturated solutions was a considerable reduction in size of the peak at 310 nm. However, the peak is not totally eliminated indicating the formation of an OH-adduct, which would be assumed to absorb at this wavelength. - 7 -

The spectrum obtained in H^O-saturated, aqueous 2-amino-l, 3, 5- trimethylbenzene solutions is shown in Fig. 3.4. The cyclohexadienyl- -type radical with absorption around 350 nm and with a fast first order decay is still formed but with Ä much smaller yield than for aniline and 2-amino-l, 3-dimethyibenzene solutions. A "blocking" of the o- and p-positions of aniline with methyl groups thus reduces the prob- ability of OH-addition. The spectrum of pulse-irradiated NLO-saturated, aqueous solu- tions of N-methylaniline is shown in Fig 3. 5. Both the cyclohexadienyl and the aniline type of radicals are formed. The pulse radiolysis study of aniline solutions has been published [4]. The examination of final products from the radiolysis of aqueous aniline solutions has been continued. The product composition has been studied with the help of thin layer and gas liquid chromatography, and spectrophotometry. Some of the main products were 2-aminophenoxasin-3-one, which in N^O-saturated solutions was formed in a yield of 0. 24, and together with its reac- tion products. Hydrazobenzene was formed with a yield of 0. 20 in N^O- saturated solutions. - 8 -

REFERENCES 1. CHRISTENSEN, H. C. , and GUSTAFSON, R. , Radiolysis of aqueous toluene solutions. 197 K (AE-414). 2. DORFMAN, L. M. , TAUB, I. A., and BUHLER, R. E. , Pulse radiolysis studies. I. Transient spectra and reaction-rate constants in irradiated aqueous solutions of . J. Chem. Phye. 36 (1962) p. 3051. 3. SAUER, M. C. Jr. , and WARD, B. , The reactions of atoms with benzene and toluene studied by pulsed radiolysis: Reaction rate constants and transient spec- tra in the gas and aqueous solution. J. Phys. Chem. 71 (1967) p. 3971. 4. CHRISTENSEN, H. C. , Pulse radiolysis of aqueous solutions of aniline and substituted . 1971. (AE-413). - 9 -

0.15

300 350 Wavelength (nrn)

Fig. 3.1 Absorption spectrum* of the benzyl radical in aqueous solution obtained by pulse radiolysis of argon-saturated 2-10" M benzyl acetate solutions. -I- : 4 y.s; 0 : 74 M>s after the pulse.

300 350 Wavelength (nm) Fig, 3.2 Absorption tpectrum of pulse irradiated acid (pH = 1) aqueous solutions saturated with toluene and N~O. + : 2 fis; A; 9 ji»; © : 83 (it after the pulse/ - 10 -

300 350 Wavelength Inm)

Fig. 3.3 Absorption spectrum of pulse irradiated unbuffered aqueous solutions saturated with toluene and N_O. + : 2 p,s; O : 82 p.s after the pulse.

300 400 500 Wav*ttngth (nm) Fig. 3.4 Absorption spectrum of pulse irradiated N,O-saturated -4 4 • 10 M N-methylaniline solutions. + : 1, 5 us; x : 1, 5 us corrected for aniline decomposition; 0 : 38 |ii after the pulse. - 11 -

0.250

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ID 4»

O. o 0.100 -

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0.010

Wavelength (nm)

Fig. 3. 5 Absorption spectrum of pulse irradiated N.O-saturated 4' 10 -4 M 2-amino-1, 3, 5-trimethylbenzene. + : 1 us; O ; 37 na after the pulse.