2132
T. J. HARDWICK
carrying dipoles (hydroxylated silica surface) or exchanging ions (porous crystals of zeolite) the heat of ether adsorption exceeds greatly the heat of npentane adsorption: namely, this excess for the hydroxylated silica is about 7.5 kcal./mole and for type 5A zeolite is even larger, about 10-13 kcal./ mole. The excess of the heat of ether adsorption above the heat of n-pentane adsorption on the hydroxylated silica surface was explained by us by the forming of hydrogen bonds between the oxygen of the ether molecule and the silanol hydroxyl groups of the surface. However, in the case of type 5A zeolite, the heat of ether adsorption exceeds the heat of n-pentane adsorption by a still greater quantity than in the case of hydroxylated silica, surface, though in the case of zeolite, the hydrogen bonds are not formed (the infrared spectrum does not show the hydroxyl groups in the zeolite 5A evacuated at the high temperature). The difference between the heats of adsorption of ether and n-pentane appears to be greatest not in the case of adsorption on the hydroxylated surface when there is the possibility of hydrogen bond forming, but in the case of the adsorption on ionic surfaces when this bond is impossible. As was mentioned before, in the case of ether adsorption by type 5A zeolite, the sharp increase of the heat of adsorption at the transfer from n-pentane to ether apparently is due to the additional electrostatic interaction of the dipole of an ether molecule with the cations, which
Vol. 66
exist on the walls of zeolite channels. This leads to the assumption that in the cases shown in Fig. 5 b and c we have a quantitative difference, rather than a qualitative one; the heat of ether adsorption rises with the increase of the contribution of the electrostatic forces in the total energy of the adsorption interaction. This comparison shows that the experimental and theoretical investigations of the adsorption on the possibly more homogeneous solid non-polar surfaces as well as surfaces carrying different dipoles and also different ions would help us to understand the nature of intermolecular interactions and particularly the interactions which usually are named hydrogen bonds. At the adsorption on both hydroxylated and cationic surfaces the quantum-mechanical effects probably are also important.38 Acknowledgment.-The authors are indebted to Dr. S. P. Zhdanov for the presentation of zeolite type 5A crystal sample and to K. N. Mikos and T. A. Melnikova for their assistance in the measurements. The authors are thankful also t o Professor J. M. Holmes and Dr. C. H. Amberg (who kindly took upon themselves the trouble of presenting this paper to the 9th International Calorimetry Conference and to the Journal of Physical Chemistry) and to Professor R. M. Barrer for his valuable discussion and comments. (38) A. V. Kiselev, Ya. Koutetski, and J. Chishek, DokE. ALad. Nauk SSSR, 187, 638 (1961); Ya. Koutetski and J. Chizhek, Zh. Fit. Khim., 36, 1508 (1962).
CHARGE TRANSFER I N THE RADIOLYSIS OF ORGANIC LIQUIDS. I. EVIDENCE FROM HYDROGEN GAS YIELDS BY T. J. HARDWICK Gulf Research & Development Company, Pittsburgh 80, Pennsylvania Received March 96. 1069
Hydrogen gas yields from the radiolysis of mixtures of saturated hydrocarbons do not necessarily follow a “law of aver) ) , may be greater or smaller than the predicted value. This effect is beages” (Ga, = z electron fraction x G H ~ ( ~but lieved to result from a charge exchange occurring on collision between the parent positive ion and a neighboring molecule of lower ionization potential. Subsequent charge neutralization results in the decomposition of a, different molecule than that which absorbed energy, Not all molecules which can decompose can transfer energy in this fashion; those which cannot transfer energy are considered to be electronically excited. On the assumption that charge transfer occurs during the interval between ionization and charge neutralization, it can be determined that the lifetime of the charge pair is about 4 X lo-’* sec. in alkane solvents.
Introduction made by Burton and Lipsky.* It is this second I n radiation chemistry, the term “protective case, where the solvent molecules are protected effect” has two general meanings. I n one case, from decomposition, that will be considered exa particular solute is “protected” from decom- clusively in this paper. Magee and Burton293 suggested on theoretical position by the presence of a second solute which is much more reactive toward the intermediates (free grounds that energy transfer would take place by radicals, ions, etc.) produced by the decomposition a charge transfer mechanism. In the radiolysis of of the solvent. Usually, this can be considered a a mixture of A and B molecules, if the ionization simple case of competing reactions. The extent potential of B is greater than that of A, a charge transfer from B + to A will occur, giving a system of solvent decomposition is unchanged. In the second case, the solute in some manner increasingly rich in A+. The radiolysis products, decreases the decomposition of the solvent mole- formed subsequent to charge neutralization, will cules. The mechanisms suggested have included (1) M. Burton and 9. Lipsky, J . Phys. Chem., 61, 1461 (1957). energy transfer from solvent to solute molecule, (2) J. L. Magee and M. Burton, J. Am. Chem. Soc., 72, 1965 negative ion formation, and charge transfer. An (1950). (3) J. L Ahgee and M. Burton, ibid., 73, 523 (1951). excellent description of these mechanisms has been
Nov., 1962
CHARGZ TRANSF~R IN RADIOLYSIS OF ORGANIC LIQUIDS
be more characteristic of A than would be expected from the gross composition of the system. I n addition t,o charge transfer, it has been shown in certain cases that energy may be transferred between electronically excited states in multicomponent systems. Dreeskamp and Burton4 have reviewed the possible consequences in radiation Chemistry. It should be noted, however, that in all systems giving evidence of this type of energy transfer, a t least one component has been aromatic in nature. Many expeirimental studies to determine the extent of such solvent protection by energy transfer have been made. It is not practical, however, to measure directly the change in the amount of solvent decomposed. Accordingly, one must select a measurable product, and relate a change in its radiolytic yield to the degree of protection. Such a step presupposes an unambiguous explanation of the mechanism1 of reaction, and it is on the validity of this assumption that most objections arise. A case in point is the radiolytic hydrogen gas yield from alkane-aromatic mixtures, particularly the system cyclohexane-benzene, which has been studied extensjvely.6-1z I n this case the hydrogen gas yield from cyclohexane is decreased markedly by small amounts of benzene. The decrease may be due entirely to energy transfer from cyclohexane to benzene. On the other hand, hydrogen atoms, which are known to be intermediates in alkane radiolysis, may add preferentially to the benzene ring. Accordingly, the decrease in hydrogen gas yield nnay result from scavenging action of the “protecting” molecule. I t is perhaps unfortunate that in most systems studied there are few cases where one can positively say that the fate of intermediate free radicals is not affected to some extent by the presence of the “protecting” material. Such cases where radical scavenging appears not to affect the product yield are : Cz gases from cyclohexane-cyclohexene; ethylene from cyclohexane-benzene ; acetylene from benzene -cyclohexene, and from benzenet01uene.~ In these systems there is good qualitative evidence for energy transfer, but the accuracy of the data is not sufficient to give quantitative results. Too, in all these systems the details of the reactions to form the measured product are only speculative. There is, however, one type of system where the presence of protective effects can be determined unambiguously. The hydrogen gas yield from mixtures of pure saturated hydrocarbons a t low conversion should give unambiguous results. (4) H. Dreeakamp and & Burton, ‘I. Dzseussions Faraday Soc., 27, 64 (1959). (5) J. P. Manion a n d M. Burton, J . Phys. Chem., 56, 560 (1952). (6) M. Burton and W. N. Patrick, ibzd., 68, 421 (1954). (7) R. H. Schuler and A. 0. Allen, J. Am. Chem. Soc., 77, 507 (1955). (8) H. A. Dewhurst, J. Chem. Phys., 24, 1254 (1956). (9) &I. Burton, J. Chang., S. Lipsky, and M. P. Reddy, ab& 26, 1337 (1957). (10) G. R. Freeman, zbzd., 33, 71 (1960). (11) J. Lamborn and A. J. Swallow, J . Phys. Chem., 66, 920 (1961). (12) P. J. Dyne a n d W. iM. Jenkinson, Can. J. Chem., 39, 2163 (1961).
2133
I n alkane radiolysis, hydrogen gas is produced by two mechanism^^.^^^^^^*
Q
RH +Hz
+ products
(1)
N
ti2
RH+H+R
(2)
k3
H
+ RH-+Hz + R
(3)
I n the first, hydrogen gas is produced directly, in yield G1, unaffected by the presence of free radical scavengers. Whether the mechanism be extraction of hydrogen from a nearest neighbor by a hot hydrogen atom, or whether a molecular detachment process takes place, is immaterial to the present argument. In the second, hydrogen atoms are produced in yield Gz and degrade to thermal energies. I n the absence of scavengers, all such hydrogen atoms react by reaction 3 to produce hydrogen gas. The total hydrogen gas yield G H ~ (is~the ) sum of GI and Gz. I n any binary mixture of alkanes B and A, the hydrogen gas yield from reaction 1 will be x X G 1 ~ (I - x) X GI*, where x and 1 - x are the eiectron fractions of B and A (representing the fraction of energy absorbed by each) and GIB, GI* are the respective vaiues of GI for B and A. Similarly, the hydrogen gas yield from reactions 2 and 3 will be x X G2B (1 - x) X G ~ Afor , it does not matter whether the thermal hydrogen atoms produced in reaction 2 react with A or B. The total hydrogen yield ( G H ~ ( ~expected )) from such a mixture therefore is x X GH~(O)B (1 x) X GH,(o)A. In other xords, a plot of the total hydrogen gas yield us. electron fraction of one component should give a straight line (law of averages6). We have found experimentally that a considerable deviation from a straight line normally is found, and in this paper a program of experiments to exploit this difference is reported. A model has been developed from the results, incorporating features of energy transfer which have been suggested previously. If the model is correct, certain values with respect to the lifetime of the excited species can be calculated. As indicated previously, apparently incontrovertible evidence for energy transfer is available in the evolution of light hydrocarbons from the radiolysis of certain hydrocarbon mixtures. This will be discussed further in a second paper in which concurring experimental data will be presented. The present paper is limited to measurement of hydrogen yields and their interpretation. Experimental
+
+
+
Materials.-Saturated hydrocarbons were Phillips Prirc Grade, and were further purified by stirring with sulfuric acid. The extent of unsaturation of the purified material, as measured by bromination, was less than 0.1 mM, except for 2,3-dimethylbutane, where the unsaturation content was 0.23 mM. Fisher Reagent isopropyl alcohol was further purified with 2,4-di henylhydrazine and distilled. Isopropyl ether (Eastman W&te Label) mas refluxed with lithium d u (13) T. J. Hardwick, J. Phys. Chem., 64, 1623 (1960). (14) T. J. Hardwick, ibid., 65, 101 (1961).
T. J. HARDWICK
2134
Vol. 66
dosimeter. Appropriate corrections were made by taking into account the relative electron densities of the solutions. Hydrogen Gas Analysis.-After radiolysis, hydrogen was pumped off the solution through a trap a t - 196". The gas then was isolated from other contaminants (air, methane) and the amount was measured on a McLeod gage. Reproducibility of the hydrogen yield was better than f1%. Systems Studied.-Hydrogen gas yields were measured for the following mixtures: (a) neohexane- n-hexane, (b) 2,3dimethylbutane-n-hexane, (c) isopropyl alcohol-n-hexane, (d) isopropyl ether-n-hexane, (e) cyclopentane-Z,3-dimethylbutane, and ( f ) 2-methylpentane-n-octane. In addition, hydrogen yields were measured for the system 7570 n-hexane-25% neohexane (by volume) containing varying amounts of methyl methacrylate. 0.2
0.4
0.6
0.8
Electron fraction neohexane.
Fig. 1.-Hydrogen
yields from the system n-hexaneneohexane. 7
R.
0.2 0.4 0.6 0.8 Electron fraction Z,%dimethyIbutane.
Fig. 2.-Hydrogen
yields from the system n-hexane2,3-dimethylbutane.
Results Throughout this paper we have assumed that energy is absorbed in proportion to the relative electron density of the components. This postulate is considered to hold true, although a suggestion to the contrary has been made in cases where n electrons are present.ll In any case, however, 2 bonding only is present in the alkanes, alcohols, and ethers studied. Hydrogen Gas Yields.-In Fig. 1-4 the radiolytic hyzrogen gas yields have Keen plotted as a function of electron fraction for various two-component systems. I n no case does this yield correspond to that expected from the electron fraction of the individual components. In the systems studied, the yields are lower than expected, but the direction of change is not necessarily always to lower yields. The direction of change in six binary systems is listed in Table I. TABLEI DIRECTIOX OF OBSERVED EXERGY TRANSFER Ionization potentials in e.v. are in parenthesesz0 Donor
Aoaeptor
n-Hexane (10.17) Seohexane (10.04) n-Hexane ( I O . 17) 2,3-Dimethylbutane (10 00) %-Hexane (10.17) Isopropyl alcohol (IO.15)" (9.3)b %-Hexane (10.17) Diisopropyl ether Cyclopentane (10.51) 2,3-Dimethylbutane (10.00) 2-Methylpentane (10.09) %-Octane (9.99)c Estimated from values Value uncertain to ic0.05 e.v. for dimethyl ether and diethyl ether. c Estimated as per method (a), Table 111.
I
I
I
0.2 0.4 0.6 Electron fraction isopropyl alcohol.
Fig. 3.-Hydrogen
I
I
0.8
yields from the system n-hexaneisopropyl alcohol.
minum hydride, distilled, and stored under nitrogen before use. Methyl methacrylate was obtained from Rohm & Haas. Solutions.-Mixtures of the various components were made up by volume a t 23 and the densities were measured. With the exception of isopropyl alcohol solutions at high alcohol concentrations, ideal behavior on mixing was observed. Where desirable, viscosities were measured a t 23"; mole fractions and electron fractions were calculated for the mixture. Irradiations.--Irradiations were carried out at 23 O in a manner described previously.13~14Briefly, a 100-ml. sample of solution was degassed and irradiated to 20-50 krad, using X-rays from a 3 MeV. Van de Graaff accelerator. The energy absorbed was monitored concurrently by the Fricke O ,
We shall consider that a transfer of energy has occurred from, for example, n-hexane to neohexane (Fig. 1). For 0.25 electron fraction neohexane. the hydrogen gas yield is that expected from the decomposition of 0.51 electron fraction neohexane. It would appear that some 0.26 electron fraction n-hexane had transferred its energy to neohexane before chemical decomposition occurred. This phenomenon may be generalized for a twocomponent system, A and B, where there is a net transfer of energy from B -+ A. Let x be the electron fraction of €3 as calculated from relative concentrations. Let zj be the mole fraction of B. Let x be the apparent electron fraction of B as indicated by the measured value of GH%. In the example given above for n-hexane (B)-neohexane (A) (Fig. 1) 1 - 17: = 0.25, 1 - x = 0.51. The amount of B which transfers energy is (I - z ) - (I - R.) = 17: - x . The fraction of B mole-
Nov., 1962
CHARGETRANSFER IN RADIOLYSIS OF ORGANIC LIQUIDS TABLE I1 PARAMETERS CALCULATED FROM DATA IN FIGURES 1-5,
Direction of charge transfer Acceptor by volume
%-Hexane 4 neohexane F NX
%-Hexane -c 2,3-dimethylbutane F NX
FOR
2135
VARIOUSBINARY MIXTURE$
n-Hexane
-+
isopropanol
Cyclopentane -+ 2,3-dimethylbutane
F NX F NX 5 0.125 2.52 0.145 2.98 0.169 2.13 2.38 .265 1.97 0.175 2.70 .290 2.28 10 ,223 2.33 1.94 ,257 2.25 .375 2.58 ,393 15 .329 ,341 2.30 1.98 .462 2.40 .484 2.18 20 ,420 2.25 2.05 .418 .541 2.26 .562 2.04 25 .485 2.15 2.15 .481 ,611 2.16 .632 1.98 30 * 559 2.13 .561 .675 2.23 .657 2.02 1.88 35 .613 2.04 2.47 .616 .713 1.92 .751 1.80 40 .664 1.89 3.00 ,729 .754 1.66 .858 1.60 50 .726 NA is the total number of collisions by B* during its lifea F is the fraction of eligible molecules which transfer energy. time.
6.0 I cules absorbing energy, and which subsequently transfer this energy, is (x - x)/x. I n Fig. 5 we have plotted the function (x - x)/x against electron fraction of A, using data obtained from Fig. 1-3. I n all cases the donor molecule is n-hexane, while the energy is accepted by neohexane, 2,3-dimethylbutane, and isopropyl alcohol. Two points are of interest: (a) the limiting value of (x - x ) / x for all three components (A) is identical within experimental error, indicating that the characteristics of the donor rather than acceptor 0.2 0.4 0.6 0.8 molecule govern the process, and (b) even a t high Electron fraction 2,3-dimethylbutane. acceptor concentrations not all of the donor type yields from the system cyclopentanemolecules can transfer energy. For n-hexane only Fig. 4.-Hydrogen 2,3-dime thylbutane. 70% of the molecules absorbing energy can transfer it to another molecule. I On making a similar plot for the system cycloB. O.* , pentane-2,3-dimethylbutane, one finds that 79% 0.7 of the excited molecules of cyclopentane molecules 3 ‘E 0.6 can transfer energy. Likewise, it is found that 0 52% of the energy absorbed by 2-methylpentane is B EJ 0.5 transferable to n-octane. This limiting number e (L) of excited molecules “eligible” for energy Ej’ 0.4 transfer appears to vary with the nature of the 0 0 donor molecule 0.3 u Knowing the limiting fraction of excited molecules which c a n transfer energy, the fraction ( F ) 0.2 of these “eligible” molecules which do transfer 2 their energy can be calculated (F = (x - x)/Lx). 0.1 Values of F for several binary systems are given 0.2 0.4 0.6 0.8 in Table I1 for various concentrations of acceptor Electron fraction acceptor.
r
.s 13
A.
I n order to transfer energy, we have assumed that collisions between excited donor molecule and acceptor molecule must occur, and in absence of evidence to the contrary, that such collisions are governed by the normal laws of diffusion in liquids. The collision frequency of an excited molecule B* with A will be proportional to the mole fraction of A. We also are assuming throughout the discussion that energy transfer occurs in unit probability on collision of B* with A. It is easily seen from the values of F in Table I1 that the excited species B* exists during several collisions with other molecules of the system. The total number of collisions ( N )which B* will undergo during its existence will be F/(l - y). Collision frequency depends on collision diameter, molecular weight, viscosity, and temperature.
Fig. 5.-Fraction of n-hexane transferring energy in various systems (z - z)/z, X, neohexane; 0,2,3-dimethylbutane; e, isopropyl alcohol.
The first two parameters are constant in a particular binary system, but the viscosity may vary. We are not entirely sure of the legitimacy of applying a viscosity correction to the total number of collisions of B* in the present instance, for can it be said that a molecule, and an excited one a t that, recognizes the viscosity of the medium within several collisions? In addition, although the gross temperature of the system is constant during radiolysis, the local temperature, or kinetic energy, of molecules near the original column of ionization may be greater than normal. A local decrease in viscosity therefore would be expected. Nevertheless, we have applied a viscosity normalizing factor
T. J. HARDWICK
2136
Vol. 66
At this time we make the suggestion that those molecules which transfer energy do so while in the ionized state, that is, in the time between ionization and subsequent charge neutralization by an electron. This energy transfer is in reality a charge transfer, and we shall refer to it as such for the rest of the paper. The remaining molecules which are not “eligible” to transfer, but which decompose to give products, are considered to be electronically 0.3 L excited, and cannot transfer energy in the time 100 200 300 400 available before unimolecular decomposition. [Solvent]/ [methyl methacrylate 1. A criterion for charge transfer is that the acFig. 6.-Kinetic plot for determining Gz in the system ceptor molecule be a t a lower ionization potential 0.25 electron fraction neohexane-0.75 electron fraction than the donor molecule. Pertinent ionization n-hexane; H atom scavenger-methyl methacrylate. potentia1 data for the compounds studied are to the systems containing n-hexane, consisting of a given in Table I. We have used photoionization term A, the ratio of the viscosity of the solution to data as these should be more accurate when conthat of the pure donor V / V O a t 23’. Values of sidering differences in ionization potential. In all Nx are given in Table I1 for various concentrations cases the ionization potential of the donor is higher than that of the acceptor, of acceptor. I n the systems chosen for detailed study, it so As the concentration of acceptor molecules increases, the “lifetime” of B”, as measured by the happens that the donor molecule has both a higher total number of collisions, is somewhat reduced. ionization potential and a higher radiolytic hydroIt would appear that two processes are instru- gen gas yieId than the acceptor. This of course mental in removing B*, and that energy transfer results in a lower hydrogen gas yield than exis predominant a t higher concentrations of acceptor pected from electron fraction data. However, ~ ) 6.18)-2-methylmolecules. The mechanisms of these processes in the system n-octane ( G H ~ ( = ~ ~4.47), the ionization potential pentane ( G H % (= will be discussed later. Effect of Energy Transfer on GI and G2.-In any of n-octane is lower than that of 2-methylpentane. binary system, factors affecting naturally As result, n-octane is an acceptor molecule, and would be reflected in values of GI and/or G,. higher yields of radiolytic hydrogen are observed Consider the case of neohexane (GI = 1.16, G2 = than would be expected without charge transfer. It is implicit in our arguments thus far that for 1.96)-n-hexane (GI = 2.12, G2 = 3.16) mixtures. A solution 0.25 electron fraction neohexane radio- n-hexane 70% of molecules which decompose were lytically decomposes as 0.51 electron fraction neo- originally in the ionized state, while only 30% were hexane (Fig. 1). As a result, for this mixture in an electronically excited state (cross sections for one would expect Gl to be 0.51 X 1.16 0.49 X absorption of energy by vibration and rotation 2.12 = 1.62; Gz = 0.51 X 1.96 0.49 X 3.16 = alone are very small, and such energy absorption processes will not be considered further). From 2.55. This prediction has been checked experimentally general considerations of energy absorption in the by measuring the radiolytic hydrogen yield from slowing down of ionizing electrons, one would exthe system 0.25 electron fraction neohexane-0.75 pect more excitation events than ionization. It electron fraction n-hexane, with varying amounts must be remembered, however, that not all exciof methyl methacrylate scavenger added. It has tation levels are repulsive, and accordingly only been shown p r e v i ~ u s l y ~that ~ ~ ~a *plot of the re- part of that energy absorbed by electronic excitaciprocal of the decrease in hydrogen yield, l/AGH,, tion leads to decomposition. Since we are measurdue to the added scavenger, us. the ratio [solvent]/ ing the relative number of events that lead to de[solute] gives a straight line, the ordinate intercept composition, it is understandable that the ionization of which is 1/G2. In Fig. 6 we show such a plot, events are in the majority, for it is likely that all using experimentally determined values for the excited molecules formed on charge neutralization decrease in hydrogen gas yield due to methyl will decompose. methacrylate. The ordinate intercept is 0.385, At the lower concentrations of acceptor, a large giving GB = 2.60. Since G H ~ (for ~ ) this system number of “eligible” molecules do not transfer is 4.17, GI = 1.57. Within experimental error energy. From our interpretation, under such conthese values agree with those predicted in the pre- ditions many ions are neutralized before charge vious paragraph. Such a result supports the con- transfer can take place. Once neutralized, energy tention that energy transfer occurs prior to any transfer is “quenched.” chemical decomposition. Lifetime of the Charge Pair.-For a particular Discussion donor (e.g., n-hexane) it, would be expected that at Nature of the “Eligible” Molecules.-One of low acceptor concentrations the number of collithe significant findings of these experiments is sions belore charge neutralization would be about that not all molecules which decompose are capa- the same, regardless of the nature of the acceptor ble of transferring energy. This limitation seems molecule. This does not appear to be so from the to be a property of the donor molecule, e.g., 70% data in Table 11. A referee has suggested an explanation of this for n-hexane, 79% for cyclopentane, etc., and is unaffected by the nature of the acceptor molecule. discrepancy. It is expected that, of the molecules
‘
1
1
1
/
+
1
+
1
Nov., 1962
CHARGE
TRANSFER I N RADIOLYSIS OF O R G A N I C h Q U I D S
decomposing through intermediate charge pair formation, only a fraction will eventually produce hydrogen gas; furthermore this fraction will vary from one compound to another, in general being large for n- and oycloalkanes, and smaller with increasingly branched alkanes. I n the transfer of charge from n-hexane to the other hexane isomers, ions which eventually would have produced hydrogen gas now have been replaced by those which form predominantly alkyl radical pairs. The resulting decrease in hydrogen gas yield is in accord with the observed decrease of NX with increasing acceptor concentration. The proper values of NX are those obtained by extrapolation to zero concentration of acceptor. For n-hexane-isohexane systems (Nh)o = 2.5 and 2.9 collisions for acceptors neohexane and 2,3dimethylbutane, respectively. I n the case of such mixtures, the collision frequency of one molecule with its neighbors can be calculated with some accuracy (6.6 >(: loT2collisions/sec.). The lifetime of a charge pair is therefore 2.5-2.9 collisions/ 6.6 X 10l2collisions/sec. = 4-5 X sec. Evidence for Charge Transfer in Previous Work. -For the cyclohexane-benzene system Freemanlo concluded t,hat the ratio of rate constants JG4/k6 k4
c-CsH,z*
+ (zeH6 +C-CeHlz f CaHe* ks
C-ceIIiz*+C-CsHii
+H
(4)
(5)
was 0.78 l./mole, where c-CeH12*represents only a specific fraction o~fthe cyclohexane molecules which absorb energy. Clearly k4 must be very fast in order to compete with reaction 5. Very likely the c-Cs&z* molecules of Freeman are similar to our positive ions, but as was remarked before, it is difficult to present unchallengeable explanations in the cyclohexane-benzene system. Ramaradhya and Freeman16 found evidence for positive ion transfer in the vapor phase from cyclohexane to benzene and propylene, although it is not clear if such a mechanism can occur in liquid systems. Dyne and Jenkinsonl'j have shown that DS is produced in the radiolysis of cyclohexane containing cyclohexane-d12. They further showed that a t low concentration of C6D12, both atoms of the Dz molecule come from the same parent molecule. Thus in any system containing small amounts of C6D12, the Dz yield based on the energy absorbed by C6Dl2 (G1(D) in their terminology) is a measure of the amount of GD12 which decomposes. If no energy transfer occurred, G1(D) should be invariant from one solvent to another. Experimentally, however, Dyne1' found that Q(D) varies widely in various saturated hydrocarbons. Some have values of G1(D) larger than that found with ordinary cyclohexane (0.32)18; others have lower values. I n the first column of Table 111 are 1iE;ted hydrocarbon solvents where (15) J. M. Ramaradhya and G. R . Freeman, Can. J . Chem., 39, 1769 (1961). (16) P. J. Dyne and VV. h l . Jenkinson, zbzd.. 38, 539 (1960). (17) P. J. Dyne, privste communication, 1962. (18) T. J. Hardwick, J . Phys. Chem,. 66 117 (1962).
2137
Gl(D) > 0.32; in the second column, those solvents where G1(D) < 0.32. Photoionization potentials for each hydrocarbon are given in parentheses. Significantly, all compounds in the first column, where Gl(D) > 0.32, have ionization potentials greater than that of cyclohexane, while those in the second column {G1(D) < 0.32) have ionization potentials less than that of cyc10hexane.l~ The explanation for the variation of G1(D) follows from our postulate of charge transfer. All compounds in the first column are donor molecules with respect to cyclohexane; hence more C6Dl2 will decompose, giving larger values of Gl(D). On the other hand, hydrocarbons in the second column are acceptor molecules with respect to C6D12,and, after charge transfer, fewer CeDlzmolecules will decompose, thus giving smaller values of Q(D). TABLE I11 COMPARISON OF GI(D) FOR SOME SATURATED HYDROCARBONS WITH GI(D) FOR CYCLOHEXANE Values of Dyne and Denhartogl? Photoionization potentials in e.v. are in parentheses20 Hydrocarbons where
Qi(D) < 0.32
Hydrooarbons where Ot(D) > 0.32 Decahydronaphthalene ( < 9 . ~ 3 ) ~ Ethylcyclohexane (
