J. Phys. Chem. 1981, 85, 3100-3105
a100
contribution of canonical structure 2 to the resonance hybrid, the more planar the radical is likely to be. This means that (CF3)2N0.is less likely to be planar than is an acyclic dialkyl nitroxide such as (CH3)2N0.. We therefore conclude that acyclic dialkyl nitroxides should be considered to be planar radicals unless and until strong evidence to the contrary is forthcoming.m I t is unfortunate that we were not able to obtain torsional. barriers for the compounds studied, but some conclusions may be drawn from our results. Firstly, it is inappropriate to analyze the torsional vibrations of compounds with two CF3 rotors as one-dimensional systems (50) For nitroxides we suggest that temperature-dependentEPR hyperfiie splittings should not be considered as “strong evidence” regarding their configuration. See: Introduction and Chatgilialoglu, C.; Ingold, K. U.; Malatesta, V. “NATO Advanced Study Institute: Fast Reactions in Energetic Systems”;Capelloe, C., Walker, R. F., as. Reidel: ; Dordrecht, in preas.
because the kinetic coupling between the two rotors is relatively large. Secondly, the barrier calculations for hexafluoropropanone reveal that, even though the observed f r e q ~ e n c i e were s ~ ~ well separated, the spacing can be accounted for by kinetic coupling alone. This indicates that top-top potential coupling is small in this and presumably in similar compounds. Thirdly, the predicted maximum for the barrier in (CF3)2NO-indicates that the torsional fundamental frequencies for this compound are probably below 40 cm-’, which may account for our inability to observe these modes. In (CF3I2NOHthe rotational barrier height is either ca. 7 or 14 kJ mol-’. For comparison, the CF3 barriers in related molecules are as follows: (CF3)2C=O, 13.5 kJ mol-’ (vide supra); CF3COCH3t34 kJ mol-’; and CF3CH2X40(X = F, C1, Br, I), -17 kJ mol-l. Acknowledgment. We thank Dr. W. F. Murphy for his assistance in recording the Raman spectra and for his helpful discussions of the results obtained.
Competltion Processes between Electron Attachment and Solvation in Dimethyl Sulfide Jean-Louis Marlgnier and Jacqueline Bellonl” Laboratoire de Physlco-Chimk des Rayonnements assocl6 au CNRS, Universl Paris-Sud, 0 1405 Orsay, France (Received: April 8, 198 1)
The main characteristic of transient absorption spectra of liquid dimethyl sulfide (DMS)consists of an intense = 420 nm which develops within the electron pulse (3 ns). A similar band and broad band centered at ,A, is also observed when electrons are produced in DMS by pulsed laser photodetachment from ethoxide anions. The 420-nm species is identified with CH3SSCHgradical anions. To judge by the results obtained under different conditions, the formation of this ion is related to the primary electrons as the only precursors which first rapidly attach onto sulfide molecules. The adduct then undergoes an ion-solvent reaction. The simultaneous existence of anionic species and solvated electrons without any filiation is discussed in terms of competition between attachment and solvation of primary electrons. The respective yields of both processes are evaluated. The dissociative character of negative adduct states is examined in relation to gas-phase data. The interaction of an electron of thermal energy with a liquid may be restricted to polarization of surrounding molecules and localization or solvation of the electron. In the case of molecules known to exhibit high electron affinity in the gas phase, electron attachment is more probable. A previous study’ of liquid hydrogen sulfide by pulse radiolysis has indicated the absence of electron solvation in this medium, related to an irreversible fast reaction of the negative adduct H2S-. The radical anion HSSH- was the product observed at the end of the pulse. However, as to the mechanism of the very fast reaction, it is not clear that the H2S- state is dissociative and that H atoms are involved. The study of a disubstituted sulfide such as the thioether dimethyl sulfide (DMS) should enable one to decide whether the fast process of transient -S-S-- formation is characteristic of the sulfides or peculiar to H2S and whether the solvated electron is an intermediate of the reaction. In any case, our observations2 on the infrared spectrum of DMS at the end of the pulse led us to conclude that at least a part of the electrons are solvated. The lifetime of these solvated species is not longer than -60 ns at low temperature.
Experimental Section The radiation sources used were a pulsed electron accelerator (Febetron 706), a 6oCosource, and a pulsed YAG laser (Quantel). Pulse radiolysis facilities and irradiation cells have been described b e f ~ r e . ~The ? ~ pulse duration is 3 ns at half-height, and the dose is around 2 X 10l8eV cm-3 pulse.-l The photon source is a Nd YAG laser delivering flashes of 3-11s duration at half-height, the fundamental wavelength being 1064 nm and the harmonics 532,353, and 266 nm. The energy of the beam amplified twice is respectively 150,80,12, or 4 mJ pulse-l. The cross section of the laser beam is 3 mm in diameter.5 The fast optical detection system was similar to that used in pulse r a d i o l y ~ i s .The ~ ~ ~fast signal is now transferred to a transient digitizer Tektronix R7912 and to a PDP8 computer in order to permit the signal processing to be made in linea2s5 The dimethyl sulfide was obtained from Merck. Its ultimate purification over sodium has been described.2 Solutions of biphenyl (up to 3 X mol L-’1, sulfur (up to 2 X mol L-’), or dimethyl disulfide (up to 3 X mol L-’) were obtained by addition of weighed fractions (3) J. A. Delaire, These 36 cycle, Orsay, France, 1973.
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(1) J. L.Marignier, J. Belloni, and J. A. Delaire, Chem. Phys. Lett., 59, 237 (1978). (2) J. L. Marignier and J. Belloni, Chem. Phys. Lett., 73,461 (1980). 0022-3654/81/2085-3100$01.25/0
(4) J. Belloni, F. Billiau, P. Cordier, J. A. Delaire, and M. 0. Delcourt,
J.Phys. Chem., 82, 632 (1978). (5) J. L. Marignier, These 38 cycle, Orsay, France, 1979. 0 1981 American Chemical Society
Electron Attachment and Solvation in DMS
Flgure 1. Transient spectra in irradiated dimethyl sulfie at 20 "C: (0) end of pulse; (X) 30 ns; (4) 70 ns; (A) 250 ns; (0)500 ns; (A) 1 pus. Dashed curve (A > 1000 nm): calculated from the 30-ns spectrum after normalization with respect to the 600-1000-nm absorbance at 3 ns.
Flgure 2. Transient spectra in irradiated DMS at -70 OC: (0) end of pulse; (+) 30 ns; (X) 50 ns; (A) 250 ns; (0)500 ns; (A) 2.5 ps. Dashed curve: as in Figure 1.
of solute or by dilution and then degassed. Potassium ethoxide was prepared in situ in a side arm of the cell before dissolution in purified DMS. In this solvent the optical spectrum of ethoxide ions consists of a UV band rising below 360 nm (t N 5 X lo2L mol-l cm-l at 300 nm). Cooling of the cell was achieved by a stream of cold nitrogen gas.
Results Pulse Radiolysis. Pure DMS. Fitst of all, no light emission has been detected, except the Cerenkov emission, during or after a pulse, in the range from 280 nm (absorption edge of the solvent) to 1440 nm (upper detection limit of our photodetectors). The evolution with time of the transient optical absorption spectrum in irradiated DMS is given in Figure 1 (2' = 20 "C)and Figure 2 (2' = -70 "C), The infrared part (6) of these spectra beyond 700 nm has been described previously.2 The temperature shift of C and its decay correlated with the increase of the Ph2- absorbance in the presence of Ph2 support its assignment to solvated electrons. In the visible, a very broad and intense band (A) is centered at 420 nm. A t the maximum the product of the yield and the extinction coefficient is GE = 1.5 X lo4 molecules (100 eV)-' L mol-l cm-l. The decay of A lasts for more than 1ms but does not obey simple kinetics. At
The Journal of Physical Chemistty, Vol. 85,No. 21, 198 I 3101
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Figure 3. Evolution with time of optical density (OD) at different wavelengths in irradiated pure DMS at -70 OC.
low temperature (Figure 2) the visible band at the end of the pulse is even broader than at 20 "C since a shoulder (D) is present around 500 nm. It decays rapidly within 50 ns (Figure 3) and therefore could not be observed at room temperature. At low temperature the evolution with time of the 420-nm peak (A) is complex (Figures 2 and 3): a short increase of the absorbance for -70 ns precedes a slow decay observable for 12 ps. In the ultraviolet range, a band (B) extends up to the solvent cutoff (280 nm) (Figure 1). It decays as fast as band A during the first microsecond and then much more slowly so that it is observable for 40 ps, the upper time limit of the detection setup used. Actually the spectrum of stable products as obtained after y radiolysis of DMS at 20 "C with a dose from 8 X 1Ol8 to 60 X 10l8 eV mL-' consists of a single UV band starting a t 350 nm and increasing to the solvent absorption edge. The optical density depends linearly on dose, and the Ge product is lo3 at 300 nm. By analogy with the end products of y radiolysis of liquid hydrogen sulfide,e sulfur or dimethyl disulfide could be responsible for the stable UV band. Calibration of optical properties of these solutes in DMS has been carried out, and, as above, the absorbance increases continuously from 350 to 290 nm. The respective extinction coefficients at 300 nm are lo3L mol-' cm-l for sulfur and only 50 L mol-' for dimethyl disulfide solutions. (This latter value is in agreement with that of solutions in cyclohexane' or waterq8) Therefore, the UV long-lived band is more likely to be due to sulfur. Apart from these radiolysis products of DMS, hydrogen and ethane have been detected by mass spectrometry. Biphenyl Solutions. Again no emission other than Cerenkov light is observed in these solutions. In contrast with the results obtained in liquid hydrogen sulfide, the addition of biphenyl solute to dimethyl sulfide has a marked effect. The transient absorption spectrum and its evolution with time are shown in Figure 4 for a solution of Ph2 of 3.1 X mol L-' at 20 "C. In the UV a new band at 360 nm, increasing within 50 ns, lasts for microseconds (Figure 4). The infrared band C is no longer observable under these conditions. As already published: its decay observed at low concentration is pseudo first order, and the value of the reaction rate depends linearly
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(6)G. S.McNaughton, Tram. Faraday SOC.,62,1812(1966). (7) F. Feher and M. Munzner, Chem. Ber., 96,1131 (1963). (8)W.Karmann,A. Granzow, G. Meissner, and A. Henglein, Radiat. Phys. Chem., 1, 395 (1969).
3102 2.1 I
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J o o L o o 5 0 0 6 0 0 X (nm)
h (nm) Flgure 4. Transient spectra in 3.1 X mol L-' biphenyl solutions in DMS ( T = 20 "C). Insert: difference spectrum between end-ofpulse spectra in Php solutions and in pure DMS. Dashed curve: biphenyl anion spectrum in THF' normalied with respect to the intensity at A, = 640 nm.
on the biphenyl concentration. In the visible range the spectrum now consists of two intense bands at 640 nm and near 410 nm (Figure 4). Measurements at the maximum of the latter, too intense, were avoided. Both bands correspond to the characteristic spectrum of the biphenyl anion: and their growth is correlated with the decay of C when observed at low scavenger concentration. The difference spectrum between the end-of-pulse spectra, with and without scavenger, is given in the insert of Figure 4. Its similarity with the Phz- spectrum (except in the UV) supports both the previous attribution of C to a solvated electron and also the lack of influence of the scavenger on species A evidenced in the pure solvent. The additional shoulder near 360 nm corresponds to the long-lived band which is still important 500 ns after the pulse (Figure 4). It can be assigned to the radical PhzH,l0with the stipulation that the yield of precursor H atoms should be low, or more likely to the triplet state of biphenyl '"Phz*.'' The yield of electron scavenging at a given concentration is derived from the corresponding absorbance at 640 nm at the end of the correlated electron decay in the infrared. The part due to the tail of the visible band A, as given at the same time in the pure solvent, is subtracted. From the extinction coefficient €ph -(640 nm) = 1.25 x lo4 L mol-' cm-' known in tetrahydrofurangand the absorbance of Ph, measured, the scavenging yield is obtained. At 15 ns, which is the reaction time of e; with Phz at mol L-' (k -+ph2 = 3.9 X 1O1O L mol-' s-l at -70 "C6),the Phz- yield an% therefore G,,-(15 ns) equals 0.25. The electron yield a t the end of the pulse, derived from the corresponding absorbances at 15 and 3 ns in the pure solvent, is then G,,(3 ns) = 0.45 at -70 "C. From the results on Phzformation at 20 and -70 "C, as obtained with identical solutions, it seems that the electron yield depends but slightly on the temperature. This point is further supported by scavenging within 1ns in 3.1 X 10" mol L-' Phz = 1.3 X lo1' L mol-l s-' (ref 5)). G,- = a t 20 "C 0.6 is found which is close to the end-of-pulse yield at low (9)J. Jagur-Grodzinski, M.Feld, S. Yang, and M. Swarc, J. Phys. Chem., 69, 628 (1965);P. Chang, R. V. Slates, and M . Swarc, J. Phys. Chem., 70, 3180 (1966). (10)A. Habersbergerova, I. Janovsky, and P. Kourim, Radiat. Res. Rev., 4, 123 (1972). (11) J. K. Thomas, K. Johnson, T. Klippert, and R. Lowers, J. Chem. Phys., 48,1608 (1968).
Flgue 5. Transient spectrum at the end of a flash of the 2 6 h m beam in ethoxide solutions in DMS ( T = 20 "C).
temperature. Values of the product (G&- in Figures 1and 2 now allow the extinction coefficient of e,- to be calculated: ~,(1440 nm) = 1.2 X lo4 and 2.7 X lo4 L mol-' cm-l, respectively, at 20 and -70 "C. Flash Photolysis. The assignment of band A, formed within the pulse, to the species CH3SSCH3-(see Discussion) raises, as for HSSH- (ref l),the difficult question of a very fast formation through a mechanism involving at least two DMS molecules. A possible process would be a successive reaction with solvent molecules of the primary electrons as sole precursors. In order to check this hypothesis, flash photolysis has been used to produce electrons in another way and under conditions distinct from those of pulse radiolysis: electrons are photodetached from ethoxide anions CH3CH20-excited by the harmonic A,, = 266 nm of the YAG laser. In the pure solvent excited by the 266-nm beam, no absorption is detected, but an emission spectrum (maximum at 305 nm and shoulder at 360 nm) is observed at the end of the flash; this does not last for more than 6 ns. No light is emitted in ethoxide solutions which absorb UV light. The transient absorption spectrum at the end of the flash (3 ns) is given in Figure 5. It consists of a single broad band centered at 420 nm which decays with 80-ns half-life at 20 "C. At -50 "C the half-life of this band is 750 ns. The shape of the band is very similar to that observed after electron pulses (A). After the flash a new band at 290 nm grows for 250 ns at 20 OC (500 ns at -50 "C) and then is stable for at least 5 ps. No absorption is observable in the range 700-1000 nm at any temperature, in contrast with pulse radiolysis experiments. However, due to the low intensity of the UV excitation beam, the absorbance of the 420-nm band is only OD, = 0.10, and the expected value in the infrared, from comparison with the spectra of Figures 1and 2, would be much lower than the detection threshold.
Discussion An identification of the different bands of the transient spectra can be tentatively proposed according to known data concerning optical properties of ionic or neutral fragments and parent species of dimethyl s ~ l f i d e . ' ~ J ~ - ' ~ The assignment of infrared band C to solvated electrons has already been discussed.2 (12)C. v. Sonntag, "The Chemistry of Functional Groups", Supplement E, Part 2, s. Patai, Ed., Wiley, Chichester, England, 1980, pp 923-34,971-93. (13)S. W. Benson, Chem. Reu., 78, 23 (1978). (14)M.H.Klapper and M. Faraggi, Q. Reo. Biophys., 12,465(1979).
Electron Attachment and Solvation in DMS
Comparison of transient spectra produced by photoexcitation of ethoxide solutions or by electron irradiation of the solvent DMS does confirm the presence of a common intense and broad band at 420 nm. The primary species in the first case are excited ethoxide anions which dissociate into electrons and ethoxy radicals. In the second case, electrons, CH3SCH3+cations, and excited DMS molecules are produced. Radicals CH3SCH2.could also be formed in both cases as secondary species either from reactions of ethoxy radicals with the solvent or from the dissociation of excited DMS molecules. However, they absorb in the UV range,15 (Arnm = 280 nm) and they cannot be responsible for the 420-nm band. Therefore, the only common species for both cases are the primary electrons which must lead to the transient A, provided the process is over within a few nanoseconds. The fastest reaction of electrons, if not solvated, is certainly an attachment onto the sulfide molecules. The high electron affinity of DMS could indeed stabilize the adduct CH3SCH3- as long as species A is alive, as was observed for C6F6-in CgFB16where the charge is stabilized by a rapid transfer between the molecules. However, A coexists with the anion Ph2- without reaction, excluding the above hypothesis which implies fast electron transfer to the molecule of higher affinity (to produce supplementary Ph, or DMS- at 420 nm). Therefore, A cannot be assigned as the adduct DMS- or as the anion resulting from a dissociative attachment, i.e., CH3S-, which is known to absorb in the UV." On account of the position of the maximum, the shape, and the intensity of the band, the transient A could well be identified with the radical anion CH3SSCH
