NOTES
4079
from consideration of all the data now available we put forward the following suggestions regarding transport in these melts. The ideas which we outline give a feasible qualitative explanation for the behavior of these data which is otherwise rather puzzling. We postulate that conduction proceeds predominantly by movements of free ions and tracer diffusion predominantly by groups of ions either neutral or charged. A corollary of this is that the concentration of free ions is very low since diffusion can proceed by both species. This first postulate asserting that essentially separate species are involved in conduction and diffusion is in accord with most of the known facts. Thus, we have shown previously’ that the activation energy for tracer diffusion for several ions in the alkali carbonates is about 11 kcal, whereas that for conductance is in the range 6-8 kcal. This can be explained in terms of the large activation energy needed for an aggregate of ions to diffuse and the small activation energy needed for free ions to migrate. Again, the equivalence of cation and anion activation energies in diffusion is strong evidence for an associated act such as aggregate movement. One can also explain why the equivalent conductance of Li2C03 is much higher than that of Nad203, whereas tracer-diffusion mobilities are in the reverse order, being higher in pure Na2C03. With the small size of the cation and its associated high polarizing power, Li2C03can be visualized as a structured meltlo with an essentially anionic lattice which would allow Li+ ion migration but inhibit aggregate movement. We next postulate that if the above picture of the two tranmort mechanisms is acceDted, the positive deviations for diffusion shown in Figure 1 Can be explained by some kind of structural breakdown as N h coois progressively added to the L ~ ~ Cmelt. O ~ hi^ loosening of the structure can be attributed basically to the differing sizes of the cations which have the effect Of providing asymmetry either by Of differing polarizing power or from purely geometrical considerations. Structural breakdown is also reflected in the negative deviation of the viscosity for many binary salt systems. In any event, the ion aggregates are able to diffuse faster in the looser melt reaching an apparent maximum at the eutectic composition. By analogy with other electrolyte systems,” the migration of free ions in the conduction process would be much less affected by structural changes. There is some controversy about t,he nature of the entities in melts at their eutectic composition, and this has recently been discussed by Antipin.12 In particular, the behavior of eutectic mixtures under pressure points to the fact that they should be regarded as the most loosely packed combinations of the *
ions or atoms which constitute them. It is also noteworthy that Jam and Saegusals found that the activation energy for viscous flow in a ternary eutectic of the alkali carbonates was 10 kcal, whereas that for the pure salts was of the order of 25 kcal. To explain the diffusion maxima one might postulate further, then, that there is maximal structural looseness at the eutectic composition. The measurement of the tracer-diffusion coefficients of Li+ ion in the above system is of course necessary to confirm the picture here presented. At the moment we do not have the facilities for mass spectrometric analysis. If our assumptions are correct, the lithium ion coefficients should show a positive deviation of the same type as Na+ and COS2- ions. These data would also allow a comparison of conductance and diffusion mobilities via the Nernst-Einstein equation and the extent of the deviations should then permit a quantitative test of the above ideas. Finally, it should be remarked that the positive deviations in the tracer-diffusion data appear to be most marked when the cations in the binary mixtures are fairly different in size, when the lithium ion is present, and when the anion is highly polarizable. Thus studies in the alkali nitrates show some evidence of the effects described but are not very definite. Similarly, our data for the K2C03-NaC03 systemJ2 though incomplete, indicate a more nearly linear relationship.
Acknowledgment. We wish to thank Dr. C. A. Angel1 for helpful discussions on this subject.
I
(10) The use of the term “structure” here and below should perhaps be qualified. Many models of molten salts assume the existence of interpenetrating anion and cation lattices which do not have the long-range order of the crystalline state yet are sufficiently real to the extent that each ion has, on the average, more nearest neighbors of opposite charge than of its own charge. This departure from a purely random state is what we imply by the term structure. (11) R. H. Stokes and R. Mills, “Viscosity of Electrolyte Solutions,” Pergamon Press Ltd., London, 1965, P 56. (12) L- N. Antipin, AECtr-594% 19639 Pa 123. (13) G. J. Janz and F. Saegusa, J. Electrochem. Soc., 110,452 (1963).
Deactivation in the Photolysis of Hexafluoroacetone at Low Pressure
by Gerald B. Porter and Kengo Uchida’ Department of Chemistry, University of British Columbia, Vancouver, Canada (Received June 13, 1966)
I n the gas phase, the dissociation of the vibrationally excited molecules in an upper electronic state Volume YO. Number 1.9 December 1966
NOTES
4080
competes with the collisional deactivation by other molecules in their ground state. In the latter process, two mechanisms have been considered:2 (i) weak (multistage) collisional deactivation and (ii) strong collision deactivation. If case i is operative, it has been shown that at low pressures a point of inflection should appear in a plot of the reciprocal of the primary quantum yield of dissociation against pressure and that the plot should lead to zero slope at zero pressure. On the other hand, if case ii is operative, the plot should be a straight line. Porter, et al.,293have examined the photolysis of ketene and have supported case ii. However, Strachan, Boyd, and Kutschke4 have suggested other treatments of the experimental data and demonstrated that the literature data on the quantum yield for the decomposition of hexafluoroacetone is better interpreted in terms of a multistage deactivation (case i). According to their method, very precise measurements of the absolute quantum yield are required. Xoreover, Bowers and Porter5 have recently studied the photolysis of hexafluoroacetone in detail at pressures above 1 mm and have assumed that the reaction occurs through case ii, since the reciprocals of the primary quantum yields extrapolate to unity at all wavelengths of excitation. However, some doubt still remains about the primary process of deactivation, since the accurate measurement of the absolute quantum yield is difficult and the extrapolated value might deviate slightly from unity. All results studied previously have been obtained at relatively high pressures. One of the best methods to confirm directly which case is operative is to examine quantum yields carefully at lower pressures than 1 mm, where the curvature could be readily seen if it occurs. In this paper, the simple photolysis of hexafluoroacetone was extended to 0.1 mm. A special reaction cell was used to photolyze hexafluoroacetone at low pressure. It is about 80 cm long and 3 cm in diameter, and is divided at the center by a quartz window into two parts, one of which is a reference filled with hexafluoroacetone at constant low pressure and is used as a monitor of the light intensity. The exciting'light at 3130 A from a mercury lamp (;\lode1 PEK-200) was focused to a parallel beam by a quartz lens and was isolated by a Corning filter 9863 and an interference filter (Jena PIL, A, 3090 A). Since it has been observed6 that products of the photolysis are simply carbon monoxide and hexafluoroethane near room temperature and the quantum yield of the dissociation has the same value as that of the formation of carbon monoxide, the product gas, after the irradiation, was passed through a The J O U Tof ~P h~y s h l Chemistry
trap cooled by liquid nitrogen, and only noncondensable carbon monoxide was collected and measured by using an automatic Toepler-McLeod gauge. All photolyses were performed at 35'. The results are shown in Table I. Previously the following primary processes have been presented and the rate constants and some energies of activation have been determined516
lA*
+ A +'A0 + A
3A0+CO
(3)
+ 2CF3
(8)
Table I : Quantum Yields at Low Pressure Pressure, mm
9
65
1.040 1.037 1.030 1.020 1.018 0.837 0.815 0.621 0.618 0.424 0.419 0.404 0.209 0.208 0.203 0.197 0.106 0.104 0.100
0.832 0.823 0.823 0.820 0.819 0.844 0.838 0.8% 0.874 0.908 0.918 0.919 0.948 0.960 0.979 0.9% 0.974 1.02 0.936
0.787 0.779 0.779 0.776 0.775 0.807 0.802 0.851 0.845 0.887 0.897 0,899 0.937 0.949 0.968 0.969 0.968 1.01 0.931
(1) On leave from Hirosaki University, Japan. (2) G. B. Porter and B. T. Connelly, J. Chem. Phys., 33, 81 (1960). (3) G.A. Taylor and G. B. Porter, ibid., 36, 1353 (1962). (4) A. N.Strachan, R. K. Boyd, and K. 0.Kutschke, Can. J. Chem., 42, 1345 (1964). (5) P.G.Bowers and G . B. Porter, J . Phys. Chem., 70, 1622 (1966). (6) P. B. Ayscough and E. W. R. Steacie, Proc. Roll. SOC.(London), A234, 476 (1956).
NOTES
408 1
where the superscripts 1 and 3 represent the multiplicity of the excited electronic state, while an asterisk denotes a molecule with more vibrational energy than a vibrationally equilibrated molecule with superscript zero. The quantum yields obtained here are relative values only. For the purpose of investigating the linearity of l/+os. [A], relative quantum yields are adequate. However, in order to represent these data, we have corrected them so that + = 1 at zero pressure. The fraction of the light absorbed in each cell was always less than 4%; therefore it could be taken as strictly proportional to the pressure in the cell. It is evident that a few per cent of the dissociation takes place by process 8, the dissociation via the triplet state, even below 1 mm. Thus, the quantum yield of dissociation from the singlet excited state directly is obtained by subtracting that via the triplet from the over-all quantum yield, with +" = 0.29. The corrected results are shown in Table I and in Figure 1, where reciprocals of quantum
I
I
0.2
I
I
I
0.4 0.6 0.8 Pressure, m m Hg
1.0
Figure 1.
yields of dissociation via only singlet state are plotted against the pressure. The straight line is drawn by a method of least squares. The experimental error is large at the lowest pressure, but there is no tendency to concavity upward. Therefore, the strong collision deactivation, case ii, is undoubtedly supported in this system.
Acknow2edgmmt' This was supported by a grant from the National Research Council of Canada.
The Isomerization of n-Pentyl and 4-Oxo-l-pentyl Radicals in the Gas Phase
by L. Endrenyi and D. J. Le Roy Laah Miller Chemical Laboratories, University of Toronto, Toronto, Canada (Received June 18, 1966)
Some years ago Kossiakoff and Rice' suggested that in many cases the activation energy for the isomerization of a long-chain free radical may be much less than the activation energy for its decomposition, and they were able to explain the products of the decomposition of hydrocarbons in terms of intramolecular hydrogen migration in alkyl radicals. Somewhat later, Sefton and Le Roy12in studying the polymerization of ethylene initiated by ethyl radicals labeled in the methylene group with C14, obtained evidence for radical isomerizations involving 1-5 and 1-6 intramolecular hydrogen migration. They showed that most of the product olefins were formed by the decomposition of alkyl radicals and that the molar activity of these was comparable to that of the ethyl radicals which initiated the polymerization. Since a simple decomposition of the long-chain alkyl radicals would have yielded inactive biradicals, and hence inactive olefins, they concluded that the radicals had undergone intramolecular hydrogen migration before decomposition. A number of additional examples have been reported more recently13although quantitative kinetic data are lacking. In the course of a study of the kinetics of the addition of methyl radicals to ethylene it became necessary to follow, in a quantitative way, all of the subsequent reactions of the propyl radicals formed by the addition of methyl radicals (from the photolysis of acetone) to ethylene. From the nature and amounts of certain of the products we were able to obtain kinetic parameters for 1-4 intramolecular hydrogen migration in n-pentyl radicals and somewhat less conclusive data for 1-5 intramolecular hydrogen migration in 4-oxol-pentyl radicals.
Experimental Section Acetone at a concentration of 3.60 X mole was photolyzed in the presence of 0.240 X mole of ethylene (0.300 X mole cm-3 was (1) A. Kossiakoff and F. 0. Rice, J . Am. Chem. SOC.,65,590 (1943). (2) V. B. Sefton and D . J. Le Roy, Can. J . Chem., 34, 41 (1956). (3) E.-A. I. Heiba and R. M. Dessau, J. Am. Chem. SOC.,88, 1589 (1966).
Volume 70,Number IS December 1966
