J. Phys. Chem. 1989, 93, 1657-1660
1657
Electron Spin Resonance Imaging Study of Spatial Distribution of Paramagnetic Species Produced by y-Irradiation in Sulfuric Acid Ices Keiichi Ohno,* Jun Yonezawa, and Yasuyuki Morita Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received: March 31, 1988; In Final Form: June 2, 1988)
Spatial heterogeneities of paramagnetic species were investigated in cylindrical samples of frozen sulfuric acid ices irradiated by y-ray, using electron spin resonance imaging. The inverse of Abel's integral was used to derive radial density distributions from observed projection spectra. The symmetric radial density distributions of hydrogen, deuteron, and SO4- radical trapped in naked and aluminum foil covered samples were observed. Especially the spectral-spatial two-dimensional method allowed us to make the first measurement of the density of the SO4- radical which has never been observed because of its nonsimple ESR spectrum. The radical density measurement has made possible a discussion on the mechanism of the heterogeneity.
Introduction The formation of hydrogen (H,) and deuterium (D,) atoms trapped during radiolysis of acid ices was first detected by Livingston et al.'-, These authors detected H, and D, by ESR method in acid ices of HCI04, HpSO4, and H$O4 irradiated at 77 K. Since then a number of subsequent papers have been published on these three acid systems. H, have been found also during radiolysis at 77 K of other acid ices; HF,4 HBI-,~HBF,, and H2SiF6.6 With regard to the mechanism of the formation of H, the following reaction was eth
+ H30+
-
H,
+ H20
(1)
A considerable decrease of G(H,) with the introduction of such effective scavengers of thermalized electrons e,as NO; and H 2 0 2 proved this reaction to be probable. However, the addition of even large quantities of methyl alcohol which is inert toward ethdoes not affect on the initial yield of H,. With increasing NO3 concentration from lo-, to 2 X lo-' M the value of G(H,) diminishes by 85%. It means that most of the H, atoms are produced according to the above reaction. The rest of the H, atoms (about 15%) are evidently formed from excited water or in the reaction of electrons with H 3 0 + ions in spurs.9 An alternative possibility for H, formation was proposed by Kevan.loJ1 The following results were known: firstly, the reverse of the relative reactivity of scavengers H202and HNO, with H atoms in water against that in acid ices; secondly, the correlation of the effect of a scavenger NaHSO, on H, with hydrated electron eaqreactivity toward the scavenger; and finally the positive effect of an electron scavenger N 2 0 on H2 yield. The overall evidence suggests that H, is formed by the reaction of mobile electron e,e,-
+ H2SO4 or HS04-
-
H,
+ HS04- or S042-
(2)
Kevan has confirmed the reaction with HS04- in ices containing NaHSO,." It was concluded that the reaction of e,,- with H 3 0 + to form H, does not seem to occur in acid ices. In radiation chemistry, no effect of macroscopic heterogeneity of radicals has so far been investigated on chemical reactions except for studies on microscopic heterogeneity, namely local (1) Livingston, R.; Zeldes, H.; Taylor, E. H. Phys. Rev. 1954, 94, 725. (2) Zeldes, H.; Livingston, R. Phys. Reu. 1954, 96, 1702. (3) Livingston, R.; Zeldes, H.; Taylor, E. H. Discuss. Faraday SOC.1955, 19, 166. (4) Zhitnikov, R. A,; Orbeli, A. L. Fiz. Tuerd. Tela 1966, 7 , 3522. ( 5 ) Belevskii, V. N.; Bugaenko, L. T. Zh. Fir. Khim. 1965, 39, 2589. (6) Ershov, B. G.; Pikaev, A. K. Radiat. Res. Reu. 1969, 2, 1. (7) Daiton, F. S.; Jones, F. T. Trans. Faraday SOC.1965, 61, 1681. (8) Livingston, R.; Weinberger, A. J . J . Chem. Phys. 1960, 33, 499. (9) Ershov, B. G.; Pikaev, A. K. Radiat. Res. Rev. 1969, 2, 1. (10) Kevan, L. J . Am. Chem. SOC.1967,89, 4238. ( 1 1) Kevan, L. Radiation Chemistry of Frozen Aqueous Solutions. In Radiation Chemistry ofAqueous System; Stein, G., Ed.; Interscience: New York, 1968.
0022-3654/89/2093-1657$01.50/0
concentration from a view point of linear energy transfer (LET). Usual samples used in radiation chemistry are enclosed in glass tubes, etc. for liquid-phase experiments. In this case, the only open surface is the top gas-liquid interface, the dimension of which is so narrow that the effect might be negligible in comparison with the whole bulk. Moreover, there has existed no method for measurement of the spatial distribution of radicals. On the other hand, in most solid-phase experiments, samples can be considered as open systems. In the past decade ESR imaging technique has been developed and simultaneously applied for many r e s e a r c h e ~ . l ~ - ESR ' ~ imaging is one of the powerful techniques for studies in physics, chemistry, biology, medicine, and even archaeology. Recently most of difficulties occurring in its application for multiline or multicomponent ESR spectra have been overcome. Lauterbur et al. developed a novel two-dimensional N M R imaging involving the frequency axis as a dimension in order to obtain shift imaging.'5s'6 Ewert and Herrling applied this method for ESR imaging1' Maltempo,I* Eaton, and E a t ~ n ' ~have . ' ~ also ~ ~ inde~ pendently developed the spectral-spatial 2-D imaging. This method has the key advantage that ESR imaging becomes applicable for even samples with considerably complex spectra regardless of relatively long data acquisition time. The first observation of different density distributions between H, and D, was made between H, and D, in naked frozen sulfuric acid ices irradiated by An experimentally appropriate algorithm for the low-distortion Abel inversion was not available at that time so that quantitative discussion could not be carried out. Lanquart presented a data approximation technique for solving numerically the Abel's integral equation which does not require any derivative of the function involved.22 The new formula can produce high-quality results which makes quantitative treatment possible. In this paper the measurements of spatial distribution about H,, D,, and SO4- radicals and the analyses of their distributions will be described.
Experimental Section Samples were prepared by pouring 6 M H2SO4 or D2S04 solution into cylindrical tubes (3 mm in diameter X ca. 15 mm (12) Ohno, K. Appl. Spectrosc. Reu. 1986, 22, 1. (1 3) Ohno, K. Magn. Reson. Rev. 1987, 11, 275. (14) Eaton, S. S.: Eaton, G.R. Spectroscopy 1986, I , 32. (15) Lauterbur, P. C.; Levin, D. N.; Marr, R. B. J . Magn. Reson. 1984, 59, 536.
(16) Bernardo, Jr. M. R.; Lauterbur, P. C.; Hedges, L. K. J . Magn. Reson. 1985, 61, 168. (17) Ewert, U.; Herrling, T. Chem. Phys. Lett. 1986, 129, 516. (18) Maltempo, M. M. J . Mugn. Reson. 1986, 69, 156. (19) Maltempo, M. M.; Eaton, S. S.; Eaton, G. R. J . Magn. Reson. 1987, 72. 449. (20) Stemp, E. D. A,; Eaton, G.R.; Eaton, S. S.; Maltempo, M. M. J . Phys. Chem. 1987, 91, 6461. (21) Ohno, K. Jpn. J . Appl. Phys. 1981, 20, L179. (22) Lanquart, J.-P. J . Compt. Phys. 1982, 49, 434.
0 1989 American Chemical Society
1658 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
in length) of aluminum foil and immersing them rapidly into liquid nitrogen. After removing the A1 foil immediately for naked samples, or keeping it for A1 foil covered samples, the cylindrical samples were put at the center of a cylindrical cage-type 6oCo radiation source to irradiate y-rays with a dose of 2.8 Mrad. Such irradiation guarantees the occurrence of symmetric radial distribution for radiolysis products. Both end surfaces of all samples were uneven and thus were shaped flat by polishing them with sand paper in liquid nitrogen. For isotopically mixed samples (H:D = 1:l) the ratios of H and D atoms were controlled by changing the ratios of HzS04, D2S04, H 2 0 , and D 2 0 . All ESR measurements were performed by JEOL JES FE-3XG X-band spectrometer with 100-kHz field modulation of 4 G and a microwave power of W. A rectangular cavity (HIo2mode, Micro Device) was used, the width of which is 16 mm and is very advantageous to generate a strong magnetic field gradient. An anti-Helmholtz coil pair was attached on the outsides of the cavity wall. When electric current of 30 A flows, it produces a maximum gradient of about 500 G/cm. The spectralspatial 2-D ESR imaging method was carried out with acquisition of 36 gradients corresponding to each 5-deg interval. The viewing angle 0 is calculated with the following equation
0 = tan-' (CD/B)
(3)
where D is the desired region of the spatial axis and B is that of the spectral axis. 0 is limited by the maximum C so that some missing viewing angles or projection spectra occur. In the present experiment, however, the maximum gradient was so intense that the distortion due to one or two missing spectra might be negligible. In practice, for given G and D the value of B was determined by the above equation at each 5-deg interval. Acquired spectra were multiplied with the square of ratio of B to Bo, the magnetic field at no gradient. Simultaneously the spectra with data points which were proportional to B (Bocorresponds to 8192 data points), were transferred to those with 256 data points. All 36 collected projection spectra were reconstructed to a pseudo-2-D image. The calculation takes about 20 min with the use of executable program compiled by Fortran. All the above procedures, control of the magnetic field sweep, data acquisition, and image reconstruction were performed with a microcomputer, NEC PC9801VX, including a 20MB hard disk and two 1MB floppy disks.
Ohno et al.
1
Figure 1. Test functions for the Abel inversion: (a) projection Z(x) for a cylindrical sample with a uniform radial density and (b) a radial density f(r) derived with the Abel inversion. The least-squares approximation was made for a function containing 22 powers of x.
The density distributionflr) can be obtained by the coefficients as follows
Thus f(r) is given by analytical integration as
5
sr6
i) . I
+ s3r4+ ls5? + + 5
where s = (x2 - r2)'12. Additional formulas are also given as k
y'(1) = C 2 i a Z i
I ( x ) = 2 S rX y ( r ) r ( r 2 - x2)-'l2d r
(4)
where x is the Cartesian coordinate and ro is the outer radius of the cylinder. To deriveflr) from I ( x ) the inverse of Abel's integral is necessary. The inversion formula used so far, however, caused some artifacts, for example, damping phenomena or singularities at the boundaries of data. Lanquart proposed a skillful derivative-free method which could almost solve the troubles.22 Z(x) can be approximated by a polynomial as k
(9)
i=O
k
f(0) = -(l/?r)X{2i/(2i i=O
Procedure for Abel Inversion The spectrum along the spatial axis obtained from the pseudoimage is a projection of a cylindrically symmetric extended source with a radial distribution of f(r), which is given by
(8)
jT1-6)
-(26)'/5'(1)
- l))CuZi
(10)
