Rare Gas Sensitized Radiolysis of Acetylene1

892

J. H. FUTRELL ASD L. 1'4. SIECK

present evidence, we are unable to suggest which of these plausible niechanisnis is the more likely explana-

tion for the conversion effects observed in the present work.

Rare Gas Sensitized Radiolysis of Acetylene'

Downloaded by NEW YORK UNIV on September 9, 2015 | http://pubs.acs.org Publication Date: March 1, 1965 | doi: 10.1021/j100887a032

by J. H. Futrell and L. W. Sieck Aerospace Research Laboratories, Ofice of Aerospace Research, Wright-Patterson A i r Force Base, Ohio (Receized September 5. 1964)

The gas phase radiolysis of acetylene has been investigated in the presence and absence of various sensitizers a t various dose rates. The polymerization reactions have been correlated with high-pressure, mass spectroiiietric studies of mixtures with rare gases, with the conclusion that the precursors for polymer propagation do not depend upon charge exchange (ionization of acetylene). quantitative investigation of benzene production and sensitization indicates that neon is unique aiiiorig the noble gases in that it alone enhances the formation. The initial interaction, NeS CzHz+ C2H+ H Ne, observed mass spectronietrically is responsible. Various photolysis and radiolysis experiments involving argon-deuterium-acetylene and deuterium-acetylene mixtures have defined the mechanism for the increase in G(CsH8) observed in this work and in previous studies a t lower dose rates.

+

Introduction The radiation-induced polymerization of acetylene and the sensitized radiolysis of acetylene early athracted the attention of workers in the field of radiation chemistry.2 The simplicity of the over-all reaction, which produces only benzene and an insoluble polymer, cuprene, in significant quantities, has, no doubt, been partly responsible for the sustained interest in this subject. Recent investigations3 have quantitatively established the radiolytic yields of benzene arid cuprene and have shown the111 to be invariant, with dose, dose rate, radiation t'ype, and pressure over a substantial range a t pressures above 20 torr and at dose rates above 2 x 1013e.v. c n r 3 see.-'. Dorfnian and Wahl studied t,he effectof rare gases as sensit,izers in an attelnpt, to assess the itnport,ance of ionic processes in t'he over-all reaction.3b JIaiIis, Xiki! and WiJnen4 and Fields have recently conduct,ed an ext,erisive study of acet'ylene iricluding isotopic st'udies the irifluence of added scavengers and sensitizing agents. Despite the T h e Journal of Physical Chemistry

+ +

long history of careful investigation, however, the mechanisiiis for formation of benzene and polymer cannot be considered as established. Recently Lindholm, Szabo, and Wilinenius have studied charge-exchange reactions of a great inany ions with acetylene,6 and it is nom possible to discuss the sensitized radiolysis in a more definitive manner. Accordingly, the radiolysis of acetylene arid the efficiency

(1) Presented at the 12th Annual Ueeting of the Radiation Research Society, Miami Beach, Fla., May 17-20, 1964. ( 2 ) See S. C. Lind, "Radiation Chemistry of Gases," Reinhold Publishing Corp., New I-ork, N. I-.,1961, Chapter 9. for summary of work and f o r references.

(3) (a) L. 11. Dorfman and F.J. Shipko, J . Am. Chem. Soc., 7 7 , 4723 (1955); (b) L. 51. Dorfinan and A. C. Wahl, Radiation Res., 10, 680 (1959). (4) G . J. ;\lains. H . Niki, and 11.H. J. Wijnen, J P h y s . C'hem., 67, 11 (1963). (5) F. H. Field, i b i d . , 68, 1039 (1964). (6) F:, I,indholm, 1, Szaho, p, rvilmenius, Arkiz. pus&, 2 5 , 417 (1984).

RAREGASSENSITIZED RADIOLYSIS OF ACETYLENE

of the rare gases xenon, krypton, argon, neon, and helium as sensitizers has been reinvestigated. Since it has been demonstrated that benzene can serve as an inhibitor of acetylene p~lyinerization,~ particular attention has been given to working at low conversion and to using pure compounds.


Experimental Irradiations were performed using 0.95-Mev. electrons produeed by a nominal 1-MeV. Van de Graaff accelerator using general procedures described prev i o u ~ l y . ~Pyrex ~ ~ flasks of ca. 1-1. volunie containing thin “bubble” entrance windows were used in most experiments while similar 200-inl. targets were used in the experiments in which only cuprene formation by rare gases was measured. The beam current during irradiation was monitored by a Varian recorder using an appropriate voltage divider network to give approximately half to full-scale pen deflection for the desired current. The total dose was determined by graphical integration, and G values (molecules produced or consunied/100 e.v. absorbed) were calculated as previously described.* Acetylene was obtained from the Matheson Co. and was purified by the following procedure. The vacuum manifold, including a Wallace and Tiernan differential pressure gauge, the targets to be filled, an -1-1. reservoir volume with trap, and the gas cylinder line were evacuated to a pressure less than 0.01 p , isolated from the pump system, and filled to a slightly greater pressure of acetylene than was required for the experiment. This sample was condensed into the reservoir-trap system and frozen out with liquid nitrogen, after which the system was re-evacuated. This sample was then re-expanded into the systein, refrozen, and the cycle was repeated until degassing was complete. The acetylene was permitted to expand into the system to the desired pressure (35 tnni. in most experiments) a t which time the reservoir containing the excess acetylene and heavier contaminants was quickly isolated froin the system. Careful inass spectrometric analysis demonstrated that this procedure effectively renioved carbon monoxide, acetone, and benzene initially present as trace impurities. Matheson research grade rare gases were analyzed mass spectrometrically and found to contain no detectable impurities except for small quantities of other rare gases. llatheson deuterium contained ca. 0.4% H D as the only impurity, and no containinants were detected in llatheson hydrogen. Accordingly, these gases were used without purification. Product yields were determined by mass spectroinetry. Low coriversion studies were possible through

893

use of an electron multiplier detector on a modified Consolidated Model 21 -103C described earlier. l 1 The multiplier gain characteristics relative to an electrometer detector were determined experimentally over the mass range of interest. The relative signal intensity of the benzene parent ion to that of the C 2 + ion from acetylene was determined empirically for particular settings of the multiplier and Wien bridge parameters. Adherence to a strict time schedule for analysis after admitting the sample and careful attention to the previous history of the source to minimize adsorption-desorption problems were necessary for acceptable precision of analysis a t low conversion levels. No attempt was made to measure quantitatively other radiolysis products after it was determined that they are formed only in very small amounts. Gas chromatography indicated the probable forination of vinylacetylene and diacetylene and possibly butadiene, but only in amounts of less than 5% of the benzene yield. These results are in general accord with earlier studies, and it is clear that benzene and cuprene are the only products forined in significant quantities. In the mercury-sensitized experiinents a quartz cell of -200-ml. volume enclosed in an insulated aluminum block wrapped with heating wires was used as the photolysis vessel. It was charged with 25 inm. of acetylene, approximately 3 nil. of liquid mercury, and various amounts of deuterium. It was then heated to 260 f 1” (at which the vapor pressure of mercury is approxiniately 96 min.) and exposed to radiation froin a, Hanovia Type 30600 mercury-resonance lamp. The intensity was adjusted by changing the lamp-to-cell distance to approximate the rate of benzene forination observed in the medium dose rate radiolysis study. Alonodeuteriobenzene, H D , and CHCD were the only deuterated products formed in significant quantities, and the deuteriobenzene-benzene ratios were determined from the reported cracking pattern for deuteriobenzene12 and calibration standards for ordinary benzene. The polymer formation was determined by assuming that the partial pressure of rare gas additive was unchanged during radiolysis. The ratios of acetylene to rare gas were then measured mass spectronietrically before and after radiolysis. A correction for the con-

s. C. Lind and P. s. Rudolph, J . Chem. Phys., 26, 1768 (1957). (8) J. H. Futrell and T. 0. Tiernan, ibid., 37, 1694 (1962). (9) J. H. Futrell and T. 0. Tiernan, ibid., 38, 150 (1963). (7)

(10) J. H. Futrell and T. 0. Tiernan, ibid., 39, 2539 (1963).

(11) K. R. (12) F. D. Petroleum Serial 538,

Ryan, L. W. Sieck, and J. H. Futrell, ibid.. 41, 111 (1964). Rossini, Ed., “Catalog of Mass Spectral D a t a , ” American Institute Project 44, Carnegie Instltute of Technology, contributed by the National Bureau of Standards

Volume 69,.Viimber 3

M a r c h 1.966

J. H. FUTRELL AND L. W. SIECK

894

version of acetylene to benzene by direct radiolysis was applied, and it was assumed that the remaining decrease in partial pressure of acetylene led to cuprene formation. A detailed study of polymer formation froni pure acetylene was not made, but a crude manonietric study of the pressure decrease gave results in agreenient with earlier workers [G(-C2Hz) = 71.93a and the fraction of that amount converted to benzene is approximately 0.201. The charge-exchange reactions of rare gas ions with acetylene were studied using the technique introduced by Cerinak and Herman.I3


Results and Discussion Polymer Formation. The charge-exchange studies of Lindholni, Szabo, and Wilinenius have established that acetylene exhibits several remarkable characteristic$ such that it seeins to resemble a diatomic molecule more than a typical hydrocarbon. The molecule ion is fornied in two isolated electronic states, the first extending froni the ionization potential, 11.4 e.v., to 12.5 e.v. and the second, rather broader state extending from ca. 15.5 to 22 e.v. I n the region froin 12.5 to 15 e.v. the cross section of acetylene for ionization by charge exchange is essentially zero, and low kinetic energy ions with recombination energies in this “window” region do not produce acetylene ions. Thus, the elementary condition that the initially ionized species have the higher ionization potential is insufficient to assure that the reaction takes place for acetylene and probably for all systems with limited internal degrees of freedom. The rare -gases have appropriate recoinbination -~ energies (Xef, 12.13 and 13.44 e.v.; Kr+, 14.0 and 14.7 e.v.; Ar+, 15.7 and 15.9 e.v.; S e + , 21.6 and 21.7 e.v.; He+, 24.6 e.v.) t,o permit verification of Lindholm’s findings by the nlodified operation of a conventional mass spectrometer as described previously.’0 The results of such a study are suniinarized in Table I and are in complete accord with their conclusions. The acetylene molecule ion signal observed for inixture A establishes the level expected for self-charge exchange. Jlixtures B arid D exhibit a substantial increase in ion signal froiii charge exchange of xenon and argon ions. 1Iixture C with krypton, by contrast, shows no increase over that produced by acetylene itself. Thus, it seems clearly established that there are at least two states of the acetylene ion separated by an energy gap. The recombination energies of both the 2Ps,2and 2Pi,l states of krypton ion fall in this region. This peculiarity of acetylene made a reinvestigation of the radiolysis of acetylene sensitized by a large excess of these rare gases pertinent to assessing the rele-

~

C?H?+intensity,

Mixture

Reservoir pressures

arbitrary units

A B

12 0 p of C2H2 12 0 p of C2Hg 520 p of Xe 12 0 p of CgHZ 498 p of Kr 12 0 p of C2HZ 570 p of Ar

210 i 3 1680 f 20

C D

208 f 5 1610 & 30

vance of ionic processes in the over-all mechanism. I n the case of xenon, only the lower 2P8,2 state can produce the molecule ion of acetylene by charge exchange. Furthermore the metastable levels of xenon lie below the ionization potential. Krypton ions, as we have seen, do not charge-exchange with acetylene, and the metastable levels of krypton a t 9.9 and 10.6 e.v. are also below the ionization potential of acetylene. hrgon, on the other hand, produces acetylene ions in the upper electronic state by charge exchange from both 2 of the argon ion, while the the 2P8,,and z P i ~states argon inetastables at 11.5 and 11.7 e.v. produce lower state acetylene ions on collision.l4 The sensitized formation of cuprene may be discussed in terms of a generalized iiiechanisin without expressing the nature of the precursors as

CzHz’--+ polymer

~

The Journal of Physical Chemistry

~~~~

Table I

X’

+ CzH2 --+ C2H2”

C2H2”

-+

(4)

polynier

(5)

where X represents a rare gas or other sensitizing agent. F~~~ this d(polymer) -

QC*H? [CZHZ IG(C&’)

dt

100

+

QX

[XlG(X’) 100

where Q i = rate of energy absorption per unit pressure per unit time (e.v. sec.-l) and G(Y) = yield of Y in iaolecules/100 e.v. of absorbed energy. Representing the first and second energy absorption terms by a: and (3, respectively, the rate of polyinerization is given by R = a:G(CzHz’) (3G(X’),and the relative efficiency of energy utilization to produce polynier by the sensitizer compared to acetylene is G ( X ’ )IG(C2Hz’) = l/PG(CzHz’) [R - aG(CzHz’)I.

+

(13) V. L. Cermak and 2. Herman, iVucleonzcs, 19, 106 (1961). (14) V . L. Cermak and 2. Herman, private communication.


Typical experimental resultls are given in Table 11. They are in agreenient with the early results of Lind and Bardwell that the rare gases are efficient sensitizers of acetylene polynierization. l5 These workers assert that, in our terniiriology, G(X’)/G(C2Hz’)is unity for low concentrations of sensitizer but that the efficiency falls with increasing ratio of rare gas to acetylene. Their Table IT^ also indicates that krypton is a more efficient sensitizer than xenon or argon. Table I1


Blix1,ure

Relative polymerization efficiency G(X’)/G(CzHz’)

24 mm. of CYHZ 145 mm. of Xe

0.44

24 mm. of CzHz 199 mm. of Kr

0.51

24 mm. of CzHs 413 mm. of Ar

0.44

35 mm. of CzHz 400 mm. of He

0.35

35 mm. of CZHZ 450 mm. of Ne

0.39

From the preceding discussion we would expect the concentration of acetylene ions from the heavier rare gases to be Ar > Xe >> Kr. Since there is no correlation of rate of sensitization of polymerization with this inequality, the precursor in reaction 4 cannot be identified as the acetylene molecule ion. It also seems unlikely that this ion is of such major importance in pure acetylene that it can be identified as the precursor in reaction 1 although some contributign of acetylene ions to polymer formation as proposed by Rudolph and Melton16 cannot be ruled out. A similar conclusion that the major polymer formation mechanism is not ionic in character has been reached by Field on the basis of the observed temperature coefficient of the reaction.5 Benzene Fomiation. It was noted in these experiments that no substantial increase in the benzene yield over that expected from direct radiolysis occurred, confirming the previous results of Dorfman and Wahl. 3b However, both of the two most recent publications4*5 on acetylene radiolysis cite evidence that energy transfer from the rare gases to acetylene leading ultimately to benzene dow occur. Accordingly, we have made a quantitative kinetic study of benzene formation in the presence and absence of rare gases. We have restricted ourselves to the low conversion region since it has been shown that the presence of moderate amounts of benzene alters the kinetics of acetylene deconiposition.7

895

It was first necessary to establish the pressure effect and the intensity effect on this reaction.” The pressure effect was investigated for a constant incident current of 0.95-Mev. electrons corresponding to a dose rate of 2.4 X 10l2P e.v. cm.? set.-', for P expressed in torr, and is shown in Figure 1. The shape of this curve is qualitatively similar to the corresponding Figure 2 in Field’s paper. The area of the niaxinium is, however, somewhat narrower in our experiments, and the constant yield region is achieved a t a much lower pressure. These differences may possibly be attributed to the different geometry of our radiolysis vessel. Dorfman and Wah13b have demonstrated the importance of cell geometry and have suggested diff usiori to the walls by a benzene precursor as an explanation for the falloff in benzene yield. Although it was not observed in their

I

0

10

20

40

30

,

50

60

PIC2Hn) IN mm

Figure 1. Effect of pressure on G(C&).

I5t

^ I

t\

--

ADDED NEON

I 2.101’

4

6

8

10110”

I 2

I4

I6

RATE OF ENERGY ABSORPTION ACETYLENE, ev/cm’-sac

Figure 2. Effect of dose rate on G(C6H8) in the rare gas sensitized radiolysis.

(15) S. C. Lind and D. C. Bardwell, J . Am. Chem. Soc., 48, 1575 (1926). (16) P. 5. Rudolph and C. E. Melton, J . P h y s . Chem., 63, 916 (1959). (17) The unsuspected presence of these effects slightly modifies the results presented at the 12th Meeting of the Radiation Research Society, Miami, Fla., May 1964.

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M a r c h 1966



896

work, it may be noted that Mains, Niki, and Wijnen predict such a maximum in G(benzene) a t low pressure. The effeci, of intensity at constant pressure is given in curve 1 of Figure 2 for a constant acetylene pressure of 35 mm. Field has observed a similar intensity effect6 while Mains, et al., observe a drastic change in G(C6H6) from Co60 ?-irradiation compared with X-rays14which may be, in part, an intensity effect. The present results differ from the work cited in that a constant G (C6Hs) is observed over a wide range of radiation intensity. Dorfman and S h i p k ~also ~ ~ found a broad range of pressure and radiation intensity where G(CeH6) is invariant, and it seems that variations are found only at rather low pressure and/or low radiation intensity. Figure 2 also summarizes the results of our investigation of the sensitized radiolysis of acetylene. In these experiments the acetylene pressure was 35 mm. , as in the intensity study, and 450 mm. of the rare gases was added. Argon, neon, and helium were studied at different intensities while krypton was studied only in the dose rate-independent region. Xenon was not reinvestigated quantitatively, but preliminary experiments gave no evidence for sensitization. Since the points for argon, helium, and krypton fall on the curve for pure acetylene, it is clear that no sensitization occurs for these gases. The same conclusion was reached by Dorfman and Wahl,3b who studied all the common noble bases except neon. In contradiction of these results, Mains, Niki, and Wijnen have reported that krypton sensitizes both benzene and polymer formation. We think a possible explanation may have been the presence of trace impurities in their experiment, as this has proven a significant factor in our own study. In preliminary experiments we used acetylene which had not been purified carefully and which contained, therefore, trace quantities of acetone detectable mass spectrometrically using the electron multiplier-amplifier system. For all systems, except krypton-acetylene, results in reasonably good agreement with those plotted in Figure 2 were obtained. Krypton, however, gave indications of sensitization and very nonreproducible results. It was further observed that the acetone impurity was greatly depleted in radiolysis. Since methyl radicals are known to produce benzene from acetylene,l8 we may reasonably attribute these results to dissociative charge exchange of krypton ions with the acetone impurity to generate free-radical precursors of benzene. The krypton system is especially susceptible to this artifact because, as previously discussed, krypton ions do not undergo charge exchange with acetylene. Although he does not discuss his results in terms of The Journal of Physical Chemistry

I60

80

PRESSURE

320

240

480

400

OF ADOED NEON (MMI

Figure 3. Yield of C6Hs in the neon-sensitized radiolysis as a function of added neon in the high dose rate region.

I

1

1

2

3

4

5

6

7

~

9

1

0

1

1

1

DOSE (ARBITRARY UNITS)

Figure 4. Effect of yo conversion in G(CsH6)in the neon-sensitized radiolysis a t high dose rate.

sensitization, Field reports data which may be taken as evidence for sensitization by both krypton and argon.5 It is difficult to estimate the magnitude of this effect as he reports his data in ternis of benzene produced per unit of energy absorbed by the system (rare gas plus acetylene). Indeed, this paper asserts that argon at low partial pressures enhances the reaction and a t high pressure inhibits it. The demonstrated effects of pressure, intensity, and geometry on the reaction may possibly be responsible for the discrepancy in our results. Nevertheless, we feel that curve 1 of Figure 2 clearly establishes that argon does not have any detectable effect, either inhibition or sensitization, on the radiolytic formation of benzene by acetylene. Since neon, alone, of the rare gases serves as a sensitizer, this system was studied further. Figure 3 shows that the sensitization is linear with pressure a t low conversion while Figure 4 shows that the effect falls off at ~

~~~

(18) C. M. Drew and A. 8. Gordon, J. Chem. Phys., 31, 1417 (1959).

2

RARE GASSENSITIZED RADIOLYSIS O F ACETYLENE

higher conversion. The falloff occurs at much higher conversions than the kinetic runs presently reported. In interpreting 1,hese results, it is of interest to examine the information on dissociative charge exchange.'j As discussed previously, xenon and krypton ions produce CzH2+ by charge exchange. Helium ions produce mostly Cz+ while the major acetylene ion from neon charge exchange is CzH+, viz.


Ne+

4-CzH2--+ iYe

+ CzH+ + H

(6)

Since H atoms are known to initiate polymerization of acetylene to benzene,Ig we may reasonably attribute the sensitization by neon to reaction 6. By the same token, since helium does not sensitize benzene formation, the charge-exchange reaction must not produce hydrogen atoms. Hence, the charge-exchange reaction observed by Lindholm must be He+

+ CzHz+He + Cz++ Hz

(7)

rather than the corresponding reaction producing H at oms. Although the H atom hypothesis is attractive, a logical alternative to explain the neon results is the proton-transfer reaction

CzH+

+ C ~ H+ Z CZH3+ + CZ

(8)

which cannot be ruled out on energetic grounds. Accordingly, we have searched for evidence for this reaction mass spectrometrically and have reinvestigated some of the previously reported ion-molecule reactions of acetylene. In Table 111we report the reactions observed previously. We have established the pre-

897

produce C4H3+ions. The latter is in agreement with Field, Franklin, and LampeZ0 and Barker, Hamill, and Williams,21who identified CzHz+as the reactant ion from appearance potential measurements. We also find evidence for proton transfer from the upper state CzHz+*ion as in reaction 2 of Table 111, but the principal proton-transfer reactant ion is CH + in reaction 4 of Table 111. The reaction of greatest importance for CzH+ is eq. 3 of Table 111,which may result simply in doubling the number of H atoms to be expected in the radiolysis system from charge exchange of acetylene with Ne +, It cannot be stated with certainty that this doubling of H atom production will occur in the radiolysis system at much higher pressure. In fact, Rudolph, Lind, and RfeltonZ2have shown that the probability of decomposition of certain ion-molecule complexes decreases with increasing pressure. The nonsensitization of benzene formation by Xe+ may be taken as further evidence that reaction l a of Table I11 does not go to completion at higher pressure, i e . , that C4H4+is formed without dissociation to the C4H3+product observed in the mass spectrometer. In an attempt to explore further the H atom mechanism, hydrogen was added in a series of experiments to the argon-acetylene system. The hydrogen may be expected to complete with acetylene for argon ions according to the well-known reaction Ar+

+ Hz +ArH+ + H

In the absence of competing reactions , neutralization would give a second hydrogen atom ArH+

Table 111: Ion-Molecule Reactions in Acetylene (la) (lb) (IC) (2) (3) (4)

---

+ + + + +

CZHz+ + CzHz C4H3+ H CzHzf C2H2 G H z + HZ(or 2H) C2Hz+* C2H2 C4HZ+ HZ(or 2H) C1Hz+* CZHZ+ CZH3' CZH CzH+ CzHz -t CaH2+ H CH+ CZHZ CZH3' C

+ + + + +

+

+

cursor ion(s) by means of the technique of comparing normalized ionization efficiency curves of primary and secondary ions." KO evidence for proton transfer from C,H+ was obtained, as only the addit,ion reaction was observed. Lindholm, Szabo, and Wilmenius6 suggest that the lower state CZHZ+ion reacts with acetylene to produce only C4H3+, as in reaction l a in Table 111, while the upper state CZHZ+*reacts as eq. I C in Table 111. Our results confirm this but, indicate that the ground-state ion also reacts as in eq. l b to

(9)

+ e +Ar + H

(10)

A pronounced sensitization of benzene formation was Indeed found to occur in the argon-acetylene-hydrogen system. An equivalent series of reactions occurs when deuterium is substituted for hydrogen, and the results from the latter study are summarized in Figures 5, 6, and 7 . With added deuterium, the benzene formed is a mixture of benzene and nionodeuteriobenzene. The increase in benzene C6H6 is shown in Figure 5 for two different dose rates. Curve 1 represents the yield obtained at high dose rate (beam current 5.0 Fa., corresponding to a dose rate in pure CzHzof 8.0 X l O I 3 e.v. (19) D. J.LeRoyandE. W. R. Steacie,J. Chem. P h y s . , 12,369 (1944). (20) F. H. Field, J. L. Franklin. and F. W . Lampe, J . Am. Chem. Soc., 79, 2665 (1957).

(21) R. Barker, W. H. Hamill, and R. R. Williams, J . P h y s . Chem., 63, 825 (1959). (22) P. S. Rudolph, S. C. Lind, and C. E. Melton, J . Chem. P h y e . , 36, 1031 (1962).

V o l u m e 69, A7?Lmber 3

M a r c h 1966

898


1

I

r

0 3 t

I

mill L+


I

1

IO0

0

200

300

ADDED TO Ar/C,Hp

Figure 7. Ratio of C6H5Dproduced to A[C,Hs] in the Ar-D2-C2H2 system as a function of added Dz: m, low dose rate; 0, high dose rate.

CzHD is very nearly ten times the rate of formation of C6H5D. The exchange reaction presumably goes via 200

IO0

mm

~2

300

ADDED

Figure 5 . Yield of C6H6in the Ar-Dz-CzH2 system as a function of added Dz: vurve 1, high dose rate; curve 2, low dose rate.

+ CzHz +CzHzD CzHzD --+ CzHD + H D

(1la) (12%)

as indicated both by the extensive H-D exchange which occurs and by the fact that the Ar-CzH2-D2sensitized radiolysis produces more C6H6 than C6H5D benzene. Figures 5 and 6 clearly demonstrate a significant dose rate effect in both C& and CGH5D production initiated by D atoms. The plateau region in a curve 2 (Figure 5 ) indicates that all argon ions are inter-03 cepted when approximately 100 nini. of Dz is added, 'I " 002 and any slight increase observed in the plateau portion Ar/C2H2/D2 of the plot can be attributed to direct absorption of 01 energy by D2 as its partial pressure is increased. The observed enhancement of the resultant benzene yields 100 LOO 300 a t lower dose rates is qualitatively similar to the effect m m D2 observed in the radiolysis of pure acetylene and in the Figure 6. Yield of C6&D and CZHDin the rare gas-sensitized systems (Figure 2 ) . The further Ar-D2-CzH2 system as a function of added Dz: implications of this data will be discussed in another curve 1, high dose rate; curve 2, low dose rate. section. The ratio [Cd&D]/A[C6H6],where A[C6H6]refers to the increase in C6H6 benzene as a result of the sensitizacm. - 3 sec. -l) whereas curve 2 resulted from experiments tion reaction, is plotted for both dose rates in Figure 7. carried out a t 0.9 Ha. (1.44 x 1013e.v. cm.-a sec.-l). The intercept of curve 1 corresponds to G(C&) = This parameter is of interest as it may be a characteristic of D atom-initiated polymerization of acetylene. 5.2 for acetylene alone or acetylene plus argon while the For comparison, the mercury-sensitized photolysis of lower current experiments are extrapolated to G(CeH6) the system Hg-Dz-CzH2 was studied. Since the lamp = 6.5 as obtained from Figure 2 . The yield of benzene used emitted a small amount of 1849-A. light, a minor C6H5Dis shown in Figure 6 as a function of added DO, correction for direct photolysis was necessary. This with curves I and 2 again representing the high and was accomplished by extrapolating the benzene yield low dose rate cases, respectively. Also shown in Figure to zero deuterium and subtracting this intercept from ti as the right-hand ordinate is the amount of CZHD each point. The resulting value for the ratio [Cf&D]j produced in t,he radiolysis for the higher dose rate ex[C&&,]for deuterium atom initiation from a series of periments. ('oincidentally, the rate of exchange to T h e Journal of Physical Chemistry



five experiments was 0.35 f 0.05. This may be conipared with the dose rate-independent value, 0.42 + 0.03, for the Ar-D2-C2Hz system. The agreement is therefore satisfactory with the postulate that D atoms are the initiators of the sensitized formation of benzene in this system. A similar preliminary radiolytic investigation of the Dz-CzHzsystem was carried out. In this case the ratio [C6&D]/A [C&] cannot be determined as accurately because the stopping power of deuterium is so much less than that of argon, and the amount of sensitization is correspondingly less. However, the approximate value for. the ratio is 1.1 f 0.2, which is substantially larger than the deuterium atom value. It therefore seems necessary to postulate an additional precursor for benzene in this system. One possibility is proton transfer according to the sequence

+ Dz +

+D D3+ + CzHz + CzHzD+ + Dz

(13)

CzHzD+ --+ CsHsD

(144

Dz+

D3+

(14)

Mains, Kilical ground. 26, 27 -4 possible alternative explanation for the nearly equal efficiency of the rare gases as sensitizers is anionic polymerization initiated by electron attachment. Although there is no experi-

The Journal of Physical Chemistry


mental evidence for such a process, it seems plausible for such a readily polymerizable substance. It is evident that no unique mechanism can be suggested, and the relatively constant rate of cuprene formation, as a number of experimental parameters is varied, remains a puzzle to those who have investigated t8hissystem. Independent of mechanistic details, this research seems to establish a duality of precursors for benzene and cuprene as suggested by Dorfnian and Wahl.3b Ionic processes do not seem to contribute at all to benzene formation in the dose rate independent region where most of our studies were carried out, and they do not appear to be determinative in the polymerization to cuprene.

Acknowledgments. The authors wish to acknowledge helpful discussions with Dr. Leonard Spialter of this laboratory, Professor G. J. Mains of Carnegie Institute of Technology, and Professor Leon Dorfman of Ohio State University concerning the complex chemistry of this simple molecule.

(25) M. Zelikoff and L. M. Aschenbrand, J . Chem. Phys., 24, 1034 (1956). (26) H. Eyring, J. 0. Hirschfelder, and H. S. Taylor, ibid., 4, 479 (1936).

( 2 7 ) J. L. Jfagee and K. Funabashi, Radiataon Res., 10, 622 (1959)

Rare Gas Sensitized Radiolysis of Acetylene1

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