THE RADI~LYSISOF ETHYLESE : DETAILS OF rrm F O m w r I o N OF DECOXPOSITIOS PRODUCTS1 BY MYRON C. SAUER, JR.,AND LEONM. DO~WMAN ChBrnislrq Daoision, Argonne National Laboratory, Argonne, Illinois Received September 86, 1061
The radiolysis of ethylene has been studied with emphasis on the decomposition products. The G-valucs of hydrogen and acetylene are independent of ethylene pressure from 150 to 1000 mm., being 1.2 and 2.4, respectively, but increase significantly below this pressure. Isotopic and scavenger data for the system C2H4-C2D4are presented. The bearing of the experimental data on the relative importance of ionic and excited states in the formation of hydrogen and acetylene is discussed.
Introduction
The isotopic purity of the CzD4 was determined mass spectrometrically to be one atom of H per 100 atoms of D. Matheson Go. CzH2 was purified by dogassing a t -196" arid distilling from -120 to -196". Mathcson Co. NO wav degassed a t -196" and distil$d from a trap a t liquid 0 2 temperature to a trap a t - 196 Irradiations were carried out \\-ith both a CoRo-ysourceo and a beam of 1 Mev. electrons from a Van de Graaff accclerator. The vessels used for Co60irradiations were Pyrcx cylinders, about 14 cm. long and 37 mm. in dianictcr, with 21 breakseal for removal of the sample after irradiation. Each vessel was used only once, and the samples were irradiated to about 5% consumption of the ct,hylene. Several similar vessels were used for the Van de Graaff irradiations. They were about 17 cm. long and 35 to 40 mm. in diameter. The beam entered through a 2 mil thick, 30 mm. diameter stainless steel window which mas either welded or silver soldered t o Kovar metal. The Kovar was scaled to 7052 Pyrex which was used for the body of the vessel. The cell mas reproducibly poshioned for each run. The current striking the window was between 30 and 60 fiamp. and a jet of air WRS directed on the window to prcvent heating. Samples were irradiated 3 to 4 min., giving about 5% consumption of the ethylene. It was found expcrimentally that, an accurate relative measure of thc beam current passing through the window could be obtained by electronically integrating, over the length of the irradiation, thc current from a lead attached to the edge of the window. The polymerization of acetylene was used as a chemical dosimeter, the value G(-C2€12) = 72 being ~ s e d . ~The J~ absorbed intensity was determined for the ?-irradiations to be 1.27 X lozoe.v. hr.-lg.-lc,H, over the measurcd range of 80 to 500 mm. A few r-irradiations w x e carried out at an intensity of 2.8 X 1019 e.v. hr.-lg.-'c,H,. The absorbed intensity in the Van de Graaff runs was approximately 2700 times the latter figure, and was shown by 15 experiments, where the acetylene pressure varied from 50 to 450 mm., to be independent of acetylene pressure. To calculate thc absorbed intcnsity for ethylene samples, t,ho experimental figure for C2H2was multiplied by the electron dcnsity ratio of ethylene to acetylene. Because of uncertainty involvcd in extrapolating G(-C2Hz) for the acetylene dosimeter to the high radiation intensity of tho Van de Graaff irradiations, the product yields obtained in Van de Graaff radiolyscs are reported in a relat,ive niannor to be described in the discussion. The niaterials were purified as follows. Mathcson C.P. The analytical method8 uscd were as follows. The gasos ethylene was degassed a t -196" and distillcd from a trap a t non-condensable a t - 196' were removed and the PV prod-150" to a trap a t -19G0, a middle fraction of about 2/3 uct, measured in a Jlc1,cod gage. RIass spectral analysis being collccted. Gas chromat,ographic analysis showed no gavo t h o pcrcontagcs of hytfrogon and methane, and in the impurities which would interfere with the product8 tu be case of isotopic expcrinicritu, the rchtive amouote of H,, analyzcd. The CED4was obtaincd from Merck and Co. and €ID and D2. Acotylonc was analyzed by gas chromatogwas urificd bv degassing a t -1196" and by a bulb-to-bulb raphy using a 25-foot, 0.19-in. i.d. column of 2570 b3' weight distiiation at "- 150'. liexadecnne on 30-60 nwsh Chromosorb Regular (from Wilkenv 1nstrumc.nt and Itcscarch, Inc.). A t room tem(1) Based on work performed under t h e auspices of the U. 8. perature and a flow rate of :&bout20 ml./m Atomic Energy Commission. Prescnted in part a t the 137th National to cleanly scparat.c, in ordcr of retention Mcetinp of t h e American Chemical Society. Cloveland, Ohio, March, and C2HRwhon an :tliquot, of tbc irradi 1980. snmllrr 1,h:tn ahoiit, 100 w.-r:m. was ustd. (2) \Ir. Bl:irid and \V. Koch,Bull. soc. chiin. Helues, 34, 119 (1925). distribution of t,hct :tcetylones w ; t x to 111 (3) S. C . Lind, D. C. Rardwell a n d J. 11. Perry, , I . Am. Chern. Soc., aretylcnc "pt~ilr" was c:ollectrtl its it, ornc:rgeti from t,ho vlrro48, 1556 (1926). matograpti, and malyzcd m:ws spc,c:t,rornc:t.rically. Thc: (4) 1'. W. Lampc, Ztadiation Research, 10, 691 (1969). pressure of ethylene \vas n~casured initially in s known ( 5 ) K. Yang and P. J. Manno, J . Phys. Chem., BS, 752 (1959).
The radiolysis of et,hylene has been studied by Mund and Koch,2 Liiid, e2 u Z . , ~ and more recently by Lampe4 and Yang and Man110,~among others. The mercury-photosensitized decomposition has been the subject of extensive investigations,B and isotopic met,hods have been used7 to establish the primary process in that react'ion. A recent study,*in our Laboratory, of the vacuum-ultraviolet decomposition, has established the primary processes in the direct photolysis, and has provided resu1t.s which show an interesting comparison with the radiolysis. The principal decomposition products in the radiolysis of ethylene are hydrogen and acetylene. The acetylene accounts for only about 10% of the ethylene which is used up, the remainder undergoing condensation reactions to form products of higher molecular weight. In attempting to understand the primary processes in this radiolysis, we have examined the details of the formation of the decomposition products. Isotopic experiments have been carried out to provide information about the molecular detachment processes, the occurrence of which already has been indicated by scavenger experiment^.^ The pressure-dependence of the yields has been examined over a broad range, particularly in the low pressure region over which collisional de-excitation effects may manifest themselves. The pressure-dependence is also of importance in connection with any possible competition between unimolecular dissociation of ions and ion-molecule reactions. And finally, it was desirable to resolve some existing discrepancies between reported yields4,5 of decomposition products. Experimental
(6) A. B. Calloar and R. J. Cvetanovif, J . Chem. Phys., '24, 873 (1958).
(7) 11.. .I. CvrtnxiriviC :m.R:riicr, S r . , :mil I,.&I. I h r f n t : u l , ihiil., 36, ,197 ( l ! l ( i l ) .
.
(9) R. A. Blomgren. "Proc. Sixth Hot Laboratorics and Equiprncnt Confcronce," p. 229, March, liiplcn, .I. Am. Chcm. S o r . , '77, 4723 (10) 1,. M. Dorfii:nn and 1,
(1955).
323
ItEsur,w Irradiated system
C2D4 C2114, CgD,b
1 MeV. e1Mev.e-
Total
TABLE I ISOTOPIC EXPERIMEN~
104 200to400 (av. of 4 runs)
-Composit,ion of products, %----------D, CzHn
I
HD
Ha
pressure (mm.)
0 2.6 4 7 . 4 f 0 . 6 5.5*1.0
97 47.1h0.7
..
..
CzHD
..
..
CnDn
.. ..
0 1.4 99 0 4.3 96 260 ... ... ... 32 f 1 45 f 1 2 3 i 1 (RV. of 2 aliquots) C2B4, GD,, 10% NOc Coco y ' ~ 180 ... ... ... 48 4 49 c2114, CzDlh COO0 7's 20 to 2!5 (nv,of2runs) 4 9 . 9 f 0 . 1 5 . 2 k 0 . 3 4 4 . 9 k 0 . 1 .. .. .. Results correcLed for 5 For mixtures containing C2H4arid C2D,, the result,s have been corrected to a 50-50 mixture. HD which results when the C1D4 is irradiated. c Results corrected for C2HD which results when the cZ134 is irradiated. C2D4,
CSII,,
10% NO
Sor1rce of radiation
OF
c?Il.,c
Co60
y's
COO0 yls
volume a t a known temperature. After radiolysis and removal of hydrogen arid methancl, the residud pressure was memured in the same volume. From this and additional information on the G-values of the volatile products, G(-CPH4)was calculated.
Results and Discussion The results of isotopic experiments, in which equimolar mixtiires of ethylene and ethylene-d4 were irradiated with both Co60 y-rays and 1 h k v . electrons, are shown in Table I. It may be seen, after correction for the I-ID originating from the isotopic impurity in the CpDa, that only 401, of the total hydrogen consists of IID. Thus it is clear that almost all of the hydrogen product is formed by molecular detachment. A significant atomic contribution to the hydrogen would not be expected, on the basis of self-scavenging of the hydrogen atoms by ethylene. Molecular detachment has been observed also in the mercuryphotosensitized decomposition' and in the direct photolysis* of ethylene. The situation with respect to the isotopic composition of the acetylene product is not as simple. The acetylene shows extensive isotopic mixiiig, consisting of 43% C2HL). However, if 10% nitric oxide is added prior to irradiation, the isotopic mixing of the acetylene is eliminated almost completely, the C2HD amounting to only 4%. The presence of nitric oxide does not lower the G-value ylene formation. Only an approximate 8 been obtained in value of G(acety1ene) ~ 2 . has the presence of KO, but it is clear that the acetylene yield is not decreased by added KO. It appears from this, along with the following observations, that the isotopic mixing of the acetylenes in the abscnce of nitric oxide is due largely to an cfiicient secondary exchange between the acetylene molecriles which also milst involve the ethylene. The acetylene is formed initially isotopically unmiued, arid the euchange apparently is inhibited hy nitric oxide. SOsuch isotopic mixing of the acetylene is ol)scm,cd i r i cithei. the mercury photosensitized tlcconipo~ition~ or the direct photolysis8 of ethylene. 'l':thle IT s h o w the results of four r~xpcriments perforincd to iiivc~stigatc the nature of the reactions Icading t o C2IIII formation. 'l'hcse runs all were carried out i n the all-glass cells using the y-source because the cells used for the electron irradiations exhibited exchange of Ihe acetylcne even in the :Lbseure of rntliat ioii, presumably because of catalysis 1)y t hc ~ne1:~I surfwcs. Experiment 1 shows
the amount of C2H2 formed when pure CJ14 is irradiated. Experiments 3 arid 3 contain C Z I ) ~ initially added. Examination of the data shows that C2D2is consumed, that there is a deficiency of CzHzformed and an excess of CzHD compared with expt. 1. These results indicsk that exchange must involve the C2Dz and the C2H2, the latter being continuously produced during the radiolysis. However, some CzHD also must be formed from an exchange between C2Ih and some species other than C2H2. This may be seen by comparison, as in expt. 2, of the amount of CzHD formed with the deficiency in C21-12. This conclusion also is indicated by the fact that, in the absence of NO, there is an isotope effect in acetylene formation (see Table I). An over-all isotope effect could not, of course, result from exchange among the acetylene molecules themselves. In expt. 4, both C2Dz and C2H2 mere added iiiitially in amounts considerably in excess of the acetylene formed in such a run. Under this condition, the cxchsnge involving C2II2 and CzDz is far more important than exchange between acetylene and some other species. Although the material balance in these runs is far from perfect, the conclusions drawn seem fully warranted. ~ORMAT'ION O F
TABLE I1 C2HI> IN THE RADIOLYSIS O F Cz&, C?H, MIXTURES5
\ Nrnolcs wr4
{
I
crrlloles CzD2
1
1880 0
;2:: EP
pmoles Final prnoles pniolcs ill1 samples same intensity.
C2D? CJID ClHZ irrttdizttcd
0
0 0
2
3
CzD2, 4
1880 1880 1870 20.2 103 99 0.31 1.6 1.5 0 0 104
84 48 21.4 110 4 4 16 64 9 3 ttir same length of time at the 0
10 1 13 0
The lack of isotopic: mixing i n the act~tylcnc~s rules out the formation of ac.ctylciic by cornhitiatinn of CIIZ r:dicals. 't'hc possibility that CZII,i mdicals arc precursors of the arrtylcne is ruled out on the basis of iriforniution on highc,r niolcculur weight products" v-hich indivatcs that CJTa is not an important radical. It appears then, that acatylenc is formed by molcculnr detachmcnt from (11) M. C Sniwr, .Jr
, 1inp111)lishrdrrsults.
RIsnos C. SAUEIZ, Jn., AND LEONPI. DORFMAN
3 24 0
I
1
I
I
I
A
'I
c 0.2
1
-
Q
CH4
-*>-v---a
I
I
1
I
I-,
*
I
I 'I
I
I
-
3.0 -e
C2H2
:\we; UJ
2
;
~
-
-
2.0
CH4 and for ethylene disappearance are shown in Fig. 1-3. The yields for Hz and CzHzclearly are independent of pressure in the region 150 to 1000 mm. As the pressure is decreased below 150 mm. these yields show a gradual increase amounting to over 40% a t the lowest pressures studied. The increase in the yield of Hz and CZHZ probably is due to a contribution from additional processes a t lower pressures. Two such processes immediately suggest themselves. Callear and Cvetanovi@ have shown, in the mercury photosensitized decomposition of ethylene, that below 150 mm. dissociation of a long-livcd excited state of ethylene occurs to give €I2 and CzH2. At higher pressurcs this dissociation is eliminated by collisional deactivation. Our data are qualitatively in accord with the postulate that decomposition of such a long-lived cxcited state is occurring in the radiolysis, and that the hydrogen and acetylene produced simply add to the amounts of these products formed in the pressure-independent processcs. Alternatively, or in addition, the incrcase in the product yields a t the lower prcssurcs could rcsult from dissociation of an ethylene molecule-ion, which a t the highcr pressures undcrgoes rcaction with ethylene rather than dissociating. That is, a t the lower pressures, dissociation occurs
1
I
Vol. 66
0
CZHZ+ U2
+
(1)
+ €1
(2)
and GHa+ = CzHa+ 0.4
7
0.3
0.2 0.I 0
followed by
t-I
C&+
CH4
I
0
20
IO
- v-
"
W
30
50'74 80
40
90
+
100
C*H&+ C2H4 = C3€16+
PCeH4 (cm).
Fig. 2.-Van de Graaff irradiations, G-values calculated relative to G(H2) = 1.2 as stated in text. l
l
I
I
l
l
20
30
40
50
60
70
I
40
-
t
-
w:
30
0
20
I
d
10 IO
80
90
+ C2Ila = CzHs+ + C2Hz
(3)
i o yield acetylene, as Lampc has ~uggestcd.~ Whereas, as the pressure is increased, thc ionmolecule rcaction
IO0
PC2H4 ( C m '
Fig. 3.-Ethylcne con;umption: 0, Cow irradiation, 2.8 X 1OlD C.V. hr.-I 0 , Cow irradiation, 12.7 X l O l 9 e.v. hr.-1 g&; 8, Van de Graaff irradiation, 4X loz2 e.v. hr.-1 g.c&; G-values calculated relative t o G(H2) = 1.2 as stated in text; W, same as @ but -78'; A, same as 8 but 10% NO present. Note: these symbols apply to all three figures.
an excited statc of ethylcne and/or by thc ioiimolecule rcaction of C*HS+with C& The dcpcndence of the product yields upon prcssurc has txcn cxamined ovcr thc prcssnrc range 13 to 1000 mm. The yields for Hz, CZH,,
+ CHJ
(4)
becomes more probable. Stcrensonl3 has discussed this competition on the basis of the statistical theory of ion dissociation, and has concluded that, in the case of ethylene at one atmosphcre prcssure, reaction 4 is strongly favored. The qucstionable quantitative validity of the quasi-equilibrium theory, . judging from recent publication^'^^^^ on the subject, would argue against placing complctc reliance on such a calculation, at least where absolute rate coilstants arc involvcd. Our observation of pressure independent yields from 150 to 1000 mm. is, however, rathcr strong evidence that a compctition bctwcen reactions 1 and 4 is not occurring ovcr that range. Thc over-all isotope effects obscrvcd are compatible with the first of the two interpretations discussed above. Table I shows that in thc rcgion where G-valucs are independent of pressure there is no ovcr-all isotope effect in hydrogen formation, the ratio bcing EIZ/D2 = 1.00 f 0.02. There is furthermore no isotope cffect in acetylene formation in the presence of nitric oxide, the ratio being C2€Iz/C2D2= 0.98 i: 0.05. In the mercury 112) D. P . Stevenson, Radiutaon Reseurch. 10, 610 (1959)
(13) E hl. Erring and A. L. Wahrhaftig, J . Chem. Phgs., 3 4 , 23 (1961). (11) (1901).
E. Stoiner, C. I". Cicsc and AI. G. Ingliraln, abtd., 34,
1%)
Feb., 1962
RADIOLYSIS OF ETHYLENE
photosensitized decomposition at 20 to 25 mm., an isotope effect of about 1.5 is o b s e r ~ e d . ~At 20-25 mm. pressure (see Table I), the radiolysis of an equimolar mixture of C2H4-C2D4 shows an isotope effect of H2/D2 = 1.11 & 0.02. This is, within iexperimental error, the predicted value obtained. by applying the isotope effect of Callear and CvetanoviB to the additional hydrogen formed at the lower pressure and combining this with the isotope effect of unity in the pressure-independent region to obtain the over-all isotope effect at the lower pressures. The following data, which appear in Fig. 2, should be mentioned briefly. The G-value of methane formation is not affected by the presence of 10% IS0 or by lowering the irradiation temperature to -78'. These facts suggest that methane is not formed via a thermal methyl radical precursor. The results of our studies of the photolysis of ethylene at 1470 A,, taken together with the coiiclusions reached by Lassettrelb in his investiga tions of excitation processes by electron impact spectroscopy, lead to the conclusion that there is some contribution to molecular hydrogen and acetylene by dissociation of an excited state of ethylene, formed by electron impact. Lassettre has shovvn that, under certain conditions of electron energy and scattering angle, the selection rules for molecular excitation by electron impact are the same as the selection rules for absorption of light quanta. Since molecular detachment of hydrogen and acetylene occurs in the photolysis,8 a contribution by molecular detachment from excited states is to be expected in the radiolysis. The quantitative extent of such a contribution cannot reliably be estimated on the basis of existing information. It is a consequence of this conclusion that a precise correlation between molecular detachment yields in ethylene and the mass spectrumla is fortuitous to the extent that the contribution from neutral excited states has been omitted, as stated, and must be included. A similar conclusion has been reached from isotopic experiments. The G-values for hydrogen and acetylene formation have been determined for the CosOirradiations (15) E. N. Lassettre, Radiation Research, Supplement 1, 530 (1959). (16) L. M. Dorfman and M. C. Sauer, J . C'hem. Phgs., 82, 1886
(196Q). (17) P. Aiisloos and R. Gorden, Jr., ibid., in press.
325
in the pressure region 150-1000 mm. and are 1.2 and 2.4, respectively. These values are based on chemical dosimetry using the value G(-CPHB) = 72 in the acetylene polymeri~ation.~J~ The hydrogen yield is in agreement with the value of Lampe4 and somewhat lower than reported by Yang and M a n n ~ who , ~ used a liquid dosimeter. The observed C2H2/Hzratio, however, is in very close agreement with the data of Yang and Manno15 and the indication, therefore, is that the acetylene G-value of Lampe is low. We do not feel justified in reporting G-values for the electron irradiations at very high intensity. The use of the acetylene dosimeter at these intensities would involve a very long extrapolation (as a function of intensity) which might be in error. All G-values for Van de Graaff irradiations therefore have been calculated relative to G(H2) = 1.2 at P c a ~=, 40 cm.
Conclusions The conclusions we have reached may be stated as follows: 1. Hydrogen is formed by molecular detachment. The data do not allow us to specify the relative importance of ionic and excited states in the formation of hydrogen. 2. Acetylene is formed by molecular detachment from an excited ethylene molecule and/or an ion-molecule reaction involving C2H3+. 3. The yields in the pressure independent region above 150 mm. are G(H2) = 1.2, G(C2H2) = 2.4 molecules/100 e.v. 4. These yields increase with decreasing pressure below 150 mm., but the role of ion dissociation or of excited state dissociation in this effect cannot be resolved by the present data. 5 . The pressureindependence of the yields above 150 mm. indicates that, in this region, a competition between unimolecular dissociation and ion-molecule reactions of the parent ion is highly unlikely. 6. The results of photolysis at 1470 8.indicate that some contribution to Hz and CaHz formation by dissociation of neutral excited states must occur in the radiolysis, a conclusion which also has been reached on the basis of isotopic studies.17 7. The precise correlation between Hz yield and ionic processes alone, based on the mass spectral pattern, must be regarded as fortuitous, to the extent that a contribution from neutral excited states must be added to the calculated values.
