2590
LARRYKEVAN
Radiolysis of Frozen Solutions.
I.
Formation of Nitrogen Dioxide
in Sodium Nitrate Ices
by Larry Kevan Department of Chemistry and Enrico Fermi Institute f o r Nuclear Studies, University of Chicago, Chicago, Illinois (Received A p r i l 10, 1964)
The principal nitrogen-containing paramagnetic species formed a t 77 OK. in y-irradiated frozen sodium nitrate solutions is the NO2 radical. The KO2 yield is linear with radiation dose to only 0.04 Mrad and is proportional to [NaN03]”*. A diffusion kinetic model in which H2O+ is the diffusing species which reacts according to H2O+ No3- + x03* + NO2 0 fits this concentration behavior. As expected from this model experiments with added electron scavengers do not affect the NO2 yield. Also, the NO3 radical has been tentatively identified as being present in minor yield.
+
+
introduction e.s.r. spectrometer was used for all resonance measurements. Hyperfine splittings were determined by comRecently, in the radiolysis of aqueous solutions1 and parison with the 41.0 gauss splitting between DPPH of frozen solution^^,^ the electron has been shown to be a and ultramarine,* and g-factors were determined by principal reactive species; and the kinetics of its reacdeviations from g = 2.0037 for DPPH.$ In the warmtion with a wide variety of solutes have been ~ t u d i e d . ~ . ~ ing experiments the temperature gradient to which the The occurrence and reactions of H20+, the stoichiosample was exposed was monitored with a copper-conmetric equivalent of the electron, have been postulatedj6 stantan thermocouple. but little evidence for HzO+ has been found. This is due to the reactivity of H 2 0 + toward water itself. Results The electron reacts slowly with water4 compared to its Identijication of Paramagnetic Species. The e.s.r. rates of reaction with niost dissolved solutes, while HzO+ spectrum a t 77°K. of 0.6 M NaN03 in ice irradiated at is believed to react rapidly according to eq. 1. 77°K. is shown in Fig. 1. This is a composite of the hydroxyl radical spectrum (dotted line in Fig. 1) proH20+ H2O --f H30+ OH (1)
+
+
In aqueous solutions reactions of the hydroxyl radical with are Observed6 rather than However, an unidentified paramagnetic Of Hzo+’ species has been tentatively attributed to H 2 0 + in ice irradiated at 77’KS7 Experimental All chemicals were reagent grade. Water was triply distilled Over alkaline permanganate solution. Identical results were obtained with degassed and nondegassed solutions and the results reported are for nondegassed solutions. The frozen samples were prepared in c y h drical form as described previ~usly.~Irradiations were carried out in liquid nitrogen in a 600-C.cobalt-60 source at a dose rate of 0.14 ,IGad/hr. A Varian 4502 The Jozcrnal of Physical Chemistry
(1) E. J. Hart and J. W. Boag, J . Am. Chem. SOC.,84, 4090 (1962); earlier references relating to chemical proof are cited therein. (2) D. Schulte-Frohlinde and K. Eiben, 2. Naturforsch., 18a, 199 (1963); 17a, 445 (1962). (3) L. Kevan, P. N. Moorthy, and J. J. Weiss, J . Am. Chem. Soc., 86, 771 (1964); Nature, 199, 562 (1963). (4) See for example: 9. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, J . Am. Chem. Soc., 8 5 , 1375 (1963); J. H. Baxendale, et al., Nature, 201, 468 (1964). (5) E. Hayon and J. Weiss, P ~ o c Intern. . Conf. Peaceful Uses A t . Energy, Bnd, Geneva, 2 9 , 80 (1958). (6) A. o, Allen, Radiation Chelaistry of Water and Aqueous Solutions,” D. Van Nostrand, New York, N. Y . , 1961. (7) J. A. McMillan, M. S. Matheson, and B. Smaller, J . Chem. Phys., 33, 609 (1960). (8) D. J. E. Ingram, “Free Radicals as Studied by Electron Spin Resonance,” Butterworth and CO.,Ltd., London, 1958, p. 99. (9) C. A. Hutchison and R. L. Pastor, Phys. Rev., 81, 282 (1951)
RADIOLYSIS OF FROZEN SOLUTIONS
2591
duced in the radiolysis of H2O and a spectrum arising from the nitrate solute. When the ice sample is warmed linearly from 77 to 148°K. over a 3.5-iniri.
period and is quickly recooled to 77"K., the hydroxyl radical spectrum decays and the spectrum shown in Fig. 2 remains. This spectrum consists of an anisotropic triplet shown by a, b, and c and a singlet designated by d. The abc triplet is definitely identified as the NOz radical by its isotropic splitting constant of 54.7 gauss and g-factor of 2.001. These values agree well with previously reported parameters for NO2: isotropic splitting constant of 52.0-56.9 gauss and g-factor of 1.998-2.001 The multiple lines and variation in line widths of the separate components of the triplet have been observed previously in a wide variety of matrices including CH4,I3 HzOl10CC14,14and argon.IE These features have been attributed to multiple trapping sited5but are perhaps better explained by AdrianI6 as due to a combination of hyperfine and g-factor anisotropy. The anisotropies reported in Table I were derived from eq. 2 and the treatment used by Adrian, Cochran, and Bowers. l 9
T(MI)=
Figure 1. E.s.r. spectrum at 77°K. of 0.6 M NaNOD irradiated t o 0.07 Mrad a t 77°K. T h e dotted line is t h e spectrum of pure ice.
A
( h ~ o- MIA) (G - 901) I MIB (BOP)
(90)
SOP
(2)
T is the traceless anisotropic field tensor, G is the gfactor tensor, I is a unit tensor, B is the anisotropic hyperfine tensor, and A is the isotropic hyperfine splitting constant. The g-factor tensor components were obtained from the central line of the triplet in Fig. 2, and the anisotropic hyperfine tensor components were obtained from the low and high field lines. The hyperfine anisotropy is smaller for the KOz radical reported here than that reported for NO2 from the photolysis of Xz04in icelo and that reported for an irradiated single crystal of KNO2.I7 Such a difference could arise if the KO2 was in some kind of motion. The singlet a t line d is due to another radical species. This is shown by the temperature dependence; when (10) P. W. Atkins, N. Keen, and M. C. R. Symons, J . Chem. SOC., 2873 (1962). (11) J. Cunningham, J . Phys. Chem., 66, 779 (1962). (12) C. Jaccard, Phys. Rev., 124, 60 (1961). (13) V. A. Sharpatyi and K. N. Molin, Russian J . Phys. Chem., 35,
a
b
Figure 2. E.s.r. spectrum of NO2 (lines a, b, and c) and NO8 (line d ) in ice a t 77°K.
G
621 (1961). (14) B. H. J. Bielski and R. B. Timmons, J . Phys. Chem., 68, 847 (1964). (15) J. B. Farmer, D. A. Hutchinson, and C. A. Mcnowell, !jth International Symposium on Free Radicals, Vppsala, 1961, preprint 44. (16) F. J. Adrian, J. Chem. Phys., 36, 1692 (1962).' (17) H. Zeldes and R. Livingston, ibid.,35, 563 (1961). (18) R. .M. Golding and IM. Henchman, ibid.,40, 1554 (1964). (19) F. J. Adrian, E. L. Cochran, and V. A. Bowers, $bid., 36, 1661 (1962).
Volume 68, Number 9
September, I064
2592
LARRYKEVAN
~
~
~
_
_
Table I : Parameters of NO$ in Irradiated 0.6 M NaN03 a t 77°K. Anitlotropic
Isotropic 0.20
g-factor
Hyperfine splitting
81 = 2.005 Qz 2.003 Q3 = 1 . 9 9 4 bllI = 2 . 5 gauss IBzzl = 1 . 7 gauss IB~SI = 4 . 2 gauss
gav = 2 . 0 0 1
Ai., = 5 4 . 7 gauss
0.15 h
8
z
G
the sample is warmed the NO2 radical decays more rapidly than the d species. The d line has a g-factor of 2.017 and a line width a t points of maximum slope of 9.8 gauss. NO3 has been reported by Golding and HenchmanIBwith g = 2.019 and a hyperfine splitting of -1.3 gauss in a single crystal of irradiated lead nitrate. Cunningham reports a species in a single crystal of irradiated potassium nitrate a t g = 2.018 and hyperfine splitting equal to 4.3 gauss which has been reinterpreted by Atkins and Symons20 as NO3. Chantry, et report NO3 in a single crystal of irradiated urea nitrate with g = 2.013; no hyperfine splitting was observed, but the line was rather broad. All other nitrogen oxide paramagnetic species have quite different g-factors and much larger splitting constants.20 By comparison with the above values the d line in Fig. 2 is tentatively identified as the NO3 radical. Because of the 10-gauss line width in the polycrystalline matrix a small splitting constant of 4 gauss or less would not be resolved. NO2 Yields. When the e.s.1. spectrum from pure ice is subtracted from the total spectrum in Fig. 1the triplet of NO2and the singlet attributed to NO3 remain. From Fig. 1 and 2 one can see that the shape of the high field line of the NOz triplet changes in the warming, cooling cycle used to obtain Fig. 2; the integrated intensity of this line remains the same, however. Since the high field line of the NOz triplet is unperturbed by the overlying ice spectrum it can be used as a measure of the NOz radical concentration. This was checked by double integration of the entire spectrum after subtraction of the ice spectrum a t several concentrations. The yield of NOz us. KO3-- concentration is shown in Fig. 3 ; the NO3- concentration is on a logarithmic scale. Notice that the NO2 yield is far from being a linear function of the Nos- concentration. The NO2 observed is not due to a direct effect of the radiation on the NOS- ions. Pure powdered NaN03 irradiated to 0.05 Mrad at 77°K. showed no resonance attributable to NOz. This agrees with Cunningham," who found that NO2 is not produced in powdered The Journal of Physical Chemistry
0.10
0.05
0.00
I 1.2
x
10-5 1.2
x
10-4 1.2 x 10-1 1.2 log [ SaNOs].
x
10-9
1.2
x
, 10-1
1.2
Figure 3. Plot of NO2 radical yield us. log [PiaNOa] in ?-irradiated ice a t 77°K. at 0.04-Mrad dose. The solid curve is a theoretical one based on the diffusion kinetic model: see text.
K N 0 3 a t 77°K. when irradiated to less than 5 n h d s , and with a single crystal study on KNO, by Zeldes and Livingston. 2 2 The SO2 yield as a function of radiation dose is plotted in Fig. 4. It is linear to only -0.04 Mrad before it begins to fall off; presumably, this is due to a secondary reaction. At all doses, however, the yield bears the same relationship to the nitrate concentration. The measurements for Fig. 3 were taken a t 0.04 Mrad. From the initial linear portion of the yielddose plot the NO2 yield in radicals produced per 100 e.v. absorbed (G) was determined as 0.24 for 0.6 M SaNO3. The G-value was obtained by comparing the integrated absorption of the NO2 spectrum with that of the hydrogen atom in 1.0 M NaHS04in which G(H) = 0.14.3 At 2 mw. microwave power the NO2 spectrum did not require correction for power saturation but such corrections were necessary for the hydrogen atom. The reported G-values are estimated to be no better than f50% accurate. (20) P. W. Atkins and M.C. R. Symons, J. Chem. Soe., 4794 (1962). (21) G. W. Chantry, A. Horsfield, J. R. Morton, and D. H. Whiffen, Mol. Phys., 5 , 589 (1962). (22) H. Zeldes and R. Livingston, J . Chem. Phys., 37, 3017 (1962).
RADIOLYMS OF FROZEN SOLUTIONS -___-
2593
solute concentration while the NO2 yields are decidedly nonlinear in solute concentration. Both of these factors, (a) no effect due to e , - scavengers and (b) nonlinearity of NOz yields, strongly indicate that e,reactions are not involved in the production of NOz. A model involving the reaction of HzO+ as outlined in (4) is in accord with the observed radical products. It is postulated that upon neutralization of NOS- the excited NO** radical can either be deactivated to yield
70
60
.R 50 0” 40 .-*I
2 30 I
&O+
+ Has-
4 No3*
’ \NOz
20
10
0
0.0
0.05
0.10
0.15 Mrads.
0.20
0.25
0.30
Figure 4. Plot of NO2 radical yield us. radiation dose a t 77°K. in ?-irradiated ice containing various concentrations of NaN03: X, 0.6 M ; 0, 0.012 M; A, 0.003 M
Discussion The principal product in the irradiation of frozen sodium nitrate solutions at 77°K. is the NOZ radical; the NO3 radical is apparently a minor product. These radicals do not arise ma a direct interaction between the nitrate ions and the radiation but rather are the result of a’species formed in the radiolysis of ice reacting with the nitrate ions. In the radiolysis of ice at 77°K. one has the following species formed; em- represents a mobile electron.
H ~ o +em-, , H~O* (3) It is suggested that the NOz and NO3 radicals are R~O
formed in a reaction involving HzO+. The other likely reacting species is e,- since it can react3 with NO3- ; however, the following evidence indicates that NO, does not arise via a reaction of enl-. Let us compare the NO2 results with a known reaction of em--. The mobile electron reacts with HSO,-, HCOs-, and HZPO4-in frozen solutions to yield hydrogen atoms. These hydrogen atom yields can be decreased by adding a second solute such as acetone which acts as an electron scavenger. Since the relative rates of em- with NO3- and acetone are reporteda as 3.5: 1 a frozen solution 1.0 M in acetone and 0.5 M in NO3- should show a 35% decrease in the NOz yield conipared to the NOz yield in 0.5 M NO3- ice. However, no change in the NO2 yield was observed experimentally. Also, the hydrogen atom yields from em- reactions are linear in
(4)
+0
NO3 or can dissociate to give NOz. The results imply that dissociation is preferred. The aonlinear NO2 yield as a function of the NO3-concentration can be understood in terms of the diffusion kinetic model that has been successfully applied to the radiolysis of aqueous solutions.23 According to this model one considers the reactive species to diffuse outward from an initial Gaussian distribution in spherical spurs. For Co60 7-radiation these spurs are formed so far apart that they do not interact. In addition to diffusion HzO+ is assumed to react via (4) and (5); reaction 5 has been postulated by Hayon and Weiss.6 2Hz0+ +HOOH
+ 2”
(5)
The diffusion kinetic equation then becomesz3
d(Hzo’) = DV2[HzO+]- k5[H20+]2dt
h[HzO+][Nos-]
(6)
where D is the diffusion coefficient of HzO+ and V 2 is the Laplacian operator. To solve eq. 6 one must first choose values for D, kq, k6, an initial spurradius, and the number of reactive species in each spur; then mathematical approximations or computer methods are necessary. For various parameters solutions to (6) have been obtained in terni~s of the fractional number of reactive species reacting with solute, N R / N o . ~ ~For , ~ a~ given set of the above mentioned parameters this fraction depends only on solute concentration. The NOz yield in Fig. 3 is the yield of the solute reaotion for our case. The solid line in Fig. 3 is calculated from the values of NR/No tabuated by Flanders and F r i ~ k e . For ~ ~ different values of the parameters the (23) A. Kuppermann, “The Chemical and Biological Action of Radiations,” Vol. V, M. Haissinsky, Ed., Academic Press, London, 1961, p. 85. (24) D. A. Flanders and H. Fricke, J . Chem. Phys., 2 8 , 1126 (1958).
Volume 68, Number 9 September, 1063
LARRYKEVAN
2594
X
x X
0.20
0.15 h
d 2
G 0.10
0.05
r 0.00
0.0
0.2
0.6 0.8 [NBNO~]*/~.
0.4
1.0
Figure 5. Cube root plot of NO2 yield from irradiated nitrate ices a t 77°K.
abscissa was shifted by subtracting the value of NR,!No a t zero solute concentration; the form of the curve is then rather insensitive to the values chosen for the paranieters. The calculated curve is that for the dimensionless parameter E = 1.54424but tor other values
The Journal of Physical Chemistry
of E the curve is similar. The agreement of the data with the diffusion kinetic model is excellent. The molecular yields of hydrogen and hydrogen peroxide in aqueous radiolysis are linearly proportional to the cube root of the solute concentration over a wide range.z5sz6 This is a fortuitous consequence of the diffusion kinetic model,Z*~Z5 and is a convenient empirical representation of the model which requires no arbitrary paranieters. Figure 5 shows that the NOz data also agree well with this cube root relationship over the lower four orders of magnitude. Stoichiometrically, eq. 4 could have been formulated equally well in terms of OH instead of HzO+. However, in ice a t 77°K. OH itself is immobile while HzO+ can move along the hydrogen-bonded network of ice by charge-transfer processes. Furthermore, the NO2 yield does not increase appreciably when the frozen samples are warmed to temperatures (>110 OK.) a t which the OH radicals are mobile. The yield-dose plot in Fig. 4 shows that the KOz yield falls off rapidly with dose above 0.04 M a d . This implies that NOz is participating in a secondary reaction, perhaps with HzO+ or em-. However, it seems surprising that secondary reactions of this type could be effective a t such low NO, concentrations M in 0.6 M NaN03). We are presently investigating other frozen solutions to see if additional evidence can be found for the presence and reactions of HzO+.
Acknowledgment. We thank the U. S. Atomic Energy Commission for support under Contract No. At (11-1)-1365. (25) H. A. Sohwarz, J. A m . Chem. Soc., 77, 4960 (1955). (26) T. J. Sworski, Radiation Res., 2 , 26 (1955).
