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J. Phys. Chem. 1986, 90,4014-4016
Disulflde Radlcal Ions in Picosecond-Pulse Radiolysis of Dimethyl Sulfide J. Belloni,* J. L. Marignier, Y. Katsumura,tl and Y. Tabatat Laboratoire de Physico- Chimie des Rayonnements, associZ au CNRS, UniversitZ Paris-Sud, 91405 Orsay Cedex, France, and Nuclear Engineering Research Laboratory, University of Tokyo, Tokai Mura, Ibaraki 31 9-1 I , Japan (Received: January 13, 1986; In Final Form: March 20, 1986)
The thioether dimethyl sulfide has been studied by picosecond-pulse radiolysis in order to investigate the fate of the primary ions and electrons and the exceptionnally fast mechanism of the disulfide bond formation. The transient spectrum is a sum of two components respectively assigned to the anion CH3SSCH3-(A,, = 420 nm) and the cation (CH3)&S(CH3),+ (Amx = 480 nm). The former is almost stable within 1 ns while the cation has decayed in the mean time. The yields are discussed in comparison with those of other species. The results obtained in dimethyl sulfide, either pure or diluted with cyclohexane, support a very fast ionsolvent reaction of the precursors CH3SCH< and CH3SCH3+,yielding the disulfide anions and cations.
Introduction
The fate of an electron with thermal energy in a liquid may lead to quite different situations. If the interaction of the electron with the solvent molecules is limited to polarization, electron solvation occurs and numerous data have been gathered about electrons solvated in various media such as water, alcohols, amines, oxyethers,. . . . I On the other band, electron attachment is the main process when molecules exhibit a high electron affinity. If the negative adduct is then involved in an irreversible reaction (dissociative attachment, ion-molecule reaction, ...) yielding secondary anions, this prevents any further transfer of the charge to a solvation trap. The secondary anion is then obviously specific to the liquid. Acetone,z certain amide^,^ halides: and hydrogen sulfide5 belong to the latter group. The thioether dimethyl sulfide (DMS) appeared in this respect as a quite unusual case.6 Actually, its behavior seems to allow both processes, electron attachment and solvation, to occur in competition, so that solvated electrons and secondary anions are observed simultaneously.' Discussion of experimental observations concluded that the common precursor, the negative adduct (CH3),S-, was able to autoionize easily into (CH3),S and an electron, thus increasing the probability of electron solvation, while a competing ion-molecule reaction would lead irreversibly to a more stable (-SS-)- anion. This implies that attachment and ion-molecule reactions occur as early as solvation, viz., within a few picoseconds, as usually happens in fluid liquidse8 Indeed, previous work6-' on nanosecond-pulse radiolysis of DMS has shown that the formation of e;, but also of the anion CH3SSCH3-, is at completion within the duration of the pulse, Le., 3 ns. This strongly supports a mechanism of (-SS-)- bonding through a direct (CH3)2S--solvent reaction, as discussed in detail in ref I . The aim of the present study is to directly investigate at shorter times the evolution toward the system observed at 3 ns and the fate of the various charged precursors. Experimental Section Guaranteed grade dimethyl sulfide and spectrophotometric grade (Uvasol) cyclohexane were purchased from Tokyo Kasei Co., Ltd., and Merck, respectively. Samples were purified by distillation over sodium mirror and prepared on a vacuum line torr. and sealed off under less than The pulse radiation source used was the 35-MeV electron linear a c c e l e r a t ~ r . ~The ~ ' ~ pulse duration is 18 ps at half-height. The dose absorbed was evaluated from the absorbance of the hydrated electron e, in pure deaerated water, measured at 500 nm and 2 ns ((5-9) X 10l6eV mL-' pulse-') and deduced for other media from the electron density. The dose is respectively 0.837 and 0.80 times the dose in water for DMS and cyclohexane. The transient 'University of Tokyo. Present address: Department of Nuclear Engineering, Faculty of Engineering, University of Tokyo, 7-3-1 Hongo Bunkyo-ku. Tokyo 113, Japan.
0022-3654/86/2090-4014$01.50/0
signal was detected by a biplaner photodiode (HTV, R 1328 U) having a rise time of 60 ps and amplified by a fast-pulse amplifier (B & H DC 3002). Amplified transient signals of the fast diode were transferred through a DC block to a transient digitizer Tektronix R 7912 or a DPO Tektronix with S-4 sampling head controlled by a PDP 11 computer systema9 Results and Discussion The correction to be applied to the signal due to the delayed response of the diode is deduced from the shape of the variation of the absorbance of e-,,, which is known to be solvated within the pulse.* However, the correction would be hazardous at times shorter than 100 ps. Anyway, even near 125 ps, the uncertainty becomes higher for signals with a shape deviating markedly from that of e-,,, as occurs around 480 nm. The different spectra in pure irradiated dimethyl sulfide at 125, 250, and 500 ps and 1 and 5 ns in the range 330-550 nm are given in Figure 1. The optical density is given relative to that of e-aq at 2 ns and 500 nm in pure water ( G c , - ~= 2.07 X lo4). The dotted line corresponds to the transient spectrum previously obtained' at the end of a pulse with 3 ns at half-width, also normalized as above. It is clear that present results coincide as regards wavelength dependence and intensity with the earlier ones at the same resolution time. At shorter times, the shape of the transient spectrum is markedly different. It is dominated by an intense band around 480 nm instead of 420 nm at 3 ns. The evolution with time indicates that the location of the maximum is shifted. Moreover, the decay around 500 nm is much faster than around 400 nm, in accord with the presence of two distinct species at least. A similar situation was found' a t low temperature (-70 "C), where a shoulder near 480 nm, less intense than at 420 nm, was still present at 3 ns and decayed faster. Again it is plausible to assign this band a t 480 nm to the dimer complexes"J2 (CH3),SS(CH3),+ of the primary cation CH3SCH3+. This primary cation has been observed in pulse radiolysis of dimethyl sulfide solutions in water.I3 The spectrum of optical absorption presents a maximum at 285 nm. It can be observed (1) Matheson, M. Adu. Phys. Chem. 1975, 7,533. (2) Chaudhri, S. A,; Asmus, K. D. J. Phys. Chem. 1972, 76,216. (3) Gavlas, J. F.; You, F. Y.; Dorfman, L. J . Phys. Chem. 1974, 78,2631. (4) Zhigounov, V. A.; Khaikin, G. I. High Energy Chem. (Engl. Transl.) 1978, 12, 21. (5) Marignier, J. L.; Belloni, J.; Delaire, J. Chem. Phys. Le??.1978, 59, 237. (6) Marignier, J. L.; Belloni, J. Chem. Phys. Le??. 1980, 73, 461. (7) Marignier, J. L.; Belloni, J. J. Phys. Chem. 1981, 85, 3100. (8) Kenney Wallace, G. A. Can. J . Chem. 1977, 55, 2009. (9) Kobayashi, H.; Ueda, T.; Kobayashi, T.; Washio, M.; Tabata, Y. Radial. Phys. Chem. 1983, 21, 13. (10) Kobayashi, H.; Ueda, T.; Kobayashi, T. Tagawa, S.; Tabata, Y. Nude. Instrum. Methods 1981, 179, 223. (1 1) Bonifacic, M.; Moeckel, H.; Banhemann, D.; Asmus, K. D. J . Chem. Soc., Perkins Trans. 2, 1975, 615. (12) Asmus, K. D. Acc. Chem. Res. 1979, 12, 437. (13) Chaudhri, S. A.; Gobl, M.; Freyholdt, T.; Asmus, K. D. J. Am. Chem. SOC.1984, 106,5988.
0 1986 American Chemical Society
Picosecond-Pulse Radiolysis of Dimethyl Sulfide
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4015
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Figure 2. Decomposition of the transient spectra of Figure 1 in the range of 0.125-1 ns into two bands: anion CH,SSCH< (A, = 420 nm); cation (CH3)2SS(CH3)2+(Am, = 480 nm) (see text). Insert: time dependence of the absorbance at 480 and 420 nm (dotted line).
band is observed (Figure l), which also decays quickly. A long-lived UV component was also present in the nanosecond study and was assigned to the primary radical CH3SCH2'(Arnx N 280 nmI6). However, the fast decay of the component in the picosecond range characterizes more probably the geminate recombinations of a charged species. Actually, certain disulfide cations could also present a second absorption band in the UV." The similar decay of the UV and the 480-nm bands in Figure 1 supports the assignment to the same ion or to ions in equilibrium (eq 2). The insert of Figure 2 presents the decay of the (>SSSSSSSSSS
