2443
THERMAL DECAY EFFECTS ON SPATIAL DISTRIBUTIONS OF RADICALS
Thermal Decay Effects on Spatial Distributions of Radicals in 7-Irradiated Poly(rnethy1 methacrylate)
by Walter Kaul Physikalisch-Technische Bundesanstalt,Braunschweig, Germany
and Larry Kevan* Department of Chemistry, Wayne State University, Detroit, Michigan 48,909
(ReceCed November 18, 1070)
Publication costs assisted by the U . S. Atomic Energy Commission and the Air Force Ofice of Scientific Research
Spin-spin, Tz, and spin-lattice, T1, relaxation times were measured for the radicals in yirradiated PMMA at room temperature us. radiation dose and us. thermal decay time after irradiation. Tz is related to the local spin Concentration around a radical. The local spin concentrations are larger than the average spin concentrations over the entire sample up to doses of about 5 Mrads. This indicates that the radicals are trapped in regions of high local spin concentration called spurs. At doses above 7 hfrads the local and average spin concentrations are comparable. During thermal decay at doses less than 5 Mrads the local spin concentration decreases fractionally faster than the average spin concentration which indicates intraspur decay. During thermal decay at doses greater than 7 %ads the local and average spin concentrations decrease at similar fractional rates.
Introduction Nonuniform spatial distributions of trapped paramagnetic species are often produced by y irradiation and by photolysis of solids. The most obvious type of nonuniformity is the production of radical pairs by radiolytic or photolytic bond scissi0n.l A more general type of nonuniformity, characteristic of y radiolysis, is the production of clusters of radicals called spurs. This is simply a reflection of the inhomogeneous deposition of radiation energy in condensed systems. In the past several years electron paramagnetic resonance methods have proved powerful for studying nonuniform distributions of trapped radicals. We have used, paramagnetic relaxation to measure T,, the spin-spin relaxation time, which is inversely proportional to the local spin c ~ n c e n t r a t i o n . ~Although ,~ the absolute value of T z may be subject to significant experimental error, relative values of Tz give an accurate picture of how the local spin concentration changes with an experimental parameter. It has been found that electrons trapped in polar solid^^-^ and some polymer radi c a l ~ ~are - ~trapped ~ in spurs in which the local spin concentration exceeds the average spin concentration over the whole sample. As the radiation dose or the total radical concentration increases, the local spin concentration remains essentially constant until the spurs begin to overlap significantly. In this study we explore how the local radical concentration decreases with respect to the average radical concentration during the process of thermal decay. By this, some insight is achieved into the relative importance of intraspur and interspur radical decay. The radicals formed in y-irradiated poly(methy1 methacry-
late) (PRIMA) form a suitable system since they are produced in spurs at low radiation doses,gthey undergo slow thermal decay at room temperature, and it is possible to measure their relaxation times by microwave saturation methods at room temperature.
Experimental Section The PMMA samples were prepared by thermal polymerization from methyl methacrylate monomer supplied by Rohm and Haas. The monomer was quoted t o be 99.95% pure and contained no inhibitors. The monomer was degassed in glass tubes and then polymerized by heating for 45 min at 90" and 24 hr at 56". The transparent PR/IMA samples were broken out of the 0.4-cm i.d. glass tubes and cut to cylinders 2 cm long. (1) For example, see A. V. Zubkov, A. T. Koritslry, and Ya. 9. Lebedev, Dokl. Akad. Nauk SSSR, 180, 1150 (1968); D. A. Wiersma and J. Kommandeur, Mol. Phys., 13, 241 (1967). (2) B. G. Ershov, Actions Chim. Biol. Radiat., 14, 191 (1970). (3) J. Zimbrick and L. Kevan, J . Chem. Phys., 47, 2364 (1967). (4) B. L. Bales and L. Kevan, ibid., 52, 4644 (1970). (5) L. Kevan and D. H. Chen, ibid., 49, 1970 (1968); D. H . Chen and L. Kevan in "Organic Solid State Chemistry," G. Adler, Ed., Gordon and Breach, London, 1969, p 183. (6) H . Hase and L. Kevan, J. Chem. Phys., 52, 3183 (1970). (7) D. Smith and J. J. Pieroni, Can. J . Chem., 43, 876 (1965). (8) B. G. Ershov, G. P. Chernova, 0. Ya. Grinberg, and Ya. 8 . Lebedev, Im. Akad. Nauk SSSR, Ser. Khim., 2439 (1968). (9) (a) A. T. Bullock, W. E. Griffiths, and L. H. Sutcliffe, Trans. Faraday Soc., 63, 1846 (1967) ; (b) A. T. Bullock and L. H. Sutcliffe, ibid., 60, 2112 (1964). (10) H . Yoshida, K. Hayashi, and S. Okamura, Ark. Kemi, 23, 177 (1965). (11) J. Zimbrick, F. Hoecker, and L. Kevan, J . Phys. Chem., 72, 3277 (1968). (12) 0. M . Taranukha, V. A. Vonsyansky, and Ya. S. Lebedev, Khim. Vys. Energ., 2, 476 (1968).
The Journal of Physical Chemistry, Vol. 76,No. 16, 1971
WALTERHAULAND LARRYKEVAN
2444 These samples contained some residual radicals from the polymerization which were removed by heating at 90" for 24 hr. Irradiations were carried out at 31" in a 6oCoy-source at a dose rate of 0.42 Mrad/hr in the dark. Measurements were made with a Varian 4502 epr system which included a dual cavity with a quartz dewar insert. The microwave bridge was operated in the lowpower mode and the automatic frequency control was locked to the sample cavity. The microwave power was variable over a 50-db range. The microwave power, P, was measured with a thermistor and power meter and the microwave magnetic field, HI, is given by HI = 2.22P1/' G with P in watts for our spectrometer. Slow passage progressive saturation measurements were made at room temperature in a quartz dewar as previously described. A magnetic field modulation frequency of 40 Hz and a modulation amplitude of 0.3 G satisfied slow passage conditions. A DPPH sample, which does not saturate within the power range used, was also measured in the dual cavity. Since the DPPH signal intensity should be linear with HI, deviations were interpreted to be due to attenuator calibration errors and the experimental saturation curves were corrected accordingly. The radical spin concentrations were determined by comparing the doubly integrated first-derivative PMMA spectra at 300°K with the uncorrected trapped-electron spectrum in r-irradiated 10 M NaOH at 77°K. Correction was made for the temperature difference. The uncorrected trapped-electron sample contains 1.3 X 10l8spins g-I Mrad-l. This corresponds to 2.1 electrons per 100 eV of radiation energy absorbed (G value). l 3
Results The normal nine-line epr spectrum associated with irradiated P M N A at room temperature14was observed in all samples. Although there has been some controversy about whether this spectrum is due to one or two radicals, it now seems clearly established that it is due to a single radical of the type RCH2C(CH3)COOCH3 with slightly different conformational angles of t h e two C-H, bonds t o the p orbital containing the unpaired electron.l5 The signal intensity (height times width squared of the first derivative curve) of the central line of the spectrum was measured vs. the microwave magnetic field, HI, to give a power saturation curve. Figures 1 and 2 show typical power saturation curves at 4.9 &!!rads for two different decay times. The shape of a saturation curve is related to the spin-lattice, T I , and spin-spin, Tz, relaxation times of the radical. The relevant theory for obtaining relaxation times from saturation curves under slow-passage conditions has been summarized in a previous paper. a The PMMA radical saturaThe Journal of Physical Chemistry, Vot. 76, No. 16, 1971
o
I
I
3.0
4.7
I
a47
1.9
HI x IO, GAUSS Figure 1. Power saturation curve for the central line in the epr spectrum of the radical in 7-irradiated PMMA a t room temperature. The radiation dose was 4.9 Mrads and the data were taken after 3 hr of thermal decay following irradiation. The relative signal intensity, V , is the height times the width squared of the first derivative epr signal and HI is the microwave magnetic field. The symbols H L 1 and H y refer to parameters used in the analysis of the saturation curve according to ref 16.
0
0.47
I
I
I
1.9
3.0
4.7
HI x I O , GAUSS
Figure 2. Power saturation curve for the central line in the epr spectrum of the radical in ?-irradiated PMMA at room temperature. The radiation dose was 4.9 Xrads and the data were taken after 54 hr of thermal decay following irradiation. The symbols are defined as in the caption to Figure 1.
tion curves were analyzed in terms of nonideal inhomogeneous saturation by Castner's treatment.16 In this analysis, groups of spins are considered t o form noninteracting lorentzian spin packets which are superimposed to form an observed gaussian line. The parameter a = 1.47 AHmsL/AHmsG,where A H m s L and AHmsG are the spin packet and observed line widths at maximum slope, is determined from the saturation curves. Then T zis given by eq 1
T , = 1.70/a(yAHmBG) (1) where y equals 1.76 X lo7 G-1 sec-'. The value of a depends on the ratio of two HI values from the satura(13) L. Kevan in "Radiation Chemistry of Aqueous Systems," G . Stein, Ed., Wiley-Interscience, New York, N. Y., 1968, pp 21-72. (14) E. E. Schneider, M. J. Day, and G. Stein, Nature, 168, 645 (1951). (15) M. Iwasaki and Y . Sakai, J . Polym. Sei. Part A-1, 7 , 1537 (1969). 116) T. G. Castner, Phys. Rev., 115, 1506 (1959).
THERMAL DECAY EFFECTS ON SPATIAL DISTRIBUTIONS OF RADICALS
2445
DOSE, MRAD Figure 3. Variation of PMMA radical concentration (0)and spin-spin relaxation time, room temperature.
2'
( 0 )us.
r-radiation dose at
tion curve and is therefore independent of the absolute value of H1. T I T z is given by eq 2 where Hl,,,,,, is obtained from the saturation curve and does depend on the H1 calibration.
It should be noted that the absolute values of TZand T1 derived from the above analysis depend on the assumption of noninteracting spin packets (ke., no spin diffusion within the observed gaussian line). This assumption is probably not completely correct, but only the relative values of T z are of principal interpretive significance. Figure 3 shows the PMMA radical yield in spins per gram and Tz vs. radiation dose at room temperature. Most of the decrease in radical yield above 7 Mrads is apparently due t o slow thermal decay of the radicals during irradiation. Data on this decay are given in Figures 2-6. The observed decay rates in Figures 2-6 do not seem fast enough to account for more than 10-20% of the observed decrease in initial radical yield above 7 Mrads. However, the decay rates seem to decrease with the time elapsed after irradiation and are presumably faster at earlier times during irradiation. We note that the maximum in the radical yield 21s. dose is phenomenologically analogous to maxima observed for electron yields vs. dose in organic glasses a t 770K,17 but we do not have a quantitative understanding of this effect a t present. The TIvalues were constant at about 1.8 X see from 0.4 to 5 Mrads and then decreased slowly to about 0.4 X see at 10.7 Mrads. The line width at maximum slope of the central line of the PMMA spectrum is 5.4 G and is independent of
0
20
40
HOURS
Figure 4. Variation of PMMA radical concentration (0) and spin-spin relaxation time, T P( 0 )us. time after 0.85-Mrad y irradiation at room temperature.
dose. At the greatest saturation achieved the line broadened to about 8 G. The line shape parameter defined by Pake and Purcellls averages 2.3 independent of dose. Since the theoretical value for a pure gaussian line is 2.2, the line shape is closely gaussian as expected for inhomogeneous broadening. Figures 4-8 show the effect of thermal decay at room temperature on the radical yields and on T z at five doses. For each dose the value of T1 was constant over the time of decay. During the thermal decay periods (17) M. Shirom and J. E. Willard, J. Amer. Chem. SOC.,90, 2184 (1968). (18) G . E. Pake and E. M. Purcell, Phys. Rev.,74, 1184 (1948). The Journal of Physical Chemistry, Vol. 76,No. 16,1971
2446
WALTERKAULAND LARRYKEVAN I
1
1W
160
HOURS
Figure 5 . Variation of PRIMA radical concentration (0)and spin-spin relaxation time, 7'2 ( 0 )us. time after 4.9-Mrad y irradition a t room temperature.
12
I
HOURS
Figure 7. Variation of PMMA radical concentration (0)and spin-spin relaxation time, TZ( 0 )us. time after 8.6-Mrad y irradiation a t room temperature.
-.-.-
.
4 -
c
1
o IW
50
0
100
60
150
150
HOURS
HOURS
Figure 6. Variation of PMMA radical concentration ( 0 )and spin-spin relaxation time, 7'2 ( 0 )us. time after 7.2-Mrad y irradiation at room temperature.
the samples were stored in the dark. Similar results were obtained for doses at 2.5 and 12.7 Mrads but are not plotted.
Discussion Since T z is related to the local spin concentration in the region of the trapped radicals, the thermal decay data in Figures 2-6 allow us to compare quantitatively how the local and average spin concentrations change. Kittel and AbrahamsIg calculated the frequency moments for spin-spin dipolar broadening from spins randomly distributed on a cubic lattice with the external magnetic field along the 100 axis. For a polycrystalline sample where the crystallites are oriented randomly with respect to the magnetic field this dipolar width is reduced by the factor 0.71.z0 The width is further reduced by g anisotropyz0which we do not consider here. When the fractional spin population is less than 0.01, as it is for radiation-produced magnetic species, the dipolar line is lorentxian and is identified with the spin-pacThe Journal of Physical Chemistry, Vol. 76, N o . 16, 1971
Figure 8. Variation of PMMA radical concentration (0)and spin-spin relaxation time, 2'2 ( 0 )vs. time after 10.7-Mrad y irradiation at room temperature.
ket of the saturation analysis. The relation between Tzin seconds and the local spin concentration Nloc in spins/cma is then given by eq 3. Nloo
= (1.23 X 10-l2Tz)-'
(3)
The local spin concentrations at each dose immediately after irradiation and after 50 hr of thermal decay are collected in Table I. These are compared with the average spin concentrations, N a y , in the same samples. Na, is converted to spins/cms with a density of 1.18 for PMMA. Note that Nloo> N,, when only two radicals per cluster are separated by rloc < rav. At low doses the ratio Nloc/Nav equals 25 at 0.42 Mrads and 12 at 0.85 Mrad. The Nloc corcesponds to an average distance between radicals of 44 A if spherical volumes around each radical are assumed. Even though our absolute value of T z , on which this ratio (19) C. Kittel and E. Abrahams, Phys. Rev., 90, 238 (1953); in the expregsion for A on the second page of this paper, g should be squared. (20) 8. J. Wyard, Proc. Phys. SOC.,Ser. A , 86, 587 (1965).
THERMAL DECAYEFFECTS ON SPATIAL DISTRIBUTIONS OF RADICALS
-~~ ~~
~
2447 ~
Table I : Local and Average Spin Concentrations (spins/cma) of Radicals in ?-Irradiated PMMA a t Room Temperature Dose
Nlooinitial Ailooafter 50 hr Decrease N,, initial N,, after 50 hr Decrease Nloc/Na,initial N1oc/.Vavafter 50 hr
0 . 8 5 Mrsd
2 . 5 Mrads
2 . 3 X 10'9 1 . 4 x 1019 39 % 1 . 9 X 10l8 1 . 6 X 1018 16% 12 8.8
2 . 3 X 1019 1 . 4 x 1019 39% 5 . 2 X 10l8 4 . 3 X 1018 17% 4.6 3.3
4.9
Mrads
2 . 3 x 1019 1 . 3 x 1019 43% 8 . 9 X 1018 6 . 6 X 1018 26% 2.6 2.0
depends, may be in error by a factor of 2 or 3, Nloo/Nav is clearly greater than 1 at doses less than 5 Mrads. This means that the radicals are formed in localized regions of higher than average radical concentration. Such regions are called spurs in radiation chemistry and reflect the nonhomogeneous deposition of radiation energy. At high doses we expect the average and local spin concentrations to become comparable. This is observed at doses above 7 R h d s where NloC/Navis of the order of unity. Bullock, et al.,9 have made similar measurements on PMMA radicals up to about 1.5 Mrads. They do not quote radiation doses but we deduce them from their ref 2. Thus they observed only the isolated spur region in which NloC/Nav> 1. Their average local spin concentration was 1.80 X 1019 spins/cm3. However, they assumed that the y factor of the radical was anisotropic (see ref 20) so their value should be decreased to 1.2 X 1019 for comparison with ours at low dose. Still, the agreement is reasonable. Our average spin concentrations are about 3 times as large as theirs at comparable doses. The initial linear portion of our yield-dose curve in Figure 3 corresponds to a 100-eV yield (G value) of 2.7 which is a typical value for radicals in nonaromatic organic solids. Bullock and Sutcliffegbalso reported that for radicals in 7-irradiated PR/lh!tA T 2 increased by about a factor of 2 when the temperature was lowered from 299 to 77°K. This suggests that the local concentration decreases when the temperature is lowered. Similar results have been observed in an analogous polymeric system.l' This temperature effect could possibly be caused by contraction of adjacent polymer chains so as to increase the distance between nearby radicals when the temperature is lowered. Our results were all taken at room temperature and do not bear on this question; however, they do indicate that Tz changes can be reasonably related t o changes in local radical concentration. One objective of this work was to compare how local and average spin concentrations change during thermal decay of the radicals. Figures 4-8 show that the radical decay rates decrease with time at room temperature at all doses studied. Table I shows the fractional decay rates, based on both local and average
7 . 2 Mrads
8 . 6 Mrads
1 . 5 X 10'9 7 . 8 x 1018 48% 10.6 X 1018 5 . 9 X 1018 44% 1.4 1.3
9 . 5 X 10l8 5 . 3 x 1018 44 % 8 . 3 X 10l8 4.4 X 47% 1.1 1.2
10.7 Mrads
1 2 . 7 Mrsds
5 . 8 X 10l8 3 . 5 x 1018 40% 4 . 2 X 10l8 2.7 X 36 % 1.4 1.3
4.4 X 2 . 3 x 1018 48% 2 . 4 X 10l8 1.2 X 50% 1.8 1.9
spin concentrations, as the per cent decrease for 50 hr of decay. At doses less than 5 Mrads the fractional decrease in local spin concentration is about 2.5 times as great as the fractional decrease in the average spin concentration. This is consistent with intraspur decay. Since the spurs are isolated and the radical concentration is highest within the spurs this result is not unexpected. However, the experimental approach used here provides a more direct observation of intraspur decay than does thermal decay kinetics alone. A consequence of intraspur decay is that the kinetics should not be second order with respect to the total (average) spin concentration, even if the decay mechanism is simple radical combination. This is best substantiated by looking at the effect of total spin concentration on the fractional decay rate. At 0.85 and 2.5 RIrads the fractional decay rates based on the total spin concentration are the same even though the spin concentrations differ by a factor of 3. Similar results are obtained for limited data at 0.4 Mrad. On the other hand, if random radical combination occurred within the spur the kinetics would be second order with respect to the local concentration. Our data do not convincingly fit either first- or second-order plots based on local concentrations below 5-Mrad dose, so we are unable to deduce unique intraspur kinetics. However, the data can be fitted to two or more concurrent firstorder decays of different rates, Below 5 Mrads the initial local concentration is constant, so the dose independence of the fractional decay rate with respect to local concentration cannot be used as a criterion for concurrent first-order decays. However, the fractional decay rates based on local concentrations are roughly independent of the local spin concentrations if the high dose data are also included. This is consistent with concurrent first-order decays within the spur. Simple radical combination within the spur is consistent with such kinetics if the combination is not random. At doses above 7 Mrads the local and average spin concentrations are comparable and show comparable fractional decay rates. In this dose range we make no distinction between intraspur and interspur decay. Again the radical decay kinetics do not fit simple first The Journal of Physical Chemistry, Vol. 76,N o . 16, 1971
FARMER, GARDNER, GERRY,MCDOWELL, AND RAGHUNATHAN
2448
or second order, but seem best represented by concurrent first-order decays. We have observed intraspur decay for radiation-produced organic radicals by spin-spin relaxation measurement of local radical concentrations. We suggest that intraspur decay is a common situation in irradiated organic solids and that it complicates the interpretation of radical decay based on total spin concentrations. Conversely, if pure second-order decay, substantiated a t several initial radical concentrations, is observed in irradiated solids these radicals cannot be trapped in spurs containing other radicals. An example of this latter situation has been observed for 3-methylpentyl
radicals in irradiated 3-methylpentane at 77 and 87"K.21
Acknowledgment, This research was supported by the Air Force Office of Scientific Research under Grant No. AFOSR-70-1852 and by the -4tomic Energy Commission under Contracts AT(l1-1)-1852 and AT(11-1)-2086. The experimental work was initiated at the University of Kansas. L. K. wishes to thank the John Simon Guggenheim Foundation for a fellowship and the Danish AEC Research Establishment Riso for their hospitality and services. (21) W. G. French and J. E. Willard, J . Phys. Chem., 7 2 , 4606 (1968).
Electron Spin Resonance of Free Radicals Prepared by the Reactions of Methylene. Deuteriomethyl and Formaldiminoxy Radicals by J. B. Farmer, C. L. Gardner, M. C. L. Gerry, C. A. McDowell,* and P. Raghunathan Department of Chemistrg, The Universitu of British Columbia, Vancouver 8, British Columbia, Canada (Received February OR, 1971) Publication costs assisted by the National Research Council of Canada
The electron spin resonance spectra of the deuteriomethyl (CHZD and CHD2) and the formaldiminoxy (CHZNO) radicals have been obtained by photolyzing diazomethane in the presence of DzO and NO, respectively, in an argon matrix at 4.2"K. Hyperfine coupling constants and g factors have been evaluated for each radical. The assignment of the structure of the formaldiminoxy radical and the correctness of the interpretation of its esr spectrum has been confirmed by experiments performed using W O . This work shows that nitric oxide may be used as a spin trap for triplet state radicals. Furthermore, it confirms the existence of the CHzNO radical which had previously been postulated as an intermediate in certain gas phase reactions.
Introduction One of the primary products of the photolysis of both diazomethane and ketene is the methylene radical (CH2). Its electronic spectrum, giving evidence of both singlet and triplet states, has been observed in the gas phase,' and the electron paramagnetic resonance spectrum of radicals trapped in a xenon matrix a t 4.2OK has confirmed that the ground state is a triplets2 We have photolyzed both diazomethane and ketene in argon matrices a t 4.2"K and have observed in many experiments the characteristic four-line spectrum of the methyl (CH3) radical superimposed on a further broad signal. Methyl radicals cannot be formed in %heprimary photolytic step of either diazomethane or ketene. There is considerable evidence, however, that methylene radicals produced in this step can abstract hydrogen atoms3 to produce methyl radicals, and this would T h e Journal of Physical Chemistry, Vol. 76,No.16, 1971
seem to be a reasonable method of generating the latter. I n an attempt to obtain further evidence for the abstraction reaction, we photolyzed diazomethane in the presence of heavy water (D20)in an argon matrix a t 4.2"K. Should abstraction be occurring, the spectrum anticipated was that of the CH2D radical. This was indeed found, and the details of the results are given in a later section. The success of these experiments suggested that the photolysis of diazomethane in the (1) G. Herzberg, Proc. Roy. Soc., Ser. A , 262, 291 (1961). (2) R . A. Bernheim, H . W . Bernard, P. 8. Wang. L. S. Wood, and P. S. Skell, J . Chem. Phys., 53, 1280 (1970); E. Wasserman, W. A . Yager, and V. J. Kuck, Chem. Phys. Lett., 7, 409 (1970) ; E. Wasserman, V. J. Kuck, R. S. Hutton, and W. A . Yager, J . A m e r . Chem. Soc., 92, 7491 (1970). (3) See, for example, G. B. Kistiakowsky and T. A. Walters, J . Phys. Chem., 7 2 , 3962 (1968); D. F. Ring and B. S. Rabinovitch, Can. J . Chem., 4 6 , 2435 (1968).
