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Publishing Co., Amsterdam, 1975, pp 67-72. (4) N. D. Chasteen, L. K. White, and R. F. Campbell, Biochemistry, 16, 363 (1977). (5) R. C. Campbell and N. D. Chasteen, J. Bb/. Chem., 252, 5996 (1977). (6) L. K. White and R. L. Beiford, J. Am. Chem. Soc., 98, 4428 (1976). (7) J. R. Pilbrow and M. E. Winfield, Mol. Phys., 25, 1073 (1973). (8) R. Wilson and D. Kivelson, J . Chem. Phys., 44, 154 (1966). (9) B. A. Goodman and J. B. Raynor, Adv. Inorg. Chem. Radiochem., 13, 135 (1970). (IO) H. A. Kuska and M. T. Rogers in "Radical Ions", E. T. Kaiser, and L. Kevan, Eds., Wiley-Interscience, New York, 1968, pp 579-745. (11) F. E. Dickson, C. H. Kunesh, E. L. McGinnis, and L. Petrakis, Anal. Chem., 44, 978 (1972). (12) R. J. Dekoch, D. J. West, J. C. Cannon, and N. D. Chasteen, Biochemisfrv. 13. 4347 11974). (13) M. A. Krystiva, J: Masurikr, G.'Spik, and J. Montrevii, FfBSLett., 56, 337 (1975).
(14) J. L. Zweier and P. Aisen, d . Bioi. Chem., 252, 6090 (1977). (15) R. C. Woodworth, R. J. P. Williams, and B. M. Alsaadi in "Proteins of Iron Metabolism", E. Brown, P. Aisen, J. Fielding, and R. Crichton, Eds., Grune and Stratton, New York, 1977, pp 211-218; T. B. Rogers, R. A. Gold, and R. E. Feeney, ibid., pp 161-168. (16) A. Bezkorovalny and D. Grohlich, Biochem. J., 123, 125 (1971). (17) N. D. Chasteen, R. I. Belford, and I. C. Paul, Inorg. Chem., 8, 408 (1969). (16) C. P. Keijzers and E. deBoer, Mol. Phys., 29, 1007 (1975). (19) J. S. GritMh, "The Theory of Transition-Metal Ions", 2nd ed.,Cambridge University Press, London, 1964, p 437. (20) A. Carrington and A. D. McLachlan, "Introduction to Magnetic Resonance", Harper and Row, New York, 1967, p 138. (21) N. D. Chasteen, R. C. Campbell, L. K. White, and J. D. Casey in "Proteins of Iron Metabolism", E. Brown, P. Aisen, J. Fielding, and R. Crichton, Eds., Grune and Stratton, New York, 1977, pp 187-195.
Paramagnetic Species in Irradiated Organic Glasses. 2. Formation and Decay of Tetramethylethylene Radical Cations and Effects of Some Additives on Free-Radical Yields'!* Takahisa Ichikawa, * 3 Nobuaki Ohta, and Hideshl Kajioka Department of Applied Chemistry, Hiroshima University, Senda-Machi, Hkoshima 730, Japan (Received duly 18, 1977; Revised Manuscript Received October 18, 1978) Publication costs assisted by Hiroshima University
Electron spin resonance (ESR) study shows that glassy 3-methylpentane (3-MP) with 0.3% tetramethylethylene (TME) y irradiated at 77 K yields the monomer radical cations of TME (TME+),the trapped electrons (e;), and the free radicals from 3-MP. Both the TME" and the e; are unstable at 77 K, and most of them disappear within 1h. When 0.2% COzis added to 0.3% TME-3-MP, TME+ together with the COz- formed is stabilized. TME" and e; in 0.3% TME-3-methylheptane (3-MHP) are bleachable by light with X >800 nm, while TME" and COz- in 0.3% TME-0.2% C02-3-MHP are stable to photoirradiation with X >800 nm and unstable to light with X C400 nm. These photo- and thermal decays are concluded to be caused by charge recombination with a mobile electron activated from its trapping site. The radical cations of TME dimer complexes (TMEz+)are formed at TME concentrations higher than 1% and disappear by neutralization with electrons as concluded -860 nm observed in in the case of TME+. Optical absorption study shows that a broad band with A, y-irradiated 1%COz-3-MP with TME higher than 0.3% is attributed to TME2" and that TME' does not give a distinct band at wavelengths longer than 500 nm. The 100-eV yields (G values) of paramagnetic species in 3-MHP glasses with and without additives (COzand TME) are obtained by ESR methods. Free-radical yields, G(R.), in CO2-3-MHP are found to decrease with an increase in negative-charge yields, G(e;) + G(COz-),the result of which indicates the importance of the charge recombination reaction between electrons and positive RH* R. + H.). The yields of TME" in the TME-3-MHP system charges to the production of R. (e- + RH' with and without 0.5% COzare found to be approximately equal to increases in G(C0,) and G(e;), respectively. It is found that TME+ in TME-R'X-3-MP (R'X = CHJ, (CH,)&Cl, CC14)decreases with decay of R'. radicals produced by dissociative electron attachment to R'X and that yields of TME" in TME-HI-3-MP are very small. Consideration is given to the TME' decay accompanying the R'. decay in relation with the ionization potentials of the R'. radicals. Radiolysis of glassy n-butyl chloride and polycrystalline CC14each containing TME yields TME+;however, polycrystalline n-heptane containing TME and C02or n-butyl chloride does not give TME'. It is observed that ESR signals from TME+ in the CC14matrix are enhanced by photoirradiation with X >800 nm or by warming. The intensity change of TME' is reversible, indicating formation of something like a complex between TME" and CC14 or a decomposition product from CC4.
- -
Introduction The formation of solute radical cations in y-irradiated organic glasses containing small amounts of olefins has been studied systematically by Shida and Hamil14 by optical spectroscopy. They have shown that positive charge resulting from ionization of the matrices can be trapped by added olefins with lower ionization potentials than the matrices. A number of radical cations due to the added olefins have been prepared in alkyl halide glasses. 0022-3654/79/2083-0284$01 .OO/O
To the knowledge of the authors, however, there exists no electron spin resonance (ESR) study confirming olefinic cation formation in organic glasses except tetramethylethylene (TME) cation formation in 3-methylpentane (3-MP) glass.5 Detailed ESR study of cations in organic glasses should give aid to understanding of the reaction of the charge produced in organic glasses by ionizing radiation. I n the present paper the ESR studies are focused on cation formation from TME in glassy alkane and 1979 American Chemical Society
Paramagnetic Species in Irradlated Organic Glasses
the additive effects of TME and C02on the yields of free radicals from the matrix alkane. 3-Methylheptane (3MHP) and 3-MP were used as glassy matrices, Effects of radiolysis on some other matrices containing TME are given briefly.
Experimental Section Materials and Preparation of Samples. 3-Methylpentane (3-MP), 3-methylhexane (3-MHX), 3-methylheptane (3-MHP), and 4-methylnonane (4-MN) were obtained from the Chemical Sample Co. with 99% purity. They were further purified; the method was described previouslya2 Tetramethylethylene-h12 (TME) of 99% purity from the Chemical Sample Co. was passed through a 15-cm column containing finely powdered y alumina and was distilled with a 30-cm Vigreux-type column. The middle fraction was stored over sodium mirrors under vacuum. Tetramethiylethylene-d12 (TME-d12)was synthesized from acetone-d6(Stohler Isotope Chemical Co. with 99.5% D) as the starting material. The procedures include hydrogenation (LiA1H4)of acetone-d6, bromination (PBr,) of (CD,),CH(OH), addition of acetone-d6 to a Grignard reagent from (CD,)&HBr, followed by hydrolysis, and dehydration (10% aqueous H,SO,) from (CD3)2CHC(OH)(CD,),. Crude TME-d12 which contains ether as a solvent and CD2=C(CD3)CH(CD3)2 by-product was fractionally distilled by the use of a packed distillation column with 20 theoretical plates. The middle fraction was found to contain 0.01% ether and 0.1% by-product from gas chromatographic analysis. The isotopic purity of the D was 96% from nuclear magnetic resonance. Carbon-12 dioxide (Takachiho, 99.99 % purity) and 13C02(British Oxygen Co., with a stated purity of 99.6% 13C) were dried over P205under vacuum. Tokyo Kasei pure grade CH31 and tert-butyl chloride (t-BuC1) were dried aver 4A molecular sieves freshly activated under vacuum. Hydrogen iodide was prepared from hydroiodic acid; the method was described previously.2 Wako pure grade CC14 and n-butyl chloride (n-BuC1) were further purified: They were washed with aqueous NaOH and water for the former and with cold H2S04and water for the latter, followed by drying over KzCO3, and were fractionally distilled. The middle fractions were dried over activated 13X molecular sieves under vacuum. Wako pure grade n-heptane (n-Hep) was further purified; the method appeared elsewhere.6 The purified alkane was further dried over fresh sodium mirrors before use, degassed by several freeze-pump-thaw cycles, distilled from an ampule to another one cooled at dry ice-methanol temperature, and sealed in quartz ESR tubes (3-mm id., Suprasil). To remove residual small amounts of ClO,, the distillation procedure involves several pumpings on the liquid. Samples containing scavengers were prepared by adding metered amounts of the scavengers to the matrix alkane in ESR tubes on the vacuum line. We took care to avoid contamination from grease throughout the preparation of samples and used greaseless stopcocks with Teflon plugs and ball joints with Teflon O-rings in thle vacuum line. All of the ESR tubes were calibrated by comparison between the intensities of ESR signals from the tubes to which benzene solutions containing known concentrations of 2,2-diphenyl-l-picrylhydrazyl (DPI?H) had been added. For use in quantitative ESR measurements to determine relative radical yields, we selected the tubes with a deviation within -11.0% in the calibration. Irradiations and Measurements. Radiolysis was performed at 77 ].( with a 6oCoy-ray source (a dose rate of 0.6
The Journal ob Physical Chemistry, Vol. 83, No. 2, 1979 285 P: 1 yw
--
1 \< >, X
250
25G
t-+
Figure 1. ESR spectra of y-irradiated 3-MHP glasses with and without additives: (A) pure :3-MHP (microwave power is indicated); (B) 0.2% 12C02-3-MHP (C) 0.3% TME-0.2 % 12C02-3-MHP (relative gains are indicated).
Mrd h-l) or a Toshiba AFX-61A-W tungsten target X.-ray tube operated a t 45 kV and 35 mA. X irradiations were used when ESR measurements had to be done immediately after radiolysis. Photobleaching was made with a 400-W medium-pressure mercury arc for ultraviolet irradiation and a 500-W point-source xenon lamp equipped with an Ushio UI-501C lamp house for visible and infrared irradiation, ESR measurements were carried out with a conventional X-band spectrometer with 100-kHz field modulation at 77 K in the dark. IJsually, the microwave power was 50 pW for measurements of free radicals and radical ions and 0.2 pW for trapped electrons. First-derivative spectra were usually presented. Methods for integration and simulation of spectra appeared previously.2 For checking of changes in the sensitivity of ESR detection a capillary tube containing MnO dispersed in MgO was fixed in a cavity (Echo Electronics ESC-lOLD, cylindrical TEoll type with the access diameter of 15 mm). The capillary tube was attached outside a quartz quide tube (0.d. 12.2 mm and i.d. 11.2 mm) which lis inserted in the cavity through its holes. The tip (0.d. 10.6 mm) of an ESR Dewar vessel was inserted and fixed in the guide tube. Though the innermost two lines in six-line Mn2+signals overlap on lines from sample signals, the outer four lines can be used as monitor signals. In this vvay, 15 n-hexane samples y irradiated at 77 K gave the result that a standard deviation of the intensities after double integrations of the ESR signals is 0.9% to the meam value. Optical absorption spectra were recorded with Shintazu D-40R spectrophotometer (190-1000 nm) at 77 K under liquid nitrogen in a conventional quartz Dewar vessel. Sample liquid and gas were introduced into a rectangular optical cell (30 mm X lOmm X 5 mm) on the vacuum line, sealed, and irradiated with y rays at a dose of 0.3 Mrd at 77 K.
Results and Discussion Formation of Tetramethylethylene Monomer Cation (TME+). Radiolysis of glassy 3-methylheptane (3-MHP) without additives gave an ESR spectrum consisting of a sharp central singlet and eight broad lines with a spacing of 22-23 G and a binominal intensity distribution of 1:7:21:35:35:21:7:1 (Figure 1A). The singlet is due to the trapped electrons (e;)' and the eight-line spectrum is attributable to 3-MWP radicals produced by C-H bond
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A
0
20
0
40
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Time, min
Figure 4. Decay curves of ESR signals: (a) peak 7 ( 0 )and (b) peaks 4 and 10 (0)in 0.3% TME-3-MP; (c) e; peak in pure 3-MP (A); (d) peaks 4 and 10 in 0.2% TME-0.3% i3C02-3-MP (0).Plots are normalized at time 5 min, and the irradiation time is 10 min. Figure 2. Observed ESR spectra (solid line) and computed spectra (dotted line): (A) y-irradiated pure 3-MHP; (B) y-irradiated 0.3% TME-0.2% '*C02-3-MHP (a strong line from 12COLis omitted). The line widths in the computations are 17.76 G for 3-MHP radicals and 4.55 G for TME'. A
n a
i81
i
3
& o L - l . - 3 0
20
40
60
80
Time, m i n
Figure 5. (a) TME' decay of peaks 6 and 8 in 0.2% TME-3-MHP (0); (b) e; decay in ure 3 MHP (0);(c) and (d) TME' decay of peaks 6 and 8 (W) and "C02- decay (0) in 0.3% TME-0.2% I3CO2-3-MHP; (e) and (f) %O2- decay (A)and 3-MHP radical decay (V)in 0.3% '%02-3-MHP. Plots are normalized at time 5 min, and irradiation time is 10 min.
Figure 3. (A) ESR spectra of 0.3% TME: 10 min (solid line) and 60 rnin (dotted line) after 10-min radiolysis. (B) ESR spectra of 0.3% TME-1 % CH31-3-MP: 10 min (solid line) and 72 min (dotted line) after 10-min radiolysis. Arrows indicate the signals from CH,.
fission.8 When 3-MHP glass containing a small amount of 12C02was y irradiated, the sharp singlet was displaced by an asymmetrical singlet (Figure 1B). The use of I3CO2 as an alternative gave a two-line spectrum with a spacing of 123 G. These signals are due to C02- anion^.^ Figure 1C shows the spectra of 3-MHP glass containing 0.2% 12C02and 0.3% TME (mol %). It is discernible that 13 lines with a spacing of 16-17 G are superimposed on the lines from the COz- anions and the 3-MHP radicals. The results suggest the formation of a TME radical ion with 12 equivalent protons. The charge may be inferred to be positive, because the 13C02-signals in 3-MHP glass intensified by 75% on the addition of 0.3% TME. If the added TME preferably scavenged electrons produced by ionizing radiation, an increase in the anion signals from COz would not be expected. On the other hand, the addition of 0.1% hexamethylbenzene to a 0.1% TME0.170 C02-3-MHP system resulted in the disappearance of the TME+ signals. Probably, hexamethylbenzene would preferably trap positive charges, because its ionization potential (7.85 eV) is lower than that of TME (8.30 eV).'O Figure 2A shows the observed spectrum of 3-MHP glass without additives (solid line) and the computed spectrum (dotted line) consisting of eight lines with a spacing of 22.2 G and a binominal intensity ratio. The observed spectrum of the 0.3% TME-0.2% 12C02-3-MHP system may be compared with the composite spectrum consisting of a computed 3-MHP radical spectrum (85%) and a computed TME+ spectrum (15%) with a spacing of 16.5 G and a binominal intensity ratio (Figure 2B). Separations among
TME+ lines in strong-acid solutions" and adsorbed states on zeolite'' have been reported to be 16.6 and 17.5 G, respectively. Decay of TME+ u t 77 K. Figure 3A shows the ESR spectra of 3-methylpenlane (3-MP) glass containing 0.3 70 TME. The sharp line in the middle of the solid line is attributable to the et- singlet superimposed on the central line of TME'. Both the central line and the side TME+ lines decreased rapidly within 1h (dotted line). The decay curve of the e< in pure 3-MP glass is comparable to those of the e; and TMEf lines in the TME-3-MP system (Figure 4). In the presence of 0.2% C02the TME+ signals are stabilized (upper curve). The simultaneously produced COT anions also were stable within 1h. These results seem to indicate that the thermal decay of TME* is caused by charge recombination with mobile electrons activated thermally from their trapping sites. It is known that e; produced in higher molecular weight matrices than 3-MP such as 3-methylhexane (3-MHX) and 3-MHP are thermally stable at 77 K in the order of 1 h.7 The TME' produced in glassy 3-MHP containing 0.3% TME was found to be stable (Figure 5); the result further confirms the above view. Radiolysis of glassy 3-MP containing 0.3% TME and 1%CH,I yielded CH3-radicals (a four-line ESR spectrum) together with TME' and 3-MP radicals (a six-line ESR spectrum2), Methyl radicals produced by radiolysis of 3-MP glass containing small amounts of CH31are supposed to be produced by dissociative electron capture of CHJL3 and are known to decay fast at 77 K.I4 The CH30 radicals in our system decayed rapidly and most of them disappeared within 1 h (Figure 3B). It was observed that the intensity of the TME+ lines does not remain constant during the CH3. decay (Figure 6). The TME' decay is slower than the CH,. decay. The TME' in the system, however, is obviously more unstable than in the TMEC02-3-MP system. Accordingly, the observed TME' decay must be related to the CH,. decay, as the TME'
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979 287
Paramagnetic !Species in Irradiated Organic Glasses
Vn
Time,min
I! I!
Figure (5. (a) and (b) TME' decay of peak 7 (0)and CH,. decay (0) in 0.3% TME-1 % CH31-3-MP; (c) and (d) TME' decay of peaks 4 and 10 (A) and fert-butyl radical decay (V)in 0.3% TME-1% t-BuCI-3-MP (e) TME' decay of peaks 5 and 9 (0) in 0.3% TME-1 % CC14-3-MP. Plots are normalized at time 5 min, and irradiation time is 10 min.
decay in the 'TME-B-MP system is caused by the e; decay. Sprague15and Willard et al.16 reported the result that most CH3. radicals in y-irradiated CH31-3-MP systems decay through abstraction of hydrogen atoms from 3-MP molecules. In our system, if sufficient amounts of CH3. radicals can diffuse to the TME+ cations without reacting with encountered 3-MP molecules, the TME+ decay will be explained by the addition of CH3. to TME' and/or the H-atom abstraction of CH3. from TME'. Both of' the reactions for rn carbonium ions of TME derivatives. If it is not the case, the possibility exists that the TME+ cations decay by charge recombination with electrons tunneling from CH3- or I-.17 The reaction due to electrons ejected by the back-reaction of CH,. within matrix cages, CH3-+ I- CH31 -1- e-, may be excluded, since the reaction of CH31 with electrons is believed to be exothermic. When various alkyl halides (RX) are used as electron scavengers, the above three possible reactions are generalized as reactions 1-3. Here, R., R', and X. are free radicals, carbonium ions, and halide atoms, respectively.
-
+
TME+ R . 4 Me2C+CMe2R TME+ TME'
and/or
Me2C+CMe=CH2 (1)
+ Re TME + R+ + X- -* TME + X* +
(2)
(3)
Figure 7A shows the ESR spectrum of X-irradiated 3-MP glass containing 0.3% TME and 0.3% HI. Weak TME+ lines are observed to overlap on broad lines from the 3-MP radicals. Asymmetrical lines discernible in the central part of the spectrum are due to color centers of irradiated quartz. The TME' signals were stable within 1 h after irradiation. The relative yield of TME+ to the 3-MP radical was about 4% from Comparison with simulation spectra of TME-3-MP systems. The yield is about one-fi€thof that obtained in a 0.3% TME-3-MP system containing 0.3% C C 4 added as an electron scavenger. The result seems to indicate that reaction 1mentioned above is a main cause for the marked decrease in the yield of TME', It seems probable that all of the H atoms produced from HI in lhe system react instantaneously after the formation, because of the absence of H-atom trapping in y-irradiated HI-3-MP systems.18 Accordingly, the observed stabiliization of the TME+ cations surviving seems likely to show a negligible contribution of reaction 3 to the decay reaction of TME+ in the TME-CH31-3-MP system. Reaction 2 is unlikely, because the ionization potential of an H atom (13.60 eV) is very high in comparison with that of TME (8.310 eV). X irradiation of a 0.3% TME-3-MP system containing 1% tert-butyl chloride (t-BuC1) gave the ESR spectrum
u! Pct,,
n
-
Figure 7, (A) ESR spectrum recorded 2 min after 10-min X irradiation of 0.3% TME--0.3% HI-3-MP. (B) ESR spectra of 0.3% TMEI-l% f-BuCI-3-MP 2 mini (solid line) and 23 h (dotted line) after 10-min X irradiation. (C) ESR spectra of 0.3% TME-1% CC14-3-MP 5 min (solid line) and 7 h (dotted line) after 5-min X irradiation.
(solid line in Figure 7B) consisting of lines from the 3-MP radical, the TME+ cation, and the tert-butyl radical (l-Bu, 10 lines with a spacing of 22.8 G). The TME+ decay in the system i s slower than that in the TME-CH31-3-MP system (Figure 6) and faster than that in the TMEC02-3-MP system (Figure 4). The ESR spectrum (dotted line in Figure 7Bl)measured 24 h after irradiation indicates the absence of the TME+ signals, while in the TMEC02-3-MP system only 15% of TME+ decayed during the same period. If the tert-butyl radical can diffuse in the 3-MP glass, TME+ decay through reactions 1 and 2 will be possible; the ionization potential of tert-butyl radiicals has been reported to be 7.42 eV.19 Neiss and Willard14 have found that about half of 3-MP radicals produced in y-irradiated pure 3-MP glass decay with second-order kinetics by a reaction following random diffusion. Their result suggests tlhat the tert-butyl radical whose molelcular size may not differ much from that of the 3-MP radical can diffuse in 3-MP glass a t 77 K. In reaction 2 direct contact of the tert-butyl radical with TME+ will not be necessary for the electron-tunneling reaction. The ESR spectra of X-irradiated 0.3% TME-1% CC14-3-MP system are shown in Figure 7C; an arrow indicates a broad singlet due to CCl,. radicals.20 Both TME+ and Wl3. decayed gradually. The TME' decay in the system (Figure 6) is slower than that in the TIMEt-BuCl-3-MP system and faster than that in the TIMEC02-3-MP system. The ionization potential of CC13-(7.92 e V 1 is somewhat higher than that for tert-butyl radicals and lower than 8.30 eV for TME+. This might be a cause of the relatively slow decay of TME' in the TMECC14-3-MP system. In the case of the TME-CH31-3-MP system, TME' is observed to decay rapidly. From the view of the ionization potential of CH3. (9.84 eV)22the reaction of CH3. with TME' will b e interpreted by reaction 1, because reaction 2 will be endothermic in this case. The observed fast CIH3. decay in comparison with the TIME" decay in the TM[E-CH31-3-MP system would perhaps be due to contribution from CH3. decay processes other than the reaction with TME+, i.e., H-atom abstraction from 3-MP molecules and recombination with each other.'" On the other hand, the decay of the tert-butyl radical is sllower than that of TME+ as observed in Figure 6 (curves c and
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A
1 In
B
r MnU
Figure 8. (A) ESR spectra of y-irradiated 0.3% TME-3-MHP before (solid line) and after (dotted line) photobleach with h >800 nm. (6)ESR spectra of y-irradiated pure 3-MHP before (solid line) and after (dotted line) photobleach with >800 nm. Peaks indicated by arrows are signals from Mn2+ in MgO in a capillary tube.
d). The cause of this is considered to be low reactivity of the tert-butyl radical in the matrix. Because the 3-MP radical is the secondary radical; abstraction of a secondary H atom from 3-MP by the tert-butyl radical will be energetically unfavorable. The dotted-line ESR spectrum in Figure 7B shows the presence of the tert-butyl radical but the absence of TME', which is probably due to difference in their radical yields; in TME-3-MHP systems yields of anionic paramagnetic species exceed TME+ yields as shown later in this text. Photoirradiation Effects on TME+. The solid-line spectrum in Figure 8A was obtained by y irradiation of 3-MHP glass containing 0.3% TME; the e< and TME+ lines superimposed on broad 3-MHP lines can be seen. The yields of TME+ and e[ were 5.7 and 14.4% of that of the 3-MHP radical from double integrals of the spectrum. When the sample was irradiated at 77 K for 5 min by filtered light (A >800 nm), the spectrum changed into the dotted-line spectrum. The e; lines almost completely disappeared, while about 35% of the TME" initially produced still survived. Figure 8B shows the corresponding change of the spectra of pure 3-MHP glass. The residual TME+ peaks were completely removed by UV irradiation (A 800 nm) and decreased by about 9% to the initial yields on 5-min irradiation with light of X >480 nm. Further 5-min bleaching (X >365 nm) resulted in 70% decrease for TME+ and 80% decrease for C02- relative to the initial yields (dotted line). The residual TME' and C02- were completely removed by UV irradiation (A >215 nmIz3as shown in Figure 9B. The COz- anion spectrum produced in a 0.3% 13C02-3-MHP system was observed to decrease in its intensity in the similar manner on photobleaching. Willard and co-workersgbhave observed that C02- produced by radiolysis of 3-MP glass containing l2CO2 is bleached by light of 480 nm and below. These results clearly show that the TME+ cation itself is stable to exposure to light with wavelengths longer than 800 nm and that the observed TME+ decay by the IR irradiation in the TME-3-MHP system is due to charge recombination
Figure 9. ESR spectra of y-irradiated 0.3% TME-0.2% "C02-3-MHP: (A) before (solid line) and after (dotted line) photobleach with h >365 nm; (6)after successive photobleach with X >215 nm.
A
Figure 10. Observed ESR spectrum (A) and computed spectrum (€3) (computed for 40 % TME and 60 % 3-MP radical) for y-irradiated 5 % TME-0.2% "CO2-3-MP. The line widths in the computations are 16.43 G for 3-MP radicals and 5.30 G for TME,'.
with electrons ejected from their trapping sites. It is to be noted that the optical absorption spectrum of TME+ in 3-MP glass is transparent in the wavelengths longer than about 500 nm, as is shown later in the text. In the TME-COz-3-MHP system most of the TME' bleached by photoirradiation would seem to decay through charge recombination with electrons from COS-, though there exists no definitive evidence. Nature of Dimer Cation TME2+. There appeared new ESR lines at TME concentrations higher than about 1.0%. Lines from TME+ were completely substituted by the new lines at 3% TME (Figures 10A and 11). The spacing of the new lines was about half of that of the TME+ lines. This suggests the formation of the dimer cation with 24 equivalent protons. Figure 10B shows the simulation spectrum composed of a computed TME2+spectrum with a spacing of 7.8 G (40%) and a computed 3-MP radical spectrum with a spacing of 22.2 G (60%). The value of 7.8 G is smaller than half of the spacing found for TME+ (16.5 G), in agreement with what may be expected for dimer cations.24 The TME2+lines were observed at a TME concentration as high as 50%; appearance of any other multilines was not discernible. Polycrystalline pure TME showed a poorly resolved broad spectrum. Solidifed 3-MP containing more than 5% TME became opaque, which seems to indicate segregation of TME. The formation of TME2+will perhaps be interpreted by capture of positive charges by preexisting neutral TME dimers, because the formation of TME+ was not observed at measurement immediately
Paramagnetic Species in Irradiated Organic Glasses
Flgure 11. ESR spectra of 1 % TME-1% CCI4-3-MP: (A) measurements 5 min (solid line) and 60 min (dotted line) after 10-min X irradiation; (B) measurement after 19-h storage at 77 K. An arrow shows a CCI3. signal.
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The Journal of Physical Chemistry, Vol. 83, No. 2, 1979 289
Flgure 13. ESR spectra before (solid line) and after (dotted line) photobleach with X >660 nm, Xi >325 nm, and 1, >215 nm: (A) y-irradiated 5 % TME-3-MP; (B) y-irradiated 3% TME-0.2% '3C0,-3-MHP. Arrows indicate presence of peaks due to the allylic radical.
u
0
20
40
60
Time, m i n
Figure 12. (a), (b), and (c) Decay of peak 0 (0),peaks f2 (m), and peaks f 3 (0)in 10% TME-3-MP; (d) and (e) decay of peaks f 2 (A) and peaks f 3 (A) in 7.5% TME-0.2% '*C02-3-MP. Plots are normalized at time 5 min, and irradiation time is 10 min.
after irradiatiion of 3% TME-3-MP systems containing C 0 2 or CC1,. Dimer cation formation due to association of neutral TME with TME' at 77 K was observed in a 1% TME-I.% CC14-3-MP system (Figure 11). Growth of TME2' peaks and decay of TME' peaks can be seen. Appearance ad weak TME2+peaks among strong TME+ peaks was oblserved after storage of a 0.3% TME-1% CC14-3-MP sample for 5 days at 77 K; however, no dimer peaks were discerned in a 0.1% TME-1% CC14-3-MP sample under the same conditions. These results indicate that association resulting in TME2+ in these systems requires times of 1 day to 1 week. Nevertheless, local heating due to the action of ionizing radiation, if effective, would transiently soften the matrix. This might be the cause of the association between TME and TME' at high TME concentrations. Decay curves of TME2+are shown in Figure 12. TME2+ in 3-MI? glass without electron scavengers decayed fast in the first 60 min after radiolysis. Line 0 which is attributable to the e< singlet superimposed on the central TME2+ peak (Figure 13A) shows an intermediate decay curve in comparison with decay curves for lines f 2 and *3. As reported p r e v i o u ~ l y ,TME,+ ~~ in 3-MP glass changes into the allylic radical Me2C=CMeCH2. a t 77 K by intramolecular proton transfer. The slow decay of lines f 2 is due to the formation of the allylic radical whose lines overlap on lines f 2 . Lines f 3 do not overlap on the allylic radical lines. When C 0 2 was added to a TME-3-MP system, the TME2+formed was stabilized as shown by the upper curves in Figure 12.26 Accordingly, the observed rapid decay of TME2' will be mainly caused by charge
OL200
'
400
I
600
800
1000
Wavelength, nm
Figure 14. Optical aibsorption spectra of y-irradiated 3 % TME- 1 % l2CO2-3-MPbefore ( 1 , -) and after (2, ---) photobleach with >365 nm and y-irradiated 10.3 % TME-1 % "CO2-3-MP before (3, ---) and after (4, --.) photobleach with X >365 nm.
recombination with electrons ejected thermally from their trapping sites. When a y-irradiated 5% TME-3-MP sample was exposed to light with h >660 nm for 5 min, both the e; and the TME2+signals disappeared (Figure 13A). The solid-line spectrum in Figure 13B was obtained from a 3% TME-0.2% 13C02-3-MP sample. Both the COf and the TME2+signals were stable to exposure to light with X >BOO nm, were diminished in their intensities by irradiation of light with XI >325 nm (dotted line), and disappeared by further irradiation of 250-nm region light (lower dotted line). It can be seen that lines from the unbleachable allylic radical overlaps ion the 3-MP radical lines as shown by arrows. As previously concluded in the case of TME', the observed TMEz+ decay due to photobleaching may be ascribed as charge recombination with electrons. Hamill and c o - w o r k e r ~ ~have ~ ~ ' found that a number of olefins form their molecular cations in y-irradiated matrices of alkyl chlorides and 3-MP. In the case of TIWE in butyl chloride glass, a broad optical absorption band with, ,A of 866 rim has been observed and attributed to TME+.4 We also found such an absorption band in a 3% TME-1% C02-3-MP system (curve 1 in Figure 14); however, its optical density decreased with decrease in TME concentrations. The absorption band became very weak at 0.3% TME (curve 3) and disappeared at 0.1% TME. Photobleaching by UV light induced a decrease in the optical density of the broad band (curve 2). These
290
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979
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T. Ichikawa, N. Ohta, and H. Kajioka
Ill1
Flgure 15. (A) ESR spectra of pure n-BuCI recorded immediately after radiolysis (solid line) and after 2-week storage at 77 K (dotted line); (6) ESR spectra of 3% TME-n-BuCI (solid line) and 0.3% TME-n-BuCI (dotted line) recorded immediately after radiolysis; (C) ESR spectrum following measurement of 3% TME-n-BuCI after 2-week storage at 77 K.
behaviors of the broad band are similar to those of ESR lines of TME2+. Therefore, the broad band is attributable to TME2' rather than to TME'. We could not find a distinct band due to TME+; however, the 290-nm-region band in curve 3 which appears by reducing the TME concentration from 3 to 0.3 % was considerably decreased by photobleaching by UV light (curve 4). We tentatively assign this band to TME+. Formation of TME Cations in Some Other Matrices. Nauwelaerts and Ceulemans have reported that in y-irradiated n-pentane containing 0.5% TME and 1.5% 2chloropentane a difference ESR spectrum between before and after photobleaching with 500 nm light gives a spectrum due to TME+.28 We examined the formation of T M E cations in a polycrystalline matrix of n-heptane (n-Hep). Differences among ESR spectra of a pure n-Hep sample and 3% 12C02-n-Hep samples with and without 0.3% TME were not apparent from their first-derivative spectra, indicating an absence of C02- and TME+. Addition of 3% n-butyl chloride (n-BuC1) to n-Hep resulted in small changes in the central part of the pure n-Hep spectrum, but no changes were observed on addition of 0.3% TME to n-Hep with 3% n-BuC1. We conclude that trapping of TME+ in n-Hep at 77 K does not occur or is undetected under our experimental conditions. Williams et al.29noted that radiolysis of n-BuC1 glass containing TME does not yield radical cations of TME. The solid line in Figure 15A shows the ESR spectrum of pure n-BuC1 glass recorded immediately after radiolysis. The spectrum is characteristic of that of the n-butyl radical.,O Spectral changes can be seen when 3 or 0.3% TME is added to n-BuC1 (Figure 15B). The formation of new lines is more evidently shown in Figure 15C recorded 2 weeks after radiolysis of the 3% TME-n-BuC1 sample. The dotted line in Figure 15A shows spectral changes of the pure n-BuC1 glass after storage for the same period. The average separation between the new lines was 16.0 f 0.5 G. The value is close to that of 16.5 G for TME' in 3-MHP glass, indicating the formation of TME'. Eleven of the thirteen lines expected for TME+ were observed. The line widths (AH) o l the component lines as measured between points of maximum slope, however, are considerably broadened, e.g., 4.1 G for line 9 in Figure 1C and 7.3 G for the corresponding line in Figure 15C. This may be a cause for the fact that the formation of TME+ in
Flgure 16. ESR spectra of y-irradiated 3 % TME-CC14: (A) measurements immediately after radiolysis (solid line) and 2 min after photobleach with A >500 nm (dotted line); (B) measurements 2 min (solid line) and 7 min (dotted line) after photobleach with A >430 nm; (C) measurement at 200 K.
n-BuC1 glass had not been recognized by ESR methods. It is to be noted that the formation of TME2+ was not observed at a high TME concentration of 3%. Our optical measurement for a 3 % TME-n-BuC1 sample also failed to detect the broad absorption band (Amm -860 nm) due to TME2+. The solid-line ESR spectrum in Figure 16A was obtained by radiolysis of polycrystalline CCl, containing 3% TME. It was observed that TME', CCl,., and unidentified species are trapped. Weak TME+ lines overlap on a strong CCl,. singlet whose center shifts to low field by the value corresponding to the g factor being 2.0132 f 0.0004. The unidentified species show a partly resolved broad singlet. The center is further shifted (g = 2.092 f 0.003) and the high-field tail of the spectrum lifts slightly the low-field side of the CCl,. spectrum. Radiolysis of pure CCl, gave these two types of spectra. Samples of TME concentrations of 0.1-10% were tested and all formed TME'. TME+ lines in the case of 10% TME were somewhat weakened. Irradiation of the 3% TME-CC1, sample by light with X >500 nm enhanced the TME" signals (dotted line in Figure 16A). The enhanced signals decreased within several minutes to the initial intensities. Such phenomena were observed by photoirradiation with a Toshiba IR-D1B (X >800 nm) glass filter. Irradiation by light with X >430 nm (Toshiba V-Y45) induced marked enhancement of TME+ (Figure 16B). Part of CCl,. and most of the unidentified species were lost by the illumination. Both radical species in pure CCl, gave a similar result t o the illumination (A >430 nm). ESR measurements during IR irradiation (A >800 nm) of the 3% TME-CC1, sample after photobleaching (A >430 nm) showed growth of TME' until a steady-state intensity was reached. Interruption of the illumination resulted in a decrease in the intensity to the intial state (dotted line in Figure 16B). Photoirradiation with Toshiba V-Y43 (A >410 nm) and V-R65 (A >620 nm) glass filters gave approximately the same steady-state intensit,y, indicating ineffectiveness of the visible light. The line widths (AH) of TME+ did not change at ESR measurement before and during IR irradiation. A 77 K ESR measurement immediately after warming of a y-irradiated 3% TME-CC1, sample to 120 K for 10 min showed results similar to those observed in the case of photobleaching,
Paramagnetic Species in Irradiated Organic Glasses
The Journal of Physical Chemistty, Vol. 83,No. 2, 1979 291 1
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EISI 8
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Flgure 17. Microwave power saturation curves for e,- (0)and 3-MHP radicals ( 0 )in pure 3-MHP and TME’ (0)and %02- (A) in 0.3% TME-0.2% ’3C02-3-MHP. The dose for y irradiation is 0.3 Mrd.
i.e., an increase of TME+ and loss of CC13. and the unidentified species. The following ESR measurement at 200 K gave sharp ITME+ lines (Figure 16C). The TME+ signals were sti3ble at this temperature and AH of line 9 indicated by an arrow was 1.63 G. When the sample was cooled back to 77 K, the spectrum changed reversibly into the initial state. These results seem to indicate formation of something like a complex. Its nature will be described as follows: The “complex” decomposes to produce TME+ on warming or upon IR irradiation and is recovered without loss by cooling or stopping of the irradiation. Irradiation by light with X >430 nm or warming to 120 K induces the “complex” for mation accompanied by loss of CC13. and the unidentified species; however, some amounts of the complex already form before these treatments (from the result obtained in Figure 16A). Experiments to identify the complex amd ESR measurements of y-irradiated C C 4 containing some other olefins are in progress.31 Yields of Ptiramagnetic Species in 3-MHP Glasses. We selected 3-MI1P for a matrix in this work because decay of paramagnetic species in 3-MHP glasses at 77 K is much slower than in 3-MP glasses as shown in Figures 4 and 5. ESR measurements in this work were made within 30 min after a 30-min y irradiation. Accordingly, errors of 5 1 0 % for e< and 1-3% for TME’, CO,, and 3-MHP radicals (R.) are expected in comparison with yields obtained by measurements about 5 min after a 10-min radiolysis. Figure 17 shows microwave power saturation curves for e; and R. in pure 3-MHP glass and TME+ and 13C02-in a 0.3% TME-0.3% 13C02-3-MHP system. It is observed that the e; signal is easily saturated at low microwave power and th,at the curve passes through a maximum a t P = 0.025 mW. Results similar to this bave been obtained by Williams el, al.’ for e; in y-irradiated 3-MP and 3-MHX glasses. It appears that the curve for TME+ reaches a maximum at siomewhat lower microwave power (P = 9-16 mW) than the curve for Re. The 13C02-signals are observed to increase linearly with the square root of power below 10 mW. The result disagrees with that obtained by Williams et al.;7they observed that signals from I2CO2-in 3-MP glass reaches a maximum at P = 0.25 mW. Our result seems consistent with the expectation that such oxygen-containing radicals are not so easily saturated at low microwave power because of relaxation through the relatively large spin-orbit coupling of oxygen atoms.32 On the basis of our results, we recorded at P = 0.2 pW to determine e; yields or otherwise at P = 50 pW. Figure 18A shows the ESR absorption spectra of yirradiated pure glasses of 3-MHX, 3-MHP, and 4methylnonane (4-MN). Subtraction of the central sharp e; signals from the absorption spectra yields broad free-radical (R.) signals. The 100-eV yields (G values) of e< and R- in these glasses can be obtained on the as-
J
1 , ;
-
Figure 18. (A) ESR absorption spectra of y-irradiated 3-MHX (-), 3-MHP (---), and 4MN (-.-) without additives. (B) ESR absorption spectra of y-irradiatiad pure 3-MHP (solid line) and 1 % 13C02-3-MHP (dotted line).
TABLE I: G! Values of Trapped Electrons and Free Radicals in Pure Alkane Glasses yIrradiated at 7 7 K alkane etR* total 3-MHX 3-MHP 4-MN
0.49 0.31 0.17
2.95 3.50 3.90
3.44 3.81 4.07
sumption that the G value of the 3-MHP radical33 is 3.5 (Table I). Relatively large errors are introduced to the values of yields from weak signals such as e; in the subtraction processes. Considering this, we estimate the errors for the values in Table I to be within f10% for e;, i5% for Re, and rt3% for the total amounts. The results seem to indicate that a decrease in the e; yields with an increase in the molecular weights induces an increase in the R. yields. Such a tendency is observed in the values of e; and R- yields in various alkane glasses reported by Willard and c o - ~ r o r k e r sthough , ~ ~ ~ ~some ~ exceptions are found in the values. A close correlation between yields of e; and R. will be presented later in the section. The e; yields for 3-MHX. and 3-MHP in Table I are considerably small in comparison with those reported by Williams et ala7(0.87 for 3-MIHX and 0.68 for 3-MHP) and Willard et al.34 (0.70 for 3-MHX and 0.53 for 3-MHP). Our values are not corrected for effects of microwave power saturation and thermal decay; however, it seems unlikely to us that all of the causes for the smallness of our values are due to these effects. Figure 18B shows the absorption spectra of pure 3-MHP glass and 3-MHP glass containing 1% 13C02. It is clearly seen that the forrnation of C02- induces a decrease in the yield of R.. The curves in Figure 19 show the effects of C 0 2 concentrations on the yields of e;, COB-,R., and the total. It is observed that G(eC) and G(R.) decrease with an increase in GI,C02-) and that G(e;) becomes zero at 0.3% COz. G(tota1) is approximately constant within the C02concentrations tested. The observed decrease in G(e;) will be due to competition of electron trapping with C02. Increases in negative-charge yields, 6(e-), and decreases in G(R-),6(R-),are presented by an enlarged scale, indicating a close correlation between the two quantities. Here, 6(e-) and 6(R.) are defined as follows: 6(e-) = G(C02-) G(e;) - Go(e;); 6(R.) = Go(R-)- G(R.); Go(e;) and Go(R.) are yields without COz. Figure 20A shows the absorption spectra of TME-13C02-3-MHP systems; appearance of TME+ peaks
+
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The Journal of Physical Chemistry, Vol. 83, No. 2, 1979
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T. Ichikawa, N. Ohta, and H. Kajioka
06
04
04
02 0
02
0.7
I_____
'0
02
04
06
08
10
[C o,] , mol */.
Flgure 19. Additive effects of COPon the yields of et: (V), C0,- (A), R-, (0),and the total (0) in y-irradiated 3-MHP; the increase in the negative-charge yields @(e-))(A)and the decrease in the R- yields (b(R.)) ( 0 )are presented by an enlarged scale.
0.1
0
0.2
0.4
0.3
[TME) ,mol
Figure 21. Additive effects of TME on the yields of TME' (V), COp(A), R. (0),and the total (U) in y-irradiated 3-MHP containing 0.5% COP; increases in the yields of TME' (S(TME')) (V)and Cop- (6(C02-)) (A)are presented by an enlarged scale. The dotted line shows changes in relative heights of the first-derivative peaks for 3-MHP radicals when TME-dlz is used in place of TME-h,,. Total
1 I
0.7 0
0
6
01
01
02
0.3
0
[TME) , m o l %
Figure 20. (A) ESR absorption spectra of y-irradiated 0.3% TME-0.5% 13C02-3-MHP (solid line) and 0.5 % 13C0,-3-MHP (dotted line). (B) Calculated spectrum consisting of 16.25% TME' and 83.75% 3-MHP radicals (solid line) with the 3-MHP radical spectrum included.
and growth of 13COz-peaks are discerned. The calculated absorption spectrum consisting of lines from TME+ (16.25%) and 3-MHP radicals (83.75%) is shown by the solid line in Figure 20B; the underlying dotted line is the calculated spectrum for the 3-MHP radicals included. Relative yields for T'ME+, GOz-, and R. can be easily obtained: The procedure includes subtraction of an assumed 3-MHP radical spectrum from the observed one, followed by further subtraction of a 13C02-spectrum (the line shapes are assumed from the observed one (Figure 18B))from the difference spectrum obtained. Thus, effects of TME concentrations on these anions, cations, and free radicals are obtained (Figure 21). Both the anions and the cations increase with an increase in TME concentrations. It is shown at an enlarged scale that the increase in G(C02-),6(C02-), is approximately equal with increase in G(TME+), G(TME+). G(tota1) increases with TME concentrations and reaches a maximum at 0.25% TME. A small decrease in G(R-)is observed; G(R.) decreases by 0.03 and 0.12 at 0.1 and 0.3% TME from the yield without TME (G,(R.) = 3.06). On the other hand, the first-derivative peak heights for lines 2 and 7 of 3-MHP radicals (Figure 2A) decrease considerably on the use of perdeuterated TME (TME-d12)in place of perprotiated TME (dotted line in Figure 21). Lines from the cations of TME-d,, do not overlap on the outer lines of 3-MHP radicals (aD= 0 . 1 5 3 ~ ~Changes ). in G(R.) may be directly
Flgure 22. Additive effects of TME on the yields of TME' (V), e; (A), R. (0),and the total (0) in y-irradiated 3-MHP; increases in the yields of TME' (b(TME')) (V)and e; @(e-)) (A)are presented by an enlarged scale. The dotted line shows changes in relative heights of the firstderivative peaks for 3-MHP radicals when TME-d,, is used in place Of TME-hlZ.
observed from changes in the first-derivative peak heights of 3-MHP radicals in TME-d12-12C02-3-MHP systems. The peak heights in a 0.5% 12C02-3-MHP sample are normalized to the value for Go(R.). The decreases in G(R.) under the above assumption are 0.25 and 0.38 at 0.1and 0.3% TME. The discrepancy of the values with those obtained from double integration is considered to be due to superposition of lines from some radicals on the 3-MHP radical lines under the condition that the lines do not overlap on peaks of lines 2 and 7 of the 3-MHP radicals. If the lines, moreover, are relatively broad, it will be generally difficult to recognize the presence of the minority radicals. It may be possible that added TME reacts with hydrogen atoms produced by radiolysis of 3-MHP glass, giving the solute radicals, MezHCCMe2 and/ or Me2CCMe=CH2. Iwasaki et al.35 found that 77 K radiolysis of neopentane containing 2 % isobutene produces the solute radicals a t the yield of 36% to the total. The curves in Figure 22 show the effects of TME concentrations on the yields of e;, TME+, Re, and the total in TME-3MHP systems without GO2. It is observed that the curves for G(R.) obtained from double integration and first-derivative peak heights have curvatures similar to those of the corresponding curves for G(R.) in Figure 21. The increases in G(eJ are observed to coincide with the increases in G(TME+). Some G values for paramagnetic species produced in 3-MHP glasses are arranged in Table 11.
The Journal of Physlcal Chemistry, Vol. 83, No. 2, 1979 293
Paramagnetic Species In Irradiated Organic Glasses
TABLE 11: :C Valuesa of Paramagnetic Species in ?-Irradiated 3 - M H P Glassed a t 7 7 Kb sample
e(
pure-3-MHE’ 0.5% CO, -3-MHP 0.3% TME-0.5% co, -3-MILIP 0.3% T M E - 3 - M H P
0.28C
R.
total 3.78
0.55
3.50 3.06 2.94
0.19
3.23
3.89
C0,- TMB 0.60
1.10 0.47
3.66 4-59
a All values were n o r m a l i z e d to t h e free-radical y i e l d in The p u r e 3 - M H P whose value was r e p o r t e d to b e 3.5.33 Values for G(e
