J . Phys. Chem. 1987, 91, 3668-3672
3668
Pulse Radiolysis of Alkane/CCI, Glasses and Liquids Norman V. KIassen* and Carl K. Ross Division of Physics, National Research Council of Canada, Ottawa, Ontario K I A OR6 Canada (Received: December 9, 1986)
The pulse radiolysis of 3-methylpentane (3MP) glasses and liquids, containing CC14 and various electron and positive ion scavengers, was studied at 75, 77, 85, 95, and 123 K. The results are consistent with a mechanism in which CCI, reacts with excess electrons to form CC4- (Lx = 370 nm) which dissociates into CCI, and C1-, followed by positive charge transfer = 470 nm). At 95 K, the decay of CCI4- and the growth of CCI3+are complete in from a cation to produce CC13+(A, less than 1 ms. A value of 1.1 X lo4 M-' cm-I was determined for cmax(CC14-) in 3MP/CC14glasses by measuring the reduction in the yield of trapped electrons which accompanied CC14- production. The assignment of the 370-nm band to CC14-, rather than CC14+as commonly accepted, removes some perplexing problems in understanding the radiolysis of cold alkane/CCI, mixtures. A suggestion in the literature that the free radical CCI, has an absorption band at 365 nm is considered to be improbable.
Introduction
report the effect of several electron and positive ion scavengers
As a model haloalkane, the radiolysis of CCI, has been widely studied. The considerable controversy which surrounds the radiolysis of CC14 and alkane/CC14 solutions revolves around the assignments of the absorption bands of the transient It has generally been assumed that CC14 reacts with excess electrons by dissociative attachment to form CCl, and C1-. However, recent publicationsM have assigned the electron adduct, CCl,-, to the intense band at 370 nm found by the pulse radiolysis of 3-methylpentane (3MP) and methylcyclohexane (MCH) glasses containing CCl,. Convincing proof for this assignment was provided by the competition between N 2 0 and CC14 for excess electrons in 3MP/CC14 glasses at 75 K.3,4 The 370-nm band in pulse radiolysis is the same band as reported at 366' and 367 nm8 for @Coradiolysis of alkane/CCl, glasses, I t seems that the most likely reaction sequence in alkane/CCl, glasses is that CCl, scavenges excess electrons to form CCl; which then dissociates into CCI, and C1-. The CCll radicals react by charge exchange = 470 nm).9 In detail with cations to form CCI,+ (A,,
on the yield and decay of CC1,- in 3MP/CC14 glasses and cold liquids. The contribution of these results to our understanding
--+ + -
3MP
3MP'
CC14
CCl,+
CC14+ CCI,
3MP e-
CC143MP' (or other cation)
+ e+ e-
CC14
+ 3MP'
CCI,- (370 nm)
+ CCI,
+ C1-
-
CCl,
3MP
+ CC1,'
(470 nm)
In view of the controversy concerning the transient species in irradiated CCI,, it is worthwhile to further strengthen our assignment of the 370-nm band to CCl,. To this end we have made a further study of CC14- in glasses and cold liquids. The production of CC14-and the accompanying reduction in the yield of trapped electrons has now been measured in 3MP/ CC14 glasses at 75 K. This leads to a value of 1.1 X 1O4 M-I cm-' for t,,(CCl;). In 3MP/CC14 solutions at 95 K we observed the conversion of CC14- into CCI3+during the first millisecond. At 123 K, 100 ns, the 370-nm band is very small and is surpassed by a broad band in the wavelength region assigned to CCI,+. We ( I ) Brede, 0.; B b , J.; Mehnert, R. Radial. Phys. Chem. 1984, 23, 739. (2) Biihler, R. E. Radiat. Phys. Chem. 1984, 23, 741. (3) Klassen, N. V.; Ross, C. K. Chem. Phys. Lett. 1986, 132, 478. (4) Klassen, N. V.; Ross, C. K. Proceedings of the Sixth Tihany Symposium on Radiation Chemistry, 1986. (5) Suwalski, J. P. Radiat. Phys. Chem. 1981, 17, 393. (6) Brickenstein, E. Kh.; Khairutdinov,R. F. Chem. Phys. Lett. 1985, 115, 176. (7) Claridge, R. F. C.; Iyer, R. M.; Willard, J. E J . Phys. Chem. 1967, 71, 3527. (8) Louwrier, P. W. F.; Hamill, W. H. J . Phys. Chem. 1969, 73, 1702. (9) Gremlich, H.-U.; Buhler, R. E. J . Phys. Chem. 1983, 87, 3267
0022-3654/87/2091-3668$01.50/0
of alkane radiolysis is discussed. The reactions proceeding from electron addition to CCl, in aqueous solution have been widely studied because CC14is a prime example of a hepatotoxic haloalkane.I0 The important intermediate CCl3I1 is conveniently produced by radiolysis by the dissociative attachment of an excess electron to CCl.,. We discuss a recent suggestion5J2that CC1, has an absorption band centered at 360 nm. The present results, in agreement with previous pulse radiolysis ~ t u d i e s , ' ~indicate -'~ that CC1, does not absorb significantly at 360 nm.
Experimental Section Philips Research Grade 3MP, Wiley 3-methyloctane (3MO) (99%), and Anachemia Reagent Grade CCl, were passed through a column of silica gel before use. Wiley cis-2-pentene (99%) was passed through silica gel and alumina and Matheson Coleman Chromatoquality 2-methyltetrahydrofuran (MTHF) through alumina. Matheson cyclopropane was used as received. Linde nitrogen, Airco Superpurified argon and Air Products N,O were dried before being used to bubble solutions. During most of the bubbling period, the solutions were cooled to prevent evaporation. Samples containing N 2 0 were saturated at room temperature before glassing. Solutions were sealed in 5-mm Suprasil cells. Solutions with volatile solutes were glassed by immersing as slowly as practical in liquid nitrogen to avoid creating a sufficient vacuum above the sample to cause the solute to bubble out. Samples were irradiated with single 40-11spulses of 35-MeV electrons from a linear accelerator. Dosimetry was based on 5 X IO-, M KSCN solutions for which G C ~ , ~ ( S C N )atC midpulse was taken to be 2.2 X IO4 M-' cm-I (100 eV)-'. The dose per pulse was about 80 Gy. The density of alkane/CCl, solutions at room temperature and the change in density caused by glassing at 77 K were measured. 3MP/CC14 solutions, from 0.00 to 0.41 M CCI,, were 27% denser at 77 K than at room temperature while pure 3-methyloctane increased in density by 21% and MTHF by 20%. Absorption measurements at X 1 8 0 0 nm were made with a Phillips XP1003 photomultiplier using a 470-0load resistor. The measuring system had a rise time of about 75 ns as determined by measuring Cerenkov emission. Measurements at X L 800 nm were made with a Barnes A-100 InAs photodiode with a 0-98% (10) Cheeseman, K. H.; Albano, E. F.; Tomasi, A.; Slater, T. F. E H P , Enuiron. Health Perspect. 1985, 64, 85. (1 1) Asmus, K.-D.; Bonifacic, M. "Radical Reaction Rates in Liquids"; Lundolt-Bornstein Numerical Data and Functional Relationships in Science and Technology; Springer-Verlag: Berlin, Vol. 13b, 1984. (12) Suwalski, J. P.; Kroh, J. Nukieonika 1979, 24, 253. (13) Lesigne, B.; Gilles, L.; Woods, R. J. Can. J . Chem. 1973, 52, 1135. (14) Packer, J. E.; Slater, T. F.; Willson, R. L. Life Sri. 1978, 23, 2617.
Published 1987 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987
Radiolysis of Alkane/CCI4 Glasses
,
0.8
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X ,nm Figure 1. Spectra measured at 123 K. 0 refers to 3MP/0.22 M CCld at 100 ns and 0 to the same sample at 800 ns. refers to 3MP/N20 at 100 ns and A to the same sample at 800 ns.
rise time of less than 80 ns. In order to reduce bleaching by the analyzing light, kinetic measurements were made with a filter before the cell, thus limiting the light through the sample to the wavelengths of interest. Samples at 75 K were irradiated in a Dewar containing liquid nitrogen, cooled below its boiling point by bubbling with helium. Samples at 77, 85, and 95 K were irradiated in a liquid helium variable temperature cryostat. Some samples at 95 K and those at 123 K were irradiated in a Dewar cooled by a stream of cold nitrogen gas. Other details of the equipment and the techniques have been p ~ b l i s h e d . ' ~ - ' ~
Results and Discussion In the 100-ns spectra of 3MP/CC14 glasses irradiated at 75 and 77 K the only absorption in excess of the blank between 300 and 800 nm is an intense absorption with its maximum at 370 (f2) The maximum was determined for a 3MP/0.22 M CCl, glass at 75 K by using a monochromator which had been calibrated against the lines of a low-pressure mercury lamp.4 The only difference between the spectra at 75 and 77 K, measured 100 ns after the start of a 40-ns pulse, was a 5% smaller Gt370 at 77 K.3v4 Decay of CC14-and Growth of CC13+. Louwrier and Hamil18 established that the 370-nm band is a precursor to the 470-nm band. We shall assume that the 370-nm band is due to CCI4and the 470-nm band is due to CC13+because these assignments seem to best explain the present results. The absorption measured for 3MP/N20 is used as a blank. The N 2 0 served as an electron scavenger in the absence of CCI4. We have two indications that this is an adequate blank: (1) in the 100-ns spectrum of 3MP/CCl, at 75 K,3 the subtraction of the blank left only the 370-nm band and ( 2 ) a t 95 K, Gt370 for 3MP/CCI, decays to that of the blank and no further. The spectrum measured at 123 K for 3MP/0.22 M CCI4 at 100 and 800 ns is shown in Figure 1. Shown as well are the measurements made at 123 K on 3 M P / N 2 0 at 370 and 470 nm at 100 and 800 ns. At 100 ns, a rather broad band centered about 470 nm is produced in 3MPl0.22 M CCI,. It seems likely that more than one species contributes to this band. By 800 ns, the band looks more like the 470-nm band ascribed to CCI3+. The absorption at 370 nm is barely greater than the blank. From spectral measurements on 3 M P / N 2 0 at 100 ns, at 75 and 77 K3s4 we know that the blank rises from 370 to 300 nm in about the same way as does the spectrum for 3MP/CCI4 in Figure 1. The 100-nsspectrum at 95 K for 3MPl0.22 M CCI, is shown in Figure 2 along with Gt370 and Gt470 for 3MP/N,O. Figure 3 shows Gt370and Gt470 for 3MP/CC14 and 3 M P / N 2 0 glasses and liquids at 77, 95, and 123 K. At 77 K, CCI, decays 1 1 1 1 1 . ~ 9 ~
(15) Klassen, N. V.; Teather, G. G. J . Phys. Chem. 1985, 89, 2048. (16) Gillis, H. A,; Klassen, N. V.; Teather, G. G. Can. J . Chem. 1977,55, 2022. (17) Teather, G. G.; Klassen, N. V.; Gillis, H. A. In?. J . Radiar. Phys. Chem. 1976,8, 471.
0
300
400
X , nm Figure 2. The 100-ns spectrum of 3MP/0.22 M CC1, at 95 K. to 3MP/0.22 M CCI,. A refers to 3MP/N,O at 95 K.
0 refers
by about 50% over the first millisecond during which time there is no evidence for the production of CCI3+. By contrast, at 123 K the yield of CCI; at 100 ns is small and the dominant absorption is in the region of 470 nm. At 123 K, Gt470 increases by 24% between 100 ns and 1 ps, followed by decay. During the same time interval, most of the absorption observed at 370 nm is due to the blank and not to CCI4-. Figure 3 shows that 95 K is more suitable than 123 K for observing the conversion of a substantial yield of CCI,- into CCI3+during the first millisecond. At 95 K, Gt470 doubles in the first 200 ps while Gt37O drops to the value of the blank. The situation at 85 K is intermediate between that at 77 and 95 K. At 85 K, 100 ns, Gt370 is 2% less and Gt470 is 15% more than the corresponding values at 77 K. The decay of CCI4- at 77 K (Figure 3) beyond 1 ps is almost linear with the log of time. One might expect the dissociation of CC1,- to be first order. Even at 95 K (Figure 3), the decay at 370 nm (corrected for the blank) is not first order. Perhaps the rate of dissociation of CCI4- into CCI3 and CI- is limited by the difficulty of separating CI- from CC13, due to the high viscosities at these low temperatures. 3MPlCC1, and 3 M P / C C I , / N 2 0 . The identification of the 370-nm band as being due to CCI, was based on a study of the competition for electrons between N 2 0 and CCI, in 3MP g l a ~ s e s . ~ It seemed prudent to follow the decay of CC1,- in the presence and absence of N 2 0 . The kinetics of Gtj70 at 75 K for 0, 0.06, and 0.44 M CCI, in 3MP and in 3 M P / N 2 0 are shown in Figure 4. The concentration of N 2 0 in saturated 3MP at room temperature was calculated to be 0.15 MI8 but seems to be equivalent to 0.09 M CCI4 in its power to scavenge electrons. The decays at 370 nm in 3MP glasses shown in Figure 4 indicate that CCI4 and N 2 0 behave independently at 75 K; Le., the fractional decay of CCI, from 100 ns onwards is the same in the presence and absence of N 2 0 . Hence, decay of CCI, by electron tunnelling to N 2 0 is insignificant. The absorption at 370 nm in 3 M P / N 2 0 is due to one or more radical and/or ionic products. The fact that Gt370 in 3 M P / N 2 0 decays in a similar manner to the CC1,~ , 3MP/CCI4 , suggests that the rates are component of G C ~ in limited by the molecular motion possible in the 3MP glass. The slower decay in 3MP/0.06 M CCI, between 100 ns and 100 ps (Figure 4) may be due to production of CC1,- after 100 ns (see below). 3MP/CC14/Scavenger. Previous studies of alkane/CCI4 glasses8 and liquids9 have examined the effects of electron and positive ion scavengers. W e also studied several scavengers in 3MP/CCI4 glasses and liquids. The presence of 0.6 M cis-2-pentene (expected to participate in charge exchange with alkane cations) in 0.44 and 0.06 M CCI4 (18) Horsman-van den Dool, L. E. W.; Warman, J. M. Interuniversitair Reactor Instituut rapport 134-81-01. Mekelweg 15, Delft, The Netherlands.
3670 The Journal of Physical Chemistry, Vol. 91, No. 13, 1987
10‘~
IO+
10.’
10.‘
IO$
Klassen and Ross
10’~
1 0 . ~IO”
10.’
10’~
IO.‘
time , s
Figure 3. The behavior of and G C , , between ~ 100 ns and 1 ms. 0 refers to G L ~and , ~0 to G~470for 3MP/0.22 M CCI,. A refers to GeJIOand A to Ge470for 3MP/N,O. I n part A the 3MP/CCI, was at 77 K but the 3MP/Nz0 results were measured at 75 K . The measurements in part B were
leu+
all made at 95 K and those in part C at 123 K. I‘
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400
500
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700
11 800
X ,nm Figure 4. The ratio of G
Lto its ~ value ~ ~at 100 ns vs. time at 75 K. 0
refers to 3MP/0.44 M CC&, 0 to 3MP/0.44 M CC1,/NzO, v to 3MP/0.06 M CC,, A to 3MP/0.06 M CCI,/NzO, 0 to 3MP/N20, + to 3MP/0.44 M CCl4/0.6O M cis-2-pentene, and X to MTHF/0.29 M CCla. in 3MP at 75 K did not change GC370 at 100 ns, within k5%. It is known that the addition of 2-methylpentene-1 did not change the earliest spectrum of 60Co-irradiated 3-methylheptane/CC14 glass.8 However, the addition of cyclohexene to methylcyclohexane/CC14 at 213 and 173 K reduced Gt470 by about 20%.9 The 100-ns spectrum of 3MP/CC14/cis-Zpentene is shown in Figure 5 . In a 3MP/0.44 M CC14/0.6 M cis-2-pentene glass, GE,,~grew 26% between 100 ns and 400 ps followed by decay. Presumably this growth represents the formation of the dimer radical cation of cis-2-pentene. The decay of Gtj70 is slower in the presence than in the absence of cis-2-pentene (Figure 4). It seems unlikely that the slower decay at 370 nm reflects a contribution by the 650-nm band. We cannot say whether the decay of CCl, is really different in the presence of cis-Zpentene, perhaps because of a change in the viscosity of the glass, or whether another absorption is intervening. 3MP/0.22 M CC14glasses were pulsed at 95 and 123 K in the presence and absence of cyclopropane (about 3% by weight). At 95 K the presence of cyclopropane reduced Gt37O at 100 ns by about 22% but increased Gt470 by 17% for reasons not understood. The growth at 470 nm over the next millisecond was reduced by about one-third by cyclopropane. At 123 K, the presence of cyclopropane had little effect on either the 100-nsvalue of Gc470 or its growth. This can be compared to a marked reduction in G Ein methylcyclohexane/CC14 ~ ~ ~ at 21 3 and 173 K brought about by adding cyclopropane which is consistent with an H or H2 transfer reaction of cyclopropane with alkane cation^.^ The presence of M T H F in a @Co-irradiated 3MP/CC14 glass increased the absorption at about 350 nm but strongly suppressed
Figure 5. The spectra of 100 ns (0) and 1600 ns ( 0 )for 3MP/0.44 M CCI4/0.6 M cis-2-pentene at 75 K.
it at 470 nm due to proton transfer from the alkane radical cation to MTHF, which traps the positive charge and prevents the The radical cation of M T H F is also formation of cc13+.8+19 believed to undergo proton transfer with MTHF. We pulse irradiated an MTHF/O.2 M C C 4 glass at 75 K. The 100-ns spectrum (Figure 6) is very similar to the spectrum in an alkane glass except that the maximum of the absorption band is 10 nm lower, at 360 nm. The decay of Gc370is shown in Figure 4. The Extinction Coefficient of CC14-. 3MP/argon glasses containing 0,0.021,0.043, and 0.400 M CC14 were pulse irradiated at 75 K. In each case the maximum of the spectrum of the trapped electron, e;, was found to occur at 2000 nm. The values of Gtzooo(e;) and Gt370 a t 100 ns are shown in Figure 7 along with previous determinations of Gt370.3 The extinction coefficient of CC14- was determined by equating the decrease in the yield of e< caused by the presence of CC1, to the accompanying production of CC14-. The yield of e; was measured at 2000 nm. The yield of CC14- was measured at 370 nm with the blank subtracted. The calculations were carried out using values of Gt extrapolated to zero decay (“midpulse”) using the measured kinetics. A value of 1.6 X lo4 M-’ cm-I was used For 0.021 and 0.043 M CCI,, values of 1.13 X lo4 for t,(e;).15 and 1.07 X lo4 M-’ cm-’ , respectively, were calculated for tmax(CC14-).Hence, we adopt a value of 1.1 X lo4 M-’ cm-’ for emax(CC14-1. This determination of tmax(CC14-)assumes that after electron addition to CCll at 75 K dissociation into CC13and C1- only takes place on a time scale longer than the pulse width of 40 ns. This assumption is consistent with several observations. (1) The dis(19)
Gallivan, J. B.; Hamill, W.H.J. Chem. Phys. 1966, 44, 2378
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 3671
Radiolysis of Alkane/CC4 Glasses I1
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Figure 6. The 100-ns spectrum of MTHF/0.29 M CCI, at 75 K. I
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2 1 ' 1 ' 0
0
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0 0
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[CCI,]
t
M
Figure 7. The values of Gt37o and G c ~ ~vs.MC) C 4 concentration for 3MP for 3MP/CCI4. A refers to Gt370 for glasses at 75 K. 0 refers to GtJTO 3MP/N20. 0 refers to GtZm for 3MP/CCI4 (note the data point at 0.4 M CCIp).
sociation of CC14- must be very small between 40 and 100 ns judging from the kinetic data in Figure 4. (2) No CC13+ (at 470 nm) is found at 100 ns which might result from "immediate" production of CC13. (3) It is shown below that the same -e, (CC14-) was determined in a 3-methyloctane/CC14glass using the alkane cation as a basis for the calculation instead of the trapped electron. This means that the same amount of immediate dissociation would have to have occurred in the much more viscous 3-methyloctane glass. In 3MP/0.021 M CCl., at 75 K the decrease in Gem between 100 and 1600 ns indicates a decrease in G(e;) of 0.24. Over the same period of time Ge370increases slightly. The increase in Gt370, if attributed entirely to CC14- production, is equivalent to an increase in G(CC14-) of 0.11, i.e., less than the loss of G(e;). However, some decomposition of CC14- and decay in the blank must also be taking place. If we assume that the percentage dissociation of CC1; for 3MP/0.021 M CCl, is the same as that measured for 3MP/0.40 M CC14,for which C(e;) is virtually zero at 100 ns (see Figure 7), we calculate that the dissociation of CC14in 3MP/0.021 M CCI, amounted to G = 0.16 between 100 and 1600 ns. The result is a calculated production of CC14- equal to G = (0.16 + 0.1 1) = 0.27 as compared to a loss of e; equal to G = 0.24. Thus, within the accuracy of our data, the loss of e; equals the production of CC14- between 100 and 1600 ns. 3-Methyloctane/CC14. In 3-methyloctane (3MO) glasses, the initial cation, 3MO+, has a broad absorption band with a maximum at 625 nm.1592*23 The extinction coefficient emax( 3MO')
IO-'
I
I
lo-6
IO-'
time
I
IO-'
,s
Figure 9. Decays between 100 ns and 1 ms in 3-methyloctane glasses. 0 refers to Gtjso (largely due to CCI,) and 0 to G c (largely ~ ~ due ~ to 3MO+) for 3-methyloctane/0.44 M CCI, at 75 K. refers to Gt62S for 3-methyloctane/N20 at 72 K (taken from ref 21).
+
is 0.88 X lO4.I5 A 3M0/0.44 M CC14 glass was pulsed at 75 K. The 100-ns spectrum (Figure 8) shows two peaks, one at 370 nm due to C C 4 - and another a t 625 nm due to 3MO+. The decays of Gt380and Gt625are shown in Figure 9 as well as the decay of 3MO' in 3 M O / N 2 0 at 72 K.*l The decay kinetics of 3MO+ are identical in both cases. A value of t,,(CCl;) was also calculated from the data in Figures 8 and 9 extrapolated to zero decay. A value of 0.88 X lo4 M-I cm-I was used for tmax(3M0+)15 and it was assumed that G(3MO') was 5% less than G(CC14-) because G(3MO') = 0.95G(e;) a t midpulse in pure 3 M 0 at 72 K.I5 The largest uncertainty in the calculation was due to the need to estimate a blank spectrum below 475 nm for pulse irradiated 3MO. A value of 1.O X 104 M-I cm-' was calculated for t-(CC&-), in agreement with the determination described above. Formation of CC13 and CCl3'. Knowing e,,,(CC14-) and adopting a value of 0.6 X lo4 for tmax(CCI3+) (or the ion pair CCI3+//CI-) as suggested by Gremlich and Buhler: we can compare the yields of these two species. We calculate from the data of Louwrier and Hamill* that G(CC13+)= 0.9 following the ~
~
(20) Klassen, N. V.; Teather, G. G . J . Phys. Chem. 1979, 83, 326. (21) Cygler, J.; Teather, G. G.; Klassen, N. V. J . Phys. Chem. 1983,87, 455.
(22) Teather, G. G.; Klassen, N. V. J . Phys. Chem. 1981, 85, 3044. (23) Gillis, H. A.; Klassen, N V.; Woods, R J. Can J Chem. 1977.55, 2022.
3672
J . Phys. Chem. 1987, 91, 3672-3676
6oCoirradiation of 3MP/0.15 M CC1,. In the present work, the maximum yield of CCI,+ in 3MP/0.22 M CC14 occurred at 95 K, 200 ws, where G(CCI3+) = 1.0. This can be compared to G(CCI4-) = 2.2 at IO0 ns at 75 K for 3MP/0.22 M CCI, (calculated from Figure 7). It seems that about one-half of the CCI4in these glasses and cold liquids is converted into CCI3+. Suwalski and stated that both CCI3 and CC1,contribute to the absorption band at 365 nm in MCH/CCI, pulse irradiated at low temperatures. SuwalskiScalculated that emax(CC13) = 0.4em,,(CCl4-). However, the reactions of CCI, in aqueous solution have been the subject of many studies and there has been no indication of an absorption band in the region of 365 nm.13*14 Accordingly, we shall examine the evidence in alkane/CC14 solutions for such an absorption. The suggestion that CCI, has an absorption band at 365 nm in irradiated methylcyclohexane glasses which contain CCI, was made on the strength of a similar behavior, with added electron and positive ion scavengers, of the ESR signal due to CCI3 and the 365-nm band.I2 If CCI,- is the precursor of CCI3, it is reasonable that their yields will sometimes change in the same way. The contribution of the blank to the absorption in the 365-370-nm region can be very significant at the highest temperatures discussed here. For example, Figure 1 shows that Gemaxat 100 ns for 3MP/CC14 at 123 K is largely accounted for by the blank and probably represents products which have nothing to do with CC1, (it is recognized that the glass transition temperature of 3MP is lower than that of MCH). It is conceivable that a similar situation existed for the MCH/CCI4 solutions at 142 K for which the absorption at 365 nm was attributed to CC13.24The value of 1.1 X 1O4 determined for- , , ,€ (CCI4-) in this study should not be affected by whether or not CCI, absorbs at the same wavelength since the calculations were
+ CH,SH
Registry No. 3MP, 96-14-0; MTHF, 13423-15-9; 3 M 0 , 2216-33-3;
CCI,, 56-23-5; NZO, 10024-97-2; CC19+, 271 30-34-3; CCId-, 59795-99-2; cis-2-pentene, 627-20-3; cyclopropane, 75-1 9-4.
(24) Suwalski, J. P.; Kroh, J. Radiat. Phys. Chem. 1982, 20, 365.
Kinetics of the OH
made using data extrapolated to "zero decay" of CC14-. The Implications for Alkane Radiolysis. The assignment by Louwrier and Hamill of the 367-nm band to CCI4+ posed two dilemmas for our understanding of the radiolysis of alkanes: ( I ) how CCI4+could be formed and (2) if formed, how it avoids charge transfer to the alkane molecules of the matrix which have an ionization potential about 1.5 eV lower than that of CC1,. With the assignment of the 370-nm band to CCI,-, these dilemmas vanish. A few words are in order about the reactions of the alkane cations. We know from pulse radiolysis studies that the initial cations in 3 M 0 and squalane glasses at 75 K have largely disas illustrated by the decay of 3MO+ in appeared by 1 ms,15921322 Figure 9, and that much of this decay is due to the formation of a second, different, cation as opposed to recombination of the initial cation with an e l e c t r ~ n . 'Yet, ~ the present results show that the formation of CC13+at 75 K takes place after 1 ms. Therefore, we can assume that charge transfer takes place between the second cation and CCI,. If CI- is intimately involved with CCI3+ in the 470-nm band, as suggested by Gremlich and B ~ h l e rthis , ~ adds another step to the reaction, but if C1- does not wander far from CCI, after formation it should be readily available for pairing with CCI3+. As long as the initial alkane cation exists, it is probably fairly mobile, but after the positive charge becomes associated with a second cation the mobility must be drastically reduced. We conclude that in alkane glasses charge exchange with CCI3 occurs from the second, less mobile, cation and must depend on its proximity for the efficiency of the reaction. We also surmise that the second cation has an ionization potential greater than the ionization potential of CCI, (8.8 eV).9
Reaction under Atmospheric Conditions
A. J. Hynes and P. H. Wine* Molecular Sciences Branch, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received: December 9, 1986)
The pulsed laser photolysis-pulsed laser-induced fluorescence technique has been employed to study the kinetics of OH reactions with CH,SH ( k , ) and CD3SH ( k 2 )in N2,air, and O2buffer gases. We find that k , and k2 are independent of O2concentration. Measured k , values are in excellent agreement with previous flash photolysis-resonance fluorescence studies, all of which employed reaction mixtures containing no 02.However, the observation of no O2 dependence is in marked disagreement with a number of relative rate studies where NO,-containing species were employed as photolytic precursors for OH and olefins were used as the reference compound. k2 is found to be 13% slower than k l , suggesting the occurrence of a minor methyl hydrogen abstraction channel. In the course of investigating complicationswhich result from photolysis of the mercaptan reactants, we found that 266-nm photolysis of either CH3SH or CD3SH in the presence of O2 (but in the absence of other OH photolytic precursors) results in production of OH.
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Introduction A number of kinetic and mechanistic studies of the reaction of hydroxyl radicals with methyl mercaptan are reported in the literature.'-1° OH
+ CH3SH
-
products
(1)
These studies have been motivated by the need to characterize the atmospheric oxidation of CH,SH, a process which is initiated *Author to whom correspondence should be addressed.
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by reaction 1 and also by reaction of methyl mercaptan with NO3 radicals. All direct measurements of k l reported to date were (1) Atkinson, R.; Perry, R. A.; F'itts, Jr., N. J. Chem. Phys. 1977,66, 1578. (2) Wine, P. H.; Kreutter, N. M.; Gump, C. A,; Ravishankara, A. R.J . Phys. Chem. 1981.85, 2660. (3) Lee, J. H.; Tang, I. N. J . Chem. Phys. 1983, 78, 6646. (4) (a) MacLeod, H.; Poulet, G.; LeBras, G . J. Chim. Phys. 1983,80,287. (b) Jourdain, J. L.; MacLeod, H.; Poulet, G.; LeBras, G. Physico-Chemical Behauior of Atmospheric Pollutants, Proceedings of the 3rd European Symposium, Varese, Italy; Reidel: New York,1984; pp 143-148. (c) MacLeod, H.; Jourdain, J. L.; Poulet, G.; LeBras, G. Atmos. Enuiron. 1984, 18, 2621.
0 1987 American Chemical Society
