PULSE AND y RADIOLYSIS OF ALKYLIODIDES
1077
Aspects of the Pulse Radiolysis and y Radiolysis of Alkyl Iodides and Their Mixtures1 by C. CapellosZaand A. J. Swallow2b Nuclear Technology Laboratory. Department of Chemical Engineering and Chemical Technology, Imperial College, London, S.W.7, England, and the Paterson Laboratories. Christie Hospital and Holt Radium Institute, Manchester d o , England (Received Novemher 2.8, 1968)
Liquid alkyl iodides have been subjected to pulse radiolysis and y radiolysis. A 2-psec electron pulse produces a broad transient absorption band with a peak near 400 nm, with GEequal to 18,000, 26,000, and 34,000 for methyl, ethyl, and isopropyl iodides, respectively. This band disappears in a second-order reaction, and is considered to be mainly due to an iodine atom alkyl iodide charge-transfer complex. Its extinction coefficient at the maximum is 9000 1. mol-’ cm-’ for all three iodides. A weaker and more rapidly decaying absorption is seen in the region 650-750 nm attributable to a positively charged species. The band with the peak near 400 nm appears with the same intensity when dissolved oxygen is present, but the decay kinetics are different. In mixtures of methyl and isopropyl iodides its formation is a nonlinear function of composition. In solutions of isopropyl iodide in oyclohexane, the concentration dependence of the band suggests that the charge-transfer complex has its origin partly in an ion-recombination process. Initial yields of iodine and HI have been determined for the radiolysis of alkyl iodides. The yield of HI falls off rapidly with dose, reaching a plateau at high doses. In mixtures of liquid methyl and isopropyl iodides, the dependence of the iodine and HI yields on composition follows a similar curve to that of the band at 400 nm in the pulse radiolysis. The 1 2 and HI yields vary with composition of mixture in quite a different way at - 196’.
Alkyl iodides are relatively simple. The molecules have a high electron affinity and contain one bond (C-I) which is much weaker than the others. They can readily be obtained in the liquid, solid, and gas phases, and striking observations can be made when the materials are irradiated under appropriate conditions and examined by techniques like electron spin resonance spectroscopy, mass spectrometry, and ultraviolet absorption spectrophotometry. For these reasons their radiation and photochemistry has received extensiye study, and the results have provided interesting information about the short-lived chemical species which can be derived from them. The literature on the radiation chemistry of the alkyl iodides up to mid-1958 has been exhaustively reviewed elsew~here.~ Among the more recent studies may be mentioned some of those by Hamill and his school4 and Willard and his scho01.~ We now report further work on the radiation chemistry of the alkyl iodides, paying attention to the phenomena occurring when mixtures are irradiated. Among other techniques, we have used pulse radiolysis to examine transient species.
Experimental Section Alkyl iodides were obtained from Hopkin and Williams or British Drug Houses and were purified in the dark by passing through a 60-cm column of silica gel to remove polar or unsaturated impurities, followed by vacuum distillation through a 60-cm fractionating column packed with glass rings, the middle cut being collected. The purified iodides contained no detectable iodine or hydrogen iodide. Potassium iodide, iodic acid, and carbon tetrachloride were all Analar, from Hopkin and Williams, and were used without further
purification. Ethylene was supplied by Ethylene Plastique. Nitrous oxide was obtained from B.O.C. For the pulse-radiolysis experiments, samples were deaerated by bubbling with argon, stated to contain less than 6 ppm of oxygen. The iodides were cooled with ice water during the degassing, to minimize evaporation. To prepare samples for y irradiation, the alkyl iodide was placed together with phosphorus pentoxide in a deaeration flask on a vacuum line and deaerated by the freezing-pumping-thawing technique. When the irradiated sample was to be analyzed for iodine, the alkyl iodide (10 ml) was distilled into an irradiation vessel connected to a spectrophotometer cell and sealed off under vacuum. Spectrophotometric measurements could be made after interrupting the irradiation without exposing the sample to air. Where the sample was to be analyzed for hydrogen iodide, the alkyl iodide (5 ml) was distilled through a constriction into an ampoule which was subsequently sealed off. (1) From the Ph.D. dissertation of 0. Capellos, University of London, England. The work reported here has been sponsored in part by the Aeronautical Research Laboratory, O.A.R., through the European Offlce, Aerospace Research, U. S.Air Force, Contract AF.61(052)-456. (2) (a) Department of Chemistry, Brookhaven National Laboratory, Upton, N. Y. 11973. (b) Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester 20 England. (3) A. J. Swallow, “Radiation Chemistry of Organic Compounds,” Pergamon Press, Oxford, England, 1960. (4) (a) R . F. Pottie, R. Barker, and W. H. Hamill, Radiation Res., 10, 644 (1959); (b) H. A. Gillis, R. R . Williams, and W. H. Hamill. J . Amer. Chem. SOC.,8 3 , 17 (1961); (c) E. P. Bertin and W. H. Hamill, ibid., 8 6 , 1301 (1964); (d) 9. Z. Toma and W. H. Hamill, ibid., 8 6 , 148 (1964); (e) J. P. Mittal and W. H. Hamill, i b i d . , 8 9 , 5749 (1967). (5) (a) H. J. Arnikar and J. E. Willard, Radiation Res., 3 0 , 204 (1967); (b) R . B’. 0. Claridge and J. E. Willard, J . Afner. Chem. S O C . 8, 8 , 2404 (1966); (c) R. F. 0. Claridge and J. E. Willard, (bid,, 89, 510 (1967).
Volume 75,Number 4 April 1969
C. CAPELLOS AND A. J. SWALLOW
1078 Pulse-radiolysis experiments were done with a 2-psec pulse from the 4-MeV electron linear accelerator at AEI, Trafford Park, Manchester, using the equipment of Keene.6 y Irradiations were performed with a 70043 6oCo source.7 When the vessel containing the spectrophotometer cell was used, the cell was shielded with lead during irradiation. The dose absorbed by the alkyl iodides was calculated on the basis of that absorbed by the Fricke dosimeter. For the electron irradiations, the modified Fricke dosimeter was used (oxygen-saturated M FeSOa in 0.8 N HzS04),*and the dose in the alkyl iodides was calculated on the basis of the relative electron densities of the iodide and the dosimeter solution. An additional correction, amounting to about 4%, was made to allow for the difference in depth-dose distribution between the iodide and the dosimeter, which caused the maximum in the depth-dose curve to shift out of the path of the light beam. For the y irradiations the relative doses in the dosimeter solution and in the alkyl iodides was calculated from the relative mass absorption coefficient^,^ so taking into account absorption by the photoelectric effect, and a correction was made for a soft component in the y irradiation.10 Iodine was determined spectrophotometrically at its absorption maximum at 480 nm. The molar extinction coefficient a t this wavelength was found to be I350 1. mol-' cm-l for solutions in all alkyl iodides used. At 360 nm the extinction coefficient was 2540 1. mol-1 cm-l for iodine in methyl, ethyl, and isopropyl iodides. Hydrogen iodide was determined by freezing the irradiated sample with liquid nitrogen, then opening the ampoule and placing 10 ml of water on top of the frozen alkyl iodide. After melting, the HI was extracted into the water, which was then washed twice with carbon tetrachloride. The efficiency of the extractions was checked by means of pH measurements M HI. In the analysis of irradiated samples using iodic acid was added to oxidize the H I to iodine, which was then extracted into cyclohexane, and from that into aqueous potassium iodide. The 13- complex so obtained was determined spectrophotometrically at 350 nm taking the molar extinction coefficient to be 26,400 1. mol-l em-'."
Results Pulse radiolysis of deaerated alkyl iodides produced broad transient absorption spectra with peaks near 400 nm. The absorption spectra at various times after a -5000-rad pulse are shown for ethyl iodide in Figure 1. Initial values of GelOO( L e . , the product of the number of molecules of the species formed per 100 eV and the molar extinction coefficient at 400 nm) were obtained by using pulses of 600 and 1000 rads in order to reduce the decay of the species during and immediately after the pulse to a negligible level. The values found were 18,000,26,000, and 34,000 (molecules/100 eV) (1. mol-' The Journal of Phgsical Chemistrg
Figure 1. Absorption spectra of ethyl iodide a t various times after a pulse of fast electrons ( 4 X O O rads, optical path lerigth 1.6 om): A, immediately after pulse; B, 5 pseo later; C, 10 pseo; D, 20 psec; E, 40 psec.
cm-') for methyl, ethyl, and isopropyl iodides, respectively. Taking all errors into account these values are considered to be accurate to about 10%. After the transient absorptions had disappeared, thc residual permanent absorption spectra were indistinguishable from the spectra of solutions of iodine in alkyl iodides and corresponded to the same G values for iodine formation as found with y radiation. The decay of the transient absorptions at 410 nm (where solutions of molecular iodine in alkyl iodides have a minimum in their absorption curve) followed second-order kinetics, independently of dose in the range 1000-5000 rads. Values of JC/e4l0 were 1.7 X lo6, 7 X lo5, and 1.1 X lo6 em sec-l for methyl, ethyl, and isopropyl iodides, respectively, where k is the rate = k[RIz, and R is the constant defined by -d[R]/dt species being observed. Gover and Porter had found k/e410 = 8 X lo5 cm sec-' in the flash photolysis of iodine in ethyl iodide.I2 Air-saturated solutions of alkyl iodides exhibited similar absorption spectra after the pulse to the air-free solutions. However, the pattern of decay was different. In the air-saturated solutions the absorption around 500 nm decreased rapidly over a microsecond or so and Keene, J. Sci. Instrum., 41, 493 (1964). (7) G. R. Hall and M. Streat, J. Imp. Coll. Chem. Eng. Soc., 13, 80 (1961). (8) J. K. Thomas and E. J. Hart, Radiation Res., 17, 408 (1962). (9) C. Capellos, Ph.D. Thesis, University of London, England, 1965. (10) W. Bernstein and R . H. Schuler, Nucleonics, 13, No. 11, 110 (1955). (11) G. Meshitsuka and M. Burton, Radtalion Res., 10, 499 (1959). (12) T.A. Gover and G . Porter, Proc. R o y . Soc., A262, 476 (1961). (6) J. P.
1079
PULSEAND y RADIOLYSIS OF ALKYLIODIDES then increased again, reaching a final level much higher than in the absence of oxygen. This observation is consistent with the higher yield of iodine in y irradiations when oxygen is present.13 Mixtures of methyl and isopropyl iodides have been pulsed, and the initial absorption at 400 nm determined as a function of composition. The effect of composition expressed as volume percentage is shown in Figure 2. Energy partition may be nearly proportional to valence electron fracti0n.1~ The greatest deviation between volume percentage and valence electron percentage occurs near 50 vol %, which is 46.3 valence electron percentage methyl iodide, so that a plot of absorption at 400 nm against valence electron percentage would differ only slightly from Figure 2.
Figure 3. Yield of transient absorbing a t 410 nm, in pulse radiolysis of solutions of isopropyl iodide in cyclohexane: 0 , without NzO; 0, NsO (120 mM).
O 0f
Me1
I
I
I
I
io
I
I
volume Per Cent lsopropyl Iodide
1
I
1 I
IW iP d
Figure 2. Product of G value and extinction coefficient a t 400 nm for mixtures of alkyl iodides ( 4 0 0 0 rads).
Pulses of radiation, -5000 rads, were given to solutions of isopropyl iodide in cyclohexane. Broad absorptions were seen as in pure alkyl iodides. The value of Gealo was determined as a function of composition and the G value for transient was calculated on the basis of 6410 8500 1. mol-' cm-' (see Discussion). The result is shown in Figure 3. Saturation with nitrous oxide produced no effect on the initial absorption at 410 nm. The formation of iodine was measured for the y irradiation of pure liquid alkyl iodides at doses of up to about 1019eV/g. Initial yields (molecules 1 2 per 100 eV absorbed) were calculated from the linear part of the yield-dose plot, and found to be G = 1.33 for methyl iodide, G = 2.01 for ethyl iodide, and G = 2.90 for isopropyl iodide, in good agreement with previous work.16 Ethylene a t a pressure of 254 mm had no
effect on the yield of iodine from ethyl iodide, but saturation of the liquid with air gave initial yields of G = 3.81 for methyl iodide and G = 4.03 for ethyl iodide. In the presence of oxygen at a pressure of 842 mm, the yield of iodine from methyl iodide was G = 4.92. A t - 196", iodine formation from crystalline alkyl iodide was found to be linear with dose up to about 1019 eV/g. For methyl iodide the yield was G = 0.37 and for isopropyl iodide, G = 0.57. Hornig and Willard have reported G = 0.32 for methyl iodide, in agreement with our result, but found G = 0.325 for isopropyl iodide.16 The discrepancy may be connected with the fact that alkyl iodides can exist a t low temperatures in glassy as well as crystalline forms: for ethyl iodide, where a comparison could be made, the iodine yields in the glassy form were three times as great as in the crystalline form.16 The formation of hydrogen iodide in the y irradiation of pure liquid alkyl iodides was linear with dose up to about 3 X lo1' eV/g but fell off at larger doses. G values were calculated from the linear part of the curve. The yields obtained for methyl, ethyl, isobutyl, and isopropyl iodides are plotted in Figure 4 as a function of the number of hydrogen atoms present in the ,8 position in the molecule. The G values are higher than those reported p r e v i ~ u s l y ,a ~presumably ~,~ because of the (13) Reference 3, p 100. (14) A. J. Swallow, Discussions Faraday Soc., 36, 273 (1963);A. MacLachlan, J . Amer. Chem. Soc., 8 2 , 3390 (1960). (15) Reference 3. p 99. (16) E. 0.Hornig and J. E. Willard, J. Amer. Chem. Soc., 79, 2429 (1967). Volume Yb, Number 4 April 1969
C. CAPELLOS AND A. J. SWALLOW
1080
I
1
I
I
3
Figure 4. Initial yield of hydrogen iodide in the y radiolysis of alkyl iodides as a function of the number of 6-hydrogens atoms in the molecule.
lower doses used here. Using ethyl iodide, it was shown that ethylene at a pressure of 760 mm reduced the yield of hydrogen iodide to about half at a dose of 3.4 X 1020 eV/ml. At -196" the initial yield of hydrogen iodide from pure crystalline methyl iodide was G = 0.55 and from pure crystalline isopropyl iodide G = 7.00. Yield-dose curves for iodine formation in the y irradiation of various mixtures of alkyl iodides were measured, and initial yields were calculated. The iodides chosen for these studies were methyl iodide and isopropyl iodide, since these show the lowest and highest yields, respectively, when the pure liquid is irradiated. The effect of composition on yield for irradiation in the liquid phase is shown in Figure 5 , and for irradiation at - 196" in Figure 6. While preparing the frozen iodides for irradiation, it was noted that whereas outside the range 5 5 4 5 % isopropyl iodide the frozen mixture was opaque, the mixtures within this range were more transparent (i.e,, tended to the glassy form) being most transparent at 70% isopropyl iodide. Yield-dose curves were also obtained for the formation of hydrogen iodide (Figure 7 ) . For the mixtures as for the pure iodides the yields of hydrogen iodide began to fall off at much lower doses than the yields of iodine. The initial yields were estimated from curves like those in Figure 7. The effect of composition on yield is shown in Figure 5 . Similar experiments were done a t -196". The effect of composition on initial hydrogen iodide yield a t - 196", calculated from the linear portion of yield-dose curves, is shown in Figure 6.
Discussion The absorption spectra seen in the alkyl iodides a t 5 psec after a radiation pulse, shown for ethyl iodide in Figure 1, resemble that already published for cyclohexyl The Journal of Physical Chemistry
La I
i
J
2
I
x
I
I
I
0
iodide.I7 An additional broad absorption above -620 nm is now seen, which decays almost to zero within 5 psec. The main band with a peak near 390 nm is attributed to a charge-transfer complex between iodine atoms produced by the radiation and the parent alkyl iodide. Gover and Porter had previously seen a similar band in the flash photolysis of iodine in ethyl iodide, attributed to the iodine atom-ethyl iodide charge-transfer complex, but also saw a separate band at 490 nm.I2 Since no such band appears in pulse radiolysis, and since the use of flash photolysis could conceivably lead to photodecomposition of the solvent, it is suggested that the separate band near 490 nm in Gover and Porter's experiments may have been due to the presence of iodine molecules produced by the flash. It cannot be excluded that some molecular iodine would be produced directly by the pulse in the radiation case, but with an extinction coefficient of only 1350 1. mol-' cm-1, any molecular iodine could only account for a small fraction of the initial absorption in the neighborhood of 490 nm. After the transients produced by the radiation have decayed, the residual absorption spec(17) M. Ebert, J. P. Keene, E. J. Land, and A. J. Swallow, Proc. Roy. Sot., A287, 1 (1965).
1081
PULSEAND y RADIOLYSIS OF ALKYL IODIDES I
V
Me1
,
1
I
I
I
I
I
I
I
(sv/5ml of Mixture)x I O
X
I
I
1
I
20
40
I
6b
I 80
0 iP d
Figure 7. Production of HI as a function of dose in the y radiolysis of mixtures of alkyl iodides a t room temperature: A, pure methyl iodide; B, 20 vol % isopropyl iodide; C, 50 vol isopropyl iodide; D, 75 vol % isopropyl iodide; E, 100% isopropyl iodide.
%
Volume Per Cent Isopropyl Iodide Figure 6. Yield of iodine and HI in y radiolysis of mixtures of methyl and isopropyl iodide a t -196": 0 ,iodine; X, HI.
trum is, as might be expected, entirely attributable to molecular iodine together with its charge-transfer complex with the solvent. The transient spectrum seen in pulse radiolysis may be compared with the spectra seen in the y radiolysis of low-temperature glasses by Claridge and Willardsblc and Hamill and c ~ l l e a g u e s . ~The ~ ~ ~low-temperature glasses, whether pure alkyl iodides or solutions of alkyl iodides in hydrocarbons, exhibit several overlapping absorption bands between the wavelength where the solvent ceases to absorb and 900 nm, the highest wavelength for which results are reported. Among the bands seen is a strong one with a peak at 760 nm which may be due to (RI)z+ or to RzI+ formed by the ionmolecule reaction.4a RI+
+ R I + RzI+ + I
It may be that the weak broad absorption with a peak above 700 nm shown in Figure 1 is due to this species. The disappearance of this band within 5 Mec suggests that most of the positive species have reacted with negative species well within 5 psec. The reason that the pulse-radiolysis spectrum is simpler than the lowtemperature glass spectrum may be that the charged species are essentially absent after a microsecond or so. If this is correct, the spectrum measured some nano-
seconds after a short pulse of radiation may resemble the spectra seen in glasses rather than the spectrum of Figure 1. It is possible to obtain an estimate for the extinction coefficient of the charge transfer complex from the present results. If it were to be assumed that the72 value for the charge-transfer complex were twice that of the Izfinally formed, the values of GE~OO would lead to extinction coefficients at 400 nm of 6800, 6500, and 6100 1. mol-l cm-' for methyl, ethyl, and isopropyl iodide, respectively. However, if some of the Iz is formed without the intermediate formation of detectable charge-transfer complex the true extinction coefficients would be higher than this. A higher extinction would also result if some of the molecular iodine is formed through the reaction of aliphatic radicals with hydrogen iodide after the pulse. Consequently, the values given above may be lower limits. A value of E490 of 13,000 1. mol-' cm-' was obtained for the ethyl iodide complex by Gover and Porter, from their flash photolysis experiments, based on the assumption that the bimolecular disappearance of transient a t 410 nm was diffusion controlled. This assumption certainly appears to be valid for the cyclohexane-iodine atom complex, where the experimental rate constant for decay17is 1.06 X 10'0 1. mol-l sec-l. From Gover and Porter's spectrum, the extinction coefficient at 400 nm would be almost identical with the extinction they give a t 490 nm. The disappearance of absorption a t 410 Volume 75, Number 4 April 1SB9
1082
C. CAPELLOS AND A. J. SWALLOW
nm in the pulse radiolysis of ethyl iodide cannot be ascribed simply to the bimolecular reaction of the charge-transfer complex, because of the likely involvement of the reaction of alkyl radicals with HI. However, it is noted that our value of k / e is very similar to Gover and Porter's, so that the reaction with HI may not greatly affect the observed decay kinetics. In the case of methyl iodide the kinetics will be even less affected by HI, because the G value for HI is so small. If the diffusion-controlled rate for methyl iodide is taken to be 1.5X 1Olo 1. mol-' see-', the extinction coefficient at 400 nm calculated from our value of k/erlo would be 10,000. Taking all these considerations into account we take our best value for the extinction coefficient at 400 nm to be 9000 f 2000 1. mol-' em-' (8500 at 410 nm) . There is no reason to suppose that the extinction coefficients for the different iodides studies are significantly different from each other within the error quoted. This is in line with the behavior of the 1 2 alkyl iodide charge transfer complexes: for these complexes the principal absorption maximum is very close t o the solvent cutoff at -350 nm, but a t 360 nm where absorption due to the solvent is not important and yet the complex absorbs strongly, the extinction coefficient for methyl, ethyl, and isopropyl iodides is the same, The apparently biphasic effect of composition on the value of GE after the pulse, shown in Figure 2, seems likely to be due t o changes in the G value for the chargetransfer complex. We assume that the primary processes in the radiolysis of the alkyl iodides are excitation to levels which lead to decomposition without ionization, and excitation to levels where the molecule becomes ionized. The ejected electrons would be captured dissociatively by different alkyl iodide molecules. It is proposed that excited and ionized alkyl iodide molecules are capable of transferring their excitation energy or charge to other alkyl iodide molecules before decomposition occurs. It is suggested that in the case of mixtures of methyl iodide and isopropyl iodide, transfer from excited molecules is in the opposite direction to charge transfer from ionized molecules RJ+
+ R2I -+ RJ + R2I+
Thus excitation energy may go from methyl iodide to isopropyl iodide and positive charge from isopropyl iodide to methyl iodide, or both transfers may be in the other direction. Excitation and ionization of methyl iodide both give rise to the charge-transfer complex in relatively low yield, and excitation and ionization of isopropyl iodide give rise to the complex in high yield, consistent with the relative weakness of the C-I bond in this molecule. When isopropyl iodide is present in a mixture containing mainly methyl iodide, one of the The Journal of Phyaical Chemistry
transfers can take place, so that the isopropyl iodide produces a more than proportionate rise in yield. When methyl iodide is present in a mixture containing mainly isopropyl iodide, the other transfer can occur, so that in this case methyl iodide produces a more than proportionate drop in yield. This explanation is of course by no means proved, and it is possible that other reactions, such as those associated with electron capture, may be involved in the nonproportionality. The shape of the curve in Figure 3 would be qualitatively consistent with the formation of the chargetransfer complex by electron capture by the alkyl iodide, followed by charge neutralization to yield an iodine atom which then gives the complex. Nitrous oxide produces no effect even at 0.05M isopropyl iodide, where there is twice as much NzO as alkyl iodide. It could be that isopropyl iodide is much more efficient than NzO a t capturing electrons in this system. Another possibility is that at least some charge-transfer complex is formed through positive charge transfer from an ion derived from cyclohexane to alkyl iodide, followed by neutralization.1s Chargetransfer complex formed in this way would be unaffected by NzO. Still more complex may be formed40 by the action of hydrogen atoms derived from cyclohexane H
+ i-PrI -+ H I -1- i-Pr. R* + H I - + R H + I
the iodine atom giving the complex. The concentrations of solute which are needed to produce appreciable yields of charge-transfer complex are higher by more than an order of magnitude than are needed to produce comparable yields of triplet states in the pulse radiolysis of solutions of aromatic compounds in cyclohexane.19 This could be because of dissociation of the charge-transfer complex at the lower alkyl iodide concentrations. However, it is not possible to obtain an equilibrium constant from these data because of the large number of processes which may be contributing to the formation of the complex. The effect of the number of @-hydrogenatoms on the yield of HI (Figure 4) resembles the effect on the yield of 1 2 in both radiolysis16v20 and photolysisz0 of alkyl iodides. It is consistent with the view that H I is formed in a "molecular" reaction following the excitation or ionization of alkyl iodide. A unimolecular decomposition of excited molecules has been proposed to account for the production of HI in the flash photolysis of ethyl iodide.21 The high yield ( G = 7.0) (18) T.S. Croft and R. J. Hanrahan, J. Phys. Chem., 66, 2188 (1962); I. Mani and R . J. Hanrahan, ibid., 71, 3301 (1967). (19) E. J. Land and A. J. Swallow, Trans. Faraday Soc., 64, 1247 (1968). (20) E. L. Cochran, W. H. Hamill, and R . R. Williams, J. Amsr. Chem. S O C . , 76, 2145 (1954). (21) B. A. Thrush, Proc. Roy. SOC.,A243, 555 (1957).
PULSE AND y RADIOLYSIS OF ALKYLIODIDES obtained from isopropyl iodides a t -196" is also consistent with this view. It is not easy to explain why the yield of H I should diminish at higher doses. Ethylene, when present in much larger quantity than the unsaturated hydrocarbons produced by the radiation, did not produce a sufficient drop in H I yield to enable a slow thermal reaction between HI and unsaturated double bonds to account for the result, unless other unsaturated hydrocarbons are greatly more reactive than ethylene. Alkyl radicals would be expected to react with HI, but the rate constant for the reaction is a little less than so that on this view the that for the reaction with 12,22 Hl yield could not drop off while the iodine concentration continues to rise. The result is thus still unexplained. I n an attempt to account for the shape of the G(I*) curve in Figure 5 , we have considered the contribution which could be due to thermal alkyl radicals, which are produced from methyl iodide with G = 4.79, but from isopropyl iodide only with G = 3.06,23reacting with hydrogen iodide which is produced in low yield from methyl iodide but in high yield from isopropyl iodide. Such a mechanism would be expected to produce higher iodine yields from the mixtures than would be expected if species produced from the two iodides did not interact with each other. We have made a calculation based on the mechanism of Hanrahan and Willard, assuming that the rate constants for the reactions of alkyl radicals with iodine and H I are equal, and independent of the nature of the alkyl radical. The calculated
1083 curve for iodine production resembles Figure 5 between 0 and 40% isopropyl iodide, but above 40y0it continues to rise smoothly toward the looyoaxis instead of levelling off and then rising again as in Figure 5. Furthermore, this mechanism would be expected to lead to H I yields lower than would be obtained if there were no interaction between the species produced from the two components, and the HI curve of Figure 5 shows that this is not borne out experimentally. We have made a similar calculation based on the mechanism of Gillis et u Z . , ~ ~ and this fits the results even less well. We therefore believe that these interactions play a relatively minor part in the system, and that the explanation of the shapes of the curves of Figure 5 may be the same as already given above to account for Figure 2. The mixtures at -196" (Figure 6) behave quite differently from the liquid mixtures, and we believe that the principal contribution to nonlinearity here is a change from the crystalline form of the solid to the glassy form in the range 1 5 4 5 % methyl iodide. This effect masks any effect of the type occurring in the liquid phase.
Acknowledgments. The authors wish to thank Professor G. R. Hall and Drs. P. G. Clay, M. Ebert, and E. J. Land for helpful discussions and for their interest in this work. (22) D. Perner and R. H. Schuler, J. Phys. Chem., 70, 2224 (1966); I. Mani and R . J. Hanrahan, ibid., 70, 2233 (1966). (23) R. J. Hanrahan and J. E. Willard, J . Amer. Chem. Soc., 79, 2434 (1957).
Volume 75,Number 4 April 1969
