NOTES
1172 experimentally observed yields are compared with yields calculated using RRKM for different values of E1 between 38 and 44 kcal mol-'. From Figure 3, u7e conclude that El = 42 kcal mol-'. Taking methyldifluoramine as the model compound, R2HCNF2, the rate parameters for H F elimination are then log (ICl/ sec-l) = 13.5 - 42/0, where 0 is 2.3RT kcal mol-', and those for C-N bond breaking2 are log (k2/sec-l) = 17.5 - 60/0. The isokinetic temperature where both rates are equal is in the region of 1OOOK. For other difluoraminoalkanes, R f; H, the C-N bond strength is significantly lower,2 which reduces the isokinetic temperature.
Acknowledgment. This work was supported by the Office of Naval Research on Contract No. Nonr 3760 (00)
-
Mass Spectrometric Study of the Reaction of Nitrogen Atoms with Nitrosyl Chloride by M. R. D u m , C. G. Freeman, M. J. RilcEwan, and L. F. Phillips Chemistry Department, University of Canterbury, Christchurch, New Zealand (Received December 9, 1970) Publication costs borne completely by The Journal of Physical Chemistry
The reaction of N with ONCI might reasonably be expected to resemble that of Tu' with NOz. The latter' is a very fast reaction ( k = 1.8 X lo-" cm3 molecule-' sec-l) which is remarkable chiefly for the variety of decomposition channels that are available to the N-N02 transition state. Thus 43% of primary reactions yield N20 0, and 33% yield NO NO, with about 10% each of Nz 0 0 and N2 0 2 . It is therefore of interest to determine whether the reaction of N with ONCl can take a similar variety of paths. Also, in view of the recent suggestion2 that the reaction with ONCl could serve as a gas-phase titration for estimating N-atom concentrations, it is useful to reexamine the stoichiometry of the reaction and to establish whether the primary reaction is sufficiently rapid to give an accurate end point in the presence of wall and homogeneous recombination of N atoms. We have studied the reaction in a fast-flow system by a combination of mass spectrometric and photometric techniques. Our conclusions concerning the stoichiometry of the reaction differ significantly from those of Biordi;2 this difference is attributable to the differing importance of wall recombination of C1 atoms in the two systems. We find that the primary rate constant is about three orders of magnitude smaller than that of the usual NO titration reaction, so that the
+
+ +
+
+
The Journal of Physical Chemistry, Vol. 76, No. 8,1971
usefulness of the ONCl titration must be restricted to systems with large N-atom concentrations and relatively slow flow speeds, preferably with a large tube diameter or an effective wall poison to minimize heterogeneous recombination. I n contrast to the reaction with NOZ,the reaction of N with ONCl does not produce detectable amounts of N20, and it appears that the observations can be satisfactorily accounted for by a mechanism based on a single primary step yielding NO KCl.
+
Experimental Section The apparatus, procedures (including poisoning the walls of the flow tube with phosphoric acid), and materials were as previously d e ~ c r i b e d . ~A) ~sample of 016NC1was prepared from 99% enriched 16N0by reaction with an excess of chlorine at 180°K and purified by repeated fractional distillation in a LeRoy still. Photometric observations were made downstream from the mass spectrometer sampling leak using a 1P21 photomultiplier in conjunction with either a Spectrolab Ptype interference filter (1.5 nm half-width a t 625 nm) to detect the nitrogen afterglow, or a Corning 7-39 filter to detect the NO p bands. The consumption of ONCl could be monitored from the height of either the NO+ peak a t mass 30 or the NCl+ peak at mass 49; the results obtained in the presence of excess N were independent of which peak was used. Both NO and NC1 are products of the primary reaction and their parent peaks might therefore contribute appreciably to the peak heights measured at masses 30 and 49. However, in the presence of excess N atoms the steady-state concentration of each of these species is expected to be low enough to cause negligible error in measurements of the ONCl concentration. Also, in a previous study of the reaction of N atoms with C120,6where there was no interference from the mass spectrum of the reactants, we were not able to detect any peak at mass 49 from NC1 radicals even though they were certainly present in the reaction system.
Results and Discussion The products of the reaction at long times (ea. 300 msec) were N2, C12, and 02. The production of N2 could not normally be observed because of the large amount of Nz already present, but was readily apparent in experiments with 016NC1. From the lack of a detectable increase in the mass 45 peak due to 'WNO in these experiments we conclude that the yield of N2O in the primary reaction is less than 0.1%. A smaller value for this limit might have been obtained but for (1) L.F. Phillips and H. I. Schiff, J. Chem. Phys., 42, 3171 (1965). (2) J. C.Biordi, J. Phys. Chem., 73, 3163 (1969). (3) C.G.Freeman and L. F. Phillips, ibid., 72, 3025 (1968). (4) M. R. Dunn, M. M. Button, C. G. Freeman, M. J. McEwan, and L. F. Phillips, ibid., in press. (5) C. G. Freeman and L. F. Phillips, ibid., 72, 3028 (1968).
NOTES
1173
the presence of about 1% of 16N02 impurity in the Ol6NC1. The stoichiometry of the reaction at long reaction times was investigated in two ways, the first being to determine the number of ONCl molecules destroyed by a known concentration of N atoms with OKCl in excess, and the second to determine the number of ONCl molecules consumed by a known concentration of N atoms at the “titration end point”2 which was indicated by the photomultiplier output signal having dropped to zero. For both methods the reaction time, from inlet to sampling leak, was 300 msec, and the N-atom presTorr. Both gave sure was in the range 4-9 X stoichiometries close to 1: 1, the mean ratio of N atoms to OKC1 molecules destroyed being 0.94 0.15 by the first method, and 1.20 f 0.12 by the second method (16 and 12 determinations, respectively). The elementary reaction steps which need to be considered are
*
+ ONCl +NO + NC1 N + NO Nz + 0 N + NCl +Nz + C1 C1 + ONCl +Clz + NO 0 + ONCl +products C1 + wall l/zClz
N
4
--t
(1) (2)
(3) (4)
(5) (6)
Here reaction 1 was found in this work to have a rate constant kl of the order of 10-14 cm3 molecule-l sec-l, whereas kz is 2 X 10-l1,‘jk3 is expected to be of similar magnitude to kz, and k4 is about 4 X 10-’2.7 Hence reactions 2-4 can be expected to be important at very short reaction times relative to the time scale of reaction 1. For reaction 5 , however, the rate constant is of similar magnitude to lcl,8 so that reactions initiated by step 5 should not be significant except at long reaction times. Reaction 6 should be almost negligible in our poisoned system, but would have been important in the unpoisoned system used by Biordi.2 Homogeneous recombination of N atoms was negligible in our system because of the low partial pressures of p\T that we used Torr). (typically 7 X Reactions 1-4 lead to the removal of four K atoms and two ONC1 molecules each time reaction 1 occurs and result in the production of two 0 atoms. Our current studies of the 0 ONCl reaction8 indicate that its stoichiometry at long times is close to 1: 1 in a poisoned system. Hence, provided sufficient ONCl is introduced to ensure that all N atoms are removed by reactions 1-4, the effect at long times would be to have 4N and 20NC1 removed by steps 1-4, and 20NC1 removed by step 5, thus giving the overall 1 : 1 stoichiometry that we observed. I n an unpoisoned system reaction 4 would have to compete with reaction 6, the recombination of C1 atoms on the wall. If C1 atoms
+
were removed entirely by 6 the result a t short times would be to remove 3N for each ONCl reacting. To predict the stoichiometry at long times in an unpoisoned system it is necessary to know the stoichiometry OKCl reaction in such a system. On the of the 0 basis of the simple mechanism (which is in accord with our current findings)
+
0
+ ONCl -+NO + C10 0 + C10 + + c1 0 2
(7)
(8)
together with reaction 6, it can be seen that two 0 atoms are required to remove one ONCl molecule, so that the effect at long times is to remove 3N and 1.5(ONCl). Thus we can account for the 2: 1 stoichiometry found by Biordi. Our experimental stoichiometry was actually slightly greater than 1: 1; this can be understood in terms of a competition between reactions 4 and 6. To measure the rate of reaction 1 we monitored the disappearance of ONCl in the presence of a known excess (typically eightfold) of N atoms, the rate constant being calculated with the assumption that 4N and 20n’Cl were removed each time reaction 1 occurred. Rate constants were calculated at times such that not less than 20% of the OKC1 had been consumed, so that sufficient time would have elapsed for steady states to be attained with respect to EO,SC1, and C1. The mean of 32 determinations at 295°K gave k, = 1.9 X 10-14 cm3 molecule-’ sec-’, with a standard deviation of 0.9 X 10-14. The rather large scatter of the values, which is reflected in the standard deviation, may have been associated with variations in the effectiveness of the wall poison. The experiments with 0l6NC1have enabled us to rule out the primary step
N
+ OSCl +NzO + Cl
(9)
However, because of the speed of reactions 2 and 3 and the presence of NO+ and NClf peaks in the mass spectrum of ONCl, these experiments could give no information about whether the initial attack by N occurs preferentially on the oxygen or the chlorine atom. Attack on the chlorine atom should be favored by the weakness of the N-C1 bond relative to the N-0 bond in ONCl. Two other primary reactions which must be considered, by analogy with the NOzreaction, are
N
+ ONCl +Nz + C10
(10)
and N
+ ONCl+Nz + C1+
0
(11)
(6) K. Schofield, Planet. Space Xci., 15, 643 (1967). (7) IT;. G. Burns and F. S. Dainton, Trans. Faraday SOC.,48, 62 (1952). (8) M. R. Dunn, C. G. Freeman, M. J. McEwan, and L. F. Phillips, unpublished work.
The Journal of Physical Chemistry, Vol. 76,No. 8,1971
1174 which are exothermic to the extent of 100 and 47 kcal, respectively (compared with approximately 50 kcal for reaction 1 and 77 kcal for reaction 9). Reaction 10 leads to exactly the same predictions of overall stoichiometry as does reaction 1. However, although we cannot distinguish between 1 and 10 experimentally, we consider that 10 can be eliminated because it requires an improbable concerted mode of decomposition of the activated complex. Reaction 11, on the other
The Journal of Physical Chemistry, Vol. 76,N o . 8,1971
NOTES hand, can be ruled out experimentally because it leads to the prediction that a t long reaction times 2N and 40NClshouldbeconsumed.
Acknowledgments. This work was supported by the New Zealand Universities Research Committee and by Grant AF-AFOSR-1265-67 from the United States Air Force Office of Scientific Research.
