N A L
OF
TiICAL CHEMISTRY
Registered in LT. S. Patent Office @Copyright, 1973, by the American Chemical Society
VOLUME 77, NUMBER 2 JANUARY 18, 1973
Shock Tukse Isomerization of Cyclopropane. I. Investigation of a Vibrationally Excited Intermediate :rnest A. Dorko,* Robert W. Crossley, Deoiirtment of Aero-Mechanical Engineering, Air Force lnstitute of Technology, Wright-PattersonAir Force Base, Ohio 45433
. Grimm, Gerhard W. Mueller, and Karl Scheller‘ Chemistry Research Laboratory, Aerospace Research Laboratories, Wright-PattersonAir Force Base, Ohio 45433 (Received June 5, 7972) Pubiication costs assisted by the Air Force lnstitute of Technology
The kinetics of cyclopropane isomerization to propylene were investigated in a shock tube by means of infrared absorption and emission analysis in an attempt to establish the mechanism. The test gas mixture was 1%cyclopropane in argon with a total gas concentiation range 2.8 >: 10-8 to 6.1 >: 10-6 mol/cc. ‘The temperature range was 1158-1323°K. Kinetic measurements were made utilizing the v g + v10 combination band of cyclopropane at 4.79 p and the carbon-carbon double bond stretch of propylene at 6.1 p. Absorption from a black body focused on the shocked mixture was detected along with the emission emanating from the tube during the reflected shock. The emission curve shows an initial buildup to a maximum followed by a decay portion. The reduced absorption curve shows only a decay from an initial max imuni. It is theorized that the absorption measurements follow the disappearance of ground-state cyclopropane and that emission measurements follow the buildup and decay of a vibrationally excited cyclopropane species. Differential equations for concentration us. time derived from a three-step kinetic mod el including excited species were integrated by a Kutta-Simpson algorithm. No steady states were assumed. The analytical concentrations were used to scale experimental infrared intensity measurements to concentration units. Values of rate constants were found for an assumed Arrhenius form based on statistical agreement of the analytical and experimental curves. The rate constants agreed with independeni, determinations within a factor of 3.
.Introduction In a previous study of the gas-phase isomerization of cyclopropane to propjlene in a single-pulse shock tubel it was discovered that the rate was slower than expected from the extrapolation of low-temperature data. In the report of this work the buildup of a reactive intermediate was discussed as a possible cause for the observed deviation from the expected rate. Presently we wish to report evidence for the existence of a vibrationally excited cyclopropane as an rntermediate in the isomerization reaction. The evidence for the vibrationally excited cyclopropane species is basod on the analysis of the emission and absorption of infrared radiation by the reacting system at 4.79 r ~ . . A further analysis is made possible by the observation of emisrrion at 6.1 p due to the product propylene.
A numerical analysis of the data was performed in order to establish the mechanism of the reaction and to determine the rate constants for the individual reactions which take part in the isomerization. 11. Experimental Section
The shock tube employed was of stainless steel (SS 304). It was fabricated from a piece of 3-in. i.d. tubing with 3/&in. walls. The tubing was rolled on a mandrill to produce a cross section which had two parallel, flat sides with a width of about 1 in. The flat sides made it convenient to attach instrumentation and the absence of cor(1)
E. A.. Dorko, D: 8. McGhee, C . E. Painter, A. J. Caponeccni, and 8 . W. Crossley,J. Phys. Chem., 7 5 , 2 5 2 6 (1971).
Dorko. Crossley. Grimm. Mueller, and Scheller
144
ners kept boundary layer problems to a minimum. The tube measured 29 ft in length with a 22-ft driven and a 7-ft driver section, The interior surface of the tube was electropolished and all valves were fabricated to fit flush with the wall. The vacuum system consisted of a LeyholdHeraeus roots pump (WS-150) in combination with a fore pump (Leyhold-Heraeus DK 25). The leak rate in the Torr shock tube a t IO-' Torr was found to he 6 X l./sec. Velocity measurements were performed with two thinfilm platinum heat gauges (0.5 mm wide by 6 mm long with 50-ohm resistance) placed 18 in. apart attached to an electronic counter which counted to 0.1 psec. The last gauge was placed 12 mm from the end flange. Preliminary experiments based on measurements obtained from seven stations spaced along the tube showed that the attenuation of the velocity was 1.2 + 0.2% in the last 5 feet of the tube in the velocity range of the kinetic experiments. Initial pressure in the driven section was measured by means of a variable reluctance type pressure transducer. The transducer was calibrated daily with a Wallace & Tiernan Bourdon gauge. Diaphragms were made of 5052H32 aluminum of differing thicknesses which were scored to different depths. The driven section was pressurized with air until the diaphragm burst to produce the shock wave. The test gas mixture used was 1% cyclopropane (99.0%) in argon (99.999%). Gas chromatographic analysis of the cyclopropane showed the impurity to he propylene. Kinetic measurements were made for the reaction occurring hehind the reflected shock by means of infrared emission and absorption measurements. For this purpose two CaFz windows were placed in the flat wall of the shock tube 12 mm from the end flange of the driven section. This location coincided with the last velocity measurement station. Shock parameters were calculatedz from the initial shock velocity assuming frozen ~ h e m i s t r y The . ~ heat capacities used in the program were calculated for the test gas mixture. In order to test the validity of the shock parameter calculations a preliminary experiment was performed with a 2.5411. long platinum heat gauge, oriented parallel to the flow, placed in the area of the CaF2 windows. The change in the resistance of this platinum strip displayed on an oscilloscope was used to measure the reflected shock velocity. In addition the shape of the trace gave an immediate indication of a possible coupling between the reflected shock wave and the chemical reaction. I t was found that the calculations of the reflected shock velocity based on the incident shock gave the same results as experimentally determined reflected shock velocities as long as the cyclopropane concentration was kept a t or helow 190,4 The total gas concentration behind the reflected shock to 6.1 X mol/cc. ranged from 2.8 X The source of infrared radiation for the absorption studies was a black body operated a t 1ooO". The emission from the black body was focused into the center of the shock tube and the emission emanating from the shock tube was focused onto the detector by means of a CaFz lens system to provide light of the appropriate wavelength to the detector. In some tests an interference filter transmitting at 4.727 p (with a half-handpass width of 0.3 p ) was used in conjunction with an indium antimonide detector (Barnes Engineering) cooled with liquid nitrogen. Other tests were made when an interference filter transmitting a t 6.238 p (with a half-handpass width of 0.4 p ) was placed in the optical path along with a mercury-doped germanium deThe J o ~ m a l oPhys,cal l Chemistry. VoI. 77. NO. 2. 1973
Figure 1. Typical oscilloscope traces lor ( a ) emlssion and absorption at 4.79 p , Ib) emission at 4.79 p . and ( c ) emission at
6.1 p . The reflected shock lemperalure was 1179°K. Each lime mark corresponds lo 5 0 psec. (Time increases from left IO right)
tector (Philco-Ford) cooled with liquid helium. During each test run the optical path was swept with nitrogen to eliminate atmospheric absorption. Cyclopropane has a us Y,O combination band a t 4.79 ,is whereas propylene has no activity in this region. Propylene on the other hand displays the carbon-carbon double bond stretch at 6.1 p6 where cyclopropane displays no activity, Measurements of the cyclopropane hand were made with and without the black body radiation passing through the shock tube in paired shocks. In the first shock the radiation from the shock tube without the black body was detected. This radiation is due to emission of the contents of the shocked mixture. The second shock was made under the same shock conditions except that the combined radiation from the black body and the contents of the shocked mixture was detected. This radiation corresponds to the emission of the black body less any absorption due to the shocked mixture plus the emission due to the shocked mixture. A determination was then made of the radiation from the black body passing through the evacuated shock tube and impinging on the detector. Measurements of the propylene hand were made only for emission from the shocked mixture. The signal from the ir detector was fed through an impedance matched preamplifier (Perry Associates) into an oscilloscope (Tektronix Model 555) which was triggered by the second heat gauge. Typical oscillograms for the three tests described above are shown in Figure 1. A time mark generator was used to determine time on the oscillogram.
+
121 0 . W. Mueller. unpublished program. ( 3 ) E. F. GrBene and J. P. Toennies. "Chemical Reactions 8" Shock Waves.'' Academic Press. New York. N. Y . . 1964. Chapter 2. ( 4 ) For a diSCUSSton of Shock lube techniques in Chemcal kinetics see R. L. Beitma and R. A. strehiow. ~ n n u ~. e v PhyS. . cnem.. 20. 247 ,lQR91 , . -,. (5) C. Brecher. E. Krikorian. J. Blanc. and R. S . Halford. J. Chem Phys.. 35. 1097 (1961). (61 R. H. Pierson. A. N. Fletcher. and E. SI. Clair Ganlr. Anal. Chem.. 28.1218 11956).
--
,
Shock Tube isomerization of Cyclopropane
145
Two tests were performed in order to ensure that the traces were not compromised by background effects. The emission from pure argon was measured behind a reflected shock. The emission from a 1% cyclopropane-argon mixture at, 5.4 IL (a region where neither cyclopropane nor propylene exhibit ir activity) was measured behind a reflected shock also. In both cases a straight line was produced. on the oscilloscope indicating a constant emission with no increase in intensity above the base line during the shock. The oscillograms were reduced to intensity us. time curves with the use of a Hewlett-Packard (Model No. 9100A) progrclmniecl calculator coupled with a digitizer (Model No. 9107A). A total of 200 shocks were obtained of which 30 were used for the analysis. 111. Analysis oEDat;a a n d Calculations
The ir amalysi~was attempted in order to obtain evidence for the existence of a vibrationally excited intermediate in the isomerization reaction. Recent work on cyclopropane using a single-pulse shock tube has led us to question the validity of analyzing the reaction by using a simple, unimcjlecdar kinetic model which assumes a twostep mecha.nir;m.l Indeed the assumption of the steady state on wikich this analysis is based may not be valid 'with the short residence or dwell times encountered in shock t,ubes,.lDuring another study of cyclopropane isomerization, Bradley and Frend7 (subsequently referred to as BF) suggested a three step-mechanism as shown in eq 1-3. En this rxiechan.ism represents ground-state cyclopropane, v * i:s?presents a low-lying vibrationally excited form of cyclopropane, V * * is another intermediate of unspecified ch,aracter, and -= represents propylene.
7 4- Ar
3 '7" + Ar &?'7"" + Ar h2
V* -f Ar
hn
sv**
(1)
(2)
-I=
(3) In order to demonstrate the existence of vibrationally excited cyclopropane, it is necessary to relate the concentrations, v anid vu, to the measured intensities. This is done as followr. Take I B and I as the black body emission and the total radiation intensity, respectively, at any point x along the path of an emitting-absorbing gas and take Ka and K, as the coefficients for absorption and emission. The incremental gain of intensity (dI)in crossing an emitting-al.jso.rbi~ggas thickness dx is written as
where, in a.ddition to the usual assumptions,8 emission has been acisurnetl proportional to v * and absorption to V, i.e., KaIKe = v / v * , If temperature, concentration, and composition are constant over a finite thickness, L, and the incident intensity at x = 0 (due to the black body emission) is IO,integrating eq 4 and rearranging yields
where I,+, i s the intensity at x = L . If there is no irradiation at x = 0, eq 4 integrates to
where I, signides intensity due to emission and self-ab-
sorption only at x = L. If the gas is optically thin as it is in these tests, eq 5 and 6 become, respectively
Ia+e = IBKeL f Io(1- KaL)
(7)
Ie = IBK,L (8) Using the assumption that Ka is propostional to v and K , is proportional to v*, and inserting eq 8 into eq 7 relates the concentrations to the measured intensities.
v
= L Y I [ ( ~ O f Ie
- .la+e)/Iol
(9)
and by analogy with eq 10 --- - ff&
(11) The constants, ai, are determined by an analytical procedure described below. The time rates of appearance of the concentrations of the four species are written as a set of simultaneous, linear, first-order differential equations. The initial value of v is the relevant experimental post-shock concentration. The other initial concentrations were assumed negligible and set equal to zero. The equations were integrated without modification using the Kutta-Simpson one-third r~le.~ Particularly, J~ no assumption of steady states is introduced. Step sizes of 10 psec were generally adequate for stable, accurate integrations over the approximately 700 psec required. The results were generally precise to a t least four digjts although two digit precision was accepted occasionally a t noncritical concentrations (Le., order of 10-9 mol or smaller). At the higher temperatures, smaller step sizes were required for stability, but never less than 1 psec. Integration and all other calculations with the shortest step sizes took approximately 1 sec of execution time on a CDC 6600 computer. The rate constants were calculated in the program from input Arrhenius parameters. The initial trial values of the Arrhenius parameters were decided as follows. The values suggested by BF were used for lzl. In order to calculate kz it was necessary to determine' the equilibrium between v * and V . A Boltzmann distribution over all the vibrational fundamental modes was assumed and the populations of all states having a contribution of 1% or more were included. In practice this required calculating up to the third vibrational level. The calculations were done for the temperature range 1000-1350°K. The equilibrium constant was determined by taking the ratio of the proportionate number of particles in the first excited state to the ground state. The calculation was performed at two different temperatures and a plot based 011K = exp(-AE/RT) was prepared. Values of the equilibrium constant could be determined at any temperature and k z was determined from the relations, kz = k l / K . BF's ratio, k a / k z , was then used for the calculation of k ~ The . fourth constant, kq, was assumed based on some preliminary integrations of the set of differential equations. Finally, kg was found from k4 and BF's value of k S / k 4 . All five rate constants were perturbed in turn and in selected groups from their initial values to obtain the best practical agreement, between the analytically predicted (7) J. N. Bradleyand M. A. Frend, Trans. FaradaySoc.. 67, 72 (1970). (8) H. C. Hottel and A. F. Sarofim, "Radiative Heat Transfer," McGrawHill, New-York, N. Y.. 1967, Chapter 1. (9) F. B. Hiidebrand, "Introduction to Numerical Analysis," McGrawHill, New York, N. Y., 1956, Chapter 6. (10) R. W. Crossley, unpublished program. The Journal of Physical Chemistry, Val. 77, No. 2, 1973
Dorko, Crossley, Grimm, Mueller, and Scheller
146
distributions of concentration with time and the experimental evidence. The criteria for judging the goodness of this agreement will be presented following discussion of the method of transforming the experimental intensity measuremenl s to units of concentration. Experimental intensities were correlated with the analytically determined concentrations at two points along each curve to solve for a using eq 9, 10, or 11. In order to test the validity of t he values of a , several pairs of points on each curve were tested so that the final calculation could be based on a pair of points that gave a representative value of a. Since a is a constant, it has no effect on the shape of either the analytical or experimental concentration us time curves. It merely places the experimental points in the units of the analytical curves. Thus, if the values of the rate constants are not proper, the shape of the analytical curve will not agree with the concentration curve based on the empirical data. Indeed a calculation of a based on an Invalid analytical curve will induce considerable separation between the curves, thus laying the basis for one criterion for judging the goodness of fit of the rate constants. Several critepia were used to evaluate the agreement between the analytical and experimental concentration curves. The correlation coefficient, p , I 1 and standard deviation, u,I1 were calculated. The agreement of the analytically predicted time of V*(max) was considered along with the agreement of the ratio of concentrations, o*(finaI)/v*(max). In addition, v** was kept at a lower concentration than v*, and dominance of v and/or -= was maintained toward the end of the reaction period. Comparisons of the analytical and experimental results were greatly facilitated by the use of computer-generated graphs.1° This techmque was especially productive in comparing cases for which the quantitative criteria had been well satisfied.
IV. Results and Discussion Plots of eq 9 and 10 as via1 and v * / w are shown in Figure 2. The plots are for the data obtained at 1231°K and are typical of the results obtained over the temperature range of the experiments, 1158-1323°K. The resulting curve for vial shows the decrease in V . The shape of the curve is as expected; that is, the concentration decreases sharply at first and more gradually as the reaction proceeds. On the other hand the curve for V * / a z displays an entirely different shape. It rises to a maximum and then decays gradually as v is depleted. The vibrational relaxation time of cyclopropane is much shorter12 than the elapsed time from the start of reaction to the maximum in the curve so the observation is not due to a relaxation process. That is, the time rate of change of the vibrationally excited species. v*. does not correspond to the expected relaxation to an equilibrium situation. BF have found that the process of populating the low lying vibrational levels of the cycloptropane system is inefficient.? The current results lead to tqe same conclusion and indicate that the buildup and decay OS the excited intermediate species is kinetically controlled because the transfer of energy is not efficient for the excita'ion of the low lying vibrational levels of the cyclopropane system. Results of cc~ncentrstionus time appear in Figures 3 and 4 for temperatures 1179 and 1277", respectively. Similar results were obtained for several temperatures over the range 1158- 1323°K. 'The figures show substantial agreement between the analytical and experimental curves over The Journal of Physical Ghernistry, Vol. 77,No. 2, 7973
1.008
b
1.006
1.004
i
1.002
I
I
li
dN
a'
IC
I
0
I
I
I
1
2co
400
600
600
TIME W E C )
Figure 2. Intensity vs. time curves for (a) V i a l and for (b) V * / a 2obtained from experiments at 1231°K.
most of the reaction period. The experimental v points appear to be scattered more than the V * or -= points. This is so because calculating v involves both I,+, and le data (eq 9). Thus, the effects of experimental scatter are magnified, especially when coupled with the relatively low precision of readings from the oscillograms and round-off errors inherent in the denominator of eq 9. Figure 4 shows a flattening in the empirical B and V * points near the end of reaction. This flattening is absent from the analytical curves. We attribute this lack of flattening to the absence of a reverse rate constant, h g , in eq 3. Work is in progress to include hg and to estimate its value from considerations exterior to the perturbation procedure before it i s included in the analysis. The fact that no flattening appears in the empirical points of Figure 3 implies that he becomes significant only at the higher temperatures, i.e., above about 1250°K. Comparing the results of the analyses of shocks in mixtures at 100 and 200 Torr showed that the same values of the k's were equally appropriate at both concentrations. Thus, the Iz's defined by the mechanism of eq 1-3 have no imbedded concentration dependencies, This is in contradistinction to rate "constants7' defined by mechanisms assuming simple, unimolecular isomerization of v .IJ3 The correlation coefficients and standard deviations associated with Figures 3 and 4 are presented in Table I. The former are indicative of how well the shapes of the analytical and experimental curves agree. The latter are indicative of how close together the curves are. The ranges (11) R. S. Burington and D. C. May, Jr., "Handbook of Probability and Statistics," Handbook Publishers, Sandusky, Ohio, 1953. (12) J. D. Lambert and J. S. Raulinson, Proc. Roy. Soc., Sec. A , 204, 424 (1951). (13) S. W. Benson and H. ,E. O'Neal, "Kinetic Data on Gas Phase Unimolecular Reactions, NSRD-NBS-21 U. S. Government Printing Office, Washington, D. C., 1970, p 15. I
Shock Tube Isomerization of Cyclopropane
147
. _ I _ -
EXPERIMENTAL POINTS EXPERIMENTAL POINTS 1.
cf
V
00.
c
4
-- -
ANALYTICAL CURVES
-9
ANALYTICAL CURVES
----
---
D
-I-
s -
----
T7
g:* %-'.=
A+
200
0
600
400
200
v*
600
400
TIME (MICROSECONDS)
TIME (MICROSECONDS) Figure 3. Plots of log concentrations vs. time for the species involved in the isomeriziation of cyclopropane at 1179'K. The lines represent the lag conc:entrations determined analytically and the points represent tho lciy concentrations determined from the experimental intensities.
Figure 4. Plots of log concentrations vs. time for the species involved in the isomerization of cyclopropane at 1277°K. The lines represent the log concentrations determined analytically and the points represent the log concentrations determined from the experimental intensities.
TABLE I: Correlation Coefficients and Standard Deviations at 1179 and 1277°K
TABLE 11: Summary of Ranges of Correlation Coefficients and Standard Deviations for All Cases Analyzed
_ I I _ ( i _ l _ _
1277°K
1179OK
Species
v v* -=
irJ
0.888 0.134 0.95
U
0.13 0.02 6.49
P
0.86 0.96 0.99
Spocias 0
P
0.54-1 .OO 0.88-1 .OO 0.95-i.oa
V
V*
0.08
-=
0.94 0.35
U
0.08-0.82 0.02-0.39 0.22-0.62
TABLE Ill: Arrhenius Parameters for the Five Rate Constants Defined by Eq 1-3 (ki = A , e - E a i i R T )
of p and u for the shocks analyzed appear in Table II. The values of p in Table I are seen to be representative with the exception that the values of p v in Table I appear high relative to the lower limit in Table 11. This lowest value occurred in 1158°K. At this temperature, the reaction is very slow, yiielding weak intensities and high relative errors. If the 1158°K case is excluded, p v ( m i n ) in Table I1 becomes 0.74. The fact that 5 v ( m l n ) is greater than Gp(mln) in Table I11 is due to the scatter in the v points already discussed. In contrast, the higher a-=(m,n) = 0.22 is due to overall separation of the analytical and experimental -_- curves,. This separation results from differences in the slopes of these curves which causes less than optimum in Table values of 013 (eq 11). However, inspecting p-= III shows that the curves actually have quite reasonable statistical agreement overall. The lack of subjective visual agreement in the -.= curves of Figures 3 and 4 results from the coupling of the overall separation as reflected in
i
A,
E a L , kcal/mol "K
1 2
1 X loi1 (cc/mol sec) 1 X 10'' (cc/mol sec)
3 4 5
6X 7x
12 8 28 20 32
5x
ioi3
(cc/mol sec) (cc/rnol sec) 109 ( s e c - I )
and from differing curve shapes in the initial reaction period. Due to reaction start up, low intensities, and rapid change of intensities, the experimental results are somewhat suspect in this region. If the experimental points are shifted downward, the visual impression is vastly improved and the curve shapes appear in good . agreement, confirming the high values of ~ - = = ( ~ j ~ ) In view of the scaling procedure used for the experimental concentrations, agreement in curve shape is deemed much more important than overall separation.
- -a
The Journal of Physical Chemistry, Vol. 77,
No. 2, 1973
148
T. Su and L. Kevan
TABLE YV: Values of kl,k !/ k 2 , k3/k2, and k 5 / k 4 Found by the Perturbation Procedure! eaf the Present Work and Compared
to Independent Sources Calculated from Table I I I
kl kl/k2 k3/kz k5/k4
Other sources
1 X lo1' exp(-l2/RT) 7.95 X 1010eXp(-11.58/RT)a exp(-4/RT) exp(-3.74/RT) = Kb 6 X 102exp(-20/RT) 1.42 X lO'ex~(-42.95/RT)~ 1 X 10-3exp(-12/RT) 1.78 X 10-3exp(-10.6/R?-ju
0 Calculated from BF's results.' of the present work.
results of the present work essentially agree with BF for kl and k5/k4. The disagreement in k3/k2 is much less serious than it might seem. First, over the temperature range of the present work, there is little difference in the values of k3/kz calculated by either expression. Further, AE, = 20 causes only a factor of 3 change in ka/kz. This does not alter the results significantly. The agreement between kl/kz and K is especially satisfying in view of the major objective of this work, establishing evidence of a low lying vibrational intermediate.
Calculated as described in section II
The results of perturbing the Arrhenius parameters are given in Table 111. Table IV shows values of 121, Kllkz, k3/k2, and k:j/k4 calculated from Table I11 and compares these with values obtained from independent sources. The
Acknowledgment. The work was supported in part by the Air Force Systems Command through the AFSC/ AFIT Research Support Fund. A portion of it was presented at the 163rd National Meeting of the American Chemical Society, Boston, Mass.
ion Cyclotron Resonance Studies of Ionic Reactions in ons. Excited Ions and Their Deexcitation Timothy Su and Larry Kevan*' Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received April 14, 1972)
Ion-molecule reactions have been studied by ion cyclotron resonance (icr) mass spectrometry in C2F6, Czl"~!,C3F6, c-C4F8, and 2-C4F8. Fluoride transfer and collision-induced dissociation reactions predominate. 'The signs of the change in rate constant with ion kinetic energy (double resonance responses) for all the reactions are consistent with the heats of reaction based on a critical compilation of heats of formation of fluorocarbon species. Charge transfer reactions indicate that the average internal energy is less for fluorocarbon ions observed by icr than for the same ions observed by tandem mass spectrometry. This difference is interpreted in terms of the different time scales for ion detection in the icr and tandem instruments. The interpretation is supported by demonstrating collisional deactivation of excited ions from pressure effects on endothermic reactions. Radiative deactivation of excited ions may also occur.
Introduction and C3F8334 have been Ion-molecule reactions in studied by tandem and high-pressure time-of-flight mass Spectrometers. High-pressure experiments on complicated systems can, at best, only specify the number of collisions involved in the production of a given ion without necessarily specifying what kind of collisions are involved or which of the possible reactants is contributing to a particular reaction. The tandem mass spectrometef unquestionably permits the most specific identification of reactantproduct relationships. However, it generally does not permit truly thermal energy reactions to be observed. In an ion cyclotron resonance (icr) spectrometer, ionmolecule reactions can be studied from thermal energies to several eV at a rather low pressure. The reaction path length and the transit time of an ion through the icr cell are much longer than in the tandem or time-of-flight mass spectrometer. Icing and Elleman5 reported a cursory icr study on ion-molecule reactions in C2F6. However, a more The Journal of P b p ca: Cneniistry, Vol. 77, No. 2, 1973
detailed investigation of icr of perfluorocarbon ionic reactions is required to develop the general outlines of perfluorocarbon ion chemistry. Here, we report ion-molecule reaction studies of several perfluorocarbons by an icr spectrometer. CZFGand C3F8 have been studied most intensively and are compared with results from tandem and high-pressure mass spectrometers to obtain rather complete information about the ionic reactions. We have found that the difference in the time scale of detection between icr and tandem mass spectrometers can produce different results if excited ions are present. Ionic reactions in c - C ~ F C3F6, ~, and 2-C4F6 were also studied. (1) Author to whom inquiries should be addressed (2) R E Marcotte and T 0 Tiernan J Chem P h y s , 54, 3385 (1971) (3) T Su. L Kevan, and T 0 Tiernan, J Chem P h y s , 54, 4871 (1971) (4) T Su, L Kevan, and T 0 Tiernan ,I Pbys Chem 75, 2534 (1971) (5) J King and D D Elleman, J Chem Phys , 48,412 (1968)
