J. Phys. Chem. 1987, 91, 3898-3902
3898
directly onto the electrode surface may also increase k,. It has been shown that adsorption of iron(II1) protoporphyrin IX onto pyrolytic graphite resulted in a value of k, which exceeded 1O4
Acknowledgment. The financial support of the Department of Energy (Grant DE-FG02-84ER13261) is gratefully acknowledged. Our appreciation is also extended to Carol F. Mason, who initiated this work, and Rebecca Schmidt, for her skillful assistance in some of the experiments. Constance Rose is thanked for her preparation of the chlorophylls used in this study.
s-I ,47
(47) Brown, A. P.;Anson, F. C. J . Electround. Chem. 1978, 92, 133.
A Flow Tube Study of Ion-Molecule Reactlons of Acetylene J. S. Knight, C. G. Freeman, M. J. McEwan,* Department of Chemistry, University of Canterbury, Christchurch, New Zealand
V. G.Anicich, and W. T. Huntress, Jr. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Cadifornia 91 109 (Received: February 20, 1987)
The reactivities of C2Hy+,C4Hy+,and C6Hy+(0 I y I 7) ions with acetylene have been investigated with a selected ion flow tube operating at room temperature. Rate coefficients and products for each reactant ion have been identified. The predominant mechanism observed was association thereby providing a pathway for the efficient conversion of C2Hy+cations into C6- and C8-based cyclic hydrocarbons. Significant differences were noted in reaction rate and product ion distribution between low-pressureand high-pressure techniques of investigating association reactions. Isomeric structures of C4H4+,C6H4+, and C6H5+are distinguished.
Introduction One of the earliest ion-molecule reactions to be studied in the laboratory was the reaction between C2H2+and C2H2by Field et al. in 1957.l Their initial investigation, in which they established the products of the reactions of Cz+,C2H+,and C2H2+with acetylene, was followed by a number of studies that encompassed a wide range of experimental techniques.2-10 Recently, however, the ion chemistry of acetylene has been studied with renewed interest: partly because of the possibility that ions derived from acetylene may participate as precursors to soot formationI1J2and partly because of the incidence of association-type processes involving ions of unsaturated hydrocarbon^.'^^^^ A very new area of interest is the link between small hydrocarbon ions and the prediction of the existence of substantial amounts of polyaromatic hydrocarbons in interstellar c10uds.l~~'~ It is quite apparent from studies of the ion chemistry of acetylene already undertaken that many unsaturated hydrocarbon ions associate with C2Hzeven under low-pressure conditions. Most of the recent studies of ion-molecule reactions of acetylene have utilized techniques such as ion cyclotron resonance (ICR) which to IO-' Torr. generally operates at low pressure: typically It is also apparent that there may be quite legitimate differences in both reaction rate and product channels between the results obtained when using low-pressure techniques (such as ICR) and those derived from higher pressure flow tube techniques, particularly when association occurs. Also it is perhaps a little surprising that few flow tube studies have been undertaken of the ionmolecule chemistry of acetylene: an omission we seek to remedy in this work. In a recent low-pressure ICR study involving a ~ e t y l e n e ,we '~ investigated reactions with C,Hy+ ions where x, y I 4. We noted that sequential condensation reactions quickly build up the carbon skeleton of the product molecules C2H2'
CP2
(C4H2+, C4H3+)
C2H2
(C6H4+, C6Hs')
C2H2
etc. In the present study we report the results of a higher pressure
* NRC-NASA
Research Associate at JPL, 1986.
0022-3654/87/2091-3898$01.50/0
TABLE I: The Source Cas or Ion-Molecule Reaction Used To Produce Various Reactant Ions Listed together with Other Ions That Are Present as Impurities in the Ion Swarm abundance of other ion reactant source gas masses relative to reactant ion or reaction" ionb 49 (1.5); 50 (0.6) C4H2c
C4H4 C4H2
C4H4c C2+ + C2H2 C4H2
C4H4e CzH++ C2H2 H2/C4H2
49 (1.8); 50 (0.8) 48 (0.5);50 (0.3) 48 (0.35);50 (0.58) 50 (0.05) 49 (0.01);51 (0.02) 48 (0.03); 49 (0.16); 51 (0.12) 49 (0.25);51 (0.20);52 (0.02) 50 (0.07); 52 (0.07)
C4+ C2H2
50 (0.50);52 (0.25) 50 (0.25);52 (0.30) 50 (0.08); 52 (0.01) 51 (0.05);53 (0.05) 51 (0.30);53 (0.30); 54 (0.05) 50 (5.0);51 (3.5) 74 (1 .O) 48 (0.05);49 (0.05);74 (0.90)
C6H6
7 3 (0.40); 75 (0.09)
C6H6
49 (0.10) 74 (0.20);76 (0.30); 77 (0.05) 75 (0.17); 77 (0.05)
C4H4c C4H6d
C2H2++ C2H2 C4H4C C4H6d
C2H2' C6H6
+ C2H2
+
C4H++ C2H2
C6H6
C4Hz+ + C2Hz C6H6
C,H,+ C6H6
+ C2H2
SO (0.07); 74 76 (0.30); 78 50 (0.03); 51 77 (0.11);79
(0.02); 77 (0.08) (0.40) (0.05);76 (0.04) (0.04)
C6H6
"The reactant ion of interest is produced by electron impact on the source gas shown or as the product of the ion-molecule reaction listed. bThe ions included in this column are introduced as impurities into the flow tube with the specified reactant ion. The parentheses represents the magnitude, e.g. 49 (1.5) indicates a signal at m/e 49 which is 1.5 times larger than that of the reactant ion. cVinylacetylene or l-buten-3-yne. 1,3-Butadiene.
(-0.3 Torr) selected ion flow tube (SIFT) investigation of these sequential reactions in order to explore differences between the 0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3899
Ion-Molecule Reactions of Acetylene
cm3 sd) k
TABLE II: Reactions of Selected Ions with C2H2(Rate Coefficients in Units of reactant ion
product
branching ratio 1.o 1 .o
C4+ C4H+
C4H+ + H C4H2++ H C4H2++ H 2 C4H3++ H C4H4+ C6H+ H C6H2+ + H
W 2 +
C6H4+
C&3+ C4H4+
C6HS+
c2+
C2H+ C2H2+
C6H' C6H2+
+
C6H6+
C6HsC + H CsH3+ CgH2' + H C8H4C
CgH3' C6H3+
C8HS+
w
C8H6
4
C6H5+
+
+H
+H C8Hs++ H C8H7f C8H6++ H CgH4:
this work
lit.
1.2 1.2
E?}
1.7: 2.6c 1.7: 1.5," 2.5: 2.4e 1.4,c.'.g1.2,d 1.5,h 1.9e
k,' 1 .2 1.2 1.2
1.2
0.08 1.o 1.o 1.O'
>28' 97%) of the less reactive isomer (Figure 5 ) . The larger number of collisions in the flow tube would favor the lower energy phenylium isomer on statistical grounds because of the density of states ratio whereas, in the low-pressure ICR cell with few collisions, the complex is continually reverting from one structure to another. Similar behavior has been observed in the CH3CNH+/CH3NCH+system where the lower energy CH3CNH+ structure is favored as the product of the reaction CH3+ HCN.29v30 Tasaka et aL3I used M I N D 0 / 3 calculations to examine structures of C6HS+and concluded that activation energies as low as 193 kJ mol-' may be required for mutual interconversion between the phenyl and acyclic structures. The internal energy available to the (C6H5+)*species formed by association from reaction 2 may be as high as 306 kJ mol-' if the value for AHf[C4H3+]of 1265 kJ mo1-I is adopted.32 There is therefore sufficient energy in the complex for mutual interconversion between a number of different structures until the excess energy is removed by collision or radiation.
+
L.
12501
,
,
,
0
5
10
15
c2y
F
I /(io17 ~ mokculr S-1)
Figure 5. Data for the reaction of C6HS+with C2H2. Semilogarithmic plots of the C6H5+signal against the flow of the CzHz. For the upper data set ( X ) C6H5+was generated by electron impact an benzene. (See the caption to Figure 2 for an explanation of the curve.) For the lower data set ( 0 )C6H5+was formed by reaction of C4H3+with CzH2.
which indicates that only 25% of the complexes formed initially were stabilized by subsequent collisions. C6H4+was generated both by electron impact on benzene and also as a product of the reaction between C4H2' and C2H2. It was evident from the ensuing reactions with acetylene that the two sources each produced single, but different, isomers of C6H4+ at m / e 76 (Figure 4). C6H4+from benzene associated with CzH2 with a rate coefficient of 7 X lo-" cm3 s-l (at 0.3 Torr) whereas C6H4+from reaction 1, C4H2' C2H2,associated much more slowly with a rate coefficient of 1 X lo-" cm3 S-I. AusloosZ6has noted that two different isomers of C6H4+were produced when benzene was subject to electron impact at low pressure: one isomer being reactive and the other unreactive toward benzene. In our study, however, the lack of any marked curvature in the semilogarithmic decay of the ion signal with added C2H2is suggestive that in each source a predominance of one isomer was present, but our experiments do not provide sufficient evidence for us to identify the isomers. Rosenstock et aLZ7have demonstrated that the acyclic C6H4+ isomers have significantly higher heats of formation than does the benzyne isomer and thus should be more reactive. It is tempting to suggest that the more reactive C6H4+ isomer we observe in this study is acyclic and that the unreactive isomer possibly has the benzyne structure, on the grounds that the acyclic isomers are more reactive with C2H2than the cyclic isomer. Such behavior has been observed for the reactant ions C3H2+,28C3H3+,12 C4H4+,22and c6H5+.24 C6H5+.Two different reactivities were observed for the reaction of C6HS+with acetylene depending on the source used to produce the reactant ion. When C6Hs+was produced by 70-eV electron impact on benzene, a curved semilogarithmic decay was found upon reaction with acetylene and this behavior is indicative of more than one isomer at m / e 77 (Figure 5 ) . The rate coefficient for the more reactive component with C2H2was 4.0 X lo-'' cm3 s-' at 0.3 Torr while the rate coefficient for the less reactive component was (2.5 f 2) X lo-" cm3 s-I. Only one product channel, that leading to the association adduct C8H7+,was observed for each isomer. However, when C6HS+was produced via reaction 2 (C4H3++ C,H2) a linear semilogarithmic decay with added acetylene was observed. The rate coefficient for the reaction of C6H5+,produced by reaction 2, with acetylene was 1 X lo-" cm3 s-l at 0.3 Torr (Figure 5 ) . Again the only product of the reaction was the association channel to yield C8H7+.Eyler and CampanaZ4
+
(27) Rosenstock, H. M.; Stockbauer, R.; Parr, A. C . J . Chim. Phys. Phys. Chim. Biol. 1980, 77, 745. ( 2 8 ) Smith, D.; Adams, N. G.In?.J . Muss Spectrom. Zon Proc., in press.
+
Conclusions It is quite apparent from the systems studied that association is a process of major importance in reactions between unsaturated hydrocarbon cations and acetylene in a flow tube. The association products appear to be new, covalently bound molecules rather than weak electrostatically bound complexes. It is also apparent that significant differences do exist in many association reactions between the results of low-pressure ICR studies and those of flow tube experiments. As a consequence there is a need for both types of measurements. These differences are related to the time between collisions which clearly influence the rate coefficients and branching ratios. For example in the reaction of C4H4+with CzH2, the present SIFT study gave only the association product, C6H6+, whereas the ICR investigations gave a number of different products of which association was one channel that occurred with low efficiency. The time taken to find these other exit channels on the potential energy surface is presumably much greater than 0.1 ps which is the time available between collisions in our SIFT study. Association between either C4H2' or C4H3+and acetylene leads to the same product at both low pressure and high pressure but in both cases the association rate is much faster in the flow tube because of the greater participation by the termolecular collisional stabilization process. Such a result requires a long-lived complex that may exist long enough to be stabilized by radiation at low pressure.I3 Finally, we note that association by collisional stabilization of the complex provides an extremely efficient way of converting small unsaturated hydrocarbon cations into less reactive larger hydrocarbons that almost certainly have ring structures.
Acknowledgment. We thank the New Zealand Universities Grants Committee and the New Zealand Lotteries Distribution Board for financial support. Registry NO. CH+H, 74-86-2; Cz+, 12595-79-8; CzH', 16456-59-0; CzHZ.+,25641-79-6;C4+,55892-20-1; C4H4-*(acy), 59699-48-8; C4H4.+ (CYC),
79105-72-9; C6H5+, 17333-73-2.
(29) Knight, J. S.; Freeman, C. G.; McEwan, M. J. J. Am. Chem. SOC.
1986, 108, 1404.
(30) DeFrees, D. J.; McLean, A. D.; Herbst, E. Asrrophys. J . 1985,293, 236. (31) Tasaka, M.; Ogata, M.; Ichikawa, H. J . Am. Chem. SOC.1981, 103, 1885.
(32) This value is based on a proton affinity for C4Hz of 738 kJ mol". Knight, J. S.; Freeman, C. G.;McEwan, M. J., to be submitted for publication.
