2048
J . Phys. Chem. 1985, 89, 2048-2053
e ~ p e r i m e n t s . ' ~The ' ~ need for the postulate of a major 1,t-halogen migration with CH,=CHBr and little or none in our present experiments with CH2=CHFis also well satisfied by its attribution to Br whose migration should be more facile than the C1 atom migration of reaction 11.
Acknowledgment. This research has been supported by Department of Energy Contract DE-AT03-76ER-70126. Registry NO,CHzzCHF, 75-02-5; j8C1, 14158-34-0; CH$H38CIF, 95590-23-1;
C H Z ~ ~ C I C H95590-24-2. ~F,
Cations and Electrons In Hydrocarbon Glasses and Llqulds Studled by Pulse Radlolysis Norman V. KIassen* and George G. Teather Division of Physics, National Research Council of Canada, Ottawa, Canada K1 A OR6 (Received: November 26, 1984)
We report the first quantitative comparisonsof the yields and decays of solvated electrons and initial cations in a pulse-irradiated hydrocarbon, in this case for 3-methyloctane glass and liquid from 6 to 293 K. The comparisons were made possible by the determination of the extinction Coefficients of the initial cation (3MO') and the solvated electron (e;) on the pulse radiolysis time scale at several temperatures. Among the extinction coefficients reported are tmx(3MO') = 0.88 X lo4 and tmax(e;) = 1.6 X lo4 at 20 ns, 72 K. At 20 ns, G(3MO') = 0.95G(e;) in the glasses but above the glass transition temperature the ratio G(3MO')/G(e;) decreases with increasing temperature. The results are discussed in the context of the possible reactions of initial alkane cations.
Introduction An understanding of the radiolysis of alkanes has long been one of the basic objectives of radiation chemistry. Despite decades of work on the radiolysis of liquid and solid alkanes,' it has, thus far, been impossible to compare quantitatively the yields and decays of the solvated electrons (e,) and the initial cations (RH'.) in irradiated, neat alkanes. For pulse radiolysis using optical absorption, this inability stemmed from a lack of accurate extinction coefficients for e; and RH+-, in those cases where RH'. has a suitable absorption band. The only determination of t,(e;) in alkanes was made by Gallivan and Hamil12 for the relaxed spectrum of the trapped electron in y-irradiated 3-methylpentane glass at 77 K. However, the unrelaxed spectrum of e;, as observed by pulse radiolysis, is much broader and probably has a lower extinction coefficients3 We reported4 a determination of emax(RH'.) for 3-methyloctane (3MO) and squalane (SQ) based on the extinction coefficient of the pyrene radical cation (Py'). However, our values for tmax(RH+.)were uncertain due to the very large range of literature values of c,(Py+). Previous publications from this l a b o r a t ~ r y ~described -~ pulse radiolysis studies of the electrons and the initial cations in alkane glasses and cold liquids. In neat alkane glasses, the cation decays much faster than the electron. The uncertainty in the extinction coefficients of the cations and of the trapped electron meant that the yields and decays of the initial cation and the electron could not be compared quantitatively. For this reason, we have re(1) G. Foldiak, Ed., 'Radiation Chemistry of Hydrocarbons", Elsevier, New York, 1981, Studies in Physical and Theoretical Chemistry, No. 14. (2) J. B. Gallivan and W. H. Hamill, J. Chem. Phys., 44, 1279 (1966). (3) G. G. Teather and N. V. Klassen, J. Phys. Chem., 85, 3044 (1981). (4) J. Cygler, G. G. Teather, and N. V. Klassen, J. Phys. Chem., 87,455 (1983). (5) N. V. Klassen and G . G. Teather, J. Phys. Chem., 83, 326 (1979). (6) H. A. Gillis, N. V. Klassen, and R. J. Woods, Con. J . Chem., 55,2022 (1977). (7) H. A. Gillis, N. V. Klassen, and G. G. Teather, 'Proceedings of the Fifth International Congress of Radiation Research, Seattle, 1974", 0. F.
Nygaard, H. I. Adler, and W. K. Sinclair, Ed., Academic Press, New York, 1974, p 443. (8) N. V. Klassen, H. A. Gillis, and G. G. Teather, J . Phys. Chem., 76, 3847 (1972).
determined the extinction coefficients of 3MO' and SQ' as well as t,,(e;) in a pulse-irradiated alkane glass. These values, while obtained independently, proved to be consistent with one another, as well as with other published results. They are used to compare the yields and decays of 3MO' and e; over the temperature range 6-293 K. A significant change in the subnanosecond reactions seems to take place on going from the glass to the cold liquid.
Experimental Section The 3-methylhexane (3MHx, 99%), 3-methylpentane (3MP, 99.9%), and 3 M 0 (99%), all obtained from Chemsampco, were passed through activated silica gel and alumina. Following this treatment, the 3 M 0 was stored over NaK under vacuum. Squalane (2,6,10,15,19,23-hexamethylcosane),specially purified by Chemsampco: was passed through silica gel before use. Pyrene (Py, 99+%), obtained from Aldrich, was recrystallized from ethanol and passed through silica gel as a solution in n-hexane. N,N,N',N'-Tetramethyl-p-phenylenediamine (TMPD, 98%), obtained from Aldrich, was sublimed under vacuum. Gold Label triethylamine (TEA, 99+%), from Aldrich, was used as received, from a fresh bottle. The argon used for deaeration was Airco, Superpurified. The N 2 0 was Linde and was passed through a trap at dry ice temperature before use. Samples were prepared and sealed off in Suprasil quartz cells, usually with a 5-mm optical path. All samples were deaerated. Deaeration was carried out by bubbling with argon or N 2 0 or, in the case of 3 M 0 , the sample was distilled into the cell under vacuum. Samples containing N,O were always saturated with 1 atm of N,O at room temperature either by bubbling or on the vacuum line. The experimental procedures and equipment have been described previou~ly.~-~ Irradiations were performed using single 40-11s pulses of 35-MeV electrons. Dosimetry was based on aqueous, 02-saturated, 5 mM KSCN, using Gt475= 2.2 X lo4 at midpulse. Variations in dose from pulse to pulse were monitored with a secondary emission monitor. The samples were irradiated in a quartz dewar containing liquid nitrogen for irradiations at 72 K or cold flowing nitrogen gas for irradiations at 153 K. Optical measurement were made with several detectors: at 450 5 X I700 nm, a Phillips XP-100photomultiplier; at 450 IX 5 1000 nm, an EG&G SHS-100 silicon photodiode, at 1000 I
0022-3654/85/2089-2048$01.50/0 Published
1985
American Chemical Society
Cations and Electrons in Hydrocarbon Glasses and Liquids
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 2049
X 2 1400 nm, a Philco-Ford L-4521 Ge photodiode; and at X L
(9) W. I. Aalbersberg, G. J. Hoijtink, E. L. Mackor, and W. P. Weijland, J . Chem. Soc., 3049 (1959). (10) H. Schomburg, H. Staerk, and A. Weller, Chem. Phys. Lett., 22, 1 (1973). (1 1) G. E. Hall and G. A. Kenney-Wallace,Chem. Phys., 32, 313 (1978). (12) E. Zador, J. M. Warman, and A. Hummel, Chem. Phys. Lett., 23, 363 (1973). (13) A. C.Albrecht and W. T. Simpson, J . Am. Chem. Soc., 77, 4454, (1955). (14) H. D. Burrows, D. Greatorex, and T. J. Kemp, J . Phys. Chem., 76, 20 (1972). (1 5) W. C. Meyer and A. C. Albrecht, J . Phys. Chem., 66, 1 168 (1 962).
1
I
1
1
-
1.0
3MO'
Results and Discussion Extinction Coefficients of Cations. Published extinction coefficients of the initial radical cations of alkanes, RH+., are scarce. The designation of the bands attributed to initial cation bands has been based on the immediate appearance of these bands in pulse radiolysis experiments and on their removal by cation scavengers such as TEA. Partial scavenging of RH+. can be used to determine the extinction coefficients of RH+.if the positive charge transfer to the scavenger results in a product with a measureable absorption and of known extinction coefficient. We = 1.6 both by previously determined that t,(SQ+)/tm,(3MO+) partial scavenging of the initial cations with pyrene (Py) and by an analysis of the charge-transfer reaction between 3MO+ and SQ! The value of em,(3M0+)/tm,(Py+) was found to be 0.19: However, it was not possible to calculate reliable extinction coefficients for 3MO+ and SQ' because of the large discrepancy which exists in the literature as to the correct value for e,(Py+). As determined by chemical methods, tmax(Py+)= 4.4 X 104.9J0 However, laser photoionization" of Py led to a value of 2.05 X lo4 and pulse radiolysisI2 led to a value of 2.4 X lo4. The absorption bands of both Py+ and Py- are very narrow and require a band-pass of 2 nm or less to be measured properly. Measurements of Py+ made with an insufficiently narrow band-pass or slightly off the absorption maximum would result in a low value of the extinction coefficient. We have pursued this matter further since reliable values of the extinction coefficients of the initial cations are needed if we wish to use pulse radiolysis to help understand the role of the initial cations in the radiolysis of alkanes. We decided to use TMPD as a positive ion scavenger to determine c,,,(SQ+) because the absorption bands of TMPD+ are broad (compared to that of Py+) and because an acceptable way to determine their extinction coefficients seems to exist. TMPD+ has two maxima of about the same extinction coefficient in the vicinity of 575 and 625 nm and a minimum at about 600 nm. The exact shape of the TMPD+ spectrum depends somewhat on the solvent and the temperature. Since TMPD' (Wurster's Blue) can be prepared as a salt, its extinction coefficient in water is readily determined. Although values for e,,,(TMPD+) in water range from 1.0 X lo4 to 1.25 X lo4, the most likely value of the average of the two maxima is 1.25 X 104,13J4and the ratio (tmax)av/emin = 1.24. For the spectrum of TMPD' in an organic glass at 77 K,I3 and using the chosen value of in water at room temperature and assuming that the oscillator strength does not change, we calculate (tmax)av = 1.5 X lo4 and (tmx),,,/tmin= 2.2. By the same method, we find = 1.8 X 104 and (tm),,,/tm = 3.4 in 3-methylpentane that (e-),, at 77 K.I5 A linear relationship exists between these values of (t,), and (t-)av/tmin. We measured the spectrum of TMPD+ in 3-MHx/N20/0.004 M TMPD glass at 72 K, 100 ns by pulse radiolysis and found (tmax)av/emm to be 1.35 (after a small correction for the background due to other products and the cell window) and, using the linear relationship, we find that = 1.28 X lo4. This value was used in the (e,,,),,(TMPD+) following determination of t,,,(SQ+). Because it is easier to separate the spectrum of SQ', than that of 3MO+, from the spectrum of TMPD+, we decided to determine tmaX(SQ+), rather than emx(3MO+),by TMPD scavenging. The
I
I
900 nm, a Barnes A-100 InAs photodiode.
8
A + .-
0
c
A0
)r
0 0 0
v)
0)
U
-0 .-CJ 0" 0.5
6,72K o 127K A 293 K 0
0
A
0
. I -
2.-!
0
-
. I -
0
O
2!
A 0
"
8
L
0
C
1000
1200
0
600
800
X,nm Figure 1. The spectrum of the initial cation of 3-methyloctane measured in 3 M 0 glasses and liquid saturated with N20:the spectrum at 6 and 72 K, 100 ns; at 127 K, 100 ns; and at 293 K, 50 ns.
TABLE I species
A,
conditions
nm
e,,,, M-' cm-'
PY+ TMPD'
3 M 0 , 6-127 K, 20 ns 3 M 0 , 293 K, 50 ns SQ, 6-72 K, 20 ns SQ,293 K, 50 ns 3 M 0 , 72 K, 20 ns 3MHx, 72 K, 20 ns
625 600 1400 1200 450 575, 622
0.88 x 104 0.88 X loQ 1.4 X lo4 1.4 x 104' 4.5 x 104 1.28 X
e, e*e,e*-
3MP, 77 K, relaxed spectrumd 3MP, 72 K, 100 ns5 3 M 0 , 153 K, 100 ns McHx, -d14 room temperaturee
1650 2000 2100 3400
3.1 X 1.6 x 2.1 x 1.9 X
3MO+ 3MO'
SQ+ SQ+
10" 104 10" 10"
'Probable value, because of the similarity to the low-temperature spectra. "Average value of 575 and 622 nm. 'Calculated as described in the text. dReference 21. 'Reference 22.
method consisted of making glasses of SQ saturated with N,O, with and without added TMPD. It was assumed that N 2 0 removes electrons with the same efficiency in the presence and in the absence of TMPD. Ge,(SQ+) was measured in the presence and the absence of TMPD and the decrease due to scavenging by TMPD SQ+
+ TMPD
-
SQ
+ TMPD+
was calculated after an extrapolation to midpulse. The loss, AG(SQ+), was equated to the production, G(TMPD+)didpulsc, and t,,,(SQ+) was calculated. TMPD at 0.015 and 0.022 M in S Q a t 72 K and TMPD at 0.01, 0.033, and 0.100 M in SQ at 173 K were used. The loss of SQ+ at midpulse caused by these concentrations of TMPD ranged from 19 to 82%. An average value of 1.41 X lo4 was determined for t,,,(SQ+) in this way. Of the five separate determinations, four gave values within 4% of the average value and one differed by 9%. We know from earlier work that (t,,,(SQ+))/(e,,,(3MO+)) = 1.6: which means that tmax(3MO+)= 0.88 X lo4. We also = 0.194.4 This leads to a know that (e,,(3M0+))/(tm,,(Py+)) value of 4.5 X lo4 for t,(Py+) which is in good agreement with the value 4.4 X 104 measured by the chemical production of P y + . ' O The spectrum of 3MO' has already been measured in 3 M O / N 2 0 at 100 ns for the glasses at 6 and 72 K4 and for the cold liquid at 127 K.6 We have now measured the spectrum at 50 ns in 3 M O / N 2 0 at room temperature. This was taken to be the difference between the spectrum at 50 ns and the spectrum at 150 ns. At 50 ns, the spectrum is dominated by 3MO+ with A-, = 600 nm with no sign of e;. At 150 ns, 3MO' has decayed,
2050 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
leaving a residual absorption which rises from 1000 nm to shorter wavelengths. At 50 ns, Gtm(3MO+) = 1OOO. As seen in Figure 1, the spectrum of 3MO+ is almost unchanged from 6 to 293 K except for a slight shift in wavelength with A, being 600 nm at 293 K and 625 nm at the lower temperatures. We conclude that the value tmx(3MO+) = 0.88 X lo4, determined at the lower temperatures, also applies to 3MO+ at room temperature. The spectrum of SQ+ was also measured at room temperature in SQ/N20 by subtracting the 150-11s spectrum from the 50-ns spectrum. At 50 ns, Gc,,(SQ+) = 4000 and Amx(SQ+)= 1200 nm. In glassy SQ, cmx(SQ+)= 1400 nm. The spectrum at room temperature is narrower than the low temperature spectrum and so might have a somewhat larger extinction coefficient, but to within 30% we expect the extinction coefficients to be the same. In Table I can be found an internally consistent set of extinction coefficients for the cations dealt with in this report. This set is also consistent with the most likely literature value for Py+. Extinction Coefficient of the Solvated Electron. We shall use the term solvated electron, e,, to denote both partially and fully solvated electrons. No direct experimental determination has been reported of cmax(e;) in alkane glasses on the pulse radiolysis time scale. The only directly determined value of tmx(e;) in an alkane glass was determined by Gallivan and HamillZ for the relaxed spectrum of e; minutes after radiolysis (see below for further discussion). However, we3 have already pointed out that the value of Gallivan and Hamill is probably larger than the value at short times, before spectral relaxation has taken place. It was reported that €-(e,); increased by a factor of 1.3 to 1.4 in 3-methylpentane and 3-methylhexane glasses when the spectrum relaxed on warming from 4 to 77 K.I6 Also, a calculation predicted cmx(e;) to be 2 X lo4 at 4 K in 3-methylpentane.” Therefore, it was important to determine cmax(e;). It seems reasonable to calculate tmx,lDO ns(e;) from the pulse radiolysis of neat 3 M 0 a t low temperatures on the assumption that G(e;) = G(3MO+) at midpulse (20 ns). The midpulse values of 3MO+ and e; a t 72 K represent only about one-third of the originally produced cations and electrons, if we accept a value of 4-5 for G(initia1 ionization). In the neat 3 M 0 glass, the decays of both electron and cation after midpulse are slow on the nanosecond time scale.5 Hence, the first two-thirds decay of these species is a subnanosecond process. The midpulse values of G for both cation and electron change very little between 118 and 6 K5v6 showing that the first two-thirds of the cations and electrons are removed by processes which are fast even a t 6 K. Both the high rate constant of neutralization and the high local concentrations of the geminate ions in the spur leads us to expect that recombination of geminate ions causes the major subnanosecond removal of cations and electrons. Assuming that recombination is the only process removing electrons and initial cations prior to 20 ns, Le., = 0.88 that G(3MO+) = G(e;) at midpulse, and that t,(3MO+) X lo4, we have determined tmax(e;) from 6 to 118 K. Measurements were made at 72 K and the relative yields and decays at 6 and 72 K were taken from experiments published previ~usly.~ = 2000 nm to its value at 625 nm was The ratio of €(e;) at A, taken to be 4.85 at both 6 and 72 K. The midpulse values of Gt,,,(3MO+) and Gcmax(e;) at 104, 109, and 118 K are taken from ref 6. The values of cmax(e;) determined in this way are shown in Figure 2. From 6 to 100 K, e,,,(e;) has a constant value of 1.7 X lo4. The glass transition temperature of 3 M 0 is 103 K.6 Above 103 K, cmx(e;) calculated in this way increases (see Figure 2), indicating that the method is no longer valid. Presumably, in the cold liquid, processes other than recombination become important on the subnanosecond time scale. We have also made a direct determination of cmax(e;) by scavenging e; with Py and equating the loss of e; to the production of Py- in exactly the same way as described above for the determination of e,,,(SQ’) by scavenging with TMPD. Two solutions of 3MP were made, one containing 0.15 M TEA and (16) M. Ogasawara, K. Shimizu, and H. Yoshida, Chem. Phys. Left.,68, 136 (1979). (17) T. Ichikawa and H . Yoshida, J . Chem. Phys., 73, 1540 (1980).
Klassen and Teather
~
0
20
40
60
00
100
120
temperature, K Figure 2. Two independent determinations of ermx(e;) at 20 ns. Data points ( 0 )refer to 3MO/N20 glasses and were calculated on the assumption that G(e;) = G(3MO’). The data point (A)was determined by Py scavenging in a 3MP/TEA glass. deaerated with argon, the other containing both 0.15 M TEA and 0.01 M Py and deaerated with argon. Both solutions were glassed and pulse irradiated at 72 K. Even though the cations in 3MP do not produce a detectable absorption band under these conditions, it was felt important to efficiently scavenge cations with TEA, because Py scavenges both electrons and cations. In the absence of more efficient cation scavenging, in this case by TEA, scavenging by Py would affect the decay rate of the cation which, in turn, might affect the geminate decay of the electron. The result would be erroneously low values for cmx(e;), We were, however, cognisant of the fact that the high concentration of TEA needed to completely scavenge the initial cations might affect the spectrum of e;. It was reported’* that the presence of triphenylamine in y-irradiated 3MP glass at 72-74 K resulted in the production of ede-, having a spectrum similar to that of For this reason, we measured the spectrum of the electron in the 3MP/TEA/argon glass and, within experimental error, found it to have the same shape as the electron in 3MP/argon measured previously at 77 K.3 On this basis, we assume that the band with A, = 2000 nm which we measured in the presence of TEA was that of e3Mp-. It was found that the presence of 0.01 M Py reduced the yield of e,- by 44% at 200 ns and by 35% a t midpulse. The absorption due to the narrow Py-band was measured over the range 490-494 nm with a bandwidth of less than 1 nm to establish the maximum. In addition, the absorption in the region of the Py+ maximum at 450 nm was measured. The ratio of absorption at 450 nm to that at the maximum of Py- established that no detectable Py+ had determined been produced. A value of 4.95 X lo4 for c,(Py-), chemically, has been reported.lg A slightly smaller value, 4.7 X lo4, was determined by pulse radiolysis using a 5-nm bandpass.20 Using the value 4.95 X lo4,we calculated a value of 1.56 X lo4for €-(e[) at 200 ns and a value of 1.46 X lo4at midpulse. The midpulse calculation involves an uncertain extrapolation. Therefore, we consider 1.56 X 104 the better value from this direct determination by Py scavenging and this value is shown on Figure 2. Thus, we have determined cmax(e;) in two independent ways for alkane glasses at 20-200 ns. The values 1.7 X lo4 and 1.56 X 1O4 differ by only 9%. We have no reason to favor one value over the other so we have chosen to use 1.6 X lo4 in subsequent calculations. The only other relevant determination of cmax(e;) was made for the relaxed spectrum of e; in y-irradiated 3-methylpentane (3MP) glass a t 77 K by Gallivan and Hamil12 using the photoshuttling of negative charge between biphenlyide ions (Biph-) and trapped electrons. They arrived at a value of 1 3 . 0 X lo4 for cmx(e;) based on a value of 3.7 X lo4for e,(Biph-) and the fact that they could not determine the efficiency with which electrons were converted into Biph- by bleaching.*’ A later determination, cmx(Biph-) = 4.0 X l@,I9 changes the Gallivan and Hamill value (18) S. C. Srinivasan and J. E. Thean, Znr.J. Radiat. Phys. Chem., 8, 589 (1976). (19) J. Jagur-Grodzinski, M. Fcld, S. L. Yang, and M. Szwarc, J. Phys. Chem., 69, 628 (1965). (20) U. Sowada and R. A. Holroyd, J . Phys. Chem., 85, 541 (1981). (21) W. H. Hamill in “Radical Ions”, E. T. Kaiser and L. Kevan. Ed.: Interscience, New York, 1968, Chapter 9.
Cations and Electrons in Hydrocarbon Glasses and Liquids
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 2051
A,micrans (
0.6
0.8
1.2
I
I
I
2.2
2.0 3 . 4 I
I
2 .o
3
-a 0) VI
a
1.8
I '0
'E
d
h 2
1.6
c3
x
1.4
IVI
al v
1.21 0
W I
I
I
50
IO0
I50
temperature, K
Figure 4. The midpulse (20 ns) values of G(3MO') and G(e;) plotted vs. temperature. 0 24
20
16
12
8
4
0
V ~ I O ,-cm-' ~ Figure 3. The spectrum of e; under three conditions. A refers to the relaxed spectrum in 3MP at 77 K.21 B refers to the unrelaxed spectrum measured at 100 ns in 3MP at 72 K.J C refers to the spectrum in methylcyclohexaned,, at short times at room temperature.2 In all case the spectrum calculated from the experimental data is represented by a solid line and the extrapolations by dashed lines. of e,,(e[) to 13.2 X lo4. The relaxed spectrum of the electron = 1650 nm as opposed to ,A, = 2000 in alkane glasses has ,A, nm at 100 ns, and the shapes of the two spectra are quite different.* Therefore, it is not surprising that our value of 1.6 X lo4 for e,(e;) a t 100 ns is quite different from the value of 13.2 X 104 determined by Gallivan and Hamill for the completely relaxed spectrum. Nevertheless, it seemed worthwhile to compare these two extinction coefficients in a simple way. We have compared the spectra of e[ in alkanes for four conditions, the relaxed spectrum of e[ in 3MP glass at 77 K,21the spectrum in 3MP a t 100 ns, 77 K,5 the spectrum in 3 M 0 a t 100 ns, 153 K (present study), and the spectrum in methylcyclohexaned,, (McIIX-di4) at short times at room temperature.22 We chose to use the latter spectrum because of the similarity of its shape to that of the 100-ns spectra at lower temperatures, but another spectrum of quite different shape has also been reported for methylcyclohexane at room temperat~re.~'Three of these spectra are plotted in Figure 3 and extended, as indicated by the dashed lines, to zero absorption on both the high- and low-energy sides. The experimental portion of the unrelaxed spectrum at 72 K is much broader than the relaxed spectrum a t 77 K. In the absence of a theoretical approach which predicts the shape of the unrelaxed spectrum, the dashed lines on the high-energy side were extended almost linearly to zero intensity. By comparing the area under these curves, by assuming that the oscillator strength of the bands are equal, and by accepting a value of 1.6 X lo4 for emsx(e;) a t 100 ns 72 K, the extinction coefficients for each spectrum were calculated. The maximum extinction coefficients are listed in Table I. The method we have used to obtain these values is supported by the similarity of our calculated value (3.1 X lo4) for the relaxed spectrum at 77 K to the value of Gallivan and Hamill 1 3 . 2 X lo4. The Yields of Initial Cations and Electrons. It is instructive to look a t the yields of electrons and initial cations at midpulse over the temperature range 6-153 K. This is shown in Figure 4. The source of the data in Figure 4 for 6 and 72 K is described above, that for 103, 109, and 118 K are taken from ref 6 and the (22) M. S. Ahmad, S.J. Atherton, and J. H. Baxendale, "Proceedings of the Sixth International Congress of Radiation Research, Tokyo, May 1979", S. Okada, M.Imamura, T. Terashima, and H. Yamaguchi, Ed., Japanese Association for Radiation Research, University of Tokyo, Tokyo, Japan, 1979, p 220. (23) F. Busi in "The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis", Proceedings of the NATO Advanced Study Institute, Capri, Sept 1981, J. H. Baxendale and F. Busi, Ed., Reidel, Dordrecht, Holland, 1981, p 417.
measurements at 153 K are from the present work. The extinction coefficients used to calculate the data are in Table I. The values of emX(e[) in Table I are assumed to be applicable to 3 M 0 . A serious problem when measuring Ge,,(3MO+), with A,, = 625 nm, is to separate its absorption band from the overlapping e; band. The magnitude of the problem is readily seen in Figure 1 of ref 5. For each set of data points in Figure 4, the absorption spectrum at midpulse was assumed to consist of only e; and 3MO+. In each case the ratio emax(e[)/t62J(e[) chosen was the value which resulted in separated e; and 3MO' spectra which best conformed to the known spectra. The ratios used were 4.85 for the glasses and 7.3 for 153 K. As explained above, we have = 1.6 X 104 for temperatures I 1 18 K. The chosen to use &(e;) result is that Figure 4 indicates that a small, constant fraction of the initial 3MO' in the glasses decays by a subnanosecond process other than recombination. A plausible hypothesis would be the rapid decomposition of excited 3MO+ ions resulting in a less mobile cation. A prominent feature of Figure 4 is the divergence of midpulse yields as the temperature of the sample is increased above the glass transition temperature. The explanation which we favor is the increasing importance, with decreasing viscosity, of subnanosecond reactions of 3MO+ to produce cations with lower mobilities than that of the initial cations. It is implied in this argument that the recombination rate depends significantly on the mobility of the cations. Resonance charge transfer has been suggested as a mechanism for enhanced hole This mode of charge movement vanishes if the initial cation reacts to become a different cation. We can estimate the extent of such cation reactions occurring a t times shorter than 20 ns a t 153 K. If we assume G(initia1 ionization) = 4.5 we set the loss of G(e[) before midpulse at 2.2. If we assume that all of the electrons lost recombined with 3MO+, this accounts for a 2.2 loss of G(3MO+). As indicated in Figure 4, we see an extra loss of G(3MO+) of 0.8. Thus, the total conversion of 3MO+ to another cation is greater than or equal to 18% of the total G(3MO'). Apparently the conversion of a significant portion of 3MO+ to another, less mobile, cation has become very rapid by 153 K and this trend is expected to continue to higher temperatures. Another possible explanation for the observed increase in the midpulse yield of electrons and decrease in the midpulse yield of 3MO+ is the presence of a large amount of an impurity, such as an alkene, capable of scavenging 3MO'. We discount this possibility for several reasons. N o absorption, clearly related to cation scavenging, was seen in any experiments with 3 M 0 or SQ. For example, we did not see the dimer alkene cations described by Mehnert, Brede, Cserep, and Nauma11n,2~.~~ but, in 3 M 0 , such spectra might be difficult to distinguish from 3MO+. Neither was there any indication of an impurity scavenger (24) M. P. de Haas, A. Hummel, P.Infelta, and J. M. Warman, J . Chem. Phys., 65, 5019 (1976). (25) R. Mehnert, 0.Brede, and Gy.Cserep, Radiochem. Radioanal. Lett., 47, 173 (1981). (26) R. Mehnert, 0. Brede, and W. Naumann, Ber. Bunsenges. Phys. Chem., 88, 71 (1984).
; : :I: 'u
Klassen and Teather
2052 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
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300
-
I5
.w
200
0
-10
150 ns
W
IO0
05
0
1
2
,
, 800
600
,
-
-
05
I
1000
X , nm
Figure 5. Spectra measured and calculated in neat 3 M 0 irradiated at
room temperature. The spectra were measured at 150 (shown) and 50 ns. The difference spectrum which resulted from subtracting the 150-11s spectrum from the 50-ns spectrum is shown. The spectra labeled e; and 3MO+ were constructed from the difference spectrum using the spectral shapes presented in this report. competing with SQ for 3MO' during our analysis of the charge transfer in 3 M O / S Q / N z 0 glasses4 The expectation that the importance of conversion of 3MO' into other cations would increase with further rise in temperature above 153 K is consistent with the results we have obtained for 3 M 0 irradiated at room temperature. Figure 5 shows the spectra measured at room temperature. The residual absorption at 150 ns does not seem to contain significant e; or 3MO'. The very marked decrease in the decay rate at all wavelengths at about 150 ns also indicates the absence, beyond that time, of a significant contribution to the spectrum by 3MO' and e;. It was concluded that the difference spectrum, 150 ns subtracted from 50 ns, contained most of the e; and 3MO' present a t 50 ns. In Figure 5 the difference spectrum is resolved into e; and 3MO+ by using the spectral shapes contained in this report. The resultant values for 50 ns are C(e;) = 0.053 and G(3MO') = 0.018, Le., an electron yield three times that of the initial cation. This result is consistent with the notion that the initial cation is being rapidly removed by reactions other than recombination. Moreover, it suggests that conductivity experiments on alkane liquids should not automatically be interpreted as if all the cations present are the initial cations. The Decay of Electrons and initial Cations in the Neat Alkane. Louwrier and Hami1127*28 were able to observe an alkane cation in y-irradiated 3MP glasses containing both an electron scavenger and a higher molecular weight alkane. This resulted from positive charge transfer from 3MP' to the higher molecular weight alkane and stabilization of the new cation by its isolation from solute alkane molecules. We have reported the kinetics of this charge exchange as examined by pulse r a d i o l y s i ~ . ~However, ?~ Louwrier and Hamill were only able to detect trapped electrons, but not initial cations, in y-irradiated single-alkane glasses minutes after radiolysis. N o initial cations were observed even with added electron scavengers. Clearly, in single-alkane glasses, the initial cation decays more rapidly than the electron. W e may now use the extinction coefficients determined for 3MO' and e; to examine the concomitant decay of the electron and the initial cation. The decays of e; and 3MO' at 72 and 153 K are shown in Figure 6 . However, it must be remembered that the spectrum of e; relaxes on the millisecond time scale in alkane glasses at 72 K.29 We have made approximate corrections for the changing spectrum of e; during its relaxation to arrive at the curves in Figure 6. It seems unlikely that a .more rigorous treatment would change the decay curves very much. The observations of Louwrier and Hami1127*28 that isolation of RH', from R H is required to stabilize it suggests that either RH'. (27) P. W. F. Louwrier and W. H. Hamill, J . Phys. Chem., 72, 3878 (1968). (28) P. W. F. Louwrier and W. H. Hamill, J . Phys. Chem., 74, 1418 (1970). (29) N. V. Klassen, H. A. Gillis, and G.G. Teather, J . Phys. Chem., 76, 3847 (1972).
I
I
I
I
I
50
200
800
t i m e , nanoseconds (log scale)
Figure 6. The decay of e; and 3MO+ is presented in units of G for 72 and 153 K.
decays by reaction with R H or that the presence of R H provides a path for charge transfer resulting ultimately in decay by reaction with something other than RH. The rate of decay of both e; and 3MO' at 100 ns is about 50% slower at 6 K than at 72 K. At 72 K, 100 ns, the absolute rate of decrease of G(eJ is much smaller than that of G(3MO'). At 153 K, 100 ns, G(e;) is decreasing a little faster than G(3MO'). At midpulse, G(e;) N G(3MO') at 72 K but G(e;) > G(3MO') at 153 K. If the extra loss of 3MO' before midpulse at 153 K is due to the same reaction which causes 3MO' to decay faster than e; between lo-' and 10" s at 72 K, then this reaction has become considerably faster at 153 K. This reaction might be proton transfer RH
+ RH'.
-
R
+ RH*+
(1)
which could very well be speeded up by faster molecular reorientation in the liquid than in the glass. Although such proton transfer is not observed in the gas phase, except for methane, recent determinations of proton affinities indicate that the reaction for n-butane is approximately t h e r m ~ n e u t r a l . ~The ~ assignment3' to Py' of the major band seen in a y-irradiated sec-butyl chloride glass containing Py was probably incorrect since the 465-nm wavelength corresponds better to the cation formed by proton transfer, namely PYH'.~~The occurrence of reaction 1 at low temperatures, as studied by ESR, has been reported by Iwasaki, Toriyama, and N ~ n o m e . ~ ~ Highly Mobile Alkane Cations. It has been known for some time that the rate constants for cation scavenging are unusually high in a few alkanes, cyclohexane, methylcyclohexane, and trans-decalin, but not in other alkanes.24 The diffusion coefficients of the mobile cations seem to be more than ten times faster than those of normal ions.12 It was suggested that the highly mobile cations were those whose geometry is very similar to that of the parent alkane molecules, thereby permitting rapid charge movement by resonance charge transfer. A possibility, suggested by the present results, is that the anomolously high mobility measured for the cations of cyclohexane, methylcyclohexane, and t r a n s - d e ~ a l i nmay ~ ~ simply mean that initial cations, in general, are highly mobile but that, in many alkanes at room temperature, most of the cations at 50 ns are not the initial cations. Mehnert, Brede, and Naumann26have taken a different approach. Their results suggest a mobile hole in n-heptane at short times. The exact nature of this mobile hole is not defined but it is believed to have an absorption spectrum similar to that of the n-heptane cation and one of its fates is to become localized as the "massive" n-heptane cation in less than s. In this picture, the mobile hole is produced by the ionizing event and decays rapidly. Very (30) P. Ausloos, Radiar. Phys. Chem., 20, 87 (1982). (31) Reference 21, Figure 58. (32) G. A. Olah, C. U. Pittman, and M. C. R. Symons in "Carbonium Ions", Vol. 1, G . A. Olah and P. von R. Schleyer, Ed., Interscience, New York, 1968, Chapter 5. (33) M. Iwaski, K. Toriyama, and K. Nunome, Radiar. Phys. Chem., 21, 147 (1983).
J. Phys. Chem. 1985,89, 2053-2058
2053
recently, Trifunac, Sauer, and Jonah3s have specifically addressed the question of the identity of the high mobility cation. They propose that the high mobility species are R+ and/or RH*+, the means of rapid charge movement being H- and H+ transfer, respectively. The production of R+ by decomposition of RH+. into R+ and H is very endothermic and must occur from excited ions. I w a ~ a k has i ~ ~also presented evidence in favor of R+ and RH2+ being mobile species. Trifunac, Sauer, and Jonah35have suggested a mechanism for cyclohexane radiolysis in which the initial ionization leads to G(R+) = G(RH+.) = 2.5. Insofar as
our low-temperature results can say anything about the room temperature situation, the subnanosecond decomposition of excited cations seems to be only a small effect judging from the almost equal yields of 3MO+ and e; which we measure at midpulse in the glasses (Figure 4). However, our results would be consistent with the significant production of R+ if most of the R+ were to recombine with e; before 20 ns. In the same vein, our finding that G(e;) > G(3MO+) at midpulse at 153 K is most simply interpreted as indicating the rapid formation of a less mobile cation, rather than a more mobile cation, but could be reconciled to the production of R+ by a more complicated mechanism.
(34) J. P. Smith, S. Lefkowitz, and A. D. Trifunac, J. Phys. Chem., 86, 4347 (1982). (35) A. D. Trifunac, M. C. Sauer, and C. D. Jonah, Chem. Phys. Lett., in press. (36) M. Iwasaki, Faraday Discuss., Chem. SOC.,in press.
Acknowledgment. We thank the authors of ref 35 and 36 for preprints of their papers. R e t r y NO. 3MO,2216-33-3; 3MO', 64156-02-1; SQ', 79054-29-8; TMPD', 95693-58-6.
Complexation Equilibria in Systems Composed of Phenols and Oxygen Bases in CCI, R. Wolny, A. KoU, and L. Sobczyk* Institute of Chemistry, University of Wroclaw. 50-383 Wroclaw, Poland (Received: April 26, 1984; In Final Form: October 30, 1984)
The complexation equilibria for a number of systems composed of oxygen bases and 2,6-dichlorophenol derivatives in CCl., have been studied. For some of them and particularly for those containing tri-n-butylamine N-oxide, the intracomplex proton transfer equilibrium has been evidenced. The linear correlation between log Km and ApK, is satisfactorily obeyed. In the case of 2,6-dichlorophenols, because of steric repulsion, a contribution of homoconjugated anions is excluded, while generally for systems containing stronger proton donors and acceptors homoconjugated OHO+ cations play an essential role. The structure of (AH)*B adducts is also discussed, and we showed that the second phenol molecule is attached to the OH group of the first one bound directly to the oxygen base.
Introduction The oxygen bases are characterized by an ability to form hydrogen-bonded complexes of various composition. The concentration ratios of particular forms depend not only on the component's concentrations but also on the electron and steric structure of the oxygen base and proton donor. Owing to two lone electron pairs on the oxygen atom, the number of possible species of (AH),B composition increases in relation to nitrogen bases. In most cases, particularly when the excess of A H is not great and carboxylic acids as proton donors are excluded, one can safely assume that n = 1 or 2. For the complexes with n = 1 the greatest probability is ascribed to configuration, where the proton-donor group is arranged coaxially with a lone electron pair, though one cannot exclude the existence of hydrogen bridges coaxial with the X=O bond (11). As far as the (AH)2B complexes are concerned, quite controversial opinions on their structure can be found in the literature. There is evidence supporting the existence of both structures I11 and IV or V.1-3 It appears that their stability depends mainly on the proton affinity of the oxygen base and steric conditions. Simple proton donors like hydrohalides or HNCS form with strong bases mostly complexes of structure 111, while phenols with substituents at the ortho positions form primarily adducts of structure IV or V. Calculations show that both structures I1 and V are energetically less stabile as compared with I and IV, respe~tively.~~ (1) S. Detoni, D. Had& R. Smerkolj, J. P. Hawranek, and L. Sobczyk, J . Chem. SOC.A , 2851 (1970). (2) A. Koll, M. Rospenk, and L. Sobczyk, Bull. Acad. Pol. Sci., Ser. Sci. Chim.,7, 735 (1972). (3) R. Wolny, A. Koll, and L. Sobczyk, Bull. SOC.Chim. Belg.. 93, 99 (1984). (4) K. Mazur, Ph.D. Thesis, University of Wroclaw, 1973.
0022-3654/85/2089-2053$01.50/0
X-
Complexation equilibria become considerably complicated when homoconjugated cations and anions [BHB]+ and [AHA]- may be formed. The oxygen bases exhibit a greater tendency than nitrogen bases to form [BHB]+ cations with stronger bases; (5) J. E. Del Bene, J. Chem. Phys., 62, 1314 (1975). (6) L. A. Curtiss and D. J. Frurip, Int. J . Quantum Chem., Quantum Chem. Symp., No. 15, 189 (1981).
0 1985 American Chemical Society
