Secondary Effec,tsof SFs in Hydrocarbon Radiolysis oxygen decreases by approximately one unit in our system. That this is reasonable is seen from results in comparable examples?' where the deuteron QCC changes considerably with the presence of ionic species, e.g., in liquid N D 3 QCC is 282 kHz,21 while the QCC of ND4+ is of the order of 180 kHz.22 If the above interpretation of Tz is correct, a measurement of the QCC of adsorbed species would be of considerable interest in acid catalysis. Acknowledgments. The authors wish to thank Professor J. Jonas for the use of the pulse spectrometer and his cooperation in the use of various facilities. M. I. C. wishes to acknowledge the continuous support of Glaverbel (Belgium) and the suggestion of Professor Plumat to undertake the study of the interaction of methanol and silica surfaces. References and Notes (1) This work was supported in part by the National Science Foundation under Grant GP 28268X. (2) M. I. Cruz, W. E. E. Stone, and J. J. Fripiat, J. Phys. Chem., 76, 3078 (1972). (3) A. Abragam, "Principles of Nuclear Magnetism," Clarendon Press, Oxford, 1961.
2053 (4) H. A. Resing, Advan. Mol. Relaxation Processes. 1, 109 (1967). (5) D.E. O'Reillyand E. M. Peterson, J. Chem. Phys., 55, 2155 (1971) (6) E. Goldammer and H. G. Hertz, J. Phys. Chem., 74, 3734 (1970). (7) Y. Lee and J. Jonas, J. Chem. Phys., 57,4233 (1972). (8) H. Pfeifer, "NMR Basic Principles and Progress," Vol. I, SpringerVerlag, New York, N. Y., 1972, p 55. (9) D. E. Woessner and 8. S. Snowden, Jr., J. Coiioid interface Sci.. 34. 290 (1970). . ,_.. - -, (IO) M. J. Tait, S Ablett, and F W, Wood, J. Collojd lnterface Sa., 41, 594 (1972). (11) . . M. 1: Cruz, K. Verdinne. J. Andre, and J. J. Frioiat, An. Quim., 69, 895 (1973). (12) M. I. Cruz. L. Van Cangh, and J. J. Fripiat. Bull. Ci. Sci. Acad. Roy. Belg., 58, 439 (1972). (13) J. Jonas, T. E. Bull, and C. A. Eckert, Rev. Sci. Instrum., 41, 1240 119701. -, (14) M. A. Vannice, M. Boudart, and J. J. Fripiat, J. Calal., 17, 359 (1970). (15) J. J. Fripiat and H. Van Damme, submitted for publication. (16) D. E. Woessner and 8. S. Snowden. Jr., J. Chem. Phys., 50, 1516 (1968). (17) B. M. Fung and Y.Wei, J. Amer. Chem. Soc., 92, 1497 (1970). (18) S. 2. Merchant and B. M. Fung, J. Chem. Phys., 50, 2265 (1969). (19) P. L. Olympia, I . Y. Wei, and B. M. Fung, J. Chem. Phys., 51, 1610 (1969). (20) Z . Luz, D. Gill, and S. Meiboom, J. Chem. Phys., 30, 1540 (1959). (21) P. Thaddeus, L. C. Krisher, and P. Cahill, J. Chem. Phys., 41, 1542 (1964). (22) R. R. Knispel, H. E. Petch, and M. M. Pintar, J. Chem. Phys., 56, 676 (1972). \
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Secondary Effects Due to the Presence of Sulfur Hexafluoride in Hydrocarbon RadioIy siI; J. Niedzielski" and J. Gawlowski Laboratory of Radiochemistry and Radiation Chemistry, Warsaw University. 02-089 Warsaw, Poland (Received June 25, 7973)
SF6, used as an electron scavenger in the radiolysis of gaseous ethylene and propane, is found to decompose with a yield close to the yield of electrons. The products of SF6 decomposition do not compete with parent molecules for electrons. The radiolysis of gaseous ethylene-cis-2-butene-1,3-butadiene-SFs mixtures reveals that secondary products, derived from SF6, react with butadiene and act as chain carriers of cis-trans isomerization. Both processes involve the same radical species. A free-radical mechanism is established on the basis of photoelectron and vacuum-uv photolysis experiments.
Introduction We have recently reported some preliminary results on the secondary reactions, due to the presence of SF6, used as an electron scavenger in the radiolysis of gaseous ethylene.l These reactions led to the decrease in the yields of higher unsaturated hydrocarbons, in particular of 1,3-butadiene. A similar observation has been reported in the radiolysis of liquid ethane.2 It was suggested in both papers that the products, resulting from the decomposition of SF6- ions undergoing neutralization, are responsible for the effect. On the other hand, there are many data in the literature on the radiation-induced cis-trans isomerization of olefins in the gas3-5 and liquid phase^.^^^ Negative SFeions and SF5 radicals have been suggested as the chain carriers. We present here evidence that both the decrease in the yields of olefins and cis-trans isomerization are due
to processes involving the same intermediary radical species derived from SFe, most probably SFs radicals. Experimental Section Materials. All hydrocarbons, ethylene, propane, cis-2butene, and 1,3-butadiene, were Fluka puriss. grade. They were rigorously purified by gas chromatography before use. Matheson sulfur hexafluoride was used after trap-totrap vacuum distillations. Vacuum-Ultraviolet Photolyses. Resonance lamps, operated with a high-frequency discharge from a 22.7-MHz high-frequency power supply, were used. The lamp filled with 10% H2 in argon at a total pressure of 1 Torr emits Lyman emission (1216 A, 10.2 eV).8 The lamp was fitted with a lithium fluoride window and a ballast volume of about 250 ml to freeze out the impurities during operation of the lamp.g Lamp intensities, measured by NO actiThe Journal of Physical Chemistry, Vol. 77, No. 24, 1973
J. Niedzielski and J. Gawiowski
2054
nometry, using a 200-ml cell equipped with a pair of rectangular 4 X 10 cm nickel electrodes positioned 3 cm apart and used in conjunction with a Unipan nanoammeter, were in the range 5 x 1011-5 x 1012 quanta sec-1. The Xe lamp was fitted with a sapphire window which eliminates the 1295-w resonance line and transmits only the 1470-A line. The lamp was provided with a titanium getter assembly similar to those described in the literature.1° Ethylene was used as a chemical actinometer, based on the quantum yield of acetylene formation 0.9.11 Lamp intensities were of about 1012 quanta sec-1. The vacuum system and 200-mi photolysis cell were mercury free. All experiments were performed a t ambient temperature. Photoelectrons. A photocell constructed from a Pyrex tube, fitted with a titanium wire anode of 1-mm diameter in the center and an aluminum photocathode made from 3 X 10-cm foil, positioned 2 cm apart, was used. Photoelectrons were generated a t the cathode by the action of ultraviolet light from a medium-pressure mercury lamp. A 0.5 cm thick filter of 2 M aqueous solution of CH3COONa was used to eliminate light below ca. 2400 A; 10 V voltage was applied to the cell electrodes during experiments. Radiol3Isis. Dosimetry, irradiation, and gas chromatographic procedures have been previously described.12 All experiments were conducted a t room temperature to about 0.1% conversion.
I
The Journal of Physical Chemistry. Vol. 74, No. 54, 1973
%VO.
+
Figure 1. Radioiysis of 3.2% cis-2-butene 0.5% sF6 in elhylene in the presence of 1,3-butadiene; total pressure 200 Torr, dose absorbed 0.1 1 Mrad. Dependence of G(trans-2-butene) on the concentration of butadiene.
5
Results
Radiolysis The yields of radiation-induced cis-trans ~ H0.5% ~ SF6 in ethylene isomerization of 3.3% C Z ' S - ~ - C f are shown in Table I. along with the yields obtained in the presence of different scavengers. The dependence of the yield of isomerization on the presence of 1,3-butadiene, added to ethylene-cis-2-butene-SF6 mixtures, is presented in Figure 1. The dependence of SF6 concentration upon dose is shown in Figure 2. From the slope of the straight line. seen in this figure, -G(SF6) = 3.65 f 0.2 may be inferred. The effects of 0 2 and KH3 on the decomposition of SF6 in irradiated ethylene and propane are shown in Table 11. Neither of the scavengers used has any appreciable effect. Photoelectrons. The yield of cis-trans isomerization of pure cis-%butene subjected to the action of photoelectrons at 2.4 Torr was found to be 1.5 molecules per electron. The tofal charge collected was 3.8 X 10-5 C. A nearly twofold increase was observed upon addition of 8% SF6 to cis-2-C& a t the same total pressure, the yield being then equal t o 2.75 molecules per electron. The total charge c01C lected in this experiment was 2.3 X Vacuum-Uu Photolysis. ( a ) 10.2-eV Photons. The yield of cis-trans isomerization of pure cis-%C4H8 photolyzed a t a pressure of 0.9 Torr was M/N+ < 3 molecules per ion pair (the limit of detection under the experimental conditions). Photolysis times were about 40 min in duration, equivalent to about 0.05% conversion. In the presence of 8% SF6 in cis-!i?-Cd& (other experimental conditions being unchanged) the yield of isomerization was greatly enhanced. M / N + = 540 molecules per ion pair. ( b ) 8.4-eV Photons. In a mixture of 0.5% cis-%Cd& in SF6 irradiated a t a pressure of 20 Torr for 20 min about 20% of the cis-2-butene underwent isomerization. When argon was used instead of SF6 the yield of isomerization
-
0.3
0.2 [1,3-C4Hs;,
0.1
Dose, Mrad
I5
15
-
Figure 2. Radiolysis of ethylene at 300 Torr in the presence SFs. Dose dependence of SF6 concentration.
of
TABLE I: y Radiolysis of Ethylene -k 3.3% cis-2-Butene -k 0 . 5 % SF6. Dependence af G (isom) on the Presence of Additivesa Additive concn. mol %
NH3
, , ,
NO 1,3-ChHs
.. .. ..
G(isom)
6700
02
~
,
.
,..
. ..
...
, , ,
... 2.2
, , ,
,
. .
.,.
... 0.1
0.1
0.7 , . . 0.1
30
