D. D. WILKEY,J. F. BRENSIKE, AND J. E. WILLARD
3580
Reaction of Dilute Aqueous CHJ with Iodine Activated by Nuclear Processes1
by D. D. Wilkey, J. F. Brensike, and J. E. Willard Department of Chemistry, Uniaersity of Wisconsin, Madison, Wisconsin (Receired March 20, 1967)
Dilute aqueous solutions of CH3IlZ7and I2(IlZ9) [or I-(IlZ9)] have been irradiated with and 11zg(n,y)1130 neutrons and analyzed to determine the fractions of the 1127(n,y)1128 events which lead to I I 2 * and to 1130 in organic combination. The organic yield of each isotope is significant, confirming an earlier report2 that organic combination of (n,T)activatzd iodine atoms can be achieved in aqueous CH31-12 systems, but showing that such combination does not require the geminate retention previously postulated.2 The yields are independent of the I- concentration from 10-5 to at least lov3mole % and increase with CH31 concentration in the range from 1.8 X mole % to the saturated solution concentration of 0.18 mole %. Like the (n,y)processes, the I130rn-+ 1 1 3 0 isomeric mole % IZ(1130m) or I-(1130rn) transition in aqueous solutions of 0.08 mole % CH31and leads to organically bound the organic yields being about 7%. The entry into organic combination of I atoms originating from I2or I-, and activated by the (n,y) or I T process, seems to be explained most plausibly in terms of the autoradiation hypothesis. Techniques have been developed for correcting for thermal exchange in the systems used and it has been shown that photochemically produced I atoms do not exchange readily with CH31in aqueous solution.
Introduction Sturm and Davis2 have reported that when dilute solutions of CH3I in HzO containing 1,- scavenger are irradiated with neutrons, 12y0of the IlZ8atoms formed by the 1127(n,y)1128 process are found in organic combination and they have attributed this to failure of rupture of the C-I bond following neutron capture by the iodine atoms. Evidence from other systems3 suggests, however, that geminate retention cannot be more than a feJy per cent. To determine whether geminate retention is required to explain the labeled organic product in this system, vie have determined the organic yields process in aqueous soluof from the 1129(n,y)1130 tions of CH3IlZ7containing I2(IlZ9) and in solutions containing, We have also determined the organic IT yields of the I130rn + 1 1 3 0 process in H20-CH31I2(I130rn) and H20-CH31-I-(1130rn)solutions. Experimental Section Aqueous methyl iodide samples (10-15 cc) were irradiated in quartz bulbs, sealed with serum bottle caps protected by Teflon inserts to prevent CH3I vapor from coming in contact with the rubber. The vapor T h e Journal of Physical Chemistry
volumes over the solutions were kept very small to minimize the fraction of the methyl iodide in the vapor phase. The CH3I used was shown by gas chromatography to be >99% pure. 1129z n-as used as received from the Oak Ridge Kational Laboratories. Known concentrations of CH31and 1 2 were obtained by dilution of saturated solutions. Tests with CH3II3l of known specific activity (prepared by illuminating a solution of 12(1131) in CH31with 2537-A radiation) showed that saturation could be achieved by vigorously shaking H 2 0 and CH3I together for 30-60 min and confirmed the. value of 0.18 mole Yc for the solubility at 2'2' reported in the l i t e r a t ~ r e . ~Aqueous 1-(1129) was prepared by reduc(1) This work was supported in part by U. S. Atomic Energy Commission, Contract AT(ll-1)-32, and by the W. F. Vilas Trust of the University of Wisconsin. (2) J. E. Sturm and D. G. Davis, Paper N o . 106 presented before the Division of Physical Chemistry a t the 140th National -Meeting of the American Chemical Society, Chicago, Ill., Sept 1961. See, for example and references: (a) J. E. Willard, "Chemical Effects of Nuclear Transformations." International Atomic EnerPv Agency, Vienna, 1961; (b) H. M. Chang and J. E. Willard, J . PhG. Chem., 71, 3576 (1967). (4) A. Rex, Z . Physik. Chem. (Leipaig), 5 5 , 355 (19OG).
(3)
REACTION OF DILUTEAQUEOUSCH3I WITH IODINE
ing I Z ( I ~solutions ~~) with SO2 gas, following which the excess SO2 was removed by bubbling nitrogen through the solution. To minimize thermal exchange, the total time for mixing of the CH31 and I2 or I- solutions, irradiation, and extraction was made as short as possible (about 6 min between mixing and extraction of the first aliquot). For analysis, aliquots n-ere withdrawn from the irradiated samples through the serum cap, using a hypodermic syringe, and discharged into a CCGaqueous S032-extracting mixture. After shaking, a known portion of each layer was placed in a screw-capped vial for counting in a well-type scintillation counter. Aliquots taken from each irradiated sample at known times following irradiation allowed correction for thermal exchange from a plot of organic incorporation of radioiodine vs. time of extraction. Similar extrapolation of the organic incorporation of the tracer activity gave the thermal exchange rate as the slope and the radiation-induced exchange as the intercept. I n determining the radiation-induced exchange for 1 I 2 8 and 1 1 3 0 from that of II3l, correction was made for the fact that the IlZ8and 1 1 3 0 are not present at the start of the irradiation, making the average time for exchange less than for the 1131 Thermal exchange rates were about O.l%/min at 25". S o exchange n-as detectable in 60 min at 0". Radiation-induced exchange increased with increasing CH31 concentration; at 0.1 mole yc it was typically 2-5%, depending somewhat on the form and concentration of the inorganic iodine. Resolution of the activities due to (23 rnin), 1 1 3 0 (12.5hr), and (8 days) was possible by making several counting rate determinations on each sample at appropriate time intervals. Irradiations were for 1 min in the pneumatic tube of the 'C'niversity of Wisconsin reactor, at a thermal neutron flux of about 1 X 10l2 n cm-2 sec-' and a ?-radiation flux of about S X lo1* ev g-l min-l.
Results Dependence of Organic Yield on Chemical Forin of the Iodine Which Captures Neutrons. The corrected organic yields of a series of samples containing 0.09 mole mole % I-, are % CH31and mole % 12,or 2 X given in Table I . The yields for I I 2 8 and 1 1 3 0 given on each horizontal line are for samples taken from the same CH31-I2 or CS31-I- solution. The yields given on line 1 of the I- section of the table are for samples made from the same aqueous CH31preparation as those on line 1 of the 1 2 section, and similarly for the other corresponding lines. In these solutions more than 95% of the IlZ8activity is formed from CHJIZ7 while more
3581
than 97% of the 1130 is formed from I2(IiZ9)or I-(I129). The monitoring of exchange by indicates that less than 3% of the inorganic iodine ever entered organic combination by thermal or radiation-induced exchange prior to the midpoint of the l-min irradiations.
Table I: Organic Yields from (n,?) Activation of Aqueous Solutions of CH311*'and I p O )or I-(I'30)a
___--
Scavenger 12
Av
I-
Av
Organic yields, %------I128
1180
8.9, 9 . 0 8.2, 8 . 1 13.2, 12.6, 12.1
5.7, 5 . 8 5 . 7 , 5.4 7 . 7 , 6.3, 6 . 3
10.1
6.1
6.2, 6 . 4 7.0, 7 . 0 6.7, 6.5, 6 . 5
3.6, 3 . 3 4.0, 4 . 0 2.7, 2.6, 3 . 0
6.6
3.3
mole 70 11, 2 X a Using 1 x mately 9 x 10-2 mole 7cCHJ.
mole % I-, and approxi-
Activation by Isomeric Transition. Yeutron irradiation of aqueous solutions of 12(1129 1131) and I-(I12O 1 1 3 1 ) before mixing them with aqueous C&I solutions made possible the study of the organic yield of the IT 1130m + 1 1 3 0 isomeric transition, in the absence of the effects of radiation-induced exchange and of the 1129(n,y)I130process. Concentrations in the mixtures were 0.085 mole % CH31 and 1 X mole I2 or 2 x moIe yc I-. The organic activity was determined as a function of time between mixing and uptake used to correct the extraction and the yield for thermal exchange, The corrected organic 1'30 activity increased with time between mixing and extraction at a rate consistent with the 9.2-min halfIT life5 of the I130m +1 1 3 0 isomeric transition. After correction for the decay of 1130m between irradiation and mixing and for the fraction of the 1 1 3 0 born in the ground state, the organic yield for the isomeric transition was estimated to be about 7y0. The results \!-ere not sufficiently precise to determine whether or not there is a difference in yields from solutions of 12(1129)and of I-(IlZg), as is indicated in Table I for n,y activation. from Concentration Eflects. The organic yield of IlZ8 neutron irradiations of aqueous 0.10 mole % CHJ containing 1--(1127 + 1131) was independent of the I- concentration from 10-3 to 10-5 mole 7c. Experiments in
+
+
( 5 ) D. D. Wilkey and J. E . i\rillard, J . Chem. Phys., 44, 970 (1966).
Volume 71, Number 11
October I967
D. D. WILKEY,J. F. BRENSIKE, AND J. E. WILLARD
3582
sample. KO observable photochemical exchange occurred. These results, together n ith evidence suggesting that cage escape of I atoms following absorption of a photon by IZin HzO is probably high,6indicate that the I* CHJ +.CH3I* I exchange in n-ater has a low probability per collision, if it occurs at all. Similar experiments in the gas phase indicate that the exchange probability is 6 per c ~ l l i s i o n . ~
Z
ZQ
+
+
Discussion I
0
I
I
l
Q4
08
l
I
.12
I
I
I
.I6
[Mal] MOLE % Figure 1. Effect of CH3I concentration on the organic yield of 1 1 2 8 produced by the Ilz7(n,y)I128reaction. Each type of point on the graph indicates a series of experiments made with different dilutions of the same saturated CHII solution. Dilution was accomplished by injecting aliquots of the CHII solutions below the surface of the scavenger solutions, through a syringe needle, except for the 0 series where normal pipetting was used. 1 X 10-3 Y scavenger was used in each case: 0, A, W, 0" irradiation, I, scavenger; 0, room temperature, I,; 0 , Oo, I-; A room temperature, I-.
which I, or I- TTas 1 X N (1.8 X mole % I-, 0.9 X mole % 1,) were done at CH3I concentrations over a 100-fold range from 2 X lod3to 1.8 X lo-' mole yo. Different techniques of analysis and different temperatures were investigated in an endeavor to minimize the loss of CH3I vapor from the sample during preparation and analysis. The results, given in Figure 1, indicate a rapid increase in organic yield with increasing CH31 concentration at the lon-est concentrations followed by a relatively slow increase at the higher concentrations. The scatter of the points is believed to reflect the difficulty of preparing and maintaining accurately known concentrations of CH31in water. The data of Table I for IlzSin the Izsystem would fall close to the curve of Figure 1 and those for the I- solutions somewhat below-. Test for Thernaal I Atom Exchange. It was important to learn rvhether 1 1 2 8 and 1 1 3 0 atoms which are neutralized and thermalized following production by the (n,r) process can exchange readily with CH31in aqueous solution. R e have illuminated aqueous solutions of 10-3 mole 95 12(Ii31)and 0.1 mole yoCH31with light of 4200-4400 -4absorbed by the I, (using a Bausch and Lomb monochromator). The length of the illumination was such that each I:!molecule absorbed, on the average, five photons. Correction for thermal exchange was determined using an identical sample which was stored in the dark for the same length of time as the irradiated The Journal of Physical Chemistry
The results given above are in agreement nith the report2 that there is a significant organic yield of 1 ' 2 8 from the 1127(n,y)1128reaction in dilute aqueous solutions of CHJ127scavenged n-ith I,. They shon in addition, that iodine atoms from inorganic iodine (I2 or I-) activated by either the ( n , r ) or isomeric transition process can enter organic combination in these systems. This indicates that failure of rupture of C-I bonds in CH31 does not account for all of the organic yield of from CH311e7,and possibly not for any of it. If the IT I127(n,y)IlZ8,1129(n,y)1130,and I 1 3 0 m + 1 1 3 0 processes are about equally effective in chemical activation, as b it apseems to be the case in n-C6H11s ~ l u t i o n . ~then pears that some geminate retention or recombination contributes to the result that the organic yields of IlZ8 (from CH3IlZ7) are invariably higher than those of 1 1 3 0 (from Izor I-) (Table I). The organic yields of both IlZ8 and 1 1 3 0 are lower when I- scavenger is used than when I? is used (Table I). This is illustrated more clearly by the data of Table I than by those of Figure 1, because the former makes comparisons of I, and I- using the same batch of CH3I solution for both, thus assuring closer identity of the difficultly controlled CH31 concentration than in the experiments of Figure 1. The lon-er yields in the Isolutions cannot be due t o differences in coniplexing lvith CH311since the latter is present in such excess that most of the must be born from unconiplexed C H S I ' ~ ~ . IT +1 1 3 0 process indiThe organic yields from the cate that a mechanism must exist by nhich an 1 1 3 0 atom which has undergone internal conversion and the Auger process in the presence of 0.1 mole %;c CHd has a probability of some 77, of encountering and reacting with an organic fragment formed from CH3I (unless the inorganic iodine is complexed vith CH31, or unless is sufficiently stable in n-ater to persist until it encounters and reacts with CH31, or unless the 1130(y-) reacts with H20t o form a species 71-hich can react nith CH31). The most plausible nieani of producing such (6) (a) F. W. Lampe and R. M. Xoyes, J . Am. Chem. SOC.,76, 2140 (1954); (b) R. L. Strong and J. E. Willmd, ihid., 79, 2089 (1957). (7) S. Aditya and J. E. Willard, J. Chem. P h y s . , 44, 418 (1966).
REACTION OF DILUTEAQUEOUS CHJ
WITH
IODINE
fragments is radiolytic decomposition of the solvent envelope around the I atom by radiations the atom emits in reaching nuclear stability or by the subsequent charge neutralization processes, as postulated by the autoradiation hypothesis.s Because the concentration of radicals formed in the volume immediately around the activated I atom must depend on the CH31concentration and because the atom may enter inorganic combination by reactions which compete with its reaction with organic radicals, the organic yield may be expected to increase with increasing concentration of CHJ, as the data of Figure 1 indicate that it does. The shape of this curve is not pre-
3583
dictable from available information. The factors determining it must include the fraction of the 1127(n,y)1128 events which are followed by internal conversion, the relative probability of emitting different numbers of Auger electrons following internal conversion, the probability of prompt formation of HI, HOI, or other inorganic species by 1130 atoms or ions before they encounter organic fragments, and the probability of formation, if any, of (CHJ)n, CH3.12, C&I.I-, and (12)n aggregates or complexes in solution. (8) P. R. Geissler and J. E. Willard, J . Phys. Chem., 67, 1675
(1963).
Volume 71, Number 11
October 1967
