T H E
J O U R N A L
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PHYSICAL CHEMISTRY Registered in U . S. Patent Office
0 Copyright, 1977, by the American Chemical Society
VOLUME 81, NUMBER 19
SEPTEMBER 22, 1977
The Self-Exchange of Dinitrogen Behind Reflected Shock Wavesiaib J. M. Bopp, R. D. Kern,* and 1. Niki Depalfment of Chemistry, University of New Orleans, New Orleans, Louisiana 70122 (Received April 21, 1977) Publication costs assisted by the National Science Foundation
The reaction of an equimolar mixture of and 30N2(4% each) diluted by a mixture of Ne (80%) and Kr (12%) was studied over the temperature and density range 4700-5400 K and 2.5-2.6 X 10+ mol ~ m -respectively. ~, The reflected shock zone was sampled by a time-of-flight mass spectrometer at 20-ps intervals during typical observation periods of 0.5 ms. The product profiles were fit to the equation (1- 2fzg) = exp(-k'tz), where fig is the mole fraction of 29N2.The time dependence z was determined to be 2.7 and the rate constants were best represented by the expression 12 = 10-'.24*0.54 exp(-138 f 10/RT) ~ 8 The . ~inert . gas dependence was assumed to be first order as reported from a previous single-pulse shock tube investigation. The rate constants from the single-pulsestudy were recalculated using z = 2.7 and were combined with the results herein. The resulting Arrhenius parameters span the extended temperature range 3200-5400 K: log A = 20.66 f 0.08; E = 140 f 1 kcal mol-l. The units of A are cm3mol-l s-2,7.The nonlinear time dependence for product formation confirms the earlier proposition of a mechanism which consists of a multistep sequence of reactions.
Introduction Most of the shock tube work during the past 12 years concerned with exchange systems has dealt with D2 re; acting with a variety of relatively simple molecules: NH3, C Z H ~CH4,* , ~ H2S,5HC1,6 HCN,7 HBr,8 and H2.9J0 The single-pulse shock tube technique has established the order for the reactants and inert gas and the activation energy for many of these r e a ~ t i o n s . ~ Dynamic , ~ ~ , ~ , ~sampling of the reflected shock zone by time-of-flight mass spectrometry for the H2-D2 exchange measured the time dependence of product formation.1° Recalculation of the single-pulse resultsgataking account of nonlinear product growth produced a single Arrhenius line for both sets of data over the temperature range 1000-3000 K.l0 Less work has been reported for heavy atom exchange. A recent studyll confirmed the earlier single-pulse workL2 on the self-exchange of carbon monoxide. The temperature range was extended to an upper value of 4650 K and once again agreement was obtained between the singlepulse and the TOF dynamic sampling technique. Furthermore, the mechanism by which the exchange was proposed to take place was tested. The original proposal stated that the requirement for product formation is a 1795
critical amount of vibrational energy (v = 7-9) possessed by one of the reactant molecules." However, population of these levels by mercury photosensitization a t room temperature failed to produce the predicted amount of exchangeall This particular photolytic result has been confirmed by others.13 The self-exchange of dinitrogen, 28N2+ 30Nz P9N2, has been studied with the single-pulse technique.14 The reaction was found to be first order with respect to nitrogen and also to argon, the inert gas. The activation energy was determined to be 116 kcal mol-' over the temperature range 3200-3800 K. The time dependence for product formation was assumed to be unity. The purpose of this investigation is to extend the temperature range, measure the time dependence and activation energy for the exchange, and to reconcile the two sets of data if possible.
Experimental Section The apparatus employed herein has been described previ0us1y.l~ Isotopic nitrogen (99% enrichment) was purchased from Stohler Isotope Chemicals and was used without further purification. Linde dry grade nitrogen was twice purified by condensation onto a type 4A molecular
1796
J. M. Bopp, R. D. Kern, and T. Niki
1 -
./
/
8
n
c
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D
Figure 2. Flt of profile generated by eq 1 to experimental points for a run at 5280 K.
B
2 82
,-=
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25
z e
2
=
1precludes the steady state approximation. This fact is consistent with the model that depicts molecules undergoing the exchange process as originating from a critical energy manifold at a rate greater than that required to replenish or maintain the population of the manifold at some equilibrium or quasiequilibrium condition. Unfortunately, the rate constants for the excitation and exchange steps are not known and the subsequent predictions for the reactant and inert gas order, the value for z, and the activation energy cannot be made. However, the data and equations presented herein do reproduce the experimental profiles for product formation and the rate constants can be reconciled with those derived by the single-pulse technique after adjustment for the observed time dependence.
References and Notes (1) (a) Paper presented at the 173rd National Meeting of the American Chemical Society. New Orleans, La., March, 1977. (b) Support of this work by the National Science Foundation, Grant CHE-7608529, is gratefully acknowledged. (2) (a) A. Lifshtz, C. Lifshitz, and S. H. Bauer, J . Am. Chem. SOC.,87, 143 (1965); (b) A. Burcat and A. Lifshitz, J . Chem. Phys., 52, 337 (1970). The Journal of Physical Chemistty, Vol. 81, No. 19, 1977
J. K. Kim, V. G. Anicich, and W. T. Huntress
1798 (3) S. H. Bauer and K. Kuritani, J . Am. Cbem. SOC., 87, 150 (1965). (4) (a) W. Watt, P. Borrell, D. Lewis, and S.H. Bauer, J. Chem. Pbys., 45, 444 (1966); (b) W. C. Gardiner, J. M.Herrmann, W. G. Mallard, and J. H. Owen, Jnt. J . Cbem. Kinef., 8, 111 (1976). (5) A. Burcat, A. Lifshitz, D. Lewis, and S. H. Bauer, J . Cbem. Pbys., 49, 1449 (1968). (6) R. D. Kern and G. G. Nika, J . Phys. Chem., 75, 171 (1971). (7) J. M. Brupbacher and R. D. Kern, J . Pbys. Cbem., 76, 285 (1972). (8) R. D. Kern and G. G. Nika, J . Pbys. Cbem., 78, 2549 (1974). (9) (a) S.H. Bauer and E. Ossa, J . Cbem. fbys., 45, 434 (1966); (b) A. Burcat and A. Lifshitz, ibid., 47, 3079 (1967). (10) R. D. Kern and G. G. Nika, J . Pbys. Cbem., 75, 1615 (1971).
(1 1) A. F. Bopp, R. D. Kern, and B. V. O'Grady, J. Pbys. Cbem., 79, 1483 (1975). (12) A. Bar-Nun and A. Lifshitz, J. Cbem. Pbys., 51, 1826 (1969). (13) A. L. Rockwood and E. A. McCullough, presented at the 172nd National Meeting of the American Chemical Society, Physical Chemistry Division, Abstract 133, 1976. (14) A. Bar-Nun and A. Lifshitz, J. Chem. Pbys., 47, 2878 (1967). (15) J. M. Brupbacher, R. D. Kern, and B. V. O'Grady, J. Pbys. Cbem., 80, 1031 (1976). (16) J. P. Appleton, M. Steinberg, and D. J. Liquornik, J . Cbem. Pbys., 48, 599 (1968). (17) R. K. Lyon, Can. J. Cbem., 50, 1437 (1972).
Product Distributions and Rate Constants for the Reactions of CH,', CH4+, C2H2+, C2H3', C2H4', C2H5+,and C2H6+ Ions with CH4, C2H2,C2H4, and C&+ J. K. Klm, V. G. Anicich, and W. T. Huntress, Jr." Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 9 1103 (Received April 13, 1977) Publication costs assisted by the Jet Propulsion Laboratory
Ion cyclotron resonance methods have been used to measure the product distributions and rate constants for the reactions of various CH,' and CZH,' ions with CH4, C2H2,C2H4, and CzH@ The product distributions for some of the reactions of C2H2+,C2H4+,and CzHs+ions are strongly affected by excess ion internal energy. Photoionization mass spectrometer experiments have been conducted in a few cases in order to obtain product distributions for reactions of these ions in the ground vibronic state.
I. Introduction A number of hydrocarbon ion-molecule reactions are examined in this work by both ICR and photoionization mass spectrometer methods. The work was initiated in order to measure product distributions for these reactions. Accurate product distributions are not available for most of the reactions studied in this work, and many of the product distributions which are available from earlier ICR and from single-source, trapped-ion or tandem mass spectrometer experiments (see section 111) are incomplete or are obtained for ions with excessive internal or kinetic energies. Parent ions obtained from CzHz,C2H4, and C2H6 by electron impact above threshold contain a large amount of excess internal energy, and this effect is readily observed in the product distributions for reactions of these ions. Careful attention is paid to the problem of excess ion energy in the present work, and photoionization experiments at threshold were conducted in some cases in order to verify the low energy, electron-impact results obtained from ICR experiments. Rate constants were also measured, and the results obtained for both product distributions and rate constants are compared to the more recent work on these reactions conducted at thermal or near-thermal ion kinetic energies. 11. Experimental Section The JPL ion cyclotron resonance mass spectrometer was used to obtain product distributions and rate constants as previously described.lS2 Product distributions were measured at constant magnetic field. The marginal os+ This paper presents the results of one phase of research carried out a t t h e J e t Propulsion Laboratory, California I n s t i t u t e of Technology,under Contract No. NAS7-100, sponsored by the National Aeronautics and Space Administration.
The Journal of Physical Chemistry, Vol. 8 1, No. 19, 1977
cillator frequency was changed in order to observe the various product ions during irradiation of the reactant ion at a single frequency. A Q-spoiler circuit was used to calibrate the sensitivity of the observing oscillator at the various frequencies weds3Rate constants were measured using the trapped ion method by observing the change in ion decay characteristics on addition of the reactant gas.2 In a few cases, only the ratio of decay rates at long times could be used, such as for the CzH4+-CzHzreaction where the reactant ion actually increases at early times on addition of C2Hzbecause of the CzHz+-C2H4charge transfer reaction. For the reaction of Cz&+ with C2Hs,and for the reaction of C2H4+with CzH6 where CZH4' is prepared from C2Hs itself, the total sum method was used for obtaining the rate c0nstant.l Various gases were used for obtaining relatively clean sources of the reactant ions in ICR experiments. The CH3+ ions were obtained from CHI at 15 eV and from CH&l at 18 eV. The CH4+,C2H2+,and C2H6+ions were obtained from their parent gases at 16, 12.5-16, and 13-16 eV, respectively. The CzH3+ ions were obtained from CzH3C1 at 16 eV, and CzH5' ions were obtained from both CzH,C1 at 16 eV and from CzHG at 16 eV. Various sources were used for CzH4' including the parent gas at 13-16 eV, CzH&l at 13-16 eV, and CzHGat 11-16 eV. No significant differences (f15%) were noted in the rate constants for any of the reactions studied for reactant ions obtained from the various sources and under the various electron impact energy conditions given above. Significant changes were observed for the product distributions of some of the reactions of C2H2+,C2H4+,and C%&+ ions, but not for any of the reactions of CH3+, CH4+,CZH3+, or CzH5' ions. Photoionization experiments were conducted using the JPL-CIT photoionization mass spectrometer4 where possible in order to obtain product distributions for those
