1702
PIETER W. F. LOUWRIER AND WILLIAM H. HAMILL
Intermediates in 7-Irradiated Solutions of Carbon Tetrachloride
in Alkanes at 77°K
by Pieter W. F. Louwrier and William H. Hamill Department of Chemistry and the Radiation Laboratory,' University of Notre Dame, Notre Dame, Indiana (Received November 7, 1068)
46566
?-Irradiated samples of CCla in alkane hydrocarbons exhibit optical absorption bands with maxima at 367 and -470 nm. Isothermal decay of 367-nm absorption and growth of 470-nm absorption and an isosbestic point between these bands provides evidence that one species is the precursor of the other. They are assumed to be CCL+ and a CT complex, CCld. C1, resulting from charge recombination with GI-. These assignments agree with positive charge migration induced by ~ 3 7 0 - n mexcitation but not by ~470-nmexcitation and by the appearance of the 470-nm band under uv photolysis.
Introduction y-Irradiated glassy solutions of CC1, in 3-methylpentane (3R.IP) at 77°K are characterized by a prominent absorption band with A,, -470 nm which is readily bleached by light and suppressed by olefins and proton acceptors2J which indicates that it is a positive ion or "hole" or has a positive ion precursor. I n pure irradiated CCL at 77°K there is no absorption at 470 nm, but there are absorption bands at -400, -340, and -270 nm which bleach slowly under tungsten lighte4 In 3MP matrices cc14 and 2-methyldecane (2MD) compete for holes. Optical bleaching of the 2MD+ band at 820 nm promotes the 470-nm band of CC14,6 supporting the proposal3 that CC4 traps excited holes in the 3MP matrix. On the other hand, and consistent with earlier observations, optically bleaching the 470nm band does not increase 2MD+ (or any other) absorption. Irradiation of 2% CCl, in 3RIP at 20°K produces absorption at 366 nm, bleached by 370-nm light. It decays rapidly in the dark below 77°K with some increase in absorption at -450 nm. It is unaffected by 2% 2methylpentene-1 (2MP-1) or by 2% Ci" and has been attributed to CC14-.6 The possibility of accounting for color centers produced by CCL in alkane matrices was offered by the recent observation that +radiated CCl, in 3MP at 77°K showed a small absorption band at 350 nm which decays with growth of 470-nm absorption. It was then found that CC14 in other alkane matrices shows the effect much more clearly. These and related systems have been examined further with particular reference to hole migration and trapping.
Experimental Section Materials. Isopentane and 3nIP were Phillips pure grade, passed twice through a 1-m column of activated silica gel; 2MP-1 was Phillips research grade, passed through an alumina-packed column; 3-methylheptane The Journal of Physical Chemistry
(3MHP), 3-methylhexane (3hIH), and 2-methyldecane (2MD) from Aldrich Chemical Co. were used as received. Fisher Certified grade CC14 and CHzClz were used as received. Fisher spectroscopic grade toluene was distilled on a spinning-band column. Tetrahydro2-methylfuran (MTHF) from Eastman was passed through an activated alumina column before each experiment. Tetramethyl-p-phenylenediamine(TMPD) was prepared from the dihydrochloride (Eastman) and sublimed. Squalane (2, 6, 10, 15, 19, 23-hexamethyltetracosane) from Baker Chemical Go. was washed with HzS04and water, dried over NazSOd, and passed through an activated silica gel column. Proceduye. Only samples with gaseous solutes were deaerated. Solutions in 1 X 1 X 6 cm Suprasil celIs were plunged into liquid nitrogen and kept there during 6oCo irradiation (1.2 X lo** eV/g min) and subsequently. The mercury resonance lamp for 2537-A photolysis was provided with a filter to remove visible light. Polarized optical bleaching was performed with Polaroid HNP-B polarizing filters and a tungsten lamp for the 470-nm band or a high-pressure mercury lamp for the 367-nm band.
Results A sample consisting of 2 mol % CC14in 3MP shows a rather small absorption at -350 nm following irradiation, better seen after bleaching at X >590 nm (Figure (1) The Radiation Laboratory of the University of Notre Dame is operated under contract with the U. S. Atomic Energy Commission. This is AEC document No. COO-38-636. (2) J. P. Guarino and W. H. Hamill, J . Amer. Chem. Sac., 86, 777 (1964). (3) J. B. Gallivan and W. H. Hamill, J . Chem. Phys., 44, 2378 (1966). (4) T. Shida and W. H. Hamill, ibid., 44, 2369 (1966). (5) P. W. F. Louwrier and W. H. Hamill, J . Phys. Chem., 72,3878 (1968). (6) F. R. C. Claridge, R. M. Iyer, and J. E. Willard, ibid., 71, 3577 (1967).
INTERRIEDIATES I N y-IRRADIATED SOLUTIONS OF
1703
CClr
I
b
P
2 .o
Figure 1. (a) (1) Spectrum of 2 mol % CCld in 3 MP, after irradiation to 1 X eV/g; (2) same after bleaching at A >590 nm; (3) difference spectrum. (b) Same as (a) but with addition of 2 mol % MTHF.
l a ) . The -350-nm absorption was also observed to decay isothermally with an accompanying small increase in the 470-nm absorption. The latter band decays very slowly in the dark, and the relative quantum yield for bleaching this band with 475-nm light is constant for -50% removal. The 470-nm absorption of CCI, in 3MP is diminished by 0.5 mol % COz or N20with 0.05 mol % CCL and by 0.12 mol % c-C4Fswith 0.1 mol % Cch. For the same conditions as Figure l a but with 2 mol % M T H F also present as hole trap the absorption at -350 nm was somewhat enhanced, but the species absorbing at 470 nm was strongly suppressed (Figure lb) When 3 mol % 3MP in CC14is y irradiated, the difference spectrum following optical bleaching shows 8 maximum at -475 nm as well as one at -350 nm. The yield of color centers at 470 nm appears to depend on viscosity, being greater in 3hIP-isopentane (70:30) than in 3MP alone, both at different doses (Figure 2) and at different concentrations of CC14 (Figure 3). This implies a precursor species, stable a t higher viscosities, and the possibility was tested using 2 mol % CCb in a matrix of 3MHP which has a much greater viscosity than 3MP.7 This system gave a clearly defined band at 367 nm which is shown by the isosbestic point to be the precursor of the species absorbing at A,, -450 nm (Figure 4a). Polarized optical bleaching of the 367-nm band does not produce optical anisotropy. Addition of M T H F depresses both bands, but less than in 3MP, and prevents isothermal conversion of one absorbing species into the other (Figure 4b). Similar experiments in squalane, which should form a much harder matrix, show both species present initially with yields comparable to those in 2MHP but with no isothermal decay (Figure 5a), while M T H F has rather little effect (Figure 5b). A sample containing 2 mol % CC14 and 2 mol % 2MP-1 in 2MHP (Figure 6) is indistinguishable from the comparable sample without 2MP-1 (Figure 4a)
Irradiation
Time (min.)
Figure 2. Optical density of 470-nm absorption for 1.5 mol 76 CCli in 3MP ( 0 )and in a 70:30 mixture of 3MP and isopentane (0)as a function of irradiation time a t 1.2 X lox8 eV/(g min).
.
OD
0
1
2 3 Mole % CCI4
4
5
Figure 3. Optical density of 480-nm absorption vs. [CCla] in 3MP ( 0 )and in 70:30 3MP-isopentane (0) at 4.8 X 10'8 eV/g.
immediately following irradiation. However, isothermal decays for these two samples gave values of -1.5 and -2.3, respectively, for the ratios AOD(367 nm)/AOD(470 nm). Also, it can be seen that absorption a t 300 nm in Figure 6 is invariant and presumably arises from a neutral species, possibly a free radical. Also, the isosbestic point is confirmed. The OD at 367 nm changes rather little with [CCId] in 3MHP, as shown in Table I and Figure 7, but the initial OD a t 470 nm increases much more with increasing concentration. The ratio AOD(367 nm)/ (7) A. C. Ling and J. E.Willard, 1.Phys. Chem., 72, 1918 (1968). Volume 75,Numbm 6 June 1068
1704
PIETER W. F. LOUWRIER AND WILLIAM H. HAMILL
'4
Wavelength, nm
Figure 4. (a) The spectra of 2 mol yo CCl4 in 3 MHP (1) after irradiation to 7.2 X 10l8eV/g; (2) same as (1), 10 min later; (3) same as (I), 90 min later; (4)same as (3) after bleach at A >620 nm. (b) The spectra of 2 mol % CCl4 and 3 mol % MTHF in 3 MHP ( I ) after irradiation to 7.2 X 10l8eV/g; (2) same, 60 rnin later.
Wavelength
Wavelength ,nm Figure 6. The spectra of 2 mol % CCla and 2 mol % 2MP-1 in 3MHP (1) after irradiation to 7.2 X 1OI8 eV/g; 2 same as (1) after bleaching at A >620 nm; (3) sample 2 after 17 min; (4)same, after 60 min; (5) same, after 135 min.
, nm
Figure 5 . (a) The spectra of 10 mol % CClr in squalane (1) after irradiat,ion to 1 X 1OI8 eV/g; (2) after bleaching at X >590 nm; (3) difference between (1) and (2). (b) As in Figure 7 but with the addition of 13 mol % MTHF.
-
.
0
300
400 Wavelength
500
600
, nm
AOD(470 nm) = -2.2 at 0.5% CCl, for isothermal decay from Figure 7, the same as that for 2% CC1,.
Figure 7. The spectra of 0.5 mol % CC14 in 3MHP (1) after irradiation to 7.2 X 10l8eV/g; (2) same, after 45 min; (3) same, after 100 min.
Table I : Absorption Bands a t Several Concentrations of CCln in 2-Methylheptane (7.2 x 10'8 eV/g)
creases the 470-nm absorption, shown in Figure 8. Holes trapped by 2MD can be optically excited in the 2MD+ band (A, 820 nm, onset E1300 nm) with enhancement of the 470-nm band. When the color centers absorbing at 470 nm were partially bleached with light polarized in a reference direction, the surviving population absorbed more strongly at 90" to this direction, shown in Figure 9. A quantitative measure of hole migration is provided by results for a sample which contained 1.2 X 1M TMPD and 2 mol % CCla in 3MHP. Following y irradiation to 7.2 X 1OI8eV/g, the sample was bleached with filtered tungsten light, X >590 nm, which had little or no effect on TRiIPD+. Subsequent illumination with unfiltered tungsten light produced the results
V O l 7% CClr
0.3 0.5 1 2
OD(367 nm)
OD(470 nm)
0.68 0.69 0.70 0.75
0.48 0.52 0.64
0.73
It has been shown that 2MD in 3MP traps holes at small concentrations and conducts holes at higher concentrations.6 When 0.5 mol % CC1, is also present the 470-nm band is first diminished and then increased as [ZMD] increases, although still higher [ZMD] again deThe JOUTnal of Physical Chemistry
1705
INTERMEDIATES IN */-IRRADIATED SOLUTIONS OF CC1, I
I
I
I
-
.3
E
$ /
c 0
I
2.
f -2-
B 'I $
I
1
B ,I
Figure 9. Optical bleaching the 470-nm band of 2 mol % CClr in 3MP, irradiated to 9.4 X 1OI2 eV/g; spectrum with polarizer in the reference direction - - - - and rotated 90'; spectrum after bleaching with filter in the reference direction and measured in the reference direction - - and rotated 90" - .- .- . ,
.I
I
I I J
1
I
I
I
I
I 2 MOL Yo 2 M D Figure 8. Optical absorption at 470 nm ( 0) and 2MD at 820 nm ( 0 )us. [2MD], dose 4.7 X 1Ol8 eV/g. U
I
+
shown in Figure 10. The initial yield of T M P D + was G E 0.35 and the increase produced by light was AG E 0.33. The 367-nm band could not be observed because of absorption by TMPD. Illumination for 30 min without prior y irradiation gave AOD(633 nm) 7 0.03. Results for a parallel experiment with 7.2 X 1 O I 6 eV/g of y irradiation but no illumination also appear in Figure 10. Other halogenated alkanes y irradiated in 3MP were CClaH, CC13F, CCI3Br, and CBr4, and the results resemble those previously reported2 except for a CBr4 band at 485 nm with weak bands a t 340-380 nm. One of these, CC13Br,prepared in a 3MHP matrix, yielded a well-defined band at 380 nm as well as a very wide band a t -480 nm. The glass was too badly cracked to permit reliable observation of growth and decay dependence. Photolysis with a mercury resonance lamp, filtered to remove light in the visible region, of 5 mol % cc14 in 3MP-isopentane (70:30) and in 3MHP gave absorption bands with A,, -470 nm. Several polyhalogenated methanes have already been reported to produce color centers when irradiated at 254 nm in an alkane matrix.*
Discussion The observations in this work, as well as in earlier work, appear to be consistent with the assumptions
m
I
I
IO
I
I 20
30
I 40
I
50
I 60
I 70
1 80
TOTAL ILLUMINATION TIME /DECAY TIME
Figure 10. Increased yield of TMPD+ due to optical bleaching with tungsten light of 0.0015 M TMPD and 2 mol yo CCld in 3MHP after irradiation to 7.2 X 1018 eV/g ( 0 ) ; optical density of TMPD increased by isothermal hole migration in 0.002 M TMPD with 2 mol % CCla in 3MHP irradiated to 7.2 X 1018 eV/g ( 0 ) . +
that a positive ion (possibly CCl4+) is responsible for 367-nm absorption, and that a CT complex (possibly CC14.C1) is responsible for 470-nm absorption. I n several respects the pattern of behavior resembles that (8)
J. P. Simons and P. E. R.Tatham, J . C h m . Soc., A , 864 (1966). Volume 78,Number 6 June 1060
1706 for alkyl iodides in alkane matrices for which the conversion of RI+ to RI.1 was also p o ~ t u l a t e d . ~ Because the species absorbing at 367 nm appears to be the precursor to that absorbing at 470 nm, earlier observations of the effectsof hole traps to depress 470-nm absorption provide evidence that the species absorbing at 367 nm is a positive ion (or has a positive ion precursor) which is provisionally considered to be CC14+. When CC14 is the only solute, CC14+ disappears by a viscosity-dependent reaction with C1- to form the C T complex, CC14.Cl. Although viscosities of the matrices used are 5 1O’O P, the charge pairs remain correlated and ion recombination is favored by the coulombic force. When all electrons have been trapped by CC14 to form C1-, all CC&+ ions necessarily recombine to form CCh-Cl by hypothesis and AOD(367 nm)/ AOD(470 nm) should be independent of [CC14], as observed. The in tercoriversion of these species also accounts for the isosbestic point. The much greater stability to isothermal decay of the 470-nm species relative to the 367-nm species is also accounted for if the former is neutral, and the latter is ionic. The optical assymetry induced by 470-rim polarized bleaching, and the lack of it for 367-nm polarized bleaching are consistent with CCl,-Cl and CC14+. The CT complex cannot be RHsC1 because 470-nm absorption is absent when other alkyl chlorides are employed. On the other hand, C1- appears to be a precursor because when electrons are trapped principally by NzO, COz, or C4Fs, which would not be expected to form CT complexes, 470-nm absorption decreases and is not replaced by another band. The constant quantum yield for bleaching 470-nm absorption follows from the model. I n contrast, the quantum yield for bleaching ionic species has always been observed to decrease with extent of bleaching, as in Figure 10 and earlier worl470 nm raise difficulties. The principal argument for the C T complex is the formation of the 470-nm band by uv photolysis of CC14, and similar effects with other polyhalides. Photoionization appears to be highly improbable. It was proposed before that hole trapping by CC1, in alkane media requires migration of excited holesI3 and this is well supported by hole transfer from 2MD+ to
CC14+by optical excitation in a matrix of 3nIP.5 This raises the problem of accounting for the comparative stability of CCL+ in alkanes with a nominal difference of >1.5 eV in ionization potentials. I n fact, only the appearance potential of CC&+ is known, but the pattern of ionization potentials of alkyl chlorides makes it appear that I(CCl4) > I(a1kane) for vertical processes. The stabilization of CC14+, either in alkane or CC14 matrices, must be attributed to intramolecular relaxation. The immeasurably small abundance of CC14+by mass spectrometry is evidence for very different configurations of the neutral and ionic species. An appreciable relaxation of CC14+ would stabilize it in the alkane matrix.
Charge-Transfer Complexes as Radiolytic Products from Alkyl Halides and Toluene in Alkane Matrices at 77°K by Pieter W. F. Louwrier and William H. Hamill Department of Chemistry and the Radiation Laboratory,l University o j Notre Dame, Notre Dame, Indiana 46666 (Receized November 7, 1968)
y-Irradiated systems of C& or CaH&H3 in a matrix of CC14 at 77’K exhibit absorption bands at -500 nm, not present for the same additives in a matrix of 3-methylpentane (3MP), which can be attributed to chargetransfer complexes Ar-C1. When both C6HbCH3 and 2-CaH7Br are present in 3MP, bands at 500 and 675 nm appear which are not present with either additive alone. The 500-nm band is attributed to CeH5CH3.Br because it increased as CaH&H3+ absorption decreased. The 675-nm band is attributed to CeH6CH3. c3H7B1-f because its decay is coupled with increased absorption by CsH&H3+. For 2-C3H7Br alone in 3MP, bands appear at 600, -400, 360, and 260 nm. By analogy with alkyl iodides in 3MP (ref 8), the dominant band in dilute solutions (400 nm) is attributed to PrBr+, and the strong band at high concentration (360 nm) to (PrBr)z+ from preexisting neutral dimer. Isothermal decay of these species enhances the 600-nm band, attributed to PrBr+ Br- + PrBrSBr. The band at 260 nm behaved very differently and may be due to Pr..
+
lntroduction A short-lived (-5 psec) absorption band with A,,
470 nm observed following pulse radiolysis of CeHsCHain CC142resembles the band at -500 nm observed by Shida3using a CC14 matrix. Rather similar bands were also observed for CaHs in CC1, by pulse radiolysis,* and in matrices of cc143 and C4HsCL4 As ions would not be detectable at see in liquid CC14 and as charge-transfer (CT) complexes have been demonstrated to be radiolytic products in rigid matrices,6 it appeared possible that the “toluene” bands in CC1, are also due to C T complexes. A preliminary test showed, in fact, little or no absorption at -500 nm for CsH,CH3 in 3-methylpentane (3MP) when COz was used to trap electrons. It appears now that a prominent 470-nm
band observed in irradiated CC14-3MP is also due to a CT ~omplex.~ It is quite appropriate that CT complexes should form under radiolytic conditions, and it is remarkable that this effect was not generally anticipated. I n fact, there is very limited evidence concerning their forma(1) The Radiation Laboratory of the University of Notre Dame is operated under contract with the U. 9. Atomic Energy Commission. This is AEC Document No. COO-38-639. (2) R. E. Buhler, T. Gaumann, and M. Ebert, “Pulse Radiolysis,” M. Ebert, J. P. Keene, A. J. Swallow, and J. H . Baxendale, Ed., Academic Press, New York, N. Y., 1965, p 279. (3) T. Shida and W. H. Hamill, J. Chem. Phys., 44, 2375 (1966). (4) T. Shida and W. H . Hamill, ibid., 44, 4372 (1966). ( 5 ) P. W. F. Louwrier and W. H. Hamill, J . Phys. Chem., 73, 1702 (1969). Volume 7S,Number 6 J u n e 1969
