1014
R. RIESSAND H.ELIAS
Kinetic Investigation of the Radiation-Induced Isotopic Exchange between Iodobenzene and Iodine in Benzene by R. Riess and H. Elias Lehrstuhl far Kernchemie, Technische Hochschule, Darmstadt, Germany (Received August 7, 1969)
The isotopic exchange between iodobenzene and radioactive elementary iodine induced by ioniaing radiation (60Co) has been investigated in benzene as a solvent. The kinetic measurements have been made at 30' in the absence and the presence of oxygen and at different dose rates. The following rate law has been derived both for aerated and oxygen-free solutions: R = (a[I~l'/~))/(lB[I2]/[PhI]). No dose rate effect has been observed. The mechanism is discussed.
+
Introduction Molecular halogens (especially iodine) are frequently used as radical scavengers by radiation chemists. It has been observed by several authors1+ investigating the radiation chemistry of organic halides that in the presence of a radioactive halogen as radical scavenger, radioactive halide is formed. In the case of aliphatic halides, R-X, the following list of mechanistic steps, as given by various authors,1-6 explains this radiationinduced isotopic exchange
Start R-X Secondary reactions
w+
R-
+ X-
Ar-X
(1) Are
+ X2 R-X + X. X * + R-X Ra + Xz X * + R-X -R-X f X. Re
bromo-, and chlorobenzene in the presence of radioactive halogen as scavenger, Choi and Willardg observed the same effect as found for aliphatic halides: during the irradiation radioactive halobenzene was formed. On the basis of the mechanistic interpretation given by Choi and Willard0 for the radiation chemistry of the halobenzenes it would appear that the observed isotopic exchange is mainly due to the following reactions, which correspond to reactions 1 and 2 for aliphatic halides
--t
(2)
(3)
exch.
(4)
Termination reactions
x*+ x . +XZ
(5)
R*
(6)
+ R * +R-R R * + X * +R-X
(7)
If radioactive halogen, *X2, is present, reactions 2, 4, and 7 can contribute to the observed formation of labeled halide, R-*X. The rate of exchange is reduced considerably in the presence of ~ x y g e n , l - ~ which is understandable on the basis of this radical mechanism and oxygen being a most powerful scavenger for organic radicals. If light is used instead of ionizing radiation,',8 the photochemical dissociation of XZ has to be considered a start reaction in addition to (1)
xz h',2 x .
(8)
Only scarce information is found in the literature on the radiation-induced isotopic exchange between aromatic halides, Ar-X, and the corresponding radioactive halogen. Investigating the radiation chemistry of iodo-, The Journal of Physical Chemistry
w+
+ *X-X
+ X. Ar-*X + X.
Ar.
---t
(9)
(10)
However, the observation of the G values for exchange in the system iodobenzene-*I2 not being reduced by atmospheric oxygenlo does not back this radical mechanism. The rate of the photochemically induced isotopic exchange between o-iodoanisol and radioactive iodine is proportional to the square root of the iodine concentration.l' This result led Anbar and Rein to postulate the exchange step Ar-I
+ * I * +Ar-*I + 1-
(11)
Reaction 11 is suggested also by other a ~ t h o r s ' ~ -to ' ~be (1) R. C. Petry and R. H. Sohuler, J. Amer. Chem. Soc., 75, 3796 (1953). (2) R. H.Sohuler and R. C. Schuler, ib&, 78, 3954 (1956). (3) E.0.Hornig and J. E. Willard, ibid., 79,2429 (1957). (4) R. 5. Hanrahan and J. E. Willard, ibid., 79, 2435 (1957). (5) R. H.Luebbe and J. E. Willard, ibid., 81,761 (1959). (6) A. H.Young and J. E. Willard, J . Phqs. Chem., 66,271 (1962). (7) M. Gazith and R. M. Noyes, J. Amer. Chem. SOC.,77, 6091 (1955). (8) I. J. Gardner and R. M. Noyes, ibid., 83,2409 (1961). (9) S. U.Choi and J. E. Willard, J.Phys. Chem., 66, 1041 (1962). (10) H. Elias, Radiochim. Acta, 6, 107 (1966). (11) M.Anbar and R. Rein, Chenz. Id.,1524 (1963). (12) R. K. Sharma and N. Kharaah, Angew. Chem., 80, (2), 69 (1968). (13) B. Milligan, R. L. Bradow, J. E. Rose, H. E. Hubbert, and A. Roe, J. Amer. Chem. Soc., 84, 158 (1962).
KINETICINVESTIGATION OF ISOTOPIC EXCHANGE the main pathway for the photochemically induced isotopic exchange between aromatic hnlides and labeled halogen. The photolytic rupture of the carbon-halogen bond (according to reaction 9) followed by its re-formation (according to reaction 10) is assumed to contribute to the isotopic exchange only to a minor extent a t low concentrations of halogen. 15-17 There are no kinetic measurements available concerning the radiation-induced isotopic exchange between aromatic halides and labeled halogen. We decided, therefore, to investigate the kinetics of the exchange between iodobenzene and labeled iodine in benzene as solvent, using the y radiation of a 6oCosource. This investigation has practical aspects, because radiation-induced isotopic exchange reactions can be applied to labeling of aromatic halides. 18,1g Experimental Section
Reagents. Iodobenzene (Schuchhardt, Munich) was distilled over Hg in an atmosphere of dry Nz a t 60" and a pressure of 12 mm. The distillation was repeated until the product was colorless and free of impurities detectable by gas chromatography (7-m Carbowax 20 M column (5 wt % on Chromosorb G) at 200"). Shortly before each run the distilled iodobenzene was finally purified by fractional sublimation in vacuo with a cooling finger (see Figure 1). Benzene (Aral AG, Essen) was purified by shaking with concentrated HzS04,washing with water and NaHC03 solution, partiad recrystallization with rejection of a 20'-70 fraction of the unfrozen liquid, drying over CaC12, and fractionation over Pz06. Iodine (Merck, Darmstadt) was not further purified. Sample Preparation. Samples with appropriate concentrations of iodine were obtained by dissolving weighed amounts of I2 in iodobenzene-benzene mixtures. These solutions were activated by shaking with 0.10.01 cc (about 30 pCi) of a carrier-free, aqueous solution of N a T (Farbwerke Hochst, Frankfurt). After shaking, these solutions were dried with anhydrous Na2S04. Samples with [I2]< 1 X M were obtained by diluting solutions of higher iodine concentration. The oxygen-free samples were obtained by degassing with the freeze-thaw-pumping technique. Irradiation. A 12,000-Ci 8OCo source (Gammacell 220, Atomic Energy of Canada, Ltd.) was used for the irradiations (ferrous sulfate dosimetry). The glass tubes containing the samples were mounted reproducibly in the irradiation chamber which had a temperature of 30". Dose calculations were corrected for the difference in electron density between the dosimeter solution and the samples. Analysis. The analytical procedure applied by Choi and Willardg in their exchange studies (shaking of the irradiated sample with an aqueous solution of sulfite for iodine reduction and separation; counting of the organic phase) appeared to be unsatisfactory. This
1015 was shown by analyzing the irradiated samples of a given exchange run in three different ways: (1) keduction and separation of the iodine according to the procedure of Choi and Willardg (as described above) and counting of the organic phase (benzene, iodobenzene, and organic radiolysis products) without further purification; (2) analysis of the organic phase by preparative gas chromatography, trapping of the iodobenzene peak, and counting of the purified iodobenzene; (3) partial sublimation of the organic phase (as described below). The comparison revealed that methods (2) and (3) gave identical results (within the limits of error of -I: 5%), whereas method (1) gave values for the fraction exchange which were 1.5-3 times (depending on the dose) higher. This effect was obviously due to the iodine containing radiolysis products diiodobenzene and 4-iodobipheny19 not being separated from the iodobenzene by method (I). The sublimation procedure (3) is as effective as the gas chromatographic separation (2) because the radiolysis products mentioned above are solids at room temperature which do not sublime upon partial sublimation of the organic phase. Method (3) is experimentally simpler than method ( 2 ) ; therefore, the analysis of the irradiated samples was carried out in the following way. The irradiated samples were shaken with a few drops of a 0.1 M Na2S2O3solution to reduce the iodine. The organic layer was separated, dried with anhydrous NazSOd, and transferred to the apparatus for sublimation where it was cooled with liquid nitrogen (see Figure 1). After the apparatus had been evacuated to a pressure of 5-15 mm, the cooling finger was filled with liquid nitrogen. The sublimation started when the liquid nitrogen cooling the sample was removed; it was stopped (by aerating the apparatus) after approximately 50% of the liquid sample had been transferred to the cold finger. The crystals of iodobenzene and benzene deposited on the cold finger were finally thawed and weighed. The 1311 activity of the sublimate was measured in a well-type scintillation counter. The concentration of iodobenzene in the sublimated iodobenzene-benzene mixture was determined with a uv spectrophotometer (Model UV 137, Perkin-Elmer) at a wavelength of 277 mp. Calculation of the Rate of Exchange, R. The equation given by McKay20 for the description of isotopic exchange reactions can be modified for the radiation-in(14) B. Miller and C. Walling, J. Amer. Chem. Soc., 79, 4187 (1957). (15) R. M. Noyes, ibid., 77, 2042 (1955). (16) R. M. Noyes, J . Phys. Chem., 65, 763 (1961). (17) J. F. Garst and R. S. Cole, Tetrahedron Lett., 19, 679 (1963). (18) H. Elias, R. Riess, and K. Muller, Naturwissenschaften, 54, 114 (1967). (19) H. Eliaa, Proceedings of the 2nd International Conference on Methods of Preparing and Storing Labelled Molecules, Brussels, 1966, EURATOM 3776 d.f.e., 881, 1968. (20) See A,., C. Wahl and N. A. Bonner, "Radioactivity Applied to Chemistry, John Wiley & Sons, Inc., New York N. Y., 951, p 7. Volume 74,Number 6 March 6,1970
1016
R. RIESSAND H. ELIAS -22.0
-22.5
log R -23.0
-23.5
-3
Figure 1. Cooling finger for sublimation.
duced isotopic exchange between iodobenzene and iodine by introducing the radiation dose instead of time log ( 1
- F)
=
-R
(2 [I21
+
[PhI])O.43430 2[LI [Phil
- 2
- 1
0
log [PhI]
Figure 2. Rate of exchange R as function of the concentration of iodobenzene. -eo-, [Iz] = 1 X 10-2 mol/l. (oxygen-free solutions); -D-D-, [I%]= 1 X 10-2 mol/l. (aerated solutions); -.-e-, [Iz] = 2 X lo-* mol/l. (aerated solutions). -22.0
(12)
where R is the rate of exchange [mol 1.-l X (eV/g)-l]; D is the radiation dose (eV/g); F is the fraction exchange, and [Iz]and [PhI] are concentrations (mol/
1.). F is given by the ratio of the specific activity of iodobenaene, a(PhI), and its specific activity a t exchange equilibrium, a(PhI),,' which can be calculated
-22.5
log A
I
-23.0
-23.5
where ~ ( 1 2 )is~ the specific activity of iodine before exchange. According to eq 12, R can be determined from the slope of the curves obtained by plotting log (1 - F ) as a function of the dose D . For most of the runs, these plots gave straight lines up to doses of approximately 1 Mrad and, correspondingly, up t o F values of 30-60%,. In some cases (especially at low iodine concentrations mol/l. and in the absence of oxygen) the like 5 X initially straight lines deviated from linearity with increasing dose (due to the radiolytic formation of iodine on the one hand and of iodine containing compounds on the other hand). In these cases only the linear, i.e., the initial part of the exchange curves (corresponding to relatively small doses and relatively small F values up to approximately 20%) was used for the determination of the slope and of R, the rate of exchange, respectively.
Results and Discussion The rate law for the radiation-induced isotopic exchange between iodobenzene and iodine in benzene was derived on the basis of the rate of exchange, R, being measured at different concentrations of the exchange partners. The experiments were carried out The Journal of Physical Chemistry
- 2.6
- 3.0
-.-.-,
-
- 2.2 log [I?]
Figure 3. Rate of exchange R as function of the concentration ofTodine. -O-C-, [PhI] 6 5 X 10-1 mol/l. (oxygen-free [PhI] = 5 X 10-1 mol/l. (aerated solutions); solutions); -A-A-, [PhI] = 5 X lo-* mol/l. (aerated solutions); -D-D-, [PhI] = 1 X 10-8 mol/l. (aerated solutions).
with oxygen-free solutions as well as with aerated solutions. The results and reaction conditions are compiled in Table I. Derivation of the Rate Law. To analyze the concentration dependence of R, log R was plotted as a function of log [PhI] (at constant concentration of 12) and as a function of log [Iz](at constant concentration of PhI). Figures 2 (runs V-VII) and 3 (runs I-IV) show these plots for aerated and oxygen-free solutions. The fact that curves are obtained instead of straight lines proves that the rate law is not a simple exponential one of the form
R
-
[PhI]"" [Iz]"
(14)
However, Figure 3 indicates that at relatively high concentrations of iodobenzene ([PhI] = 5 x lo-' mol/
KINETICINVESTIGATION OF ISOTOPIC EXCHANGE
1017
Table I: Rate of Exchange Rarb for the Radiation-Induced Isotopic Exchange in the System Iodobenzene-1811z-beneeneas a Function of Concentration [Id, mol/l. [Phil, mol/l.
Run
5
x
lo-'
I IIC I11
IV
1
x
10-1
2.2(2.3d)/ 1.36O
5 x 10-1 5 X 10-8 1 lo-*
2.04 1.18 0.74
x
2.6 x lo-'
3.21
1
x
5
4.6 1.31e
10-0
7.5
x
5.13 1.65 0.39
1
10-3
x
10-2
5.9
5.3
1.126
4.2 1.71 0.63
1.54 0.74
7
1,158 6.07 1.40 0.35
1.136 6.67
-
[PhI], mol/-.----
---~-I---
[Ll,
Run
V VI
I
mol/l.
2 1
x lo-* x 10-8
1 X 10-2
x
5
"3 1f
0.65 *
VI10
x lo-'
10-8
1.52
1 ,366
1
x
5
10-9
1
2.3
'"g
1.248
x
10-1
2.55
5
x
10-1
3.07
-
1 .45e
1.528
2.38
0.34
10-2
5113
2.03
1.15a
x
-
a R in mol 1.-1 (eV/g)-l X 1028. b Unless otherwise stated, the data refer to irradiations a t 30" in presence of atmospheric oxygen and a t a dose rate between 3.12 X 1019 and 3.36 X 1018 (eV/g) hr-1. c Oxygen-free solutions. d At a dose rate of 1.20 X 10'9, 0.776 X l O 1 9 , and 0.252 X 1019 (eV/g) hr-1. e Ratio R (without air)/R(with air).
1.) the dependence of log R on log [Iz]is practically linear with a slo.pe of approximately 0.5, which means [I2]'/'holds under these conthat the relationship R ditions. By plotting the R values compiled in Table I as a function of various concentration parameters it was finally found that the data can be represented by the relationship
-
For the determination of a and P one could plot [IzI1/'/R against [Iz]/[PhI]. This procedure would put all the points on a single plot. It was found, however, that in such a plot most of the points are concentrated in a rather narrow range of [Iz]/[PhI], so that the slope of the curve is determined by only a few points at higher values of [Iz]/[PhI]. Therefore, a and P were determined separately for each of the seven runs by using eq 16, a rearranged form of eq 15
Run V and run VI refer to exchange experiments in the presence of atmospheric oxygen; the agreement between CYVand VI and also between PV and PVI is good. The runs which were made at various concentrations of I2with [PhI] = constant (runs I, 11, 111, and IV) can be used now for an independent determination of a from the slope of the straight line obtained by plotting R ( l P [Iz]/[PhI]) against [I2].ll2 This plot is based on eq 17
+
R(l
+ P[Iz]/[PhII) = a[IzI1/'
With a mean of p = 2.24 (from @V and PVI; aerated solutions) and P = 1.99 (= ~ V I I ; oxygen-free solutions) one obtains ~IJIIJV = a11 =
(mol/l.)'/' X (eV/g)-l
6.29 X
(m01/1.)~/*X (eV/g)-l
7.23 X
By averaging the various a values one ends up with the following set of constants. Aerated solutions a = 6.16 X
By plotting 1/R against l/[PhI], a results from the intercept on the ordinate axis at l/[PhI] = 0, whereas P can be evaluated from the slope of the straight line. The results as obtained by the least-squares method are av = 5.90 X
(rnol/l.)'/' X (eV/g)-l;
~ V = I
6.29 X
= 2.18
(mol/l.)l'z X (eV/g)-';
PVI = 2 . 3 0 ~ V I I=
7.14 X
(m01/1.)'/~ X (eV/g)-l = 2.24
Oxpgen-free solutions a = 7.18 X
(18) (mol/l.)"' X (eV/g) -l
p
pv
(rnol/l.)'/' X (eV/g)-l; PVII
= 1.99
(17)
= 1.99
These numbers have an error of about 5%; they differ slightly from the preliminary results published previously.21 To prove the validity of eq 15 and the numbers derived for a and b, Figures 4 and 5 show a summarking plot of R as a function of the right side of eq (21) R. Riess and H. Elias, Radiochim. Acta, 10, 110 (1968). Volume 74, Number 6 March 6 , 1970
1018
R. REISSAND H. ELIAS This simple rate law is in agreement with the results of runs I and I1 ([PhI]/ [Iz]1 50; see Figure 3) which give practically straight lines with a slope of 0.5 in the log R VS. log [Iz]plot. Discussion of the Mechanism. If the mechanism of the radiation-induced isotopic exchange between iodobenzene and labeled iodine is of the radical type, one would suggest the following sequence of reaction steps. PhI +P h Ph. Ph* 61.6
''
[I,]"*
2.24 [
$1
+
I.
[Phl]
Ph.
Figure 4. Rate of exchange R as a function of the expression (c~[I2]~/~/1 p[Iz]/[PhI]); measurements in the presence of atmospheric oxygen.
+
1
2
3
4
5
6
7
11.6 [12]"2
+
15 for all concentrations investigated. As one can see, the data are well represented by eq 15. There are only very few points which deviate from the straight line (slope = 1.0) by more than 10%. One finds, therefore, that as well in the presence a s in the absence of oxygen the results follow the same type of rate law. The only difference is CY being about 17% greater and p being about 11%smaller for oxygen-free solutions. Equation 15 and the numbers given for CY and p in (18) imply that for the ratio [PhI]/[Izl 2 200 the contribution of the denominator term p [I&' [PhI] becomes negligibly small ( 51%). Under these conditions, i.e., in the presence of a large excess of PhI as compared to Iz, eq 15 is reduced to eq 19, in which R is just proportional to the square root of the iodine concentration =
(Y[I~]'/~
The Journal of Physical Chemistry
12
+PhI
+ I.
----t --j.
(20) (21)
+ 1.
Is
Ph-Ph
(22) (23) (24)
According to the scavenger experiments done by Fessenden and Schuler,22oxygen and iodine are competitive scavengers for organic radicals. Therefore, in the presence of air, according to reaction 25
Figure 5 . Rate of exchange R as a function of the expression (c~[It]'/~/l P[Iz]/[PhI]); measurements in the absence of oxygen.
R
+ 1. + P h I
+ Ph.
Ph*
0
+ I*
(19)
+ 02 +PhOz.
(25)
oxygen would compete for phenyl radicals together with iodine according to reaction 22, which-in the presence of radioactive iodine-represents the exchange step. As a consequence, oxygen should reduce the rate of exchange. As can be seen from Table I (runs I/II and VI/VII), there is a small rate-reducing effect of atmospheric oxygen being dissolved in the samples. If one assumes that iodine and oxygen are equally effective scavengers for phenyl radicals and that [OZ]dissolv = 3 X mol/l. (estimated), then, a t [Iz] mol/l, one would expect rate and 1 X =5 X ratios R(without OZ)/R(with 0,) of 1.6 and 1.3, respectively. The actually observed values, 1.57 and 1.36, respectively, are in good agreement (see Table I; runs 1/11). On the basis of the results obtained by Fessenden and Schuler,22 the rate-reducing effect of oxygen should decrease and finally become negligible with increasing iodine concentration. The results of run 1/11 (Table I) indicate this trend. However, a t [Iz]= 2.5 X 5X 7.5 X and 1 X mol/l. one observes the rate ratios 1.31, 1.12, 1.15, and 1.13 instead of 1.12, 1.06, 1.04, and 1.03, as to be expected. So, at higher iodine concentration the OXYgen effect is stronger than it should be. Even more inconsistent with the picture of radical scavenging by oxygen are the results of run VI/VIII (Table I). At [I21 = 1 x lo-' mol/l. and [OZ]dissolv. = 3 x lo-* mol/l., one would expect a rate ratio of 1.03 and observes clearly higher values with a mean of 1.35. It follows, therefore, that the results as discussed on the basis of an entirely radical mechanism according to reactions 20-25 are not unambiguous. On the one (22) R. W. Fessenden and R. H. Sohuler, J . Amer. Chem. Soc., 79, 273 (1957).
1019
KINETICINVESTIGATION OF ISOTOPIC EXCHANGE hand, the results do not follow strictly the consequences of the suggested radical mechanism with respect to the role of oxygen as scavenger; on the other hand, it cannot be ruled out definitely that there is some contribution of such radical processes. According to the observed rate law (see eq 15) the rate of exchange is proportional to the square root of the iodine concentration. It has been shown by Hamill, et al.,23,24 and by Noyes15 that the competition of a strong radical scavenger like Iz with geminate recombination reactions like (21)can lead to a square-root dependence for the scavenger. Therefore, the denominator of the observed rate law could be in agreement with the radical mechanism described by reactions 20-25. It is hard to see, however, how this radical mechanism should lead to the observed denominator term (1 /3 [I2]/[PhI]) (see eq 15). This term obviously describes the competition of iodine molecules and iodobenzene molecules for some relevant species which the radical mechanism does not account for. For the photochemical exchange between o-iodoanisol and labeled iodine Anbar and Rein'l suggested as a rate determining step PhI
+ *I*
Ph-*I
+ 1.
is formed by energy transfer from excited benzenez8because the solvent benzene is the major constituent in the solutions irradiated (the percentage by weight of iodobenzene in the solutions irradiated was 11% for the maximum concentration of [PhI] = 5 X lo-' mol/l. and less for all other solutions). Taking into account that there can be energy transfer from excited benzene not only to iodobenzene but also to iodine and oxygen, one could suggest the following set of reactions as a mechanism for the observed radiation-induced isotopic exchange I
Excitation
(I
CsH6 =
CeH6* Ce&*
the resulting rate law has the form
R
-
[PhI]
(28) However, there are several arguments against this mechanism being responsible for the radiation-induced isotopic exchange between iodobenzene and iodine. One of these arguments is the observationz5 that the rate constant for exchange reaction 26 is very low. Another argument can be derived from photochemical studies.26 If a solution of iodobenzene and labeled iodine in benzene is exposed to a certain number of light quanta from the visible part of a high-pressure mercury lamp spectrum, the degree of isotopic exchange taking place is very small. However, if the same solution is irradiated under identical conditions with the same number of quanta from the uv part of the mercury lamp spectrum, the degree of exchange is about 8 times higher as compared to the irradiation with visible light. This experiment demonstrates that iodobenzene in the ground state plus iodine atoms (as produced by visible light) causes only a slow exchange, whereas excited iodobenzene (as produced by uv light) plus iodine atoms exchanges much faster. Very similar results have been published by Macrae and Shaw.27 If excited iodobenzene molecules cause the radiationinduced isotopic exchange according to eq 26,one has to consider the question of how they are produced. It appears reasonable t o assume that excited iodobenzene [12]'/'
+ + oz
k2
+ CaHa + oz*
-
CeHe
1 2 -+x-
kS
(30)
(31)
12*
(32)
kc
1% -21. 7
Equilibrium
(33)
k-4
Deexcitation
(27)
= excited state)
dose rate;
ki
+21
*
(29)
* + PhI +C6Ha+ PhI *
Ezchange step
I2
*
Energy transfer CBHB
(26) If the concentration of iodine atoms is given by a dissociation equilibrium according to --j
CaHs
m ~ . - )
PhI*
+ *I.
ks
Ph-*I* ka
+ I.
(34)
(*I = radioactive iodine)
-% CaH6 PhI* -% PhI
CeHe*
(35)
(36)
The concentration of radioactive iodine is extremely small as compared to inactive iodine. Therefore, the equilibrium concentration of inactive and radioactive iodine atoms, respectively, is
[ * I * ]= K1" X [I-*I]/[Iz]1'2
(38)
Stationary-state treatment of the concentration of CoHa* mo~eculesgives (1 = dose rate in (eV/g) sec-'; d = density of the solution in g/cc; G = number of the c6&*molecules per 100 eV; NL = Loschmidt number)
(23) J. C. Roy, R. R. Williams, Jr., and W. H. Hamill, J. Amer. Chem. SOC.,7 6 , 3274 (1954). (24) H. A. Gillis, R. R. Williams, Jr., and W. H. Hamill, ibid., 83, 17 (1961). (25) R. M. Noyes, private communication. (26) R. Rieas and H. Elias, unpublished data. (27) J. F. C. Macrae and P. F. D. Shaw, J. Inorg. Nucl. Chem., 24, 1327 (1962). (28) W. G. Burns, R. B. Cundall, P. A. Griffiths, and W. R. Marsh, Trans. Faraday Soc., 64, 129 (1968). Volume '74?Number 6 March 6 , 1870
1020
R. RIESSAND H. ELIAS
With the abbreviation a = ~OGI~/NL one obtains [C&*]
= a/(h[PhIl
+ kz[Iz] 4-
k3[02]
+ ks)
*
It turns out, therefore, that /3 is actually not a constant (40)
The stationary-state concentrations of PhI and Ph*I *, respectively, can be determined from eq 41
[PhI *] = ! ![c&*] [PhI]
(42)
k7
[Ph*I *] = [PhI *] [Ph*I]/ [PhI]
(43) The rate of formation of radioactive PhI molecules, Ph*I, is given by the rate of formation of excited radioactive PhI molecules, Ph*I *, according to reaction 34 d [Ph*I] - d [Ph*I $1 dt dt
=
kj([PhI*] [ " I - ] [Ph*I *I [I *
1)
(44) Introducing eq 37, 38, 42, and 43 in eq 44 one obtains (s = specific activity = concentration of radioactive species/concentration of inactive species)
(45) Therefore, the rate of exchange, R, is 2o
The introduction of eq 40 and rearrangement gives
IZ*S221.
(47)
R is determined in units of mol 1.-' (eV/g)-l from the slope of the curves obtained by plotting log (1 - F ) as a function of the dose D ( = I t ) (see Experimental Section). Using the abbreviation a =
k,K'/'Gd* 10 k7NL
(48)
eq 47 can be rewritten,.therefore
In the presence of oxygen, 0represents the expression
whereas in the absence of oxygen one obtains
The Journal of Physical Chemistry
but a parameter depending on the concentration of oxygen and iodine. I n agreement with eq 50 as compared to eq 51 the observed value for p is higher for aerated solutions than for oxygen-free solutions. The small difference of only 11% indicates, however, that the contribution of the term k3/kl' [O,]/(IZ) (and as well of the term k 6 / k l *1/ [L])to p is small, so that p =kz/kl = constant. The rate law 49 as derived from the mechanism described by reactions 29-36 would fit the experimental data. It is obvious, however, that the ratedetermining step 34 consisting in the collision of excited iodobenzene molecules with iodine atoms is open to discussion. With the assumptions that lcs = 1Olo l./mol sec and that the mean lifetime of excited iodobenzene molecules is approximately 10-6 sec, one can calculate from the observed rate that in a solution with an iodine mol/]., the equilibrium concentration of 2.5 X concentration of iodine atoms should be 0.69 X mol/l. This would mean that during the irradiation approximately 1% of the iodine molecules should be M dissociated into iodine atoms. For a 2.5 X solution of iodine, one can estimate a thermal equilibrium concentration for iodine atoms of approximately mol/l.299a0 One has to conclude that processes other than thermal dissociation lead to the formation of iodine atoms. It is possible, of course, to assume that the forward reaction of equilibrium 33 is not thermal but radiation induced in the sense that excited iodine molecules, Iz*, dissociate (52)
Since kq' >> kq, the steady-state concentration of iodine atoms would be increased as compared to the thermal case, This assumption would imply, however, that the rate of exchange, R, is proportional to the square root of the dose rate I, i e . , a dose rate effect should be observed. To check this, the rate of exchange, R, in units of (mol/l.) (eV/g) -l was determined at four different dose rates I ( I = 3.31 X 1019,1.20 X lOl9, 0.77 X 0.252 X loi9 (eV/g) hr-l) under constant conditions of concentration ([PhI] = 5 X 10-1 and [Iz]= 1 X 10-% mol/l.). As can be seen from Table I, R = 2.2 X for the highest dose rate and R = 2.3 X for the other dose rates. This means that a variation of the dose rates by a factor of 13 does not reveal a dose rate effect, It follows from these results, therefore, that the suggested mechanism with reaction 34 as rate-determining step does leave an open question with respect to the mechanistic interpretation of how the relatively (29) J. E. Mayer and M. G. Mayer, "Statistical Mechanics," John Wiley & Sons, New York, N . Y., 1950, p 216. (30) J. Zimmermann and R. M. Noyes, J . Chem. Phys., 18, 658 (1950).
1021
PHOTOCHEMISTRY OF AZIDO-AMMINE COMPLEXES OF COBALT(III) high concentration of iodine atoms participating in this reaction is formed, Another point is that the reaction scheme 29-36 cannot explain the fact that a(oxygenfree solution) > a(aerated solution) (one can see from eq 50 that the concentration of oxygen enters only the expression for B). As indicated above, this could
possibly arise from a small contribution of the radical mechanism (see reactions 20-25) to the exchange. Acknowledgments. The authors wish to thank Professor R. M. Noyes and Professor R. W. Dodson for helpful discussions and the Deutsche Forschungsgemeinschaft for financial support.
The 2537-A Photochemistry of Azido-Ammine Complexes of Cobalt(II1) in Aqueous Solution: Products, Stoichiometry, and Quantum Yields' by John F. Endicott, Morton Z. Hoffman, and Laszlo S. Beres2 Department of Chemistry, Boston University, Boston, Massachusetts 08,916 (Received January 17, 1969)
The principal products of the 2527-A irradiation of C O ( N H ~ ) ~are N ~Co2+, ~ + Nz, and probably CO(NH~)~OHzNa2+. The yields of Co2+and Nz have been determined directly under a variety of conditions and exhibit a peculiar dependence on the intensity of absorbed radiation. This dependence of product yields on I. as well as the spectral features of irradiated solutions imply the formation and subsequent photolysis of a different mido-ammine complex of cobalt(II1). The source of product N2 has been shown to be coordinated azide and experiments with C O ( ~ ~ N H ~show )~N that ~ ~Co(NH8)50H2*+ + cannot be an important product of photolysis. C O ( N H ~ ) ~ O Hexhibits ~N~~+ kinetic behavior very similar to that of CO(NH~)~N~Z+, The photochemistry of Co(tetraen)Na2+,cis-Co(NHs)4(Ns)z+,and I ~ u ~ s - C O ( N H ~ ) ~appears ( N ~ ) ~to+ be much more straightforward and may not involve photolabilization of an ammine ligand.
Introduction Although there have been several investigations of the photochemistry of cobalt(II1) complexes, there is still some dispute about the specific role of the excited states which are generated on absorption of radiation.a Some of the ambiguity concerning the photochemical mechanism arises from a lack of accurate detailed information regarding the over-all products and the over-all reaction stoichiometry of the photoprocess. The identification of all the final products of reaction and the determination of product yields are complicated by the fact that three experiment ally distinguishable reaction paths are possible in the photochemistry of simple cobalt(II1) c~mplexes.~ (I) Photolabilization of weakest ligand bonds
+ + HzO
Co(NH3)6X2+ hv
---f
+
C O ( N H ~ ) ~ O H ~ SX+-
(11) Photolabilization of the strongest ligand bond6J
+ hv + HsO+ +
Co(NHs)5X2+
+ NH4+
Co(NHs)40HzX2+ (111) Photoreduction6 Co(NH3),X2+
+ h~ + 5H+
---f
Co2+
+ 5NH4+ + X *
The photolabilization processes I and I1 are often difficult to establish experimentally since (1) the chemical and physical properties of any Co(NHs)40H2X2+ formed are often so similar to the properties of the original Co(NH&X2+ complex as to make in situ spectral analyses ambiguous and separations difficult; and (2) X- is frequently formed from subsequent (1) This research was supported by the National Science Foundation under Grant GP 7048. The mass spectrometer used was purchased from funds provided by the National Science Foundation under Equipment Grant GP 6561. (2) Participant, National Science Foundation Undergraduate RB search Participation Program, Summer 1966 and the 1966-1967 academic year. (3) (a) For recent reviews see E. L. Wehry, Quart. Rev. (London), 21 213 (1967); (b) V. Balzani, L. Moggi, F. Scandola, and V. Carassiti, Inorg. Chem. Acta Rev., I , 7 (1967);(0) A. W. Adamson, Coord. Chem Rev., 3 , 169 (1968): (d) V. Balzani, L. Moggi, and V. Carassiti, Ber. Bunaenges. Phys. Chem., 72, 288 (1968); (e) L. Moggi, V. Balzani, and V. Carassiti, ibid., 293 (1968); (f) D. Valentine, Jr., Advan. Photochem., 6 , 123 (1968). (4) For simplicity we confine the present discussion to pentaammine complexes containing simple unidentate ligands X (e.g., C1-, NS-, NHs, OHZ,etc.). (5) A. W.Adamson, Discussion8 Faraday SOC.,29, 163 (1960). (6) L. Moggi, N. Sabbatini, and V. Balzani, Gazz. Chim. Ital., 97, 980 (1967). (7) J. F.Endicott and M. Z . Hoffman,J . Amer. Chem. ,Yococ., 90, 4740 (1968). Volume 74, Number 6 March 6, 1970
