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Electron scavenging at very low temperature
Int. J. Radiat. Phys. Chem. 1974, Vol. 6, pp. 393-400. Pergamon Press. Printed in Great Britain
ELECTRON
SCAVENGING
AT
VERY
LOW
TEMPERATURE TAKENOBU HIGASHIMURA Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka-fu, Japan (Received 11 October 1973; in revised form 10 June 1974)
Abstract--Results of electron scavenging experiments for gamma irradiation at 4 K are reviewed. An anomalous increase in the scavenging efficiency at 4 K is discussed. Intermediate anions which have very short lifetimes at usual temperatures are very stable at 4 K. As an example of these anions, the optical absorption spectrum of the benzyl chloride anion is described. 1. INTRODUCTION THE DISCOVERY of the hydrated electron in 1962 (1) was an epoch in radiation chemistry. Studies on the yields and the reaction of the hydrated electron and the solvated electron in a variety of organic media have been most fruitful in this field. Very m a n y data have been accumulated on the reaction constants of the solvated electron. Much experimental and theoretical work on the physico-chemical structure of the solvated electron has been published (2). But for a long time there was no experimental evidence on the state of electrons before solvation. The first work on the electron prior to solvation was done in 1967 by employing g a m m a radiolysis at 4 K and E.S.R. measurements at the same temperature ts~. The linewidths of the E.S.R. signals of the electron is some glasses, ethanol, 3-methylpentane, etc., irradiated at 4 K showed remarkable differences from the corresponding linewidths obtained by irradiation and measurement at 77 K. These differences in E.S.R. linewidths were not due to their reversible dependence on temperature, but to an irreversible change upon warming to 77 K t4). It became highly plausible that the electron observed at 4 K is a trapped electron around which permanent dipoles of solvent molecules are not oriented.* In 1970, the optical absorption spectra due to such unsolvated electrons were obtained by two different methods, i.e. microsecond pulse radiolysis at 77 K (5~, and g a m m a radiolysis at 4 K (e~. After that, trapped electrons were found in a variety of matrices and their physico-chemical properties have been investigated in detail. However, pre-solvated electrons are not observable in some matrices. The solvation time in these substances may be too short to observe electrons in their unsolvated states. The mechanism of electron scavenging in the liquid and in the glassy phases has been investigated for a long time, and fairly recently interesting hypotheses have been presented: dry electron by Hamill (7~, mobile electron by Hunt (s~ and electron tunnelling by Miller tg~ and other groups tl°~. Electron scavenging at 4 K was studied by our group and anomalous phenomena were found. This paper is an autoreview on the electron scavenging experiments at 4 K.* It includes results on the very low * In this article, this pre-solvated electron is named the "trapped electron" and the electron toward which surrounding permanent dipoles are oriented is called the "solvated electron", though the latter appears in the solid phase. Solvated electrons are observed at 77 K. In some kinds of matrices, such as methanol and monodeuterated ethanol, they can be observed at 4 K. 393
394
TAKENOBU HIGASHIMURA
temperature effect on electron scavenging efficiency and intermediate species observed when scavenger molecules capture electrons at 4 K. 2. EARLY EXPERIMENTS ON TRAPPED ELECTRONS AND ELECTRON SCAVENGING AT 4 K Ethanol glass irradiated with g a m m a rays at 77 K shows an absorption spectrum with a m a x i m u m at 545 nm. When the ethanol glass is irradiated at 4 K and measured without warming, it shows an intense absorption in the I.R. region with a m a x i m u m at 1490 nm (Fig. 1). When this is warmed rapidly to 77 K, the absorption in the I.R. i
l
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FIG. 1. Optical absorption spectra of the trapped electron and the solvated electron in ethanol. (a) Irradiated and measured at 4 K. (b) Irradiated at 4 K and measured at 77 K. (c) Irradiated and measured at 77 K. (d) Irradiated at 77 K and measured at 4 K. disappears and the optical densities in the visible region increase remarkably. The latter spectrum (?'max = 540 nm) is very similar to that of the ethanol glass irradiated at 77 K. The I.R. band does not reappear if the irradiated glass is cooled again to 4 K. The E.S.R. spectrum of this species shows a similar irreversible change from a sharp singlet with a width of 6.5 + 0.5 G at 4 K to a broad singlet with a width of 1 5 . 0 + 2 . 0 G at 77 K (11,12). Such results, consistent with the results obtained by pulse radiolysis at 77 K ~5,13~, lead us to the conclusion that, when irradiation is carried out at 4 K, the electron is trapped at a site surrounded by molecular dipoles with r a n d o m directions and, when irradiation is carried out at 77 K, the electron is surrounded by glass matrix molecules favourably oriented to relax the Coulombic force~3L In the course of our optical study on the trapped electrons at 4 K, an interesting phenomenon was found concerning the yield of trapped electrons in 2-methyltetrahydrofuran ( M T H F ) glassmL When the glass irradiated at 4 K is warmed rapidly to 77 K, the yield of solvated electrons is only about one-quarter of that produced by irradiation and measurement at 77 K. In the case of E.S.R. experiments, however, the yield of solvated electrons in M T H F glass, produced by irradiation at 4 K and * Scavenging experiments were carried out by Hirotomo Hase, Takenobu Higashimura, Akira Namiki, Masato Noda, Masaaki Ogasawara, Hiroshi Yoshida and Tetsuo Warashina of Kyoto University, except M. O. and H. Y., of Hokkaido University.
Electron scavenging at very low temperature
395
then warmed to 77 K, is about the same as that in a sample irradiated at 77 K. In our E.S.R. experiment, M T H F was purified by repeated fractional distillation, dried with a sodium potassium mirror and sealed in v a c u o into the sample tube. In the case of optical absorption measurements, reagent grade M T H F was used without further purification. Therefore, a discrepancy in temperature dependence of the yield of solvated electrons might be caused by impurity molecules in the optical absorption sample. However, the concentration of impurities in this sample must be very small because the yield of solvated electrons in the sample irradiated at 77 K was not much different from the reported value ~14~. Only the yield in the sample irradiated at 4 K was much smaller. Hence it seemed plausible to assume that the electron scavenging efficiency of the impurity molecules is remarkably larger at 4 K than that at 77 K. In order to obtain quantitative data on this apparent increase of scavenging efficiency at 4 K, scavenging experiments were carried out, first with biphenyl solution in M T H F by E.S.R. measurements (15~, and then with benzyl chloride solution in ethanol by optical absorption measurements t~e). 3. EXPERIMENTAL Samples
MTHF was purified by repeated fractional distillation in vacuo and was dried with a sodium potassium mirror. Other solvents and all solutes of reagent grade were used as received. For E.S.R. measurements, samples were sealed in vacuo into Suprasil silica tubes; glassy specimens were prepared by immersing the sealed tubes into liquid nitrogen, and then transferring them into liquid helium. MTHF sample tubes frequently broke when cooled in liquid helium. For preparing samples of alcohol for optical absorption measurements, a stainless steel disc with a circular aperture of 2 cm in diameter was put on to a metal block and both were immersed into liquid nitrogen near to its surface layer. A sample solution was put dropwise into the aperture and pressed with a cold metal trowel. A clear glass was obtained and held fast in the frame ~ta~, avoiding the presence of a silica window. Dewar systems
A few cryostats for optical spectroscopy were made and used by our group for very low temperature radiation chemistry. An all-glass cryostat, described in Ref. (6), although satisfactory for absorption measurements, became fragile due to radiation damage to the glass. A much more satisfactory metal-glass cryostat has been used throughout the scavenging study described here. It consists of an inner glass tube and an outer metal Dewar vessel and it will be described in Ref. (17). Gamma ray irradiation
A 10 kCi e°Co source delivers a maximum dose rate of 3 x 10e rad h -1 for irradiation at 77 K, and of 3 x 10~ rad h -x for irradiation at 4 K. The safety control system was such that both the E.S.R. and optical measurements could be started within 1 min after terminating the irradiation. 4. VERY LOW TEMPERATURE EFFECT IN ELECTRON SCAVENGING B i p h e n y l in M T H F (15)
M T H F glasses with various concentrations of biphenyl were irradiated with gamma rays at 4 K or at 77 K and the concentrations of electrons and biphenyl anions estimated by E.S.R. Because the paramagnetic relaxation time of the trapped electron prior to solvation is very long, it is somewhat difficult to determine the concentrations of trapped electrons at 4 K. Therefore, the concentration is measured after the sample irradiated at 4 K is warmed to 77 K which transforms the trapped electron to the solvated electron.
396
TAKENOBU HIGASHIMURA
For pure M T H F , the irradiated glass shows signals of the radical and the solvated electron. Irradiated M T H F with a sufficient concentration of biphenyl shows signals of the radical and the biphenyl anion. By subtracting the seven-line spectrum of the radical from the observed signals in both cases, spectra of the solvated electron and the biphenyl anion are determined. For intermediate concentrations of biphenyl, signals of radical, solvated electron and biphenyl anion are superimposed. By comparing the observed signal with the expected one synthesized from the spectra of the trapped electron and the biphenyl anion, the intensity of each component is obtained within an accuracy better than 10 per cent. The yield of solvated electrons is plotted as a function of the glass irradiated at 4 K, the electron is scavenged much more efficiently than in the case of irradiation at 77 K. The scavenging efficiency, which is given by the initial slope of the scavenging curve, is four times larger at 4 K than at 77 K. Hereafter we shall denote the ratio of these efficiencies at 4 K and 77 K by R.
---
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g 0.5
2 ~"
0
OI
Concentrotion of biphenyl,
02
mot %
FiG. 2. Yields of trapped electrons or solvated electrons in M T H F containing various concentrations of biphenyl. (a) Irradiated at 77 K . (b) Irradiated at 4 K , a n d m e a s u r e d at 77 K.
In this experiment, however, the concentration of solvated electron and biphenyl anion in the glass irradiated at 4 K is determined after warming the glass to 77 K. If the trapped electron in the glass escapes from its trap upon warming to 77 K and is captured by a biphenyl molecule, this additional capture causes an increase in the scavenging efficiency for irradiation at 4 K compared with that at 77 K. Further investigation on the scavenging efficiency without warming the glass is required. Benzyl chloride in ethanol a6)
The trapped electron in ethanol glass irradiated at 4 K has been studied in detail n2). In this glass, the trapped electron observed at 4 K is the unsolvated one and shows an absorption spectrum extending from the I.R. to the visible. The trapped electron transforms to the solvated electron when the irradiated glass is warmed to 77 K and then a broad absorption spectrum is observed in the visible.
Electron scavenging at very low temperature
397
Irradiation of ethanol glass containing benzyl chloride (BzCl) at 77 K gives benzyl (Bz) radical resulting from dissociative electron capture by BzC1. Bz radical shows a sharp absorption band around 318 nm. In this spectral region, ethanol radical, trapped electron and solvated electron show no noticeable absorption and also the fundamental absorption of ethanol does not disturb the estimation of the optical density of this absorption band. Therefore, yields of Bz radical, trapped electron and solvated electron can be obtained unambiguously. For irradiation at 4 K, the trapped electron spectra (measured at 4 K) in glasses containing different concentrations of BzCl are shown in Fig. 3. When the optical densities at any wavelength of the trapped electron band are plotted as a function of i
i
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I 2 d
0 0.5
.3
/ 6 0 5
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FIG. 3. Optical absorption spectra measured at 4 K of trapped electrons in ethanol containing different concentrations of benzyl chloride, irradiated at 4 K. Curves 1 to 6 are those of ethanol containing, respectively, 0, 0.006, 0.015, 0.03, 0.05 and 0.10 tool ~o of benzyl chloride. I
I
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D -g
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0'1 Concentrofion
of b e n z y l c h l o r i d e ,
0'2 . mot %
FIG. 4. Y i e l d s o f t r a p p e d e l e c t r o n s o r s o l v a t e d e l e c t r o n s i n e t h a n o l c o n t a i n i n g different c o n c e n t r a t i o n s o f b e n z y l c h l o r i d e . (a) I r r a d i a t e d a n d m e a s u r e d at 77 K. O p t i c a l densities at 540 n m . (b) I r r a d i a t e d a n d m e a s u r e d at 4 K . O p t i c a l densities at 540 n m . (c) I r r a d i a t e d a n d m e a s u r e d at 4 K . O p t i c a l densities at 1500 n m .
398
TAKENOBU HIGASHIMURA
the concentration of BzCI, the initial slope of this scavenging curve gives the scavenging efficiency. The scavenging efficiency thus obtained is much larger than that for irradiation at 77 K (Fig. 4). Another interesting feature of this experiment is that the I.R. part of the trapped electron band is quenched more efficiently than the visible part, as the ratio, R, of scavenging efficiencies at 4 K and 77 K are 5 for 1500 nm absorption and 3 for 540 nm absorption. The scavenging efficiency is also obtained from the initial slope of the Bz radical band, and R is then 6.
Other glasses For glasses with other scavengers, the whole scavenging curves have not been estimated but the yields of the trapped electrons produced by irradiation at 4 K are measured by optical absorption at a concentration of the scavenger. R-values are obtained as: 10 for biphenyl in ethanol, 4 for BzCI in M T H F and 2 for BzCI in methanol. 5. DISCUSSION Conclusively, electron scavenging efficiency increases remarkably for irradiation at 4 K. The ratio R differs from system to system. Let us call such an increase the "very low temperature effect of electron scavenging". The mechanism of electron scavenging in polar glasses has not been well elucidated. According to a simple competitition mechanism, a secondary electron has independent chances to be trapped, scavenged or recombined. The yield of solvated or trapped electrons is expressed as (1)
1/[et-] = (1/[et-]0) {1 + c~[S]},
where [et-] 0 is the yield of trapped electrons in the matrix without scavenger, [S] is the concentration of scavenger, and ct is the scavenging efficiency. The occurrence of the very low temperature effect of electron scavenging means that c~ is a function of the temperature of the matrix. The competing mechanism has been proven for solutions in some alcohols, e.g. acetone and phenol in ethanol, methanol and isopropanol (ls~. Quite recently, the tunnelling mechanism was advocated and was verified in the system of benzene in 3-methylpentane (9) and other systems (1°). The electron is first trapped and then it moves to the scavenger molecule which is close to the trap. The yield of the trapped electron is expressed by (2)
[et- ] = [et-]0exp ( - c~[S]).
Therefore, the logarithm of the yield of the trapped electron is proportional to the concentration of the scavenger, which is the case for biphenyl in M T H F (19~ and biphenyl in ethanol, and others (~°~. In the case of the trapped electron in BzCI solution in ethanol at 4 K, our experimental plots seem to fit both a reciprocal and a logarithmic curve because our results do not extend to large enough concentrations of the scavenger. In an earlier paper, results were analyzed by the competing process (x6~by assuming that the cross-section of electron capture by BzC1 molecules is inversely proportional to the velocity of the thermalized electron and that the cross-sections for electron trapping and for recombination with positive charges are independent of temperature. The ratio R
Electron scavengingat very low temperature
399
of the scavenging efficiencies at 4 K and at 77 K calculated was (77/4)J = 4.3, which is consistent with the experimental value. Thereafter, it became clear that R-values are not always close to 4.3, as mentioned in the preceding subsection, and it is therefore difficult to interpret this phenomenon by the competing process. The R-value obtained from the I.R. absorption band is larger than that obtained from the visible absorption band. The I.R. absorption is attributed to the electrons trapped in shallow traps. Such trap depth dependence of scavenging efficiency is difficult to interpret by the competing mechanism, but is easily elucidated by the tunnelling mechanism. Because the tunnelling probability decreases exponentially with the product of the square root of the height of the potential barrier and its width, the electron in a shallow trap has a larger probability to tunnel to the scavenger molecule than that in a deep trap. However, the tunnelling mechanism only gives a qualitative explanation in our case. The height of the potential barrier, its width and, especially, the density of the resonant states of the scavenger molecule are not known but they can be adjusted to fit experimental results. In conclusion, the mechanism of electron scavenging at 4 K remains obscure. Scavenging experiments with sufficient precision are needed with different combinations of solutes and solvents over a wide range of scavenger concentrations. 6. PRECURSORS IN DISSOCIATIVE ELECTRON CAPTURE Low temperature radiolysis, mainly at 77 K, was used very successfully for studying the solvated electron and also for spectroscopy of intermediate anions and cations {2x). Radiolysis at 20 K proved that very low temperature radiolysis is an excellent method for studying reactions which proceed very fast at normal temperature ~22). Radiolysis at 4 K was used for the study of intermediates by our group. In the case of ethanol glass containing BzC1, the absorption spectrum contains after irradiation at 77 K, a sharp peak at 318 nm and a small doublet peak around 305 nm. These are due to the absorption of the Bz radical which results from dissociative electron capture by BzC1. When the glass is irradiated and measured at 4 K, a broad peak appears at 340 nm, in addition to the aforementioned peaks {z6). This new peak disappears at about 55 K in the course of warming the glass from 4 K, while the sharp peak increases. At 77 K, the intensity of the 318 nm peak reaches about twice its initital intensity. The 340 nm peak does not reappear if the glass is cooled again to 4 K. This irreversible change proves that a species giving the 340 nm peak is a precursor of Bz radical. As the 318 nm peak appears after irradiation at 4 K, it must be admitted that some BzC1 molecules dissociate to give Bz radicals and some other BzCl molecules do not dissociate but remain in the anionic state. Because the electron affinity of C1 is 3.84 eV and the bond dissociation energy of BzCI is 2.88 eV{~3), the BzC1 molecule dissociates to give Bz radical and C1 anion. In the condensed phase, however, solvent molecules around the BzC1 anion may act as a potential wall for the C1 anion. At 77 K, solvent molecules or small atomic groups in the molecules, such as OH or CH3, will fluctuate to some degree and the wall will be soft enough for the BzCl anion to dissociate. At 4 K, the solvent molecules are completely frozen and the potential wall will be so hard that the anion is prevented from dissociating. Some BzCI molecules may be located in sites where solvent molecules are not packed so hard; these may dissociate to give Bz radical at 4 K.
400
TAKENOBU HIGASHIMURA
M a n y o t h e r molecules w h ic h u n d e r g o dissociative electron capture in the solid phase at 77 K show new a b s o r p t i o n spectra when they are irradiated an d measured at 4 K. H a l o b e n z e n e s are a g o o d e x a m p l e o f them. As a result o f dissociative electron capture, they p r o d u c e a h a l o g e n a n i o n a n d a neutral benzene molecule which are f o r m e d by h y d r o g e n a b s tr a c t io n f r o m the solvent molecules by p h e n y | radicals p r i m ar i l y f o rm e d . N o a b s o r p t i o n due to the p r o d u c t s appears in the U.V. an d visible. In the case o f i r r a d i a t i o n at 4 K, an a b s o r p t i o n peak is o b s e r v e d in the U.V. at wavelengths d e p e n d i n g on the h a l o g e n a t o m o f the halobenzene. These results will be r e p o r t e d in detail in the near future. Acknowledgement--The author would like to express his sincere thanks to Dr. J. R. Miller for his kind correspondence about the tunnelling mechanism. A part of this work was done with financial support from the research funds of the Ministry of Education, Japan.
REFERENCES 1. E. J. HART and J. W. BOAG,J. Am. chem. Soc. 1962, 84, 4090. 2. J. E. WILLARD,Fundamental Processes in Radiation Chemistry, Wiley, New York, 1968, p. 599; L. KEVAN, Actions Chimiques et Biologiques des Radiations edited by M. HAISStNSKY, Masson, Paris, 1969, Vol. 13, p. 57; L. KEVAN, ibid. 1971, Vol. 15, p. 81. 3. D. R. SMITH and J. J. PIERONI, Can. J. Chem. 1967, 45, 2723. 4. H. YOSHIDAand T. HIGASHIMURA,Can. J. Chem. 1970, 48, 504. 5. J. T. RICHARDSand J. K. THOMAS,J. chem. Phys. 1970, 53, 218. 6. T. HIGASHIMURA,M. NODA, T. WARASHINAand T. YOSHIDA,J. chem. Phys. 1970, 53, 1152. 7. W. H. HAMILL,J. chem. Phys. 1968, 49, 2446. 8. R. K. WOLFF, M. J. BRONSKILLand J. W. HUNT, J. chem. Phys. 1970, 53, 4211. 9. J. R. MILLER,J. chem. Phys. 1972, 56, 5173. 10. A. 1. MIKHAILOV,R. F. KHAIROUTOINOV,K. ]. ZAMARAEVand V. I. GOLDANSKY, Radiation Chemistry, edited by J. DoBo and P. HEDVIG, Akademiai Kiado, Budapest, 1972, p. 1201; J. KROH and Cz. STRADOWSKI,Int. J. Radiat. Phys. Chem. 1973, 5, 247. 11. H. HASE, M. NODA and T. HIGASHIMURA,J. chem. Phys. 1971, 54, 2975. 12. H. HASE, T. WARASHINA,M. NODA, A. NAMIKI and T. HIGASHIMURA,J. chem. Phys. 1972, 57, 1039. 13. L. KEVAN,J. chem. Phys. 1972, 56, 838. 14. T. SHIDA,J. phys. Chem. 1969, 73, 4311 ; J. P. GUAR1NOand W. H. HAMILL,J. Am. chem. Soc. 1964, 86, 777. 15. H. YOSHIDA, M. OGASAWARA,T. WARASHINAand T. HIGASHIMURA,J. chem. Phys. 1972, 56, 4238. 16. T. HIGASHIMURA,A. NAMIKI, M. NODA and H. HASE,J. phys. Chem. 1972, 76, 3744. 17. M. NODA, Ann. Rep. Res. Reactor Inst. Kyoto Univ. 1974, 7, 75. 18. T. SAWAI, Y. SHINOZAKIand G. MESHITSUKA,Bull. chem. Soc. Japan 1972, 45, 984. 19. P. J. DYNE and O. A. MILLER, Can. J. Chem. 1965, 43, 2696. 20. M. NODA, private communication. 21. W. H. HAMILL, Radical Ions, edited by E. T. KAISERand L. KEVAN, Wiley, New York, 1968, p. 321. 22. R. F. C. CLARIDGE,R. M. IYERand J. E. WILLARD,J. phys. Chem. 1967, 71, 3527; A. EKSTROM and J. E. WILLARD,ibid. 1970, 74, 1708. 23. E. BENSON, The Foundations o f Chemical Kinetics, McGraw-Hill, New York, 1960, p. 670. R6sum6--On passe en revue les r6sultats d'exp6riences de capture d'61ectrons au cours de la radiolyse 7' a 4 K. On discute l'accroissement anormal de l'etiicacit6 de capture h 4 K. Les anions transitoires, dont la dur6e de vie aux temp6ratures habitueUes est tr6s courte, sont tr6s stables 4 K. Comme exemple on d6crit le spectre d'absorption optique de l'anion du chlorure de benzyle. Zusammenfassung--Eine (]bersicht fiber Versuche mit Elektronen-Abf~ngern bei der Gammaradiolyse bei 4 K wird gegeben. Die bei 4 K beobachtete anormale Erh6hung der Wirksamkeit des Abfangens wird diskutiert. Die instabilen Anione, deren Lebensdauer bei iJblichen Temperaturen sehr kurz ist, sind bei 4 K sehr best~indig. Als Beispiel wird das optische Absorptionsspektrum des Benzylchloridanions beschrieben.
ELECTRON
SCAVENGING
AT
VERY
LOW
TEMPERATURE TAKENOBU HIGASHIMURA Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka-fu, Japan (Received 11 October 1973; in revised form 10 June 1974)
Abstract--Results of electron scavenging experiments for gamma irradiation at 4 K are reviewed. An anomalous increase in the scavenging efficiency at 4 K is discussed. Intermediate anions which have very short lifetimes at usual temperatures are very stable at 4 K. As an example of these anions, the optical absorption spectrum of the benzyl chloride anion is described. 1. INTRODUCTION THE DISCOVERY of the hydrated electron in 1962 (1) was an epoch in radiation chemistry. Studies on the yields and the reaction of the hydrated electron and the solvated electron in a variety of organic media have been most fruitful in this field. Very m a n y data have been accumulated on the reaction constants of the solvated electron. Much experimental and theoretical work on the physico-chemical structure of the solvated electron has been published (2). But for a long time there was no experimental evidence on the state of electrons before solvation. The first work on the electron prior to solvation was done in 1967 by employing g a m m a radiolysis at 4 K and E.S.R. measurements at the same temperature ts~. The linewidths of the E.S.R. signals of the electron is some glasses, ethanol, 3-methylpentane, etc., irradiated at 4 K showed remarkable differences from the corresponding linewidths obtained by irradiation and measurement at 77 K. These differences in E.S.R. linewidths were not due to their reversible dependence on temperature, but to an irreversible change upon warming to 77 K t4). It became highly plausible that the electron observed at 4 K is a trapped electron around which permanent dipoles of solvent molecules are not oriented.* In 1970, the optical absorption spectra due to such unsolvated electrons were obtained by two different methods, i.e. microsecond pulse radiolysis at 77 K (5~, and g a m m a radiolysis at 4 K (e~. After that, trapped electrons were found in a variety of matrices and their physico-chemical properties have been investigated in detail. However, pre-solvated electrons are not observable in some matrices. The solvation time in these substances may be too short to observe electrons in their unsolvated states. The mechanism of electron scavenging in the liquid and in the glassy phases has been investigated for a long time, and fairly recently interesting hypotheses have been presented: dry electron by Hamill (7~, mobile electron by Hunt (s~ and electron tunnelling by Miller tg~ and other groups tl°~. Electron scavenging at 4 K was studied by our group and anomalous phenomena were found. This paper is an autoreview on the electron scavenging experiments at 4 K.* It includes results on the very low * In this article, this pre-solvated electron is named the "trapped electron" and the electron toward which surrounding permanent dipoles are oriented is called the "solvated electron", though the latter appears in the solid phase. Solvated electrons are observed at 77 K. In some kinds of matrices, such as methanol and monodeuterated ethanol, they can be observed at 4 K. 393
394
TAKENOBU HIGASHIMURA
temperature effect on electron scavenging efficiency and intermediate species observed when scavenger molecules capture electrons at 4 K. 2. EARLY EXPERIMENTS ON TRAPPED ELECTRONS AND ELECTRON SCAVENGING AT 4 K Ethanol glass irradiated with g a m m a rays at 77 K shows an absorption spectrum with a m a x i m u m at 545 nm. When the ethanol glass is irradiated at 4 K and measured without warming, it shows an intense absorption in the I.R. region with a m a x i m u m at 1490 nm (Fig. 1). When this is warmed rapidly to 77 K, the absorption in the I.R. i
l
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number,
20
25 x IOs
c m -~
FIG. 1. Optical absorption spectra of the trapped electron and the solvated electron in ethanol. (a) Irradiated and measured at 4 K. (b) Irradiated at 4 K and measured at 77 K. (c) Irradiated and measured at 77 K. (d) Irradiated at 77 K and measured at 4 K. disappears and the optical densities in the visible region increase remarkably. The latter spectrum (?'max = 540 nm) is very similar to that of the ethanol glass irradiated at 77 K. The I.R. band does not reappear if the irradiated glass is cooled again to 4 K. The E.S.R. spectrum of this species shows a similar irreversible change from a sharp singlet with a width of 6.5 + 0.5 G at 4 K to a broad singlet with a width of 1 5 . 0 + 2 . 0 G at 77 K (11,12). Such results, consistent with the results obtained by pulse radiolysis at 77 K ~5,13~, lead us to the conclusion that, when irradiation is carried out at 4 K, the electron is trapped at a site surrounded by molecular dipoles with r a n d o m directions and, when irradiation is carried out at 77 K, the electron is surrounded by glass matrix molecules favourably oriented to relax the Coulombic force~3L In the course of our optical study on the trapped electrons at 4 K, an interesting phenomenon was found concerning the yield of trapped electrons in 2-methyltetrahydrofuran ( M T H F ) glassmL When the glass irradiated at 4 K is warmed rapidly to 77 K, the yield of solvated electrons is only about one-quarter of that produced by irradiation and measurement at 77 K. In the case of E.S.R. experiments, however, the yield of solvated electrons in M T H F glass, produced by irradiation at 4 K and * Scavenging experiments were carried out by Hirotomo Hase, Takenobu Higashimura, Akira Namiki, Masato Noda, Masaaki Ogasawara, Hiroshi Yoshida and Tetsuo Warashina of Kyoto University, except M. O. and H. Y., of Hokkaido University.
Electron scavenging at very low temperature
395
then warmed to 77 K, is about the same as that in a sample irradiated at 77 K. In our E.S.R. experiment, M T H F was purified by repeated fractional distillation, dried with a sodium potassium mirror and sealed in v a c u o into the sample tube. In the case of optical absorption measurements, reagent grade M T H F was used without further purification. Therefore, a discrepancy in temperature dependence of the yield of solvated electrons might be caused by impurity molecules in the optical absorption sample. However, the concentration of impurities in this sample must be very small because the yield of solvated electrons in the sample irradiated at 77 K was not much different from the reported value ~14~. Only the yield in the sample irradiated at 4 K was much smaller. Hence it seemed plausible to assume that the electron scavenging efficiency of the impurity molecules is remarkably larger at 4 K than that at 77 K. In order to obtain quantitative data on this apparent increase of scavenging efficiency at 4 K, scavenging experiments were carried out, first with biphenyl solution in M T H F by E.S.R. measurements (15~, and then with benzyl chloride solution in ethanol by optical absorption measurements t~e). 3. EXPERIMENTAL Samples
MTHF was purified by repeated fractional distillation in vacuo and was dried with a sodium potassium mirror. Other solvents and all solutes of reagent grade were used as received. For E.S.R. measurements, samples were sealed in vacuo into Suprasil silica tubes; glassy specimens were prepared by immersing the sealed tubes into liquid nitrogen, and then transferring them into liquid helium. MTHF sample tubes frequently broke when cooled in liquid helium. For preparing samples of alcohol for optical absorption measurements, a stainless steel disc with a circular aperture of 2 cm in diameter was put on to a metal block and both were immersed into liquid nitrogen near to its surface layer. A sample solution was put dropwise into the aperture and pressed with a cold metal trowel. A clear glass was obtained and held fast in the frame ~ta~, avoiding the presence of a silica window. Dewar systems
A few cryostats for optical spectroscopy were made and used by our group for very low temperature radiation chemistry. An all-glass cryostat, described in Ref. (6), although satisfactory for absorption measurements, became fragile due to radiation damage to the glass. A much more satisfactory metal-glass cryostat has been used throughout the scavenging study described here. It consists of an inner glass tube and an outer metal Dewar vessel and it will be described in Ref. (17). Gamma ray irradiation
A 10 kCi e°Co source delivers a maximum dose rate of 3 x 10e rad h -1 for irradiation at 77 K, and of 3 x 10~ rad h -x for irradiation at 4 K. The safety control system was such that both the E.S.R. and optical measurements could be started within 1 min after terminating the irradiation. 4. VERY LOW TEMPERATURE EFFECT IN ELECTRON SCAVENGING B i p h e n y l in M T H F (15)
M T H F glasses with various concentrations of biphenyl were irradiated with gamma rays at 4 K or at 77 K and the concentrations of electrons and biphenyl anions estimated by E.S.R. Because the paramagnetic relaxation time of the trapped electron prior to solvation is very long, it is somewhat difficult to determine the concentrations of trapped electrons at 4 K. Therefore, the concentration is measured after the sample irradiated at 4 K is warmed to 77 K which transforms the trapped electron to the solvated electron.
396
TAKENOBU HIGASHIMURA
For pure M T H F , the irradiated glass shows signals of the radical and the solvated electron. Irradiated M T H F with a sufficient concentration of biphenyl shows signals of the radical and the biphenyl anion. By subtracting the seven-line spectrum of the radical from the observed signals in both cases, spectra of the solvated electron and the biphenyl anion are determined. For intermediate concentrations of biphenyl, signals of radical, solvated electron and biphenyl anion are superimposed. By comparing the observed signal with the expected one synthesized from the spectra of the trapped electron and the biphenyl anion, the intensity of each component is obtained within an accuracy better than 10 per cent. The yield of solvated electrons is plotted as a function of the glass irradiated at 4 K, the electron is scavenged much more efficiently than in the case of irradiation at 77 K. The scavenging efficiency, which is given by the initial slope of the scavenging curve, is four times larger at 4 K than at 77 K. Hereafter we shall denote the ratio of these efficiencies at 4 K and 77 K by R.
---
t
>,
~5
g 0.5
2 ~"
0
OI
Concentrotion of biphenyl,
02
mot %
FiG. 2. Yields of trapped electrons or solvated electrons in M T H F containing various concentrations of biphenyl. (a) Irradiated at 77 K . (b) Irradiated at 4 K , a n d m e a s u r e d at 77 K.
In this experiment, however, the concentration of solvated electron and biphenyl anion in the glass irradiated at 4 K is determined after warming the glass to 77 K. If the trapped electron in the glass escapes from its trap upon warming to 77 K and is captured by a biphenyl molecule, this additional capture causes an increase in the scavenging efficiency for irradiation at 4 K compared with that at 77 K. Further investigation on the scavenging efficiency without warming the glass is required. Benzyl chloride in ethanol a6)
The trapped electron in ethanol glass irradiated at 4 K has been studied in detail n2). In this glass, the trapped electron observed at 4 K is the unsolvated one and shows an absorption spectrum extending from the I.R. to the visible. The trapped electron transforms to the solvated electron when the irradiated glass is warmed to 77 K and then a broad absorption spectrum is observed in the visible.
Electron scavenging at very low temperature
397
Irradiation of ethanol glass containing benzyl chloride (BzCl) at 77 K gives benzyl (Bz) radical resulting from dissociative electron capture by BzC1. Bz radical shows a sharp absorption band around 318 nm. In this spectral region, ethanol radical, trapped electron and solvated electron show no noticeable absorption and also the fundamental absorption of ethanol does not disturb the estimation of the optical density of this absorption band. Therefore, yields of Bz radical, trapped electron and solvated electron can be obtained unambiguously. For irradiation at 4 K, the trapped electron spectra (measured at 4 K) in glasses containing different concentrations of BzCl are shown in Fig. 3. When the optical densities at any wavelength of the trapped electron band are plotted as a function of i
i
i
i
I 15
I 20
i
1.0-
I 2 d
0 0.5
.3
/ 6 0 5
I0
Wove
number,
I 25 X 103
crn -~
FIG. 3. Optical absorption spectra measured at 4 K of trapped electrons in ethanol containing different concentrations of benzyl chloride, irradiated at 4 K. Curves 1 to 6 are those of ethanol containing, respectively, 0, 0.006, 0.015, 0.03, 0.05 and 0.10 tool ~o of benzyl chloride. I
I
1.0 q E
D -g
o •~-~ 0.5
u
0
0'1 Concentrofion
of b e n z y l c h l o r i d e ,
0'2 . mot %
FIG. 4. Y i e l d s o f t r a p p e d e l e c t r o n s o r s o l v a t e d e l e c t r o n s i n e t h a n o l c o n t a i n i n g different c o n c e n t r a t i o n s o f b e n z y l c h l o r i d e . (a) I r r a d i a t e d a n d m e a s u r e d at 77 K. O p t i c a l densities at 540 n m . (b) I r r a d i a t e d a n d m e a s u r e d at 4 K . O p t i c a l densities at 540 n m . (c) I r r a d i a t e d a n d m e a s u r e d at 4 K . O p t i c a l densities at 1500 n m .
398
TAKENOBU HIGASHIMURA
the concentration of BzCI, the initial slope of this scavenging curve gives the scavenging efficiency. The scavenging efficiency thus obtained is much larger than that for irradiation at 77 K (Fig. 4). Another interesting feature of this experiment is that the I.R. part of the trapped electron band is quenched more efficiently than the visible part, as the ratio, R, of scavenging efficiencies at 4 K and 77 K are 5 for 1500 nm absorption and 3 for 540 nm absorption. The scavenging efficiency is also obtained from the initial slope of the Bz radical band, and R is then 6.
Other glasses For glasses with other scavengers, the whole scavenging curves have not been estimated but the yields of the trapped electrons produced by irradiation at 4 K are measured by optical absorption at a concentration of the scavenger. R-values are obtained as: 10 for biphenyl in ethanol, 4 for BzCI in M T H F and 2 for BzCI in methanol. 5. DISCUSSION Conclusively, electron scavenging efficiency increases remarkably for irradiation at 4 K. The ratio R differs from system to system. Let us call such an increase the "very low temperature effect of electron scavenging". The mechanism of electron scavenging in polar glasses has not been well elucidated. According to a simple competitition mechanism, a secondary electron has independent chances to be trapped, scavenged or recombined. The yield of solvated or trapped electrons is expressed as (1)
1/[et-] = (1/[et-]0) {1 + c~[S]},
where [et-] 0 is the yield of trapped electrons in the matrix without scavenger, [S] is the concentration of scavenger, and ct is the scavenging efficiency. The occurrence of the very low temperature effect of electron scavenging means that c~ is a function of the temperature of the matrix. The competing mechanism has been proven for solutions in some alcohols, e.g. acetone and phenol in ethanol, methanol and isopropanol (ls~. Quite recently, the tunnelling mechanism was advocated and was verified in the system of benzene in 3-methylpentane (9) and other systems (1°). The electron is first trapped and then it moves to the scavenger molecule which is close to the trap. The yield of the trapped electron is expressed by (2)
[et- ] = [et-]0exp ( - c~[S]).
Therefore, the logarithm of the yield of the trapped electron is proportional to the concentration of the scavenger, which is the case for biphenyl in M T H F (19~ and biphenyl in ethanol, and others (~°~. In the case of the trapped electron in BzCI solution in ethanol at 4 K, our experimental plots seem to fit both a reciprocal and a logarithmic curve because our results do not extend to large enough concentrations of the scavenger. In an earlier paper, results were analyzed by the competing process (x6~by assuming that the cross-section of electron capture by BzC1 molecules is inversely proportional to the velocity of the thermalized electron and that the cross-sections for electron trapping and for recombination with positive charges are independent of temperature. The ratio R
Electron scavengingat very low temperature
399
of the scavenging efficiencies at 4 K and at 77 K calculated was (77/4)J = 4.3, which is consistent with the experimental value. Thereafter, it became clear that R-values are not always close to 4.3, as mentioned in the preceding subsection, and it is therefore difficult to interpret this phenomenon by the competing process. The R-value obtained from the I.R. absorption band is larger than that obtained from the visible absorption band. The I.R. absorption is attributed to the electrons trapped in shallow traps. Such trap depth dependence of scavenging efficiency is difficult to interpret by the competing mechanism, but is easily elucidated by the tunnelling mechanism. Because the tunnelling probability decreases exponentially with the product of the square root of the height of the potential barrier and its width, the electron in a shallow trap has a larger probability to tunnel to the scavenger molecule than that in a deep trap. However, the tunnelling mechanism only gives a qualitative explanation in our case. The height of the potential barrier, its width and, especially, the density of the resonant states of the scavenger molecule are not known but they can be adjusted to fit experimental results. In conclusion, the mechanism of electron scavenging at 4 K remains obscure. Scavenging experiments with sufficient precision are needed with different combinations of solutes and solvents over a wide range of scavenger concentrations. 6. PRECURSORS IN DISSOCIATIVE ELECTRON CAPTURE Low temperature radiolysis, mainly at 77 K, was used very successfully for studying the solvated electron and also for spectroscopy of intermediate anions and cations {2x). Radiolysis at 20 K proved that very low temperature radiolysis is an excellent method for studying reactions which proceed very fast at normal temperature ~22). Radiolysis at 4 K was used for the study of intermediates by our group. In the case of ethanol glass containing BzC1, the absorption spectrum contains after irradiation at 77 K, a sharp peak at 318 nm and a small doublet peak around 305 nm. These are due to the absorption of the Bz radical which results from dissociative electron capture by BzC1. When the glass is irradiated and measured at 4 K, a broad peak appears at 340 nm, in addition to the aforementioned peaks {z6). This new peak disappears at about 55 K in the course of warming the glass from 4 K, while the sharp peak increases. At 77 K, the intensity of the 318 nm peak reaches about twice its initital intensity. The 340 nm peak does not reappear if the glass is cooled again to 4 K. This irreversible change proves that a species giving the 340 nm peak is a precursor of Bz radical. As the 318 nm peak appears after irradiation at 4 K, it must be admitted that some BzC1 molecules dissociate to give Bz radicals and some other BzCl molecules do not dissociate but remain in the anionic state. Because the electron affinity of C1 is 3.84 eV and the bond dissociation energy of BzCI is 2.88 eV{~3), the BzC1 molecule dissociates to give Bz radical and C1 anion. In the condensed phase, however, solvent molecules around the BzC1 anion may act as a potential wall for the C1 anion. At 77 K, solvent molecules or small atomic groups in the molecules, such as OH or CH3, will fluctuate to some degree and the wall will be soft enough for the BzCl anion to dissociate. At 4 K, the solvent molecules are completely frozen and the potential wall will be so hard that the anion is prevented from dissociating. Some BzCI molecules may be located in sites where solvent molecules are not packed so hard; these may dissociate to give Bz radical at 4 K.
400
TAKENOBU HIGASHIMURA
M a n y o t h e r molecules w h ic h u n d e r g o dissociative electron capture in the solid phase at 77 K show new a b s o r p t i o n spectra when they are irradiated an d measured at 4 K. H a l o b e n z e n e s are a g o o d e x a m p l e o f them. As a result o f dissociative electron capture, they p r o d u c e a h a l o g e n a n i o n a n d a neutral benzene molecule which are f o r m e d by h y d r o g e n a b s tr a c t io n f r o m the solvent molecules by p h e n y | radicals p r i m ar i l y f o rm e d . N o a b s o r p t i o n due to the p r o d u c t s appears in the U.V. an d visible. In the case o f i r r a d i a t i o n at 4 K, an a b s o r p t i o n peak is o b s e r v e d in the U.V. at wavelengths d e p e n d i n g on the h a l o g e n a t o m o f the halobenzene. These results will be r e p o r t e d in detail in the near future. Acknowledgement--The author would like to express his sincere thanks to Dr. J. R. Miller for his kind correspondence about the tunnelling mechanism. A part of this work was done with financial support from the research funds of the Ministry of Education, Japan.
REFERENCES 1. E. J. HART and J. W. BOAG,J. Am. chem. Soc. 1962, 84, 4090. 2. J. E. WILLARD,Fundamental Processes in Radiation Chemistry, Wiley, New York, 1968, p. 599; L. KEVAN, Actions Chimiques et Biologiques des Radiations edited by M. HAISStNSKY, Masson, Paris, 1969, Vol. 13, p. 57; L. KEVAN, ibid. 1971, Vol. 15, p. 81. 3. D. R. SMITH and J. J. PIERONI, Can. J. Chem. 1967, 45, 2723. 4. H. YOSHIDAand T. HIGASHIMURA,Can. J. Chem. 1970, 48, 504. 5. J. T. RICHARDSand J. K. THOMAS,J. chem. Phys. 1970, 53, 218. 6. T. HIGASHIMURA,M. NODA, T. WARASHINAand T. YOSHIDA,J. chem. Phys. 1970, 53, 1152. 7. W. H. HAMILL,J. chem. Phys. 1968, 49, 2446. 8. R. K. WOLFF, M. J. BRONSKILLand J. W. HUNT, J. chem. Phys. 1970, 53, 4211. 9. J. R. MILLER,J. chem. Phys. 1972, 56, 5173. 10. A. 1. MIKHAILOV,R. F. KHAIROUTOINOV,K. ]. ZAMARAEVand V. I. GOLDANSKY, Radiation Chemistry, edited by J. DoBo and P. HEDVIG, Akademiai Kiado, Budapest, 1972, p. 1201; J. KROH and Cz. STRADOWSKI,Int. J. Radiat. Phys. Chem. 1973, 5, 247. 11. H. HASE, M. NODA and T. HIGASHIMURA,J. chem. Phys. 1971, 54, 2975. 12. H. HASE, T. WARASHINA,M. NODA, A. NAMIKI and T. HIGASHIMURA,J. chem. Phys. 1972, 57, 1039. 13. L. KEVAN,J. chem. Phys. 1972, 56, 838. 14. T. SHIDA,J. phys. Chem. 1969, 73, 4311 ; J. P. GUAR1NOand W. H. HAMILL,J. Am. chem. Soc. 1964, 86, 777. 15. H. YOSHIDA, M. OGASAWARA,T. WARASHINAand T. HIGASHIMURA,J. chem. Phys. 1972, 56, 4238. 16. T. HIGASHIMURA,A. NAMIKI, M. NODA and H. HASE,J. phys. Chem. 1972, 76, 3744. 17. M. NODA, Ann. Rep. Res. Reactor Inst. Kyoto Univ. 1974, 7, 75. 18. T. SAWAI, Y. SHINOZAKIand G. MESHITSUKA,Bull. chem. Soc. Japan 1972, 45, 984. 19. P. J. DYNE and O. A. MILLER, Can. J. Chem. 1965, 43, 2696. 20. M. NODA, private communication. 21. W. H. HAMILL, Radical Ions, edited by E. T. KAISERand L. KEVAN, Wiley, New York, 1968, p. 321. 22. R. F. C. CLARIDGE,R. M. IYERand J. E. WILLARD,J. phys. Chem. 1967, 71, 3527; A. EKSTROM and J. E. WILLARD,ibid. 1970, 74, 1708. 23. E. BENSON, The Foundations o f Chemical Kinetics, McGraw-Hill, New York, 1960, p. 670. R6sum6--On passe en revue les r6sultats d'exp6riences de capture d'61ectrons au cours de la radiolyse 7' a 4 K. On discute l'accroissement anormal de l'etiicacit6 de capture h 4 K. Les anions transitoires, dont la dur6e de vie aux temp6ratures habitueUes est tr6s courte, sont tr6s stables 4 K. Comme exemple on d6crit le spectre d'absorption optique de l'anion du chlorure de benzyle. Zusammenfassung--Eine (]bersicht fiber Versuche mit Elektronen-Abf~ngern bei der Gammaradiolyse bei 4 K wird gegeben. Die bei 4 K beobachtete anormale Erh6hung der Wirksamkeit des Abfangens wird diskutiert. Die instabilen Anione, deren Lebensdauer bei iJblichen Temperaturen sehr kurz ist, sind bei 4 K sehr best~indig. Als Beispiel wird das optische Absorptionsspektrum des Benzylchloridanions beschrieben.