NOTES
4076
the shock front is probably XeF, but no detailed kinetic system that we could find is consistent with all of the observations.'
4(H), from (1) is related to the measured Hz quantum yield, cb(Hz), as
Conclusions The dissociation of Fzmolecules proceeds more slowly in the presence of Ar as a third body than would be expected by comparison with other halogens.2*8 The addition of K r does not appear to greatly affect the rate in Ar, so we conclude the bond energy in IirF is not especially large (certainly less than 10 kcal/mole). The addition of Xe affects not only the rate but the course of the reaction. At least one stable, colored Xe-F species is formed between 1100 and 1600"I~; probably this is XeF2. This one species, however, is not sufficient to account for the initial differences in the apparent rate of change of the F2 concentration, and it would appear that XeF is an important intermediate. Acknowledgment. We thank the U. S. Army Research Office (Durham) for support of this work. (7) Since no detailed conclusions are given, we will omit the detniled discussion of the possibilities. This is available from the authors if desired. (S) D. J. Seery, J . Phys. Chem., 7 0 , 1684 (1966).
Hydrogen Atom Yield from Benzene Photolyzed at 1849 A' by Frank Mellows and Sanford Lipsky Department of Chemistry, University of Minnesota, Minneapolis, Minnesota (Receiced May 2G, 196'6)
The quantum yield for the disappearance of benzene vapor a t 1849 A has been recently determined to be 0.25 f 0.02 a t 1 torr and to extrapolate to 1.0 at zero pressure.2 The present study was undertaken to determine the contribution to the disappearance yield from process 1.
PIT?vious f3tdies by Wilson and xoYes3 using a group of A1 lines between 1855 and 2000 A have indicated that process 1 is not an important primary process in view of a low H~ quantumyield and no evidence for production Of deuterated benzenes in the presence Of D2. Our results confirm this conclusion. The technique employed involved the measurement of Hzyields in the presence and absence of saturated hydrocarbons (RH). The H atom quantum yield, The Journal of Physical Chemistry
where 4o(H2) is the H2 yield from pure benzene and k3 and li-4 are rate constants for reactions 3 and 4.
H H
+ CsHs
+CsH,
+ R H +R + Hz
(3) (4)*
All photolyses were carried out in Pyrex cells to which were sealed 1-mm thick Suprasil quartz windows. Cell volumes for liquid and gas phase photolyses were 10 and 200 ml, respectively. An electrodeless lamp was employed containing Hg 2 torr of Ar. Power was supplied from a Raytheon 2450-If c microwave generator. For improved stability the lamp was air cooled during operation. The emission spectrum of the lamp, determined with a 0.5-m Seya-Samioka grating monochromator, was found to contain below 2600 A only two lines at 2537 and 1849 A. S o attempt was made to remove the 2537-A line. A lamp output of 1.7 X lo1* quanta sec-' was determined using XH3 at 100 torr as a~tinometer.~This value compares well with a value of 2.0 X 10l6 quanta sec-' obtained with an ethanolwater actinometer.6 To keep conversions low, irradiation times ranged from 10 to 100 see. HZ was determined by gas-solid chromatography using Ar as the carrier gas. Complete separation of HZ, 0 2 , and N2 was achieved with a 1-m column of Nolecular Sieve 5A. With a thermistor detecting unit, H2 yields as low as 5 X 10-'0 mole could be determined. Natheson Coleman and Bell Spectrograde benzene was purified by recrystallization from the melt three times. Spectrograde isooctane was purified by repeated passage through silica gel columns. Spectrograde cyclohexane mas used without further purification. Optical densities of 1 em of pure deoxygenated liquid cyclohexane and isooctane at 1850 A were 0.75 and 0.5, respectively.' Instrument grade propane and ammonia were further purified by bulb-to-bulb distillations. The propane had an extinction coefficient at 1850 and 1750 A of 0.015
+
(1) Research supported by U. S. Btomic Energy Commission COO913-7. (2) K. Shindo and S. Lipsky, J . Chem. Phys., 45, 2292 (1966). (3) J. E. Wilson and W. A. Noyes, Jr., J . Am. Chem. soc., 63, 3025 (1941). (4) In the derivation of 2 it is assumed that the reaction of H with
CaHa to produce Hz is of negligible importance compared to reaction 3. (5) W. Groth and H. Rommel, 2. Physik. Chem. (Frankfurt), 45, 96 (1965). (6) J. Barrett, M. F. Fox, and A. L. Monsell, J. Phys. Chem., 69, 2996 (1965).
NOTES
4077
and 0.025 atm-l cm-', respectively.' The extinction coefficient of benzene was determined to be 707 atm-' cm-I for the 1849-A line as emitted by our lamp. All substances were deoxygenated prior to photolysis. The temperature was 27 f 2". Photolysis of both 1 and 2 torr of benzene gave an HZ quantum yield of 2.5 f 0.5 X This value is in good agreement with a value of 3 X reported in the earlier work over the pressure range from 200 to 760 torr.s I n Table I are presented the results of all experiments with added hydrocarbons. Table I: Quantum Yields of Hz and H from Benzene-Hydrocarbon Mixtures at 1849 A
and Teller" that H atoms may arise from benzene owing to absorption into a repulsive state underlying the T-T* El, + A1, transition. However, the major benzene disappearance route appears to be predissociative. (7) For an extremely wide range of olefins, Jones and Taylor ( A d . Chem., 27, 228 (1958)) report extinction coefficients a t 1850 A of ca. 7500-10,000 l./mole cm. If we therefore assume that all of the alkane optical absorption is due to olefin, we obtain upper bounds to the olefin concentration of ca. 0.001% in cyclohexane and isooctane and of ca. 0 . 0 0 5 ~ 0in propane. With the exception of one of our measurements at 0.0025% benzene, these olefin levels are considered sufficiently low to be neglected. (8) K. Yang, J. Am. Chem. SOC.,84, 3795 (1962). (9) H. Schiff and E. Steacie, Can, J. Chem., 29, 1 (1951). (10) D. F. DeTar and R. A. J. Long, J. Am. Chem. SOC.,80, 4742 (1958). (11) G. Nordheim, H. Sponer, and (1940).
E. Teller, J. Chem. Phys., 8 , 455
Partial Alkane
CsHe
4(Hz) bO(H2)
550 torr of propane 500 torr of propane 10 torr of propane 4 torr of propane 40 torr of isooctane 100 torr of cyclohexane Liquid cyclohexane Liquid cyclohexane
0.027 0.25 2.0 5.0 2.5 1.0 0.0025 0.025
0,010 0.015 0.005 0.002 0.003 0.007 0.006 0.006
vol.
%
4(H)
0.010 0.020 0.015
0.010 0.006
An Interpretation of the Concentration Dependence of Mobilities in Fused Alkali Carbonate Mixtures
0.009
0.006 0.006
The H atom quantum yields presented in Table I for benzene-propane mixtures were calculated with the value of k3/k4 = 100 as determined by Yang.* For both cyclohexane and isooctane, a value of k3/k4 = 33 was used based on a ratio of collision yields for H atom abstraction from propane and cyclohexane ofg = 3. ~~(CGHIZ)/JCI(CQHB) The low value of ca. 0.01-0.02 for +(H) indicates clearly that processes other than C-H bond rupture are mainly responsible for benzene disappearance at 1849 A. This is further confirmed by the fact that no biphenyl has been detected in the vapor-phase photolysis,2,3whereas it has been demonstrated, at least in solution, that reaction of phenyl radicals with benzene produces biphenyl and dihydrobiphenyl as major products. l o Owing to some H2 production from polymer buildup on the photolysis window (and this mas kept minimal by virtue of the low conversions), we feel that little significance can be placed upon the variation between individual yields in Table I. Within our uncertainties, therefore, there does not appear to be evidence for any important effect of total gas pressure on +(H). This contrasts markedly with a very high sensitivity to ) ~ tends foreign gas pressure exhibited by +( - C ~ H O and to support an early suggestion of Nordheim, Sponer,
by R. Mills and P. L. Spedding Diffusion Research Unit, Research School of Physical Sciences, Australian National University, Canberra, Australia (Received May 31, 1966)
I n a recently reported study from this laboratory' which was concerned primarily with the temperature dependence of tracer diffusion in alkali metal carbonates and their eutectic mixtures, we observed that diffusion coefficients at the eutectic compositions were considerably higher than in the component pure salts. With a view to exploring this behavior further, we have now made a more detailed study of the concentration and temperature dependence of the tracer-diff usion coefficients of Na+ and GO3*- ions in Li~C03-NazCOs mixtures. The experimental techniques used in this study have been described fully elsewhere.lv2 The data are tabulated in abbreviated form in Table I and for completeness we have included also the preliminary data obtained previously. 1;2 With these data we have calculated sets of isothermal diffusion coefficients and graphed them against composition as shown in Figure 1. Two not.able features of the graph which invite interpretation are the marked
~~
(1) P. L. Spedding and R. Mills, J . Electrochem. Soc., 113, 599 (1966).
(2) P. L. Spedding and R. Mills, ibid., 112, 594 (1965).
Volume 70, Number Id
December 1966
