J . Phys. Chem. 1986, 90, 6722-6726
6722
the final numbers were not 4.75, nor were they all the same because of the added noise, and only three digits are significant. Nevertheless, this trial demonstrates that the convergence is extremely rapid. Precisely the same behavior occurred when no random noise was added. We conclude that the suggested pro-
cedure will work well in practice. Acknowledgment. This work was supported, in part, by NSF Grant C H 8214688. We thank Dr. G. Rajabali for some helpful preliminary work on this problem.
Rate Constant Determination of the Reaction of Metastable N(2D,2P)with NO, Using Moderated Nuclear Recoil Atoms Ren Iwafa,+ Richard A. Ferrieri,* and Alfred P. Wolf Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973 (Received: May 28, 1986)
The kinetics of the reaction N(2D,2P) + NO2 were studied by using moderated nuclear recoil atoms to generate thermalized metastable atomic nitrogen, ,D and ,P, at a [NO,]/[N] ratio of Branching ratios were obtained for the following channels of reaction: N + NO2 N 2 0 + 0 (kla); N + NO, 2 N 0 (klb);N + NO, N2 + 0, ( k l c ) N ; + NO2 N2 + 2 0 (kid). The distribution of channels la:lb:(lc + Id) was measured to be 1.00:0.65:0.27. An upper limit for the overall rate constant for reaction was also evaluated at 3.3 X cm3 molecule-' S-I (293 K) by using competitive kinetic measurements of the formation of radioactive nitrogen-13 labeled products. This rate constant is at least an order of magnitude smaller than previously reported values for the ground-state N(4S) + NO2 reaction.
-
-
Introduction Considerable interest exists in the reactions of N atoms the nitrogen oxides primarily because of their importance for modelling NO, pollutant emissions during the air burning of hydrocarbon and secondly because of their importance in upper atmosphere chemistry.' Reaction of nitrogen atoms with NO2 can proceed by one of four primary N
+ NO2 N
- + + - + N20
+ NO,
+ NO, N + NO2
N
.-f
0
AH = -42 kcal mol-' ( l a ) AH = -78 kcal mol-'
2N0
AH = -121 kcal mol-' (IC)
0,
N,
N2
(1b)
AH = -2 kcal mol-[
20
(Id)
Unfortunately, previous kinetic investigations have only probed reactions of ground-state N(4S) atoms, and even so, no direct determinations of the individual rate constants for the above channels have been made. Relative rates of these channels have been It was first suggested by Kaufman,8and later supported by the work of Clyne and Thrush,lo that channels l a and 1b are the predominant processes. An inherent problem exists in measuring even the overall rate constant of this system. This is attributed to the rapid catalytic cycle that is initiated by channel l a . N
+ NO,
0 + NO2
+
+0 NO + 02 N2O
-
-
TABLE I: Overall Rate Constants for N(?S) + NO2 Reaction initial IN0,l /IN1 k.... cm3 molecule-' s-' ref 1 8.8 X 15 1.9 X lo-''
1 20
80 >80 >80
7.5
x
10-12
3.8
X
lo-''
1-2 x 10-12
3.0
X
9 15 16 17 14
presumably is due to the experimental complications. The reactions of metastable atomic nitrogen, 2D(2.38 eV) and *P(3.57 eV), with NO2 are also of interest relevant to the chemistry in the perturbed thermosphere. There exists a general interest in measuring reactivities of these metastable states because they may well play significant roles in phenomena associated with energy deposition. Little work has been done toward understanding the chemistry of these species, and, in particular, the N(2D,2P) NO2 reactions have not been investigated. Rate coefficients for quenching in a number of gases have been addressed, but with considerable discontinuity observed in measured values amongst investigators.18-20 Kaufman also established that
+
( I ) Sawyer, R. F. Symp. ( I n t . ) Combust., [Proc.] 18th, 1980 1980, 1. (2) Tang, S.-K.; Churchill, S. W.; Liar, N. Ibid. 1980, 73. (3) Claypole, T. C.; Syred, N. Ibid. 1980, 81. (4) Hahn, N. A.; Wendt, J. 0. L. Ibid. 1980, 121. ( 5 ) Roper, G.; Smith, D. B. Ibid. 1980, 133. (6) Gaydon, A. G.; Wolfhard, H. G. Flames; Wiley: New York, 1979. (7) Heicklen, J. Atmospheric Chemistry; Academic: New York, 1976; pp
(la)
Reactions 2 and 3 are exceedingly fast with rate constants in the low 10-I'cm3 molecule-' s-' (293 K) range.l2~l3Clyne and Ono pointed out in a recent publication that this cycle could yield erroneously high overall rate constants for N NO2 if the nitrogen atom concentration were sufficiently high.I4 In fact, one can see from published rate constants in Table I for the N(4S) NOz reaction, that values ranged over a power of 10 as the initial [NO,]/[N] concentration ratio was varied.'"" In fact there does not appear to be any set trend to the rate constant variation. This
141-143, 185-195. (8) Kaufman, F. Symp. ( I n t . ) Combust., [Proc.] 7th, 1958 1958, 5 3 . (9) Phillips, L. F.; Schiff, H. I. J . Chem. Phys. 1965, 42, 3171. (10) Clyne, M. A. A.; Thrush, B. A. Trans. Faraday SOC.1961, 57, 69. (11) Kistiakowsky, G. B.; Volpi, G. G. J . Chem. Phys. 1957, 27, 1141. (12) Lee, J. H.; Michael, J. V.; Payne, W. A.; Stief, L. J. J . Chem. Phys. 1978, 69, 3069. (13) Bermand, P. P.; Clyne, M. A. A.; Watson, R. T. J . Chem. SOC., Faraday Trans. 2, 1974, 70, 564. (14) Clyne, M. A. A.; Ono, Y. Chem. Phys. 1982,69, 381. (15) Verbeke, G. J.; Winkler, C. A. J . Phys. Chem. 1960, 64, 319. (16) Husain, D.; Slater, N. K. H. J . Chem. Soc., Faraday Trans. 2 1980, 76, 606. (17) Clyne, M. A. A,; McDermid, I. S. J . Chem. SOC.,Faraday Trans. I 1975, 71, 2189.
Present address: Cyclotron and Radioisotope Center, Tohoku University, Sendai 980, Japan.
1972. 53. 201. (79) Husain, D.; Mitra, S. K.; Young, A. N . J . Chem. Soc., Faraday Trans. 2 1974, 70, 1721.
N
+ NO
+
+
N2
(2)
+0
(3)
+
+
0022-3654/86/2090-6722$01.50/0
(18) Husain, D.; Kirsch, L. J.; Wiesenfled, J. R. Discuss. Faraday Soc. ~
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 6723
Reaction of Metastable N(2D,2P)with N O 2 reaction rather than quenching was the principal channel for the interaction of metastable N(2D,2P) with a number of atmospheric components.2'sz2 In view of the fact that the published rate constants for the N(4S) NO2reaction are so diverse, and that the metastable N(zD,ZP) NO2 reactions have never been investigated, a series of experiments were initiated to probe the mechanisms and kinetics of reaction for the N(4S) ground-state and metastable N(2D) and N('P) states with NO,. In the first of a series of papers, the results of the metastable nitrogen atom studies using the moderated nuclear recoil method are reported. This method employs nuclear reactions to generate radioactive recoil nitrogen-13 atoms. In an excess of inert moderating gas, these atoms can be thermalized prior to chemical reaction and kinetic measurements can be made by monitoring the stable radioactive end products arising from competing reactions. In the present case, NOzand N2 were used as competing substrates for the atoms. This combination permitted elimination of the reactions of 13N(4S)since this state does not undergo thermal exchange with N,. A unique advantage of the present approach to measuring rate constants for the N NO2 reaction is that the nuclear recoil method yields on the order of lo8 atoms/cm3 of gas which results in extremely high [N02]/[13N] ratios (on the order of 1013). The inherent problems of the catalytic cycle in reactions 2 and 3 would not affect the kinetic measurements of the I3N-atom reactions as they would occur only on the same molecular scale as the recoil atoms. It is felt that the present methods may provide more accurate values.
+
+
+
Experimental Section Materials. All gases utilized in the preparation of the targets were purchased from Matheson Gas Co. and had the following minimum purities: Ne (99.997%), N 2 (99.97%), and NO2 (99.5%). N o pretreatment of the Ne and N2gases was included in the procedure. The NO, gas was pretreated with O2 to convert trace NO to NO,. The excess O2was removed by conventional outgassing procedures. Sample Preparation. Quartz irradiation cells of approximately IO-mL volumes were equipped with a thin 0.25-mm quartz window for beam penetration and a vacuum seal Teflon brand stopcock. The cells were evacuated, filled to 8 Torr with a mixture of NO, and N2, then equilibrated to 760 Torr with Ne gas. A Barocell electronic manometer was used to monitor gas pressures. Irradiations. All irradiations were carried out at the Brookhaven 60-in. cyclotron, where a 15-MeV proton beam was degraded in energy to 12 MeV as it traversed an aluminum degrader and the front quartz window of the cell. Typically, a beam intensity of 0.1 pA was applied for only 30 s in order to minimize radiation-induced reactions. Target cells used in the kinetic studies were irradiated at the ambient temperature of the cyclotron vault which remained fairly constant at 20 OC. The above irradiation conditions initiated the I4N(p,pn)l3N and I60(p,pa)l3Nnuclear reactions from nitrogen-14 and oxygen- 16 contained within the gaseous constituents of the target. The quartz window also contributed to the pool of recoil nitrogen-13 atoms from the beam interaction with oxygen-16 in the quartz. To ensure that this in situ source yielded nitrogen atoms, a similar quartz cell containing only ethylene was irradiated under the same conditions. These samples yielded a single source of nitrogen-1 3 that recoiled out of the quartz. The only observed product in this test was HCI3N. This is consistent with published studies on nitrogen atom chemistry with hydrocarbon^,^^ and strongly suggests that the quartz window yields a clean source of bare recoil atoms. Radioassay of I3N Activity. The target cell was removed immediately after bombardment and placed on a vacuum rack that allowed for sample extraction through a sealed port with a Teflon-brand gas syringe (Precision Sampling Corp.). Samples (20) Young, R. A,; Dunn, D.J. J . Chem. Phys. 1975, 63(3), 1150. (21) Lin, L.; Kaufman, F. J. Chem. Phys. 1971, 55, 3760. (22) Iannuzzi, M. P.; Kaufman, F. J . Chem. Phys. 1980, 73(3), 4701. (23) Brocklehurst, B.; Jennings, K. R. Progress in Reaction Kinetics; Pergamon: New York, 1967; Vol. 4, pp 1-36.
i
I3N0
20
1
t 0;
-
1
i 20 1
'
40
1
60 1
1
80 1
1
700
VOLUME % OF NEON
-
Figure 1. Effect of kinetic energy on the reaction channel distribution; I3N NO "NN 0 and "N + NO 13N0 + N.
+
+
comprising between 2- and 3-mL aliquots of the radioactive gas were subjected to routine radio gas chromatography in analysis, where a 2.45 m X 6.35 mm 0.d. glass column packed with molecular sieve SA (30-60 mesh) and a 3.65 m X 6.35 mm 0.d. glass column packed with Porapak Q (80-100 mesh) provided separation of I3NN, I3NO, and I3NNO. The radioactivity from these components was monitored by using a Wolf-type flow proportional counter24interfaced with appropriate counting electronics and a Hewlett-Packard 85 computer. The detector had an active counting volume of 10.7 mL, with a cylindrical window constructed of 0.013-mm-thick titanium foil. The entire detector housing was maintained a t 100 'C to eliminate the possibility for labeled compounds to stick inside the active zone of the counter. Absolute activities of each product were obtained by correcting the net peak counts for radioactive decay, detector counting efficiency, and sampling fraction. These values were converted to absolute yields by dividing each value by the total induced nitrogen- 13 activity in the sample.
Results and Discussion Moderator Studies. The ability of neon to act as an efficient moderator of the nitrogen atom's translational energy in this system was tested. The energy requirements of the I3N + N O reaction were used as a probe, where the two possible reaction channels can lead to different labeled products.
+ NO I3N+ NO I3N
--
+0 I3NO + N
I3NN
(4a) (4b)
Under thermal conditions channel 4a is the only observed channel of reaction.23 In fact, this reaction is often used as a chemical titrator to measure N-atom concentrations in flow reactors. On the other hand, the reaction of nonthermal recoil nitrogen-13 atoms with N O can produce substantial yields of 13N0.25The effect of neon as an energy moderator can be seen in Figure 1. Channel 4b yielding I3NOaccounts for 20% of the total nitrogen atom chemistry; however, this channel is completely suppressed at 99% neon. On the other hand, channel 4a yielding I3NN accounts for 80% of the total nitrogen atom chemistry but increases to 100% at high moderator concentration. This strongly suggests that at 99% neon composition, the majority of the nitrogen atoms react as thermal species. It is implicit in the mechanistic hypothesis that provides a basis for the proposed treatment that the I3N + N, exchange used as the competitive probe reaction, proceeds only for metastable N(2D,2P)atoms at thermal energy. I3N(*D,*P)+ N2 13NN + N (5)
-
Such exchange reactions have not been observed for N(4S) atoms at 293 K. Bar-Nun and Lifshitz have reported a rate constant of 5 X cm3 molecule-' s-l at 3400 K,26whereas Back and (24) Welch, M. J.; Withnell, R.; Wolf, A. P.Chem. Instrum. (New York), 1969, 2, 177. (25) Dubrin, J.; MacKay, C.; Wolfgang, R. J. Chem. Phys. 1966,44,2208. (26) Bar-Nun, A,; Lifshitz, A. J . Chem. Phys. 1967, 47(3), 2878.
6724
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986
Iwata et al.
2.0
,---.
LL
0
5
0
1.5
E
, z z '0
w 1.0
60
0.5
1 20 40 60 00 IO(
i/
400
VOLUME % OF NEON
Figure 2. Effect of kinetic energy on the "N
exchange reactions.
+ Nz
-
"NN
1 8
1 4
+N
[Ne
TABLE 11: Product Yields in NO2 + N2Competition for Thermal l3N(*D?P)
target compositiond NO2
N2
0.80 0.60 0.40 0.20 0.10 0.05
0.20 0.40 0.60
0.80 0.90 0.95
36.0 f 0.3 33.9 f 0.3 35.8 f 0.3 38.8 f 0.4 33.4 f 0.3 28.6 f 0.4
Mui report an upper limit of 6.6 X cm3 molecule-' s-l at 1000 K.27 The above hypothesis was tested by measuring the 13NN yield from I3N-atom exchange with N, as a function of neon moderator. Results in Figure 2 show that under nonthermal conditions all of the nitrogen atoms generated react via this exchange reaction. Above 80% moderator, this level of exchange drops off drastically to approximately 60% at 99.9% neon concentration. Since the statistical counting errors were prohibitive at higher moderator concentrations, it was not possible to ascertain a true thermal cutoff limit for the exchange pathway. However, the drastic decrease does imply that the hot 13N(4S)exchange pathway is considerably suppressed where these atoms are now presumably lost to the target walls. It also implies that a dominant portion of the nitrogen atoms generated by the nuclear recoil method are electronically excited. It could not be ascertained whether the metastable state population was solely at the zD level, or involved a mixture of states which included the 2P level as well. Kinetic Treatment. In Table I1 the yields of I3NN and 13NN0 are presented as a function of the binary gas composition of NO2-N2. These measurements were obtained from samples containing 99% neon moderator concentrations to ensure complete thermalization of the nitrogen- 13 recoil atoms. Rate expressions for the production of 13NN and I3NNO can be written for the two competing reactions as eq 6 and 7 , respectively. The product yield ratio of I3NN to I3NNO can be k5[N21[l3N1
R I ~ N N=Ok~a[NOzl[l3N1 connected through eq 6 and 7 .
Rl3" -Y =13"- K~NNO R ~ ~ N N O
LN21
(6)
(7)
+-
klc
-t
kl,
(27) Back, R.A,; Mui, J. Y. P. J . Phys. Chem. 1962, 66, 1362
I
[ NO2 1
A plot Of YI~NN/I+JNO VS. [NZ]/[NOz] should yield a straight line whose slope is equivalent to ks/kl,, and intercept equivalent to (klc + kld)/kla. Likewise, a rate expression for the production of 13N0 via channel 1b can be written as eq 9 The absence of
"Fraction compositions relative to 1% (NO2 + N2) and 99% Ne.
R"NN = ( ~ I Ckid)[N021 [l3N1
1
1 16
Figure 3. Kinetic treatment by eq 8.
radiochem yields "NN I'NNO 14.2 f 0.3 16.9 f 0.2 22.3 f 0.2 32.3 f 0.3 41.1 f 0.4 70.7 f 0.6
1 I2
(8)
R~~N = oklb[N02] [I3N] (9) I3NO suggests that the following reaction was sufficiently rapid to remove I3NO from that portion of the analyzable product spectrum. 13NO* NO, F= l3"o3 (10)
+
Clough and Thrush have reported on various levels of vibrational excitation for N z O formed from channel la.28 It would be expected that similar excitation may exist in N O formed in channel l b and that this could drive the equilibrium in reaction 10 toward complete formation of nitrous anhydride. It is assumed that the unaccountable nitrogen- 13 activity arises from losses initiated by channel lb. Thus the yield of I3NO can be translated into measurable terms through eq 11. The product yield ratios of Y'"o = Y(t0tal induced I3N) - (53" + Y I ~ N N O(1) 1) I3NN to I3NO can be connected through eq 6, 9, and 11. Y13NN
= -R"NN -
-
Ytotal - ( Y I ~ N+N Y ~ ~ N N o )R I ~ N O
+
A plot Of Y""/(Ymd - (Y'3" K3"o)) VS. [N2]/[NO2] should again yield a straight line whose slope is equivalent to k5/klband intercept equivalent to (klc kld)/klb. Treatment of the data in Table I1 using eq 8 and 12 yielded the plots shown in Figures 3 and 4. A linear regression analysis was applied to this treatment to obtain appropriate slopes and intercepts. A value of 1.9 X cm3 molecule-I s-l (293 K) for kS was taken from the recent work of Slanger and Black29which provided an upper limit for our calculated rate constants. Their work measured the rate of disappearance of N(2D) by monitoring the NO(P)-band signal in the presence of N,. This signal arose from the following sequence of reactions initiated by the photodissociation of N z O to generate N(2D) atoms.
+
N 2 0 -kN(2D) + N O
( 1 3a)
N('D) + N 2 0 N0(P2x) + N2 NO(@'*) -.+ NO(X2*) hv(3000-4000 A)
( 1 3b)
-+
(1 3 ~ )
(28) Clough, P. N.; Thrush, B. A. Discuss. Faraday SOC.1967, 44, 205. (29) Slanger, T. G.; Black, G.J . Chem. Phys. 1976, 64(11), 4442.
Reaction of Metastable N(2D,2P) with N O 2
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 6725 TABLE I V Target Temperature Effects on the 2N02 == N204 Equilibrium
-
1.6
temp, OC
0
z m >-
+
0% neon 1.2
z
ro
-F
. I
c
99% neon
0.0
z
m
>-
v 0.4
product yields" WNO "NN
10.8 f 0.9
*
10.0.f 0.4 12.3 i 0.9 11.4 f 0.5 19.4 i 2.2
(n = 3) (n = 3) (n = 2) (n = 2)
43.9 f 2.5 41.4 f 1.6 47.4 i 9.8 40.6 f 4.0
12.1 i 3.3 8.1 f 0.3 10.2 f 2.3 13.5 f 2.8
( n = 3) (n = 2) (n = 2) (n = 2)
27 142 24 1 350
21.0 i 0.4 24.3 i 1.8 16.9 0.1
27 142 241 350
#Errors report the standard deviation on the number of samples (n).
Figure 4. Kinetic treatment by eq 12. TABLE III: Upper Limit to Rate Constants for Metastable N(*D?P) Reactions with NO,
N2O + 0 2N0 N2 + 02, N2 + 2 0 overall
1.7 x 10-13 1.1 x 10-13 (3.1-6.6) X 3.3 x 1043
The rate constant obtained for the rate of disappearance of N(2D) under these conditions represents a cumulative value composed of the exchange reaction and physical quenching process. There have been no studies to date that have evaluated the specific rate of exchange for either of the ZDor zPmetastable states. In general, ,P quenching rates tend to be lower than those measured for the 2D The value selected for k5 should at least set an upper limit in the present system for the actual rate constants of reaction. Table I11 lists the rate constants obtained in the present study representing the individual channels and the overall reaction. An overall rate constant limit for metastable N(2D,ZP) atom reactions cm3 molecule-' with NO2at 293 K was calculated to be 3.3 X s-I. While there are no reports in the literature on this reaction, it is interesting to note that this value is at least an order of magnitude smaller than any reported rate constants for the N(4S) NO2 reaction. Clyne and Ono have demonstrated that little difference is observed in the measured values when [N02]/[N] ratios ranged between 100 and 400. They claim that the extrapolated value cm3 molecule-I for the ground-state reaction of 3.01 X reflects minimal effects from the rapid catalytic cycle of reactions 2 and 3.14 However, slight deviations in this rate constant may go undetected in the small range of [NO,]/[NO] utilized. This might be especially true if the N N O reaction were perhaps 2 orders of magnitude faster than N + NO2. The condition under which the present experiments were carried out provided a [NO,]/[N] ratio of I O l 3 that virtually eliminated the catalytic cycle. Mechanistic Considerations. Branching ratios for channels la: 1b:( l c Id) were calculated as 1.00:0.65:0.27 from the rate constants in Table 111. An averaged rate constant for the combined channels (IC + Id) was used because slight differences in the measured intercepts were obtained from the data in Figures 3 and 4. It is interesting to note that no direct evidence for reaction channels l b through Id has ever been presented. The present studies have shown conclusively that I3N atoms will react with NO2 to yield not only I3NNO but "NN as well. This demonstrates the Occurrence of channels ( I C Id) in addition to the previously observed l a channel. As pointed out earlier, I3NOwas not observed in this system; however, it was assumed that the nonaccountable nitrogen- 13 activity was due to losses from channel 1b followed by reaction 10, and that the driving force for shifting the equilibrium in 10
+
+
+
+
toward formation of N z 0 3 might be the vibrational excitation of NO* formed in 1b. Under the conditions of the present experiment, the majority of the gas composition comprised N e moderator, while only about 8 Torr of gas comprised a mixture of NO2 and N2. Unfortunately, N e is not a very good energy sink for internal molecular excitations. It was thought that N 2 might provide a better sink. A series of experiments were carried out using various percentages of NO2 and N2 irradiated at 760 Torr total pressure. However, I3NO was not observed even under what was thought to be ideal conditions for collisional cooling. At 0.995 mole fraction levels of N 2 nearly a l-in-103 difference is attained in the magnitudes of the 13NO*collision frequencies between NO2 and N2, respectively. Thus the present work provides no evidence supporting the occurrence of channel 1b, but does not rule out that possibility. It is interesting to note that Clyne and McDermid computed an upper limit of 0.4 for klb/kI, using a simulation model that matched the best fit for the N 2 0 yield from la." If the present assumptions concerning the fate of 13N0are correct, an experimentally measured value of 0.65 for klb/kl,, is obtained in the present studies. NO2 Equilibrium. A final concern in this study involved the effect of N O 2 dimerization on the measured rate constants.30 2NO2
N2O4
(14)
A number of experiments were performed using gaseous targets comprising 760 Torr of neat NO2 and 1% NO2 in N e to determine whether shifts in equilibrium due to the different NO2 concentrations can affect the product spectrum. Since Ne will alter the primary atom-molecule chemistry because of its kinetic energy moderating capability, a comparison of product yields between the two cases could not be made. By comparing yields within the same case at a number of elevated temperatures, it was possible to determine if any additional right-to-left shift in equilibrium occurred. A temperature elevation should shift the equilibrium in (14) in favor of NO2. In other words, it should be possible to determine whether N2O4 contributed to the observed atommolecule chemistry under the conditions of the kinetic study. Results from these experiments are presented in Table IV. In the top half of the table, 760 Torr pressure of NO2 was maintained during the sample irradiation a t temperatures ranging between 27 and 350 OC. A trend of decreasing 1 3 N N 0 yields is quite apparent. The 13NN yield also showed a sharp rise between 241 and 350 OC. Therefore, under the conditions where a neat target of NO, is employed, it must be concluded that N2O4 is present in significant concentration. Results in the bottom half of Table IV represent the temperature effects on targets maintained at 760 Torr pressure comprising 99 mol % of neon. This would simulate the upper limit of the NO2 concentration utilized in the actual kinetic studies. Between the temperature range of 27 and 350 "C no change was detected in the I3NNO and I3NN yields. This leads to the conclusion that the equilibrium is shifted well in favor of NO2 under the conditions of extreme dilution in neon. Therefore, N2O4 is not a significant reactant under the conditions which the kinetic studies were carried out. (30) Harris, L.; Churney, K. L. J . Chem. Phys. 1967, 47, 1703.
J . Phys. Chem. 1986, 90, 6726-6731
6726
Conclusions The present investigation has reported on the kinetics of the reaction of metastable N(2D,2P)atoms with NO2. Previous investigations on the ground-state N(4S) NOz reaction have yielded a spread of rate constants apparently due to low [NO2]/ [N] ratios initiating rapid catalytic cycles involving N and NOz. Under the conditions of our experiments a [NO,]/[N] ratio in the range of lOI3 is achieved which virtually eliminates this problem in the kinetic measurement. The overall rate constant of reaction was evaluated as 3.3 X cm3 molecule-' s-l (293 K). This value is an upper limit because the rate constant used for the N + N2 exchange reaction, the competitive probe in this system, may actually represent both reaction and quenching channels for the atom-molecule encounter. In any event, the above rate constant for metastable N(2D,ZP) + NO, reaction is a t least an order of magnitude smaller than reported values on the ground-state N(4S) + NO2 reaction. In
+
future studies, it will be interesting to see if the present experimental methods applied to the N(4S) NO, reaction will indeed yield a smaller rate constant. Branching ratios for the distribution of channels la:lb:(lc Id) were evaluated as 1.00:0.65:0.27. Previous studies have yielded direct evidence only for channel la. The remaining channels were postulated from reaction stoichiometry. The present investigation demonstrates additional validation for this channel, and also provides direct evidence for some reaction channel or channels yielding N2, presumably by I C and Id.
+
+
Acknowledgment. This research was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the U S . Department of Energy and supported by its Office of Basic Energy Sciences. Registry No. N, 17778-88-0; NO2, 10102-44-0; Ne, 7440-01-9; N,, 7727-37-9.
Chemkal and Phase Transformatlons of Cyanogen at High Pressures Choong-Shik Yo0 and Malcolm Nicol* Department of Chemistry and Biochemistry, University of California. Los Angeles, California 90024 (Received: May 8, 1986)
Chemical and phase transformations of CzNzat 293 K and pressures as high as 12 GPa have been studied by Raman and Fourier-transform infrared spectroscopy. Three phases of the monomer have been identified. C2N2I freezes at 0.3 GPa and transforms to another solid, C2Nz11, at 0.5 GPa. At 2 GPa, C2N2I1 transforms to a third solid phase of the monomer, C2N2111. Between 3.5 and 10 GPa, C2N2reversibly converts to what is identified as a linear chain of dimers, (-(C2N2)2-)m. Above I O GPa, further irreversible reactions convert these chains to a very chemically and thermally stable material that can be recovered at atmospheric pressure. Vibrational spectra indicate that the linear chain is a poly(2,3-diiminosuccinonitrile) and the stable product is a paracyanogen ladder of fused pyrazine rings that may be highly cross-linked.
Introduction Pressure accelerates the polymerization of unsaturated molecules. Thus, organic polymer chemistry a t high pressures,',z including applications to electroconducting polymers and high strength material~,33~ is of considerable interest. Chemical reactions at high pressure have been observed with many types of unsaturated chemical bonds including those in alkene^,^,^ alkyne^,^ nitriles: carbonyls: and carbon sulfides.IO Indeed, all unsaturated molecules are expected to be unstable at high pressure with respect to associative, cross-linking reactions which form denser, more saturated species." Very detailed information concerning these reactions can be obtained by combining diamond-anvil cell methods, which generate high, quasi-hydrostatic pressures at which even the strongest multiple bond should be unstable,', with spectroscopic and other sub-microanalytical technique^.'^,^^ At the very high pressures (1) Katz, A. M.; Schiferl, D.; Mills, R. L. J . Phys. Chem. 1984,88, 3176. (2) Babare, L. B.; Dremin, A. N.; Perskin, S. V.; Yakovlev, V. V. Proceedings the First International Symposium on Explosive Cladding Institute of Chemical Physics: Moscow, 1971; pp 239-57. (3) Heiman, R. B.; Kleiman, J.; Salansky, N. M. Carbon 1984, 22, 147. (4) Bundy, F. P. J. Geophys. Res. 1980, 85, 6930. (5) M e a , R.; German, A. L.; Heikens, D. J. Polym. Sci. 1977, 15, 1765. (6) Holmes, W. S.; Tyrrall, E. Trans. Faraday Soc. 1956, 52, 47. (7) Rice, J. E.; Okamoto, Y. J . Org. Chem. 1981, 46, 446. (8) Yakushev, V. V.;Nabatov, S. S.; Yakusheva, 0.B. Dokl. Akad. Nauk. SSSR 1973, 214, 879. (9) Dremin, A. N.; Babare, L. V. J . Phys. 1984, 45, C8-177. (IO) Ginsberg, A. P.; Lundberg, J. L. Inorg. Chem. 1971, 10, 2079. (11) Nicol, M.; Yen, G . Z. J . Phys. 1984, 45, C8-163. (12) McMahan, A. K.; LeSar, R. Phys. Reu. Left. 1985, 54, 1929. (13) Agnew, S. F.; Swanson, B. I.; Jones, L. H.; Mills, R. L.; Schiferl, D. J . Phys. Chem. 1983, 87, 5065.
used in this work, polymerizations tend to occur as bulk solid reactions that proceed without participation of a solvent or catalytic initiators. Reactivity and active centers are created by a combination of mechanical deformation, topological arrangements of the solid reagents, and reduction of the free energy of activation. The reactions either proceed a t constant pressure or, because transport and steric considerations often are important at intermediate stages of the reactions, occur in stages as the pressure increases. Cyanogen, C2N2,is one of the simpler carbonitriles, a class of molecules whose common feature is a strong carbon-nitrogen triple bonds. Cyanogen also is a relatively energetic m o l e c ~ l e and ' ~ can be polymerized to an electrically conducting material.16 Much of the chemistry, spectroscopy, and industrial applications of the nitriles near atmospheric pressure have been explored,I7 and the behaviors of some nitriles at high densities have been determined. Most halonitriles and alkyl nitriles trimerize symmetrically to aromatic 1,3,5-trisubstituted triazines at ambient]*and high static pressures. The trimerizations can be promoted by specific solvents and catalyst^,'^ and the product yields also depend strongly on the group attached to the nitrile moiety. Cyanogen has not been reported to trimerize, although it is known to polymerize at high temperatures.,O (14) Yin, G.2.;Nicol, M. J . Phys. Chem. 1985,88, 1171. (15) Knowlton, J. W.; Prosen, E. J. J. Res. Natl. Bur. Srand. 1951, 46, 4x9. .. .
(16) Whangbo, M.; Hoffman, R.; Woodward, R. B. Proc. R. SOC.London, See. A 1976, A366, 23. (17) Cordain, B. Coord. Chem. Reu. 1982, 47, 165. (18) Cook,A. H.; Jones, D. G . J . Chem. Soc. 1941, 278. (19) Cairns, T. L.; Larcher, A. W.; McKusick, B. C. J . Am. Chem. SOC. 1952, 74, 5633. (20) Perret, A,; Krawczynski, A. Bull. SOC.Chim. 1932, 51, 622
0022-3654/86/2090-6726$01.50/00 1986 American Chemical Society
