R. H.JOHNSEN, N. T. BARKER, AND M. BURGIN
3204
Sin'ce the contribution of torsional strain is the same or nearly the same for both compounds it is fair to say that the causes of the increase of the barrier to bond shift must be sought in the angle strain and (or) in the resonance energy. The relative importance of these two contributions cannot be evaluated a t the present stage of sophistication. Any such evaluation must be
postponed at least until better functions for angle-strain energy are available. Acknowledgments. This work was performed with the support of a grant from Consiglis Nazionale delle Ricerche of Italy. We wish to thank Dr. A. Segre for running the nmr spectra a t 100 MHz and for a useful discussion.
Studies on Iodine as a Scavenger in Irradiated Hydrocarbons and Hydrocarbon-Alcohol Solutions1 by Russell H. Johnsen, Norman T. Barker,2and Mary Burgin Chemistry Department and the Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida (Received December 27,1968)
32806
The yield of those species produced in X-irradiated hydrocarbon-iodine solutionswhich give rise t o 1 3 - upon the subsequent addition of ethanol has been measured. While it has been generally assumed that 1 2 reacts with thermal hydrogen atoms, it is suggested here that in fact Izreacts preferentially with electrons and that G(I3-) = G(e-). This view is in part suggested by the behavior of solutions to which alcohols are added prior to irradiation. The extrapolated yields of G(I8') in n-hexane, cyclohexane, and 3-methylpentane are 2.42,1.94,and 1.56, respectively.
Introduction Thermal hydrogen atoms are generated when hydrocarbons are irradiated with high-energy ionizing radiation. The yield of these atoms is, however, the subject of considerable disagreement. Historically the method of measuring the yield of hydrogen atoms is to add a scavenger. The yield of the product resulting from reaction with the hydrogen atoms may then be measured and directly related to the hydrogen-atom yield. Alternately the decrease in the hydrogen gas yield may be measured as a function of scavenger concentration, leading to an indirect measure of the hydrogen atom yield. It has been shown that when iodine is used as a scavenger in cyclohexane the depletion in hydrogen gas yield is a function of the iodine concentration such that a against [Izl-' competition plot of [G(H2),,, - G(H2)]-' produces a straight line with intercept assumed to equal G(H). This is not the case for a number of other scavengers such as benzene, carbon tetrachloride, and tetrachloroethylene so studied. a It has generally been believed that this depletion in the hydrogen yield is directly due to the reaction of IZwith the hydrogen atom. A generalized reaction scheme is RH-w.)H. The Journal of Physical Chemistry
+ Hz +- -. ...
(14
RH--+RH+
+ e-
RH*
R*
+ RH +H:! + R* Ha+ HI + I
Hd
I2
+ H**
(lb)
(2)
(3)
G(H2),ax is the hydrogen gas yield in the absence of iodine and eq 2 and 3 are the competitive reactions. The alternative method for finding the limiting value for G(H),assuming that all H atoms react with 1 2 , is to directly measure G(H1); then a plot of G(HI)-l against [Iz]-' should provide the same limiting value for G(H). The yield of hydrogen iodide should be measurable if the H I is converted into Is- by the addition of a polar solvent, e.g., ethanol, to the irradiated hydrocarboniodine mixture and the resulting ions measured spectrophotometrically a t 3600 A. Equation l a represents the generation of thermal atoms without the involvement of charged species and (1) This research was supported in part by the U. S. Atomic Energy Commission under Contract AT(40-1)-2001. This is A.E.C. Document ORO-2001-9. (2) On leave from the University of New South Wales, Kensington, N. 6 .W., Australia. (3) C. E. Klots, Y. Raef, and R. H. Johnsen, J . Phys. Chem., 68,2040 (1964) I
IODINE AS A SCAVENGER IN IRRADIATED HYDROCARBONS eq l b represents the formation of hot hydrogen atoms as a consequence of geminate recombination. The work of Cramer and Piet5 clearly demonstrates that the yield of thermal hydrogen atoms is quite low, being 50.8 in liquid cyclohexane, which is quite inconsistent with the view that the diminution in the hydrogen yield by Iz or the yield of H I is a measure of thermal hydrogen atom production. Thus, some part of the hydrogen yield must arise via the electrons produced in reaction l b . This may be the result of recombination to produce hot hydrogen atoms which would not be scavengable. However, iodine could interfere with the recombination reaction thus lowering the yield of hydrogen. I n addition to studies in which ethanol was added following irradiation, a number of experiments were conducted in which various concentrations ranging from 0.1 to 1.8 M ethanol or n-amyl alcohol were added prior to irradiation. It was anticipated that in these experiments the increased polarity of the solution would result in changed behavior of the electron as a result of the degree of solvation achieved. A further effect to be considered in these experiments is the complexing of the iodine by the alcohol and the effectiveness of this complex as a scavenger.
Experimental Section (a) Materials. Hydrocarbons from various sources, e.g., Matheson Coleman and Bell, Fisher, or Phillips, were used throughout the experiments. The hydrocarbons were used directly as received or subsequent to treatment with sulfuric acid, washing with water, and then drying in order to remove unsaturated impurities. If a solution of hydrocarbon and iodine together with hydrogen iodide and ethanol produced a stable spectrum with respect to time, unsaturates were asat 3600 sumed to be absent and the material was used as received. The treatment with sulfuric acid produced stable solutions using cyclopentane and cyclohexane; 3methylpentane and n-hexane did not require treatment. The iodine used was Merck resublimed U.S.P. The alcohol was U.S.I. absolute Pure Ethanol, U.S.P., N.F., Reagent quality, benzene free. (b) Procedure. Two separate hydrocarbon-iodine samples each containing approximately 5.5 ml of solution, were thoroughly degassed by 8-10 freeze-thaw cycles via 500-ml expansion bulbs. The residual pressure above the samples was less than mm. The final volume of each solution amounted to approximately 5 ml. The vials containing the samples were weighed after sealing off and one sample was irradiated with X-rays generated from 3-NIev electrons originating from a Van de Graaff accelerator. Total doses were of the order of 2-3 X lo1* eV g-l. After irradiation the sample was immediately frozen in liquid nitrogen, the vial was opened, and about l ml of alcohol was added. The sample containers were initially
3205
washed with chromic acid, water, absolute alcohol, and the solvent hydrocarbon used for irradiation. The tubes were outgassed overnight prior to use. The nonirradiated sample was treated in a similar fashion. Sufficient ethanol was added to the sample to convert all the radiolytic hydrogen iodide and/or iodide ions to solvated protons and triiodide ions in the presence of excess iodine. Control experiments with hydrogen iodide showed that the addition of 1 ml of ethanol to 5 ml of solution was adequate for total conversion to the ions. The extinction coefficient calculated from these experiments was 22,657 a t 3600 A. After warming the mixture to 25")the opticaldensity of the s2lution was measured spectrophotometrically a t 3600 A. The optical density at this wavelength did not change with respect to time. At this wavelength the Is- ions absorb without interference from other species. The charge-transfer spectrum has its maximum at 4600 A. The absorption of the 1 3 - ions may then be directly related to the hydrogen iodide and/or I- ion concentration. A nonirradiated sample provided the blank for each run and its absorption was substracted from the total absorption of the irradiated sample. The empty vial, after weighing, was reused immediately to determine the dose received by the sample with 5 ml of Fricke ferrous sulfate dosimetry solution being used for this purpose.
Results and Discussion Table I shows the effectof iodine concentration on the yield of 1 3 - ions. Figure 1 shows a linear relationship between G(Ia-)-l and [Izl-l over the measured concentration range of iodine with an extrapolated maximum Table I : Yield of Iodide Ions from Irradiated Solutions of Iodine in Hydrocarbon for Various Iodine Concentrations Optical density a t 3600 A of Ia- ions Total Blank Sample
Total dose, eV g-1 X 10-18
Iodine concn, mol 1.-1 X 108
G(1s-)
0.492 0.600 0.442 0.670 0.780 1.030 0.990
0.104 0.108 0.106 0.110 0.168 0,220 0.232
In 3-Methylpentane 0.388 2.41 0.492 2.90 0.336 1.90 0.560 2.55 0.612 2.68 0.810 2.90 0.758 2.73
1.07 1.38 1.38 2.21 3.59 4.20 5.80
0.669 0.765 0.774 0.969 0.990 1.23 1.23
0.735 0.795 0.867 0.930 1.510
0.068 0.072 0.087 0.117 0.196
In n-Hexane 0.667 2.87 0.723 2.76 0.784 2.79 0.813 2.71 1.314 3.06
1.15 1.35 1.65 2.01 5.66
1.05 1.19 1.27 1.36 1.93
(4) P. J. Dyne, Can. J . Chem., 43,1080(1965). (5) W. A. Cramer and C. J. Piet, Trans. Faraday SOC.,63, 1402 (1967).
V o l u m e 78, Number 10 October 1969
R. H. JOHNSEN, N. T. BARKER,AND M. BURGIN
3206
gators used perfluorocyclobutane which was previously shown to scavenge electrons12predominately as a solute in n-hexane. An additional yield of 1.4 thermal hydrogen atoms scavenged by benzene or hexene is also reported. These results would suggest, then, that 1 2 is scavenging electrons rather than H atoms and HI is formed only subsequently. The role of iodine in reducing the hydrogen yield may then be ascribed to a reaction such as RH+ lVz M 1
10-3 M 2
3
4
5
6
7
8
9
10
1 x 102 Iodine Figure 1. Change in 1 3 - ion yield with change in iodine concentration for (a) X-irradiated 3-methylpentane; (b) X-irradiated n-hexane.
value for G(Ia-) of 1.56 in the case of 3-methylpentane and 2.42 in the case of n-hexane. If it is assumed that G(II-) is a measure of G(H1) as is the case for ethanol6 then these values are equivalent to G(H1) which in turn is a measure of G(H.).’ The limiting yield of 1 3 - ions found from irradiation of n-hexane-iodine mixtures is in fair agreement with the value reported by Meshitsuka and Burton for G(HI) = 2.5 from n-hexane when H I was determined from an aqueous extract after radiolysis.’ This value is only in fair agreement with Sauer and M a n W value of 1.8 f 0.4 and agrees rather poorly with Rajbenbach’sOvalue of 1.4 for thermal H atoms in hexane. The limiting value of 1.56 for G(13-) obtained for 3methylpentane lies in the same region as the yield of G(H) reported for other branched chain hydrocarbons by Holroyd. For 2,2,3-trimethylpentane G(H) = 1.6 and 2,4-dimethylpentane G(H) = 1.8 where G(H) was determined from the yield of ethyl radicals formed by the addition of hydrogen atoms to ethylene and measured with C-14 labeled methyl iodide.1° Thus, in the case of n-hexane and as will be seen later, cyclohexane, the direct measurement of Is- by this method produces apparent H-atom yields which are in agreement with those obtained by previous workers but are much higher than the values obtained for G(H.) obtained by direct methods such as using ethylene as a scavenger. The possible dependence of the hydrogen atom yield upon an ionic mechanism has previously been suggested as an alternative to the exclusive free-radical mechanism. l1 Thus, although the apparent hydrogen atom yields found in the present series of experiments agree with many of those previously described in terms of a free-radical mechanism, the yield of 1 3 - from irradiated n-hexane-iodine mixtures is also in fair agreement with the yield of G(e-) = 2.6 for scavengeable electrons as found by Rajbenbach and Ka1dor.Q These investiThe Journal of Physical Chemistry
+ I---tR*
+HI
(4)
which does not involve freely diffusing thermal hydrogen atoms while still resulting in the production of HI. This reaction is similar to one proposed by Geissler and Willardla to account for the effect of alkyl iodides on the radical yield in hydrocarbon-iodine solutions. These authors suggested that in the absence of alkyl iodide reaction l b took place, while as a result of dissociative attachment of electrons to the alkyl iodide resulting in I-, the hydrocarbon ion could be neutralized without dissociation. If 1- is formed from Iz as suggested here, then the role of the alkyl iodide must be somewhat different, perhaps acting to neutralize the hydrocarbon ion directly by charge transfer. Pulse-radiolysis studies of 3-methylpentane containing iodine as a solute show that at an iodine concentration of M the iodide ion yield, G(1-) is approximately unity.14 This is of the same order as the yield found in the present series of experiments and strongly supports the role of 1 2 as an electron scavenger. A measure of the 13- yield from irradiated cyclohexane-iodine mixtures proved to be a particularly difficult study. There was a random variation in the results which could not be eliminated despite an exhaustive investigation. Over 40 separate determinations of Iswere made for cyclohexane-iodine solutions under a variety of conditions, e.g., from t,he anhydrous state to water-saturated solutions, variation in total dose and dose-rate, pretreatment of the irradiation vessel, purity of starting materials, etc. An analysis of a large M number of results gave a value for G(I3-) a t iodine = 1.0 0.3 with an extrapolated [G(L-)-l 11s. (Iz)-’] value for (?(Is-) of between 1.8 and 2.2. The failure to obtain reproducible results in this system seems to be a common problem. Other workers, for (6) N.T.Barker and J. H. Green, Nature, 204,872 (1964). (7) G. Meshitsuka and M. Burton, Radiat. Res., 10,499 (1959). (8) M. Sauer and I. Mani, J . Phys. Chem., 72,3856 (1968). (9) L.A. Rajbenbach and U. Kaldor, J . Chem. Phys., 47,242 (1967). (10) R. A. Holroyd, J . Phys. Chem., 70,1341 (1966). (11) L. M. Dorfman and R. F. Firestone, Ann. Rev. Phys. C h m . , 18, 189 (1967). (12) L. A. Rajbenbach, J . Amer. Chem. SOC.,88,4276 (1966). (13) P.R. Geissler and J. E. Willard, ibid., 84,4627 (1962). (14) R. Cooper (Argonne National Laboratory), private communica-
tion.
IODINE AS A SCAVENGER IN IRRADIATED HYDROCARBONS
3207
I
I
1~~t.4
10-zM
1
2
3
4
5
& I
0,O
I
0,2
04
Ob
0.8
10
C?
L4
16
I
ETHANOL mole liter-’
Figure 2. Variation in 1 8 - ion yield for increasing ethanol concentration in n-hexane-iodine solutions. The iodine concentration is approximately 1 X 10-3 M .
example ref 7 and 13, have reported G(H1) = 2.2,2.1, 2.0, 1.8 & 0.3. Figure 2 shows the effect on the 1 3 - yield of adding ethanol to the n-hexane-iodine mixture prior to irradiation. The iodine concentration is approximately 1X M . It may be noted that there is little effect on the yield below an ethanol concentration of about 10-1 M . The value of G(Ia-) at zero ethanol concentration is 1.05. The data are summarized in Table 11.
Table 11: Yield of Iodide Ions from Irradiated IodineCyclohexane Solutions Containing Varying Amounts of Ethanol Optical density a t 3600 of Is- ion
Total dose, eV g - * X
0.390 0.727 0.615 0.725 0.662 0.832 0.640 0.842 0.957 0.945 0.815
10-18
[I21 X 108 from O.D. a t 5200
[EtOHl, mol L - 1
@(Is-)
1.72 1.85 1.74 1.66 1.59 1.86 1.34 1.59 1.73 1.66 1.34
1.01 1.08 1.01 1.02 1.02 1.08 1.07 1.02 1.50 1.02 1.18
0.116 0.481 0.514 0.773 0.874 0.961 0.977 1.044 1.384 1.586 1.770
1.09 1.82 1.65 2.01 1.91 2.06 2.11 2.38 2.29 2.33 2.40
The yields in Figure 2 are corrected for G(13-) in ethanol-iodine solution = 5.6.15 This correction factor is less than 5% of the total yield a t an ethanol concentration of 1 M. Above concentrations of about 1.2 M the slope of the curve approaches zero with a limiting value of G(Ia-) = 2.4. Figure 3 illustrates the effect of ethanol at a number of concentrations on the 1 3 - yield as a function of iodine concentration. It is clear that when ethanol is present in concentrations in
6
7
8
9
10
’02
Figure 3. Effect of ethanol on yield of 1 3 - ions from n-hexane-ethanol-iodine solutions expressed in terms of iodine concentration: (a) 1.39 M ethanol; (b) 1.77 M ethanol; (c) 1.59 M ethanol; (d) no ethanol.
the neighborhood of 1.5 M the 1 3 - yield becomes indeconcentration. pendent of 1% The factor causing the random variation in the yield of 1 3 - from cyclohexane-iodine solutions appears to be suppressed when ethanol is added to the cyclohexaneiodine mixture prior to irradiation as shown in the following results for low9M iodine in cyclohexane. These Molarity of ethanol
1.1 1.4 1.7
Q(Ia-)
1.81 1.88 1.94
figures are in agreement with the extrapolated competition plot value using 1 2 as a scavenger, and also agree with the accepted value for G(H1) obtained for cyclohexane-iodine mixtures by Hamill. l6 More recently, Freeman” has investigated competitive charge and hydrogen-atom scavenging in irradiated hydrocarbons using nitrous oxide and propylene as scavengers. An attempt to distinguish between these two types of scavenging on the basis of a nonhomogeneous kinetic treatment was not successful, but on the basis of supplementary evidence it was concluded that a decrease in hydrogen yield caused by these scavengers is the result of interaction with charged species. The results obtained in the present work involving the prior addition of alcohol can be interpreted in several ways. If one adopts the view that, in fact, iodine is scavenging electrons primarily, then the alcohol can be involved in several different ways. It is clear from spectroscopic studies that a t these concentrations the alcohol exists as hydrogen-bonded clusters and all of the iodine is complexed with Furthermore, the alcohol-solvated electron has been observed by (15) N. T. Barker, Ph.D. Thesis, University of New South Wales, Kensington, N.S.W., Australia. (16) 6 . Z. Toma and W. H. Hamill, J. Amer. Chem. Soe., 86, 1478 (1964).
(17) M.G. Robinson and G. R. Freeman, J. Chem. Phys., 48, 983 (1968). Volume 78, Number 10 October 1969
JEROMEGREYSONAND HARRIET SNELL
3208 pulse radiolysis in cyclohexane containing as little as 4% methanol.lsb The yield of electrons is six times greater than that expected from the mole fraction of methanol. This strongly suggests efficient trapping by alcohol aggregates. I n these systems then, the alcohol, present in concentrations sufficient for spur penetration, would serve to stabilize the electron, preserving it for reaction with Iz. Alternatively, the alcohol could be suppressing recombination by undergoing reaction with the parent hydrocarbon ion in a proton-transfer reaction. These reactions would lead to species ROH2+ and 13- which should be quite stable in the systems under investigation. I n some preliminary experiments in which n-amyl alcohol was used, the limiting yield of 1 3 - was not reached until somewhat higher concentrations of alcohol were employed than those used in the ethanol experiments. In fact, at very low concentration of amyl alcohol the yield of IS-was reduced from the zero alcohol value. Spectroscopic studies and an nmr inves-
tigation show that amyl alcohol is considerably less hydrogen bonded at low concentration than is ethanol in the same solvents. Furthermore, in ternary solutions containing iodine the iodine is not fully complexed until significantly higher concentrations of alcohol are reached compared to those which produce total complexing in the ethanol case. These observations suggest that both the polar aggregate of alcohol molecules and the iodine-alcohol complex play a role in these ternary systems. These effects are under continued investigation. Acknowledgments. The authors wish to acknowledge the assistance of Messrs. D. Pritchett and D. Mullins in performing the irradiations, as well as the Institute of Molecular Biophysics for provision of experimental facilities . (18) (a) Unpublishedresults, this laboratory. (b) T. J. Kemp, C. A. Salmon, and P. Wardman in “Pulse Radiolysis,” M . Ebert, J. P. Keene, A. J. Swallow, and J. H. Baxendale, Ed., Academic Press, London 1966, p 247.
The Influence of the Alkaline Earth Chlorides on the Structure of Water1
by Jerome Greyson and Harriet Snell North American Rockwell Corpordwn, Cunogu Park, Cult&rnk
91804 (Recdved JUnUUTy 3, 1969)
Entropies of transfer between heavy and normal water for MgCh, CaC12, SrClz, and BaClz have been determined from combination of free energies of transfer obtained from cell measurements with heats of transfer obtained from calorimetric measurements. The free energy and heat values indicated a spontaneous and exothermic transfer of the salts from DzO to HzO. Solvent-structureinfluence, established from the sign and magnitude of the entropy, indicated that the alkaline earth chlorides behave as structure-breakingsalts. The order of structure-breaking influence for the cations was found to be Ba2+ > Sr2+ > Ca*+ > Mg*+with Mg2+ behaving as a structure maker. The structure influence of the cations has been attributed to their long-range interaction with the bulk solvent.
Introduction Recent studies of the nature of the structure of water and aqueous solutions have been carried out by investigating differences in the properties of heavy and normal water solutions*2-’ For example, rneaswements of Walden products for solutions of quaternary alkyl ammonium halides in each of the solvents have yielded the structure-making sequence of these salts.4 have taken a different among preach by determining entropies Of transfer for various species passing between HzO and D2O and attributing The Journal of Physical Chemistru
the sign and magnitude of the entropy values to structure influence. The entropy values were obtained by (1) This research was supported by the Research Division of the Offioe of Saline Water, U.S. Department of the Interior, under Contract No- 14-01-0001-1701. (2) J*C k w O n * J . Phw*Chern*966, 2218 (1962)* (3) J* Greyson*ibid*i 713 2210 (1967). (4) R. L. Kay and D. F. Evans, ibkl., 69,4216 (1965). (5) Y.C. Wuand H. L. Friedman, ibid., 70,166 (1966). (6) R. E. KerAn, Ph.D. Thesis, University of Pittsburgh, PittS burgh, Pa., 1964.
