Reaction of CzNz and HP behind Reflected Shock Waves
1329
Reaction of Cyanogen and Hydrogen behind Reflected Shock Waves' J. M. Brupbacher and R. D. Kern* Department of Chemistry, Louisiana State University in New Orieans, New Orleans, Louis/ana 70722 (Received September 29, 7972) Publication costs assisted by the National Science Foundation
+
The metathetical reaction, C2N2 H2 e ZHCN, has been studied by shocking equimolar amounts of the reactants in the presence of an inert gas diluent over the temperature range 1850-2650°K. A complementary shock tube facility was utilized to obtain the data from the reflected shock zone by recording the infrared emission of HCN and the time-resolved mass spectra at m / e 27 (HCN) and 52 (C2N2). The total density variation was 1.8-4.4 X 10-6 mol/cm3. Observation times were typically 500 psec during which period an equilibrium condition was established for the higher temperature experiments. The growth of the mole fraction of HCN, ~ H C N ,was found to be a nonlinear function of time and to depend upon the inert gas concentration, [MI. The data from the two independent techniques of infrared emission and mass spectrometry were fitted to the bimolecular rate expression that includes the back reaction, ln j[(K - 4)fHCN- ( K 2K1l2)]/[(K - 4)fHCN - ( K - 2KT/2)])= ~ ~ I [ C ~ N ~ ] C J [ M ] ~In. ~[ (~KKi-~/~~~ 2K1/2)/(K - 2K1/2)], where the forward rate constant is givenby k l = 1035.35*0.21exp(-61,610 A 2070/RT) cm3 mol-1 sec-2 [M]-O.75. These results rule out the direct bimolecular reaction. This complex reaction is discussed in terms of an atomic mechanism -and a mechanism involving vibrationally excited species. Experiments on an equimolar mixture of cyanogen and deuterium revealed a lowering of the preexponential factor in reasonable agreement with that predicted from the square root of the inverse ratio of reduced masses.
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Introduction Reactions of hydrogen with various halogens and their reverse decompositions have played an important role in the development of chemical kinetics. The most notable of these reactions is that of hydrogen with iodine which was first investigated by Bodenstein.2 This reaction was most recently demonstrated by Sullivan to occur by an atomic pathway after having been for some 70 years the classic example of a reaction proceeding uia a four-center transition state.3 One interesting feature of this reaction is that in applying the symmetry rules proposed by Pearson for predicting possible reaction mechanisms, it is found that the direct molecular channel is symmetry forbidden.4~5 The same prediction holds for all of the halogen-hydrogen reactions. An examination of the coefficients of the atomic orbitals which contribute to the highest occupied molecular orbital (HOMO) of cyanogen discloses that it is antibonding while the lowest unfilled6 molecular orbital (LUMO) is bonding.7 Both the transfer of electron density from HOMO of cyanogen (rg*)to the LUMO of hydrogen ( uu*) or from HOMO of hydrogen ( u g ) to LUMO of cyanogen (n)occur with positive orbital overlap. Hence, when the entire molecular orbitals are considered with respect to positive overlap, the forward reaction appears to be symmetry allowed. It also follows that the same prediction holds for the reverse reaction. However, this argument is incomplete since the bonds made and broken during the course of reaction have been ignored.8 A previous investigation of the reaction of cyanogen and hydrogen was performed in a silica vessel9 over the temperature range 550-675" and was reported to fit reasonably well with the expression developed by Bodenstein and Lind for the H2-Br2 system.1° The reaction was found to
+
take place in an essentially homogeneous manner with an activation energy of 72 kcal/mol. The decomposition of cyanogen has been extensively investigated in shock tubes.11-14 Kinetic data are not available for the decomposition of hydrogen cyanide. The purpose of this work is to study this metathetical reaction under truly homogeneous conditions at elevated temperatures and to test for the existence of the direct bimolecular reaction mechanism. Furthermore, the discussion of the role of cyanogen as a possible intermediate D2 exchangel5 was limited due to the lack in the HCN of data on the reaction of cyanogen and hydrogen in the temperature region 2000-3000°K. The extent and nature
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(1) (a) Support of this work by the National Science Foundation under Grants No. GP-23137 and GP-33949 X is gratefully acknowledgeid. (b) Paper presented in part at 163rd National Meeting of the American Chemical Society, Boston, Mass., April 1972. (2) M. Bodenstein, Z. Phys. Chem., 13, 56 (1894):22, 1 (1897):29, 295 (1899). (3) J. H. Sullivan, J. Chem. Phys., 46,73 (1967). (4) R. G. Pearson, Chem. Eng. News, 48,66 (1970). (5) R. G. Pearson,Accounts Chem. Res., 4,152 (1971). (6) W. L. Smith, J. Chem. Educ., 49,654 (1972). (7) A. D. McLean and M. Yoshimine, "Tables of Linear Molecule Wave Functions,'' International Business Machines Corporation, San Jose. Calif.. 1967 We are g r a t e f i to Professor Roald Hoffman for sending us a correlation diagram which shows a crossing of electronic levels when the carbon-carbon u bond of cyanogen and the Hz bond are included. This more complete analysis reveals that the direct bimolecular pathway is forbidden. N. C. Robertson and R. N. Pease, J. Amer. Chem. SOC., 64, 1880 ~~
(1942). M. Bodenstein and S. C. Lind, Z.Phys. Chem., 57,168 (1907). K. T. Knight and J. P.Rink, J. Chem. Phys., 35, 199 (1961). W. Tsang, S. H. Bauer, and M. Coperthwaite, S. Chem. Phys., 3Ci,
1768 (1962). M. W. Slack and E. S. Fishburne, J. Chem. Phys., 52, 5830 (1970). M. W. Slack and E. S. Fishburne, J. Chem. Phys., 54,1652 (1971). J. M. Brupbacher and R . D. Kern, J. Phys. Chem., 76,285 (1972). The Journal of Physicai Chemistry, Voi. 77, No. 7 1, 7973
1330
J. M. Brupbacher and R. D. Kern 70
DETEC TOR AMPLIFIER BlOMATlON 61 O B TRANSIENT RECORDER OSClL LOSCOPE STRIP CHART RECORDER
T
I NTERFACE TELETYPE WITH PAPER TAPE PUNCH AND READER' PDP-IO COMPUTER
TIME! USEC
Figure 2. Plot of signal at 5.05 p arrival at observation station.
YS.
time to determine shock
Figure 1. Schematic of infrared emission analog to digital conversion system.
of isotopic effects are examined with data collected from an equimolar mixture of cyanogen and deuterium.
Experimental Section The complementary shock tube facility upon which all data were obtained has been described previously.16 The TOF shock tube system remains to a large extent unchanged. However, the ir system has been significantly improved in the data reduction phase of operation. Two Biomation 610B transient recorders have been acquired which utilize a very high-speed six-bit analog to digital converter to digitize the amplified ir profiles. These recorders accept full-scale signals from 50 mV to 50 V with a frequency response of dc to 2.5 MHz. Sampling rates are available from 0.1 psec to 50 msec for 256 words. The digitized output of the Biomation units is then fed uia a Pivan Data Systems Model B103 interface to a 33ASR Teletype equipped with an acoustic coupler, paper tape punch, and reader. A permanent paper tape record is made for each experiment. The output for each trace from the interface is formatted in a 10 X 25 array of two-digit numbers followed by a line of five numbers and a terminating colon. Each number corresponds to the relative emission intensity on a scale of 1 to 63 where 63 represents the full-scale voltage setting. The Biomation units can also output a smoothed form of the ir signals on an oscilloscope or strip chart recorder for visual analysis or a permanent paper record, respectively. The additional equipment does not interfere with the previous method of oscilloscope analog analysis if it is desired. The ir profiles can now be analyzed in minutes as compared to several hours by earlier techniques. Figure 1 shows a diagram of the analog to digital conversion system. The remaining portion of the ir shock tube facility has been reported.le The experiments were performed using Matheson cyanogen (98.5%) which was distilled several times keeping only the middle fraction in each distillation. Liquid Carbonic cylinder hydrogen (99.99%) was used without further purification as was the Matheson C P deuterium (99.5%). Calibration experiments on the ir system consisted of doubly distilled hydrogen cyanide which had been prepared for an earlier study.15 The diluent gas for the TOF experiments was a mixture of Matheson Research The Journal of Physical Chemistry, Vol. 77, No. 11, 1973
Grade 99% Ne-1% Ar. For the ir experiments Liquid Carbonic argon (99.99%) was used as the diluent. Vacuums during mixture preparation were typically 1 x 10-6 Torr and outgassing rates were less than 0.5 plmin. The partial pressures of all gases were measured with Wallace-Tiernan differential pressure gauges 0-10 and 0-400 in water. The gases were stored in 5-1. bulbs and analyzed using the TOF mass spectrometer. The mixtures were found not to contain any contaminant in a concentration greater than background. The 0 2 level was less than 25 ppm as compared to a standard 0 2 mixture. Mixtures were allowed to stand at least 24 hr before shocking. In the TOF experiments, mole fractions of HCN and CzNz were obtained from careful measurement of the peak heights corresponding to m / e 27 (HCN) and 52 (C2N2). For each experiment mass spectra were recorded a t 20-psec intervals on Polaroid 10,000 speed film. Due to the unequal ionization cross sections of CzNz and HCN, an equimolar mixture was analyzed at room temperature in order to establish a sensitivity factor. A value of 1.054 was obtained using an ionization voltage of 35 eV for the equimolar ratio CzNz/HCN. It was also noted that this value did not change appreciably over the range 25-45 eV. A small contribution to m / e 27 resulted from isotopic contributions from the CN cracking peak. A value of 0.062 for CN (27)/CN (26) was determined by performing several shock experiments with a mixture of 2% CzNz in diluent gas at 35 eV. The effective HCN peak height in the metathetical reaction experiments then resulted from the difference between the measured value a t m / e 27 and 0.062 times the measured value of m / e 26. This value was used in conjunction with the peak height at m / e 52 divided by 1.054 to obtain the mole fraction of product a t any time. Corrections of this type were not necessary for those experiments in which deuterium was used instead of hydrogen. Peaks at m / e 28 (DCN), 40 (Ar), and 52 (CzNz) were measured and used directly to calculate the mole fraction of DCN as a function of time. Argon served as an internal standard for the ion source pressure. In the TOF experiments mixtures were analyzed for 0 2 within 20 sec of shocking. Experiments were not performed on any mixture which had an oxygen content greater than 25 ppm. (16) R. D. Kern, Jr., and G.G . Nika, J. Phys. Chem., 75, 171 (1971).
1331.
Reaction of C2N2 and H2 behind Reflected Shock Waves
ried out on equimolar HCN-H2 mixtures of the same concentration as the reacting experiments. From these experiments a plot of log of the steady calibration intensity as a function of temperature was constructed. The mole fraction of HCN, f H C N , at any time was calculated using the relationship HCN,
fHCN =
Flgure 3. Polaroid record of ir experiment: lower trace at lefthand side is emission from HCN at 3.0 p ; upper trace is emission from C2N2at 5.05 p .
WCN,
where HCNl is the reacting HCN signal height at some time t and HCN, is the signal obtained from the calihration plot a t that temperature. Several experiments were performed on a 290 CzNz in argon mixture. The Polaroid record of one of these runs a t 5.05 p is depicted in Figure 4. These photographs display the achievement of vibrational equilibrium for CzNz at shock temperatures and pressures similar to those recorded for the metathesis experiments. An approximate value for vibrational equilibration of CzNz was found to he 130 psec at 2500°K. Exneriments on the TOF shock tube facility were carried but with equimolar mixtures of CzNz and Hz a t 5 Torr starting pressure. For reasons which will become apparent in the Discussion section, a number of experiments were performed on the TOF in which DZ was used instead of Hz. The mole fraction of product, f P , a t any time is given by ~
A narrow-band Infrared Industries interference filter with center wavelength at 3.0 p was selected to monitor the emission from the production of HCN. A suitable filter could not be found for cyanogen which would permit meaningful kinetic analysis. However, a second filter a t 5.05 p was chosen to observe the emission from the C-N stretch of both CzNz and HCN. Measurements of this emission were taken at 0.2-psec intervals for all ir experiments. The emission at 5.05 p was plotted as a function of time and the rising portion was extrapolated to the base line. This resulted in an accurate determination of the time of shock wave arrival a t the slits. One of these plots is displayed in Figure 2. Measurements at the 3.0-p filter were taken at I-, 2-, or 5-psec intervals depending upon the reaction rate. No attempt was made to study the c2Nz-D~system on the ir shock tube because of complicated emission a t 3.8 p where DCN emits. The slit width for both filters was kept constant at 0.5 mm. The possibility of emission from CN radicals in these experiments is discounted by the work of Slack and Fishburne who studied the decomposition of C2N21' by recording CN emission. The lowest temperature included in their report, 2750"K, was limited by weak emission and exceeds the upper extent of the range investigated herein, 2650°K. Calibration experiments described in the next section confirm the stability of the species HCN and CzNz with regard to pyrolysis within the observation period and temperature range of this work. The temperature for each experiment was determined as mentioned in an earlier work.15 Hydrogen was used as the driver gas for all experiments. All calculations were accomplished with the aid of a DEC PDP-10 computer. Plotting of experimental results was carried out by a Complot Model DP3 plotter.
Results Reacting experiments on the ir system were performed on equimolar CzNz-Hz mixtures. A Polaroid record of the emission time history for one of these runs is shown in Figure 3. The trace shows clearly the nonlinear growth of product and the establishment of an equilibrium value in the 500 psec of reaction time. Calibration runs were car-
where h c 2 N 2 is the observed peak height of CzNz and h, is the corrected peak of HCN if Hz is used. For those experiments where Dz is a reactant, f,, and h, are the mole fractions of DCN and the peak heights attributed to DCN, respectively. The calculated mole fractions for both the ir and TOF experiments were fit to the second-order rate equation that includes the back reaction
(K
- 4)f, - ( K
+ 2!m
where fp is the mole fraction of product, z is the time power for product formation, and K is the equilibrium constant of the reaction. Equilibrium constants were obtained from a van't Hoff plot which was prepared using thermodynamic data from JANAF.'? The kinetic parameter k' is given by the expression
k'
=
k,[Rlo[M]Y
(4)
where kl is the rate constant for the forward reaction and y is the order with respect to the total density. The symbol [Rjo refers to the initial concentration of either reactant. Equation 3 may he recognized in its standard form by setting L = 1 and y = 0. As a consequence of the exothermicity of reaction (AH"zooo K = -12 kcal/mol), the reflected shock zone was heated approximately 40°K for a 2% equimolar mixture. T o minimize the effects of this heating on the reaction (17) "JANAF Thermochemical Tables.'' The DOWChemical Ca.. Midland. Mich.. 1971
The J ~ u m a l o l P h y ~ i c aChemiSBy, l Voi. 77, No. 11. 1973
1332
Figure
4. Experimental record Showing ir emission from 5 05 p : 2% C2N2in argon at 2500°K
J.
C2N2
M. Erupbacher and R. D. Kern
at Figure 6. Arrhenius plot for data in Table I : V, mixture 0.mixture E.
Figure 5. Reaction profile lor HCN generated from a n ir experiment at 2123°K and fit to a quadratic time dependence: every fifth point is plotted. rate, only those mole fractions less than 50% of the equilibrium amount were used in the profile calculation. The equilibrium mole fraction of product was determined with the following equation (5) The values calculated from eq 5 ranged from 0.91 a t 1800°K to 0.86 at 2500'K and agreed satisfactorily with the experimental value obtained from both the ir and TOF facilities. Thus, the validity of sampling a low enthalpy change reaction from the reflected shock zone is supported by equilibrium constants obtained from tables of thermodynamic data. This is not the first demonstration of such agreement.lJJ6 Plots of the log of the left-hand side of eq 3 us. the log of reaction time displayed slopes of z = 2 for those mole fractions less than the limit described in the preceding paragraph. For all experimental profiles of which Figure 5 is but one example, the data could he adequately fit with a quadratic time power. The value of k' for each experiment was determined by variation of k' until a minimum The Journal of Physrcal Chemrslry. Yo1 77, NO 1 1 , 1973
A;
and
standard deviation was obtained by the method of least squares between the experimental mole fractions and those generated using eq 3. The resulting best fit line and the experimental mole fractions were drawn with the aid of a digital plotter as shown in Figure 5. The order with respect to reactants was confirmed by shocking a 1% CZNZ-I% Hz and a 2% CzNz-2% HZ mixture on the TOF a t a n initial pressure of 5 Torr. Arrhenius plots of I n k ' us. T-1were constructed for these mixtures. The resulting parallel lines differ by a factor of two. Thus, doubling the initial concentration of reactants had the effect of doubling the value of the kinetic parameter k' within the experimental error associated with k ' . This indicated that the reaction was second order with respect to the reactants. The total density dependence was determined by performing experiments on the ir shock tube system on a 2% CzNz-2% HZ mixture at both 5 and 10 Torr initial pressure. Several runs were accomplished on a 1% CzNz-l% Hz mixture a t 10 Torr. The value of y was arrived a t by variation of y until a minimum standard deviation was obtained in an Arrhenius plot of the data with the rate constants on a total density basis. Using a value of 2 for z, a value of 0.75 was found for y. The total density rate constants for the ir experiments are given in Table I and the Arrhenius plot in Figure 6. The least-squares treatment yielded values of log A = 25.29 0.23 and E* = 61.02 i 2.29 kcal/mol. The units of A a n d ' [ M ] are consistently cm3 mol-' sec-Z (M)-Q-75and mol cm-3, respectively. Using values of 0.75 and 2 for y and z. the TOF rate constants were likewise converted to a total density hasis and fit to the Arrhenius equation. Least-squares treatment of this data gave values of log A = 25.80 0.57 and E* = 65.72 i 5.38 kcal/mol. The TOF rate constants are given in Table I1 and the Arrhenius plot in Figure 7. The ir and TOF experiments agree within one standard deviation. Combination of the data followed by leastsquares treatment to the Arrhenius equation gave values 0.21 and E* = 61.60 i 2.07 kcal/mol. of log A = 25.35 The Arrhenius plot for the combined data is displayed in Figure 8.
*
*
1339
Reaction of C Z Nand ~ H2 behind Reflected Shock Waves TABLE I: Rate Constants for the c2Nz-H~Reaction from I r Experiments kl X Mixtuae
Argon diluent A. 2% C2N2-2?0' PI = 5Torr
6. 2% C2N2-2% P , = 10Torr
T, O
H2
H2
K
1944 2041 2075 2123 2175 2212 2224 2279 2586 2668 1838 1891 1989 2037 2082 2097 2101 2120 2128 2143 2237
p x 106, mol ~ m
1.95 1.99 2.03 2.03 2.05 2.09 2.07 2.09 2.19 2.22 3.85 3.87 3.96 4.00 4.06 4.04 4.04 4.06 4.08 4.12 4.15
cm3 molsec-* - ~(M)-0.75
3.94 3.75 7.10 8.18 10.5 24.5 15.5 25.1 194 286 1.18 2.14 2.91 4.24 11.4 9.16 6.90 13.2 13.2 15.9 14.6
I A , " " 1
5 0
4.0
6 0
1 0 %4/T( ~ . K)
Figure 7. Arrhenius plot for data in Table II: X , mixture A; 0 , mixture B; and 4,mixture C.
TABLE II: Rate Constants for the c2N2-H~Reaction from TOF Experiments ki X lo-", p
Mixtllre
Ne-l% Ar diluent A. 2% C2N2-2% H2 PI = 5 Torr
B. 1 % C2N2-1% H2 P , = 5Torr
x
106,
cm3 mol sec-*
T, "K
mol c w 3
(M)-0.75
1962 1983 1990 2009 2024 2039 2059 2069 2095 2122 2144 2160 2251 1844 1892 1989 1989 2003 2121 2285 2303
1.95 1.98 1.96 1.98 2.00 2.02 1.97 2.03 2.04 2.04 2.09 2.07 2.10 1.79 1.81 1.89 1.85 1.85 1.93 1.97 1.98
2.77 1.70 5.06 3.53 6.44 6.66 3.22 9.89 9.53 13.9 15.2 19.8 398 1.35 1.59 3.48 4.65 4.73 7.95 27.4 27.8
0
1 0 ~4*/ T (
K)
Figure 8. Combined Arrhenius plot for data in Tables I and II
in place of Hz. Least-squares treatment of this data gave values of log A = 25.11 f 0.33 and E* = 61.48 f 3.27 kcal/mol. Table I11 gives the rate data obtained for the czN2-D~system. A few experiments were performed on the TOF on a 2% CzN2-2% H2 mixture in which the TOF was adjusted for high mass analysis ( m / e 40 m / e 100). In the 500 psec (of reaction time no species could be observed other than
-
CzN2. Discussion
A series of experiments were performed on the TOF using a 2% CzN2-2% Dz mixture at 5 Torr. These experiments were treated in a manner similar to that of the c2N2-H~system. The results of these experiments compared to the C2N2-H2 Arrhenius dashed line are shown in Figure 9. These experiments show a marked decrease in the values of k l by a factor of about 0.6 when D2 is used
The results obtained on the complementary shock tube facility for the czN2-H~reaction indicate that the mech,anism is complex. Both the quadratic time dependence fix product formation and the order dependence on the total density argue effectively against the direct four-centler mechanism which predicts a linear time dependence and zero order with respect to inert gas. The Journal of Physical Chemistry, Voi. 77, No. ? I , 19T3
1334
J. M. Brupbacher and R. D. Kern
TABLE I l l : Rate Constants for the czN2-D~Reaction from TOF Experiments
p
Ne-1% Ar diluent 2% C2N2-2% D2 PI = 5 Torr
106,
mol
T, "K
Mixture
x
1857 1984 2062 21 09 2213 2383 2393 2469 2482
kl X lo-'', om3 mol sec - * (M)-0.7g
1.93 1.95 1.99 2.01 2.08 2.10 2.12 2.14 2.13
0.751 2.18 3.25 6.78 12.6 18.7 33.1 50.7 58.2
An atomic mechanism may contain the following steps.
+ M
H, C2N, CN
+
H
+ +
L 2 H
+
M L 2 C N k
H, 4?. HCN
CZN,
HCN
i
101:
M
5 8
b O
1 Ox x 4 / T ( K )
+
M
+
H
+
CN
(b) (c)
Invoking the condition of low conversions and neglecting the back reactions, allows the following relations.
Figure 9. Arrhenius plot of the data in Table I l l ; X , solid line. Dashed line taken from Figure 8.
where u implies one or more quanta of vibrational energy. If the time required for C2N2 to achieve vibrational equilibration is long compared to the metathesis reaction time, the following equation is applicable [C,N,I" = ~uz[C,N,lo[Ml"t
(11)
The rate of formation of product, neglecting the back reaction, is given by
Substitution of eq 11 into 12 followed by integration yields Equation 7 has been shown to be applicable under the reaction conditions hereids and a similar argument may be made for eq 8.14 Substitution of eq 7 and 8 into 6 followed by integration yields
[HCNIt = k,h,z[C,N210[H,]o[M]Y t 2
(13)
A vibrational excitation mechanism (VEX) may also be written for this reaction system. VEX would predict a rate expression of the form
[HCNIt = ( ~ , , ~ d , [ M I-Iy k,,kd,[MJY')[CzN,]o[H,l,t2' (9)
[HCN], = k[C,N,]o"[H,]o"'[M]Y t z = f ( t )
(14)
The relative importance of the terms in parentheses in eq 9 may be estimated with regard to their activation energies. For reactions b and c, the sum is (69 l4 + 7 19) kcal/ mol whereas for reactions a and d the minimum value of 2 10 17) is 106 kcal/mol. Equation 9 may the sum (96 20 then be approximated by
The reader may fill in the individual steps for this mechanism by referring to an earlier work by Bauer.21 It is sufficient here to say that in this mechanism C2N2 and H2 undergo a set of elementary excitation steps whereby they reach their critical vibrational levels. The rate of reaction is controlled by the population of molecules in these criti[HCN]t = ~ ~ ~ ~ ~ I , [ C ~ N ~ ] O [ H ~ ] O(10) [ M I "cal~ ~levels; for instance, H2 in u = 3 is the threshold level for the H2 D2 exchange according to VEX. The value of y' may be less than 1 if reaction b is taking A theoretical treatment of the C2N2 Ha system would place in the fall-off region. require solution of a large set of coupled differential equaThe metathesis of C2N2 and H2 may also occur via a vitions with many unknowns. For the simpler Hz + D2 brational energy chain mechanism (VEC). For the temreaction, these equations have been solved with the result perature interval spanned in this study, a reaction mechathat the value of z changes from 1.2 to 3 without an innism is written employing the following sequence duction period.22 k" It has been suggested that the reaction of C2Nz with Dz H, M d z H," M (fast) (e) to form DCN would be faster than C2N2 with H2 if VEX
+
+
+
CzN,
+
M k C z N
'CzN2" HCN"
+
+
C,N,
Hz
-
+ t +
M (slow)
BHCN'
CzN2" + HCN
The Journalof Physical Chemistry, Vol. 77,
No. 11, 1973
(0 (9)
(h)
(18) (19) (20) (21) (22)
+
R. D. Kern, Jr., and G. G. Nika. J. Ph'ys. Chem., 75, 2545 (1971). P. Hartel and M . Polanyi, J. Chem. Phys., 11, 97 (1930). A. L. Myerson and W. S. Watt, J. Chem. Phys., 49,425 (1968). S. H. Bauer and E. Ossa, J. Chem. Phys., 45,434 (1966). S. H. Bauer, D. M . Lederman, E. L. Resler. Jr., and E. R. Fisher, lnt. J. Chem. Kinet., in press.
Reaction of TrifliJoromethyl Radicals with Nitric Oxide
1335
were the dominant mechanism.23 This prediction is related to the observation reported in the single-pulse shock tube study of the Hz Dz exchange that the rate depended more strongly on the concentration of Dz than on H2.21,24 In the temperature range 1050-1200”K, the rate of Dz with CzNz was found to be faster than that of HZ with CzN2 although the rate decreased with increasing temperat~re.~~ According to VEC, the important species is CzNz” and not H2U or D2U. Consequently, there should be a decrease in the rate constant as expected from the normal isotope effect when D:l rather than Hz is reacting with CzN2. The magnitude of the isotope effect, A D J A H ~calculated , from collision theory, is listed below for both atomic and VEC mechanisms. H,, D, CN HCN, D C N H, D 0.73 (15)
+
-
H,
+ + D + C2N2HCN, D C N + CN 0.715 H,, D z + C,N2” BHCN, DCN 0.72
-
(16)
(17) The experimental ratio (kD2/kHz) is about 0.6 which agrees with either the atomic or VEC mechanism within experimental error. The quadratic time dependence and the order with respect to reactants and total density are facts which can be rationalized in terms of an atomic or vibrational energy chain mechanism. The experimental activation energy of 62 kcal/mol is less than the 76 kcal/mol predicted by eq 10. This discrepancy plus the lack of any direct evidence for the presence of radicals argues somewhat against the atomic route. The observation of an appreciable vibra-
tional equilibration time for CzNz compared to the metathesis reaction time lends support to VEC. However, the activation energy predicted by eq 13 is not known and it is possible that it is high. A more definite statement may be made with regard to DZ exchange.15 The the role of cyanogen in the HCN proposal involved the following steps.
+
C2N2
+
D2
-
2DCN
( j) This work has demonstrated that the rate of reaction j is much slower than the exchange process and therefore cyanogen is not an important intermediate in the exchange sequence. The evidence is conclusive with respect to the complex nature of this metathetical reaction. The direct bimolecular combination of the reactants with the formation of a four-center transition state does not occur to any appreciable extent and provides another example of a “simple” chemical reaction that avoids the four-center pathway.
Acknowledgments. The authors would like to thank Mr. Tim Dupuy for his assistance with data reduction, MY. Darryl Olivier who helped maintain the equipment, and Professors Robert Flurry and Peter Politzer for fruitful discussions. We appreciate very much a critical review of the manuscript by Professor S . H. Bauer. (23) D. K. Lewis, Thesis, Corneli University, 1970; University Manu(24)
scripts, Ann Arbor, Mich. D.K. Lewis, private communication.
A Mass-Spectrometric Study of the Reaction of Trifluoromethyl Radicals with Nitric Oxide’ Hiok-Seng Tan and F. W. Lampe* Whifmore Laboratory, Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received December 18, 1972) Publication costs assisted by The U.S. Atomic Energy Commission and The Petroleum Research Fund
Trifluoromethyl radicals, formed by the photolysis of hexafluoroazomethane, react with nitric oxide by successive addition to yield trifluoronitrosomethane and perfluorotrimethylhydroxylamine. Trifluoronitrosomethane is the sole product until the nitric oxide concentration has been reduced to very low levels, at which time the addition reaction to form perfluorotrimethylhydroxylamine can occur. After the trifluoronitrosomethane has been depleted the trifluoromethyl radicals can react with hexafluoroazomethane to yield perfluorotetramethylhydrazine and with each other to yield hexafluoroethane. Kinetic analysis o f the data subsequent to depletion of nitric oxide permits evaluation of a lower limit to the specific reaction rate for the addition of trifluoromethyl radicals to trifluoronitrosomethane. The value found is ks 2 9.7 i 0.7 X cm3 molecule-1 sec-I at 56”.
The photolysis of hexafluoroazomethane in the visible and near-ultraviolet regions of the spectrum has been well studiedz-? and often used8-12 as a convenient source of tri-
fluoromethyl radicals at near room temperature. The products of the photolysis are nitrogen, hexafluoroethane, perfluorotetramethylhydrazine, and perfluorohexamethylThe Journal of Physical Chemistry, Voi. 77, No. 11, 19:73
