M. S.B. MUNSON
572
Ionic Reactions in Gaseous Acetylene'
by M. S. B. Munson Humble Oil and Refining Company, Research a d Development, Baytown, Texae
(Received September 81* 1984)
Mass spectrometric studies of ionic reactions in acetylene have been made which gave rate cc./molecule-sec. for reaction of C2H2+and C2H+. Conconstants of 0.9 and 1.3 X secutive ionic reactions were observed giving ionic species up to Cl2 and perhaps (21,. C,H4+ was observed as a third-order product ion with no indications of chemi-ionization reactions being involved in its formation. No appreciable concentration of CsHs+ ions was observed. Product ions were observed which were apparently formed by reaction of excited states of the acetylene ion. Analogous reactions were observed in CzDz and the rate constant for reaction of C2D2+was within experimental error the same as the rate constant for reaction of C2H2+.
Introduction The radiation chemistry of acetylene has been studied for many years; the major products are benzene and an uncharacterized polymer, called "cuprene. " 2 s At various stages in the history of these studies the reactive internwdiates have been considered as radicals or ions and it appears that both may be Recently, work has been done in these laboratories on the radiation chemistry of acetylene3 which made it seem worthnrhile to continue the studies of ionic reactions in gases with experiments on acetylene with the mass spectrometer available a t Humble a t pressures as high as possible. l l a s s spevtrometric studies of ionic reactions in acetylene have been made which gave values of rate constants for the formation of some second-order product ion,^,.^-^ In work on the bombardment of C2Hz with positive ions, second-order products ions were also ~ b s e r v e d . ~Experiments have been made with acetylene i n a mass spectrometer a t a pressure of about 100 p which showed the presence of some higher-order product^,^^^ but no kinetic data were reported.
Experimental The mass spectrometer was that described by Fieldlo and was used with a Faraday cup or an electron multiplier as an ion detector. Appearance potentials mere detenuined by the modification of the retarding potential difference technique" which has been discussed previously with regard to operation at high The Journal of Physical Chemistry
pressures.12 The source pressure was determined in the previously described manner. l 2 The source temperature was varied between 130 and 220' in different experiments and was maintained constant within a few degrees during each experiment. The residual source pressure was about 1 x 10-6 nim. The electron current was several hundredths to a few tenths of a microampere (Fa.) using the electron multipler as the ion detector and larger electron currents were used in the experiments with low electron voltage to increase the observed ion current. When the Faraday cup was used, electron currents of about 1 pa. were used. Pressure studies were made with (1) Supported in part by Project SQCID under Contract Nonr-3623 (5-18). (2) (a) S. C. Lind, "The Radiation Chemistry of Gases," Reinhold Publishing Carp., New York, N. Y., 1961; (b) L. 11. Dorfman and A. C. Wahl, Radiation Res., 10, 680 (1959). (3) F. H. Field, J . Phys. Chem., 6 8 , 1039 (1964). (4) F . H . Field, J. L. Franklin, and F. W. Lampe. J . A m . Chem. Soc., 79,2665 (1957). ( 5 ) R. Barker, W. H. Hamill, and R. R. Williams, Jr., J . Phys. Chem., 6 3 , 825 (1959). (6) R. Fuchs, Z.Yaturforsch., Ida, 1026 (1961). ( 7 ) E. Lindholm, I. Szabo, and P. Wilmenius. Arkio F y s i k , 2 5 , 417 (1963).
(8) P. S. Rudolph and C. E. Melton, J . P h y s . Chem., 6 3 , 916 (1959). (9) A. Blooh in "Advances in Mass Spectrometry, 11." R. M.Elliot, E d . , Pergamon Press, New I-ork, N. T.. 1963, p . 48. (10) F. H. F i e l d , , J . Am. Chem. SOC.,8 3 , 1523 (1961). (11) R. E. Fox, W. M.Hickam, D. J. Grove, and T. Kjeidaas, Jr., Rev. Sci. Inatr., 2 6 , 1101 (1955). (12) M. S. B. Munson, J. L. Franklin, and F. H. Field, J . Phys. Chem., 6 7 , 1642 (1963).
IONIC 12EACTIONS IK
GASEOCSACETYLENE
573
ionizing electrons of nominal energy of 10-50 v. in the presence of repeller voltages of 2.5 or 5 v. The acetylene was obtained from the Matheson Co. and mas purified by passing through two traps kept in a slurry of COZ arid acetone and then frozen and sublimed twice with liquid nitrogen, discarding the first and last fractions from each sublimation. Acetone, air, and water were the only observed impurities in conc-entrations of the order of 0.01%. CzDz was obtaincld from llerck & Company of Canada and contained only a few per cent of CzDH and was used without further purification. D 2 0 was allowed to flow through the vacuum manifold, gas reservoir, and mass spectrometer for several hours before the experiments with C2Dz were performed.
Results In the primary mass spectrum of acetylene a t low pressures these data confirm the previous reports of the formation of CHz+ as a rearrangement ion and the presence of C2H2+2and C2H+2.l3I4 These data indicate that CzHz+2is about 4oy0 of the total intensity a t mass 13 in acceptable agreement with the other values of 73%,13 and 4370.14 A(CH2+) and A(13) were determined as 21.1 + 0.1 v. and 21.8 f 0.2 v., respectively, the latter in good agreement with earlier work.'5 From the heats of formation of CH2+,l6C,17and CH,18 and the spectroscopic ionization potential of CH,I6 one may calculate that appreciable excess kinetic energy is involved in these decompositions. A kinetic analysis of the sequence of second-order reactions involved has been given previously. l 9 With the assumption that the observed ion current is proportional to the ion concentration with the same proporrionality constant for all ions and that the fragmentation pat tern for decomposition of the initially excited C2H2+ is independent of pressure, then for prin1ar.y ions, since (C2Hz) >> (CZHZ+) In (1,/211)= in A , - k,(CzHZ)t,
(1)
for which the initial concentration of P,+, is A , z I , ; t , = (2dM,/eFS)I", in which d is the ion path in the source, d l , is the mass of P I + ,e is the electronic charge, FS is the repeller field strength in the source; arid k , is the rate constant for reaction of P I + with acetylene. From the slopes of plots of the logarithms of relative intensity against source pressure one can obtain rate constants for reaction of the primary ions. Using an electron multiplier as the ion collector, the rate constant for the reaction of C2H2+ with acetylene, kC.H?A,was determined as 1.03 f 0.07 X low9cc./ molecule-sec. (since kt = &d, & = 48 X cm.a
a t FS = 12.5 v./cm.). This value is the average of rate constants determined from experiments a t temperatures from 130 to 220', with nominal energy of the ionizing electrons of 9-50 v., and at repeller field strengths of 6.3 and 12.5 v./cm. (corresponding to final ion energies of 1.3 and 2.5 v.). Sone of these paranieters had any significant eff ect on the rate constant. The activation energy for this reaction must be less than 1.5 kcal./inole, if one assumes that a 4Oy0 increase in rate constant between 130 and 220' could have been detected unequivocally if it had occurred. From experiments with 50-v. electrons, k C I H + is cc./molecule-sec. (Q = 72 X 10-l6 1.6 f 0.2 X cnx2 at FS = 12.5 v./cm.) arid kc,. is 1.7 i 0.2 X cc./molecule-sec. (& = 75 X 10-l6 cni.2 at FS = 12.5 v./cm.). Plots of log ( 1 1 3 / 2 1 1 ) against source pressure gave reasonable straight lines from which a "rate constant" of 2 X lop9 cc.,'molecule-sec. could be estimated. The residence time of CH+ and C2H2+2 would be the same, but since mass 13 probably consists of roughly equal amounts of C H I and CzH2+2and the C H + is formed with excess kinetic energy, this number should not be considered as other than an indication that these ions react rapidly. From data of rate constants for reaction of CH+ with D2, CH,, and CzHB for which the rate coristarits vary approxiniately as the polarizability of the a value of about 2 X cc./molecule-sec. would be expected for kCH in acetylene. Appearance potentials of product ions are shown Table I. Appearance potentials of some product ions were determined in C2D2 because of interferences from carbon isotopes. Agreement with earlier data for the major ions is good. No niajor secondary ion was observed of appearance potential rorrcsporidirig to C2H+ (the concentration of C,H+ was small compared to that of C2H+),but the C2H+ reacted rapidly with acetylene. The ratio 150/Z51 increased with increasing electron energy so that in addition to (13) C. E. Melton, A I . h1. Bretscher, and R . Baldock. J . Chem. P h y s . , 26, 1302 (1957). (14) F. L. llohler, V. H. Dibeler, L. Willianlson, nnd FI. Dean, J . Res. S a t l . Bur. Std., 48, 188 (1952). (15) J. T. Tate, P. T. Smith, and A. L. Vaughn. P h y s . Reu., 48, 525 (1935). (16) F. H. Field and J. L. Franklin, "Electron Iinpact Phenonrena," Academic Press, New York, N. Y.. 1957. (17) L. Brewer and A . \V, Searcr, A n n . Rer. P h y s . C h e m . . 7 , 2 i 1 (1956). (18) R. G. Brewer and (19)
F. L. Kester. J . Chem. P h y s . , 40,
812 (1964).
F. W. Lampe, J. L. Franklin, arid F. H. Fielri. "Progress i n Iieart-
tion Kinetics," Vol. 1 , Pergarnon Press, Oxford, 1961, 1111. 7:3--79. (20) 11. S. B. Munson, J. L. Franklin, and F. H. I'ield, J . PhUs. Chem.. 68, 3098 (1964).
M. S. B. M C ~ O N
574
CzHz+
+ CzHz
i' C4Hz+ C4H3+
+
HZ
(2)
+H
C4H2+is produced by another reactions as well, probably CzH+
+ CzH2 +C4Hz+ + H
small concentrations: C2H3+, C3H+,C3H2+,and C4H+. The relative concentration of these ions passed through a maximum with increasing pressure: hence, they all react rapidly with acetylene, IC of the order of 10-'0 cc./molecule-sec. The appearance potentials suggest that these ions are formed to some extent from reaction of excited acetylene ions
(3)
From experiments bot-nbarding CzHz with positive ions, it has been suggested that C4H3+ comes from a low
Table I : Product Ions in Acetylene A P , e.v.
Lowest energy precursor
Order
CzHz CzHz 1563501 CzHz+* 1553501 CzHz +* 1 7 4 3 ~ 0 4 CzH 12 5-13 CzDz +* 11 6 * 0 I CzHz 11 4 3 5 0 1 CzDz 11 4 + 0 1 CzHz Low Probably CzHz Low Probably CzHz+ 11 4 * 0 1 C4HZ' 11 3 r t o 1 C4H3 Low Probably CZHZ Low Probably CZHZ 11 3 * 0 1 11 5 3 5 0 1
+ +
+
+
+
+
+
+
+
+
Second Second Second Second Second Second Third Third Third Third Third Fourth Fourth Sixth ( ? ) Sixth (?)
energy state (probably the ground state) and that C4H2+conies from a state around 15.5 v.' However, in the present experiments appreciable concentrations of C4H2+ were observed a t electron energies below was 0.44 f 0.04 for electrons with nomi15 v. 160/l,1 nal energy up to 15 v. and then increased with increasing electron energy to 0.85 35 0.10 for 50-v. electrons. The values a t low voltage compare reasonably well with the ratio 0.61 f 0.05 obtained by Rudolph and Melton with their a-particle spectrometer for which C.!H2+was the only primary ion.8 The ratio at 50 v. compares well with the other values of this ratio deterinined with 70-v. electron^.^-^ Recent measurements have been reported on appearance potentials of ions in acetylene21 which indicate a sharp break a t about 17.0 v. in the ionization efficiency curves of C4H3+and C2H2+but not C4H2+, which is interpreted in terms of reaction of CZH2+* to give C4H3+ and not C4€12+. This observation is a t variance with these data for which C4H2+is forined by the higher energy -. species. . Other second-order product ions were detected in The Journal of Phvsical Chemistry
CzHz+*
+ CzHz i' C3H2+ C3H+
+
CH2
(4)
+ CHI
Both reactions were exothermic for an excited state of this energy. The 15.5-v. state of acetylene corresponds to the suggested state of Lindholm and co-workers7 and is perhaps indicated by the data of Collin,z2 although he did not interpret his data in this fashion. The previous studies gave appearance potentials of 20-22 v. for these ions, corresponding to CH+ as a primary ion. It is very probable that higher energy processes other than the reaction of C2H2+* are also involved and the lower appearance potentials in this paper are merely the consequence of a higher sensitivity which enables the lower energy processes to be seen. I n C2D2, A(C2D3+) was determined as 12.5-13 v. The shape of the plot of ion current against electron energy indicated that the ion is formed by more than one process. The vinyl ion can be formed exothermally from a second-order reaction of an excited acetylene ion of about 13 v. energy and Collin22reports an excited ionic state a t 13.0-13.2 v. Two other product ions were observed which could be second order based on their stoichiometry, C3H3+ and C4H4+. These two ions, however, were actually third-order ions. No evidence was found for a chemiionization reaction leading to the formation of C4H4+ or C4D4+. These ions can be formed exothermally by a reaction of the (CzH*.CzHz+)complex ion kza/l
C2Hz+
C4&+
/
+ CzHz
(C4H4+) ,k2b CJL+
(C4H4+)
+H
+ CZHZ
Y
(C&+)
C3H3+
+ HZ
+ C3H3
2 C4H4+ + CZHZ \
kro \
C6H3+
(54
(5b)
+ HZ + H
(21) C. E. Melton and W. H. Hamill. J . C h e m . Phus., 41, 1469 (1964). (22) J. E. Coliin, BUZL.SOC. chim. Belges. 71, 15 (1962) ~I
IONIC REACTIONS IN GASEOUS ACETYLENE
575
If the second-order product ions, C4H3+ and C4H2+, do not, undergo appreciable reaction then,lg for example
tion. The ions were formed from low energy processes, probably from CXHz+ or one of its product ions. Very small concentrations of C7H3+ ( q ’ e = 87) were also
A t pressures up to about 100 p there is no evidence for appreciable reaction of C4H3+ and a plot of 13g/I51 is linear in acetylene pressure. C6H3+ is included in reaction 5b since it exhibits the same pressure behavior as C3H3+ and C4H4+. k3/k2a is approximately 2 X l O - I 7 cc./molecule. Unfortunately, there is no way a t preseni of establishing the values of and k3 separately. kza cannot be as low as 106 set.-' because no C4H4+was observed as a second-order ion nor were any spontaneous decompositions of C4H44 to C4H3+ observed as “metastable ions” in the mass spectrum. (139 152 4- 175)/211, the sum of relative concentrations of the third-order products from reaction 5b, never amounts to more than a few per cent of the total ionization, although the concentration of the fourth-order ions becomes much greater than this. The low value for the relative concentration of thirdorder ions suggests that (kZa k2b) is large and that the complex (C4H4+) disappears predominantly by dissociation. The ions, C6H4+ and C8HB+,are the major higherorder ions a t these pressures. At about 300 p, the sum 176 17, was about 30% of the total ionization (EV = 50 v. and FS = 12.5 v./cm.); the processes forinirig these ions are therefore very rapid. The ratio 177/176 increases with increasing pressure, so that the two ions cannot be formed from dissociation of the same complex. Although their stoichiometry requires only three acetylene molecules, the pressure dependence is fourth order; that is, C6H5+/C4H3+ increases approximately with the square of the acetylene pressure. These results are best explained by
Ions of m/e = 100 through 104 %?ereobserved which probably came from more than one source since the ratios of ion currents varied with electron voltage a t constant pressure. They were forined partly from C2H2+ since these ions were found in the low voltage experiments when C2H2+ was the only primary ion. The most abundant of these ions were 102, C8H6+,and 103, C8H7+, perhaps formed by sixth-order processes since 1 1 0 2 / 1 7 6 increased approximately with the square of the acetylene pressure. Very small amounts of Cg species were found; the most abundant was a t m / e = 114, C9H6+. Larger concentrations of Clo ionic species were present up to 129, CloHg+. Very small amounts of C12ions were observed, and it is possible that the trace concentrations of ions at m / e = 180 (CI4Hl2+)were products of ionic reactions. Typical data for relative concentrations of ions in acetylene are shown in Figures 1 and 2 . The wrve for C2H2+ was calculated using the average value of the rate constants. The dashed curves for C4H3+ and C4H2+ are calculated assuming that C2H2+reacts to give only these two ions in the ratio C4H2+/C4H3+ = 0.44 and that these ions do not undergo further reaction. The deviations of the experimental data from this curve show the extent of higher order reactions. The sequential reactions, C2H2++ C4H3++ CsH5+ are readily apparent in Figure 1. The higher order of reaction for formation of C6H4+than C3H3+or CsH4+ and the large ratio of fourth-to third-order products are apparent in Figure 2. Some experiments were done with a Faraday cup as an ion detector. The general pattern of results was the same with the two types of detectors; however, there + , the was a difference in rate constants. k ~ ? ~ ?using Faraday cup as an ion detector, was 0.86 f 0.12 X cc./molecule-sec. (to be compared with 1.03 X using the electron multiplier). This difference in rate constants is not the result of discrimination in the multiplier for ions of different masses since a comparison of ion currents between the Faraday cup and the electron multiplier for primary fragment ions of masses 27 to 8-2 indicated a decrease i n sensitivity of less than 20y0 over this threefold mass range. These effects would produce a change in rate constant within experimental error and in the wrong direction. I t was suggested by one of the reviewers that saturation at high ion currents could be the cause of this difference.
+
+
+
C4H3 CpHe (CeH5+)
CeH5+
(74
The complex formed by conibination of the ion with a molecule should not be stable since it is 60-70 kcal./ mole above the ground state of the ion. The dissociation of the (C6H6+) complex to C6H4+ or C6H3+ is endothermic. No significant concentration of C6H6+ was observed as a product ion at these pressures, although there was generally a slightly greater ion current a t mass 78 than cciuld be accounted for from I3C isotopes (?O.lY, of the total ionization). CsH2+ and C5H3+were observed as third-order ions, maximum concentration about 1% of the total ioniza-
Volume 69, Number 2
February 1966
0.3 0.2
*
0.1
‘0
curvature) for the experiments using the Faraday cup compared with those using the electron multiplier. I n previous work it has been observed that high electron currents (necessary in this case to allow detection of ions with reasonable accuracy) gave lower rate constants a t high pressures than those obtained from experiments with a smaller extent of ionization. I t is likely that the results with the Faraday cup are soniewhat low. The differences in experimental conditions were not great and a personal preference for one set of data does not seem sufficient reason for discarding the other. These two values should be ronsidered as extremes and the average of 0.85 X cc./moleculesec. (with a probable accuracy of *0.20 X loV9) should be taken as the “best” value. kc2H+from the experiments with the Faraday cup was 1.0 0.2 X cc./molecule-sec. (to be compared with 1.6 f 0.2 X lop9 using the electron multiplier). The average of these two values, 1.3 f 0.3 X cc./molecule-sec. should be taken as the “best” value of these data. Experiments were performed with C2Dz to obtain appearance potentials of C2D3+and C4D4+. Pressure studies showed no evidence for chemi-ionization reactions in the formation of C4D4+,which was a thirdorder ion from CzDz+. These experiments also confirmed the observation that CzH3+was a second-order product ion. The assignment of the higher mass ions in CzHzwas confirmed by the experiments with CZDz. Studies were made on CzDz and mixtures of CZHZ and CzDz to see if an isotope effect were observable for these reactions, but essentially none was observed. kc*^^+ was 0.53 f 0.05 X cc./molecule-sec. with the cc. /molecule-ser. Faraday cup and 0.98 0.07 x with the electron multiplier, for reaction with CZDZ. These values are perhaps lower than the values for the rate constant for reaction of C2H2+but any difference in rate constants is within the experimental precision of the two numbers. .Li pressure study was made on a mixture (about 1 : l ) of C2Hz and CzD2. Since the reactions
20
40
60
80 100 P (CzHz), p
120
140
160
180
Figure 1. Relative concentrations of ions in C2H2: El‘ = 11 v.; F S = 6.3 v./cm.
*
CzHzf
Figure 2. Relative concentrations of ions in C2Hn: E l i = 11 v.; FS = 6.3 v./cm.
There is appreciably more scatter in the experimental data and thc plots of log (Zz6/ZZ,) us. P(CzHz)are not linear over as wide a pressure range (with upward The Journal of Physical Chemistru
{ ‘‘HZ CzDzkH, products
both occur in this mixture, from eq. 9
(8%)
IONIC REACTIONS IN GASEOUS ACETYLENE
577
(neglecting the 2% difference between t 2 6 and tzs) one Table 11: Comparison of Rate Constants Obtained by may calculate the difference in rate constants for C2H2+ Different Workers for Reaction of C2H2+ with CZH2 cc./ ) 0.00 f 0.06 X and C2D2+. (ICH - k ~ was molecule-see. ; that is, within experimental error there kCyH2+ x 10-". QCYH;C x F S , v./cm. Emel. v. cc./moleoule-sec. cm.2 is no isotope effect. The total rate constant for disappearance of C2H2+ or C2D2+ in the mixture was the 2 0.10 4.9 68" 6 0.30 8.1 90 same as the rate constant for reaction in C2Hz or C2D2, 8 0.40 8.7 87 respect ively . 10 0.50 8.2 77 An interesting question arises about the decomposi20 1.0 5.9 41 tion of the C4H4+complex. For mixtures of CzDz 0.74 0.1 25 56Sb and C2Hzall of the possible C4H,BPn+and C ~ H , D Z - ~ + 7.4 1.0 12 85 species are observed, but the overlapping of the two 37 5.0 13 44 species prevents any statement about the preferential 6.6 0.66 22 172c modes of decomposition, if any, of the complex. If 13.3 1.33 20 97 there is appreciable reversible decomposition of the 20.0 2.00 17 66 complex, then one would expect C2H2+
CD2+
+ C2D2
12.5
\
(C4HtDz+) --+CzHD+
+ C2Hz 7
+ C2HD
See ref. 4.
2.5 a
See ref. 5.
8.5 f 2.0 See ref. 6.
40d
These data.
The earlier values for the other rate constants were based on the forination of relatively low intensity secondary ions and there is some doubt about the assignas well as decomposition of CzH2+ or CZD2+. It was nients of the product and reactant ions. Field, possible to reduce the energy of the ionizing electrons Franklin, and Lainpe* report kc*+ as about 7 X 10-10 low enough to eliminate practically all of the C2H3+. cc./molecule-sec. based on the formation of C4H+. ConThere was a small concentration of CzHD present in sidering the probable complications in mechanisms, one the C2D2 which gave mass 27. The ratio CzHD+/ should consider this value as acceptably close to the CzD2+was small, about 0.03 and independent of prespresent value of 17 X lo-'" cc./molecule-sec. sure, so that essentially no C2HD+ was formed. An The general pattern of reactivity in this paper conestimate can be made that less than 5% of the (C4H2D2+) firms that indicated by Rudolph and ,Ifeltons and it complexes dissociate according to reaction 10. was possible to establish a inore detailed mechanism Conclusions than they were able to do. In particular: C2H3+ is Table I1 gives a summary of the available data for formed by a second-order reaction of excited C2H2+ rate constants or cross sections for reactions of C2H2+ (ruling out reaction of C4H3+), C4H4+ is a third-order with acetylene. The earlier results were deterinined ion froni C2H2+ (one would certainly expect the secondfrom the formation of C4H3+ and C4H2+ with electrons order (C4H4+)coinplex to be unstable), and the formaof about 70 v. energy and are necessarily slightly large tion of C6H4+ and C6H5+ from C4H2+ and C4H3+is not since part of the C4Hz+ is formed froin C2H+. The a direct ion-molecule association, but requires a third pressure measurements are the critical problem, howbody for stabilization. In addition, even higher-order ever. The values of Fuchs6 are notably higher than product ions of the polymerization reactions are rethe others but since he considers the absolute value of ported in this paper. his pressure calibrations as correct to within about a KO evidence was found for chemi-ionization reactions factor of two, this disagreement is not surprising. in acetylene as has been reported for the formation of There is no marked variation in rate constant over the C4H2+ and C4H3+ in photochemical experimentsz4 range of ion energies given in this table. The value of and it is very difficult to understand why photoexcitak C z H 1 +of 8.5 2.0 X cc./niolecule-sec. appears tion should show this but not excitation by electron to be a good choice for the absolute value of the rate impact. The formation of these products froin acetylconstant. This number is not as well established as the values for k C H , + and k C H a + in 1nethane,~3but the (23) F. H. Field, J. L. Franklin, and M .s. B. Munson, J . A m . Chem. agreenient is satisfactory. Soe., 8 5 , 3575 (1963). S o (bornparison can be made for k c ?+ ~since the other (24) I. Koyano, I. Tanaka, and I. Omura, J . Chem. P h y s . , 40, 2734 experiments reported no values for this rate constant. (1964). (10)
*
Volume 69, Number 2
February 1965
eiie excited to 10.2 v. is endothermic unless the heats of formation of C4H2+ and C4H3+ are much lower than the presently accepted values. The obswvation that C2H3+,and C3H+, and C3Hz+ may be fornicd by reaction of excited ions is further evidence that reactions of excited ions may be more conimoii than has been considered in the past. S o evidence was observed for different reactivities of excited states of acetylene; however, unless the rate constants were greatly different and the relative concentrations of two states were about the same, one would not expect to be able to observe this phenomenon. The curves for the disappearance of iiiass 13, C H + and CzH2+’, indicate that the doubly charged ions react rapidly, but quantitative data are not available. The failtire to observe c&+ as an important product ion militates against benzene being formed by a purely ionic process. The heat of reaction of Cd&+ f CzH2
---f
C6Hs+
+ C?H
(11)
is not known with sufficient certainty to say anything about the permissibility of this reaction. Xeutralization and hydrogen abstraction a t the wall or in the gas phase could produce benzene. However, the gas phase abstraction of hydrogen from acetylene by phenyl radicals is probably endothermic and would not compete effectively Tvith addition. On the other hand, the rapid ionic polymerization can accouiit for the disappearance of soiiie of the acetylene and the formation of high molecular weight polymer. After the initial loss of H or H? in this process (to give C,H3+ and C4H,+) the reactions seem to involve only addition. This addition is stabilized by collision for siiiall aggregates, but for high molecular weight polymers one expects that “sticky” ion-molecule collisions would oc(w. Little hydrogen would be produced by this niechanisni and little hydrogen is observed in the early stages of reaction.2a The major primary ions in any radiolysis of C2H2 should be C?H2+ and to a lesser extent C2H+,both of which will fall into this polymerization sequence. Remarkably little is known about the polymeric material (“cuprene”) or any diff erenres whiclh might occur from diff erelit methods of fortnation. If one takes the temperature coefficient for G(-C2H2) from Field3 in the radiation of acetylcnc v;ith 2->rev. electrons and calculates an artivation cticrgy, a value of 1.5 kcal./mole is obtained. This rather small teinperature coefficient does not seem
T h e .Journal of Physical Chcmistry
to be incompatible with a complicated, predominantly ionic reaction mechanism. The confusion about the formation of benzene in radiolysis of acetylene to essentially coniplete reaction may be helped by observations made on ionic reactions in benzene. In a brief and qualitative pressure study of benzene in the mass spectrometer it was observed that C?Hz+ fragment ions from benzene reacted rapidly with benzene but that C6H6+ions were relatively inert. Many ions of mass greater than 78 were observed so that ion-niolecule reactions giving higher molecular weight product ions are probable in addition to charge transfer reactions established by Rudolph and Nelton.*j One would expect that the higher iiiolecular weight ions might also react with acetylene in a manner analogous to the ionic polymerization reactions in acetylene arid the inhibition of acetylene radiolysis by benzene should be less than 100% efficient. Also, the benzene may be removed by reaction with acetylene ions although benzene is relatively unreactive with itself under radiation. I t is of interest to consider previous work from this laboratory about reactions of acetylene and oxygen in t e r m of effects on acetylene radiolysis.*6 Reactions of Oz+with C2Hz to give oxygenated product ions were very slow, of the order of lo-” cc. /molecule-sec. and no evidence was observed for reactions of C2H2+ with 0,. Consequently, it seems reasonable that ionic polymerization of acetylene should occur readily in the presence of oxygen although termination could easily occur at different stages to give different polymers. According to Lind,2a a polymer containing very little oxygen is produced in radiolysis of oxygenacetylene mixtures. S o mention was niade of any benzene being observed under these conditions. Since no COz+ and very little CO+ were observed, it seems likely that the oxidation of C2H2to CO, and CO occurs through a predominantly radical process.
Acknowledgments. The author is very grateful to A h . W. C. Gieger for performing these experiments in his usual enthusiastic and competent manner as well as for his help with many of the calculations, and to J. L. Franklin for his helpful discussion. (25) P. S. Rudolph and C. E. l l e l t o n , J . Chem. Phys., 32,586 (1960). (26) J. L. Franklin and N . S. B. Munson, “ S t h International Combustion Symposium,” Cambridge, England, Aug 1964.
