3088
TIMOTHY JOHNSTON AND JULIAN HEICKLEN
Table IV : Rate Constant Data kaka/ [ki(kii
Temp,
OC
22
95 150
ka/ (kll
+ ka)
0.15 11 -0.10
*-o.
w I,
+
mm-1
0.24
... a
108 X ks/kz'/z &/mole sec)'/z This work Ref 4
10.0 31 130
4.3
40 130
Similar to value at 22'.
brational energy, CzF40* would never be stabilized under our conditions and would always dissociate. Consequently, CzF40* must be an excited electronic level, presumably a triplet in accordance with the spin conservation rules. Finally we return to the problem of the multiplicity
of the CFz radicals formed in reaction 11. Originally, the suggestion that the CF2 radicals in this system were triplets was based on their excessive reactivity with C2F4 and 0 2 . However, the results of this study have shown that their reactivity toward CzF4 is identical with that for singlet CFz radicals. Thus if they are triplets, the evidence must rest on the reactivity with 02. The appropriate experiments are being conducted in the Aerospace Laboratories, and will be the subject of a future report.
Acknowledgment. This research was supported by the U. S. Air Force under Contract No. A F 04(695)-669. The authors wish to thank Mr. Richard Frey for performing many of the analyses and experiments and Xrs. Znnette Kohlmeyer for assistance with the manuscript.
Photolysis of Methyl Iodide in the Presence of Nitric Oxide
by Timothy Johnston and Julian Heicklen Aerospace Corporation, El Segundo, California
(Received March 7, 1966)
Methyl iodide was photolyzed in the presence of NO a t room temperature. Iodine is both a primary and secondary product. Nitrosomethane is a primary product, while (CH3N0)2, NOz, CH30N0, Nz, and NzO are secondary products, and CH8ONO2, CHzO, and HNOz are tertiary products. The complete reaction sequenc'e is given. Methyl iodide enters the chain step to give CH30 radicals. The important steps in removing CH3NO are NO 3 CH30 N20. The rate constants for 2CH3NO -t (CH3NO)Z and CH3NO both reactions were obtained and are tabulated with several other rate constants. Where comparisons could be made with existing results, agreement was good.
+
I. Introduction The methyl radical addition to nitric oxide has and the rate 'Onstant is been studied reasonably well known.1-6 The adduct has been observed by the use of infrared analysis' as well as mass spectral analysis.8 Nitrosomethane can be removed in a number of reactions. If the NO pressure is sufficiently low that all methyl radicals are not scavenged by NO, then two The Journal of Physical Chemistry
+
methyl radicals can react with CH3NO. Such a reaction was proposed by Home6 to account for the fact (1) R. W. Durham and E. W. R. Steacie, J . Chem. Phys., 20, 582 (1952). (2) F. P. Lossing, K. V. Ingold, and A. W. Tiokner, Discussions Faraday SOC.,14, 34 (1953). (3) W.A. Bryce and K. V. Ingold, J . Chem. Phys., 23, 1968 (1955). (4) M.I. Christie, Proc. Roy. SOC.(London), A249, 248 (1958). (5) W. C. Sleppy and J. G. Calvert, J . Am. Chem. Soc., 81, 769 (1959).
PHOTOLYSIS OF METHYL IODIDE IN THE PRESENCE OF NITRICOXIDE
3089
that up to three methyl radicals can be removed per CF3N0, the same reaction occurs; it too is found to be first order in CF3N0arid second order in nitric oxide molecule in the gas phase. The reaction was confirmed by the experiment of Maschke, Shapiro, However, Christie'? and Christie, Gilbert, and Voiseyl* found that when a great excess of N O (100and Lampe,*who actually observed CDsNO and (CD3)2NOCD3in a mass spectrometer. 600 mm) was added to CHINO, the amount of NO, If the NO pressure is sufficiently great that the methyl produced was proportional to the first power of NO, radical is completely scavenged, then C&NO must be even though all of the CH3IC'O was consumed. Furremoved by different routes. At temperatures above thermore, Christie, Frost, and Voiseylbfound that CH, 200", CH3N0 can isomerize, and the isomer can d e NO can be consumed in a reactionfimt order in CH3N0. compose both homogeneously and heterogeneo~sly.~~'~ They report a rate constant of 2.45 exp(-7700/RT) However, both the isomerization reaction and the subsec-l and attribute this reaction to isomerization on sequent decomposition have been reported to occur the walls. This explanation is inconsistent with the with an activation energy of about 40 kcal/mole.l1Pl2 activation energy for the isomerization reaction and Consequently, they cannot be important a t lower with the fact that the isomer definitely is not a product temperatures. At room temperature, nitrosoalkanes of the reaction at room temperature.18 Finally, in are quite stable,'^ l 2 , I 3 but can disappear slowly by studies in this laboratory,20X20 was found to be an polymerization. Thompson and Linnett14 found a important product a t low intensities. The results white, solid deposit when mercury dimethyl was could only be interpreted if all the CH3NO mas conphotolyzed in the presence of nitric oxide, probably the sumed by the reaction polymer of CH&O. Calvert, Thomas, and Hanst' CHsNO NO +HsC-N=O +CH 3 0 NzO measured the dimerization rate constant for CHBNO ,.. ... to be 87 I./mole sec at room temperature. Christie, O=N Frost, and V o i ~ e y 'report ~ the dimerization constant to be 0.63 X lo6 exp(-4600/RT) I./mole sec, which It is also clear that the results of Christie and cogives 25 I./mole sec a t room temperature in approximate workers imply such a reaction. Consequently, we agreement with the Calvert, Thomas, and Hanst value. initiated a study to establish this reaction and to measure its rate constant. I n the presence of a great excess of NO, BrownI6 found thai alkyl radicals catalyze the conversion of II. Section NO to N, and NO2. He proposed the reaction seA . Materials. Matheson Co. research grade N2, quence 02, NO, and N2O were used, the N2 and 0% without N=O N-0 further'purification. Both the NO and k20were
+
/
RNO
+ 2NO -+R--N
/
/
I
+R-N
\
0-N=O
\
0-N-0
N
R
+ NZ + sym-NOa +R-N
//
\
0 0-N
+
/ \
0
The alkyl radicals then react with nitric oxide to regenerate the nitrosoalkane, and the NO, reacts with NO to produce NO2. A number of recent reports show that N2 and NO2 can be formed in very great yields.12~16*17-20 Christie and her co-workers12~16,1s show that the above reaction is first order in RNO and second order in NO. I n ref 15, the rate constant is reported to be 2.6 exp(+1800/RT) 1.2/mole2sec. With
(6) D. E. Hoare, Can. J . Chem., 40, 2012 (1962). (7) J. G. Calvert, 8. J. Thomas, and P. L. Hanst, J . Am. Chem. SOC., 82, l(1960). (8) A. Maschke, B. S. Shapiro, and F. W. Lampe, ibid., 85, 1876 (1963). (9) G. L. Pratt and J. H. Purnell, Trans. Faraday Soc., 60, 371 (1964). (10) L. Batt and B. G. Gowenlock, ibid., 56, 682 (1960). (11) B. G. Gowenlock, L. Batt, and J. Trotman, Special Publication No. 10, The Chemical Society, London, 1957, p 75. (12) M. I. Christie, Proc. Roy. SOC.(London), A249, 258 (1958). (13) F. W. Dalby, Can. J . Phys., 36, 1336 (1958). (14) H. W. Thompson and J. W. Linnett, Trans. Faraday SOC.,33, 874 (1937). (15) M. I. Christie, J. S. Frost, and M. A. S'oisey, ibid., 61, 674 (1965). (16) J. F. Brown, Jr., J . Am. Chem. SOC.,79, 2480 (1957). (17) E. G. Burrell, Jr., J . Phys. Chem., 66, 401 (1962). (18) 0. P. Strausz and H. E. Gunning, C'un. J. Chem., 41, 1207 (1963). (19) M. I. Christie, C. Gilbert, and M. A. S'oisey, J . Chem. SOC.,3147 (1964). (20) J. Heicklen, Aerospace Corp.. Report No. TDR-469(5250-40)-9 (March 15, 1965). (21) J. Heicklen, J . Phys. Chem., 70, 112 (1966).
Volume 70, Number 10 October 1966
3090
degassed a t -196" before use. In addition, the NO was distilled previously from a bath at -186" to remove any NOz and NzO impurities. Eastman Organic Chemical C:H31 and Peninsular ChemResearch Co. CF31were used after degassing at -196". B. Procedure. CH31 and NO were mixed in a Pyrex T-shaped cell. The cross of the T was 5 cm in diameter and 10 cm long, had NaCl windows at each end, and was situated in the sample beam of a Beckman IR-4 infrared spectrometer. The stem of the T was also 5 cm in diameter and 10 cm long. At the end was a quartz window through which the ultraviolet radiation entered the cell. The incident radiation came from a Hanovia U-shaped, Type SH, mediumpressure mercury lamp and passed through a Corning 0-53 filter (to remove radiation below 2800 A) and appropriate screens (to reduce the intensity) before entering the cell. During exposure, the infrared product bands were monitored continually. After exposure, the bands were monitored until reaction was complete. Then the reacted mixture was expanded through a trap at -212' into a calibrated volume, and the pressure was measured after equilibrium was attained. At -212", the only noncondensible gas is Kz;thus, this measurement permitted calculation of the nitrogen yield. The YJ2 and S O then were removed at - 186", the products were warmed to -160", and the SzO was collected for chromatographic analysis. C. Actinometry. To determine the incident intensity, mixtures of 300 mm of CF3I and 300 mm of Oz were photolyzed, and the C F 2 0 was monitored by infrared analysis. Under these conditions, all radiation between 2800 and 3200 A entering the cell is absorbed, and the quantum yield of CF2O formation is unitySz1 The absorbed intensity for any CH31-NO run, then, was computed from the absorption coefficients of CH31 a t various wavelengths, the transmission characteristics of the Corning 0-53 filter, and the relative lamp intensities a t the various mercury lines. The absorption coefficients were found directly on a Cary Model 15 ultraviolet spectrophotometer; the transmission characteristics of the Corning filters are well known. and the relative lamp intensities had been found previously.22 D. Calibrations. For Nz and NzO we made direct calibrations. For NOz, measured amounts of 02 were mixed with CH31and NO to correspond to actual runs. The 0 2 completely converted to NOz. Detailed calibration curves for the 6.16-p band were made for every condition because of the complexity of the NO2 system. In the first place, KO2 dimerizes to form a system in equilibrium with Nz04;secondly, NO2 reacts with NO The Journal of Physical Chemistry
TIMOTHY JOHNSTON AND JULIAN HEICKLEN
to form a system in equilibrium with Nz03; and thirdly, the N02-Nz04 equilibrium is nonideal. The extinction coefficient depends on the total pressure of the system, even if an inert gas (argon) is used. However, once the calibrations were made, NO2 could be monitored reliably. Our reported values for NOz correspond to total NOZ,i.e., (NOZ) 2(Nz04) (NzOa). For CH,ONO and CH3ONO2, we did not make calibrations. The extinction coefficients used were extracted, respectively, from TarteZ3and from Brand and C a w t h ~ n . The ~ ~ infrared peaks used for the product identification and analysis of KOZ, CH30N0, CH3ONOz, CHZO, and HKOz were, respectively, at 6.16, 5.88,239.85,245.75,26and 12.65 p e Z 6
+
+
111. Results The infrared product bands were found for NOz (including Nz04 and Nz03), C H 3 0 x o , CH30N02, CHzO, and HN02. The infrared band reported for CH3n'O7lies under the large NOz band at 6.16 p ; thus no analysis could be made for CH3N0. The dimer of CH3N0 goes to the wall and polymerize^,^ so none of its infrared bands would be detected. Iodine and nitrogen have no infrared spectra, and that for NzO is extremely weak. S o analysis was made for iodine, but Nz and NzO were measured after the reaction was completed. It is possible that H N 0 3 was also formed as a minor product, but its infrared spectrum is similar to that for HN02, and analysis would be difficult. We carefully looked for the CH3TOebands at 6.35 and 7 . 2 p,27 but did not see them. Consequently, if CH3iY02is formed, it cannot bean important product. The important products of the reaction that can be followed spectrometrically are NOz, CH30N0, and CH30NO2. For a typical run, the results are plotted in Figure 1. It should be understood that NO2 refers to total NOz including Sz04and K20J. The CHzO and HKOZ bands were quite small. I n many cases no values could be obtained; in other cases only final values were found; in a few cases CHzO could be monitored throughout the run. The curves in Figure 1 show that the NOz, CHsONO, and CH30NO2 all have induction periods; those for C H 3 0 N 0 and NO2 are similar, but the one for CHs(22) D. Dutton and J. P. Heicklen, unpublished work at the University of Rochester, 1956. (23) P.Tarte, J . Chem. Phys., 20, 1570 (1952). (24) J. C. D. Brand and T. 31.Cawthon, J . Am. Chem. SOC.,77, 319 (1955). (25) R . H. Pierson, E. N. Fletcher, and E. Yt. C. Gantz, Anal. Chem., 28, 1218 (1956). (26) L. H. Jones, R. M. Badger, and G. E. Moore, J . Chem. Phys., 19, 1599 (1951). (27) D. C . Smith, C. Pan, and J. R. Nielseri, ihid., 18, 706 (1950).
PHOTOLYSIS O F METHYLIODIDE IN THE PRESENCE OF NITRIC OXIDE
3091
CHsNO. That 1 2 is also formed as a secondary product has not been proposed previously. However, our results require that CH31 be decomposed in a chain step, and thus part of the I2formed would be secondary. Our results clearly show that NO2 and C H 3 0 N 0 are secondary products. Since the N2 formation is associated with NO2 production, it too must be secondary. The dimer of CH3N0 is, of course, secondary. For the reasons stated in the Introduction, we believe N20 to be secondary. For the runs in which CH20 was monitored, its curves of growth had induction periods similar to CH3O?JO2; thus, both CH30N02 and CH20 are tertiary products. Nitrous acid can be formed only from X 0 2 , so it too must be a tertiary product.
Table I : Products of the Reaction Product
TO
Primary and secondary Primary Secondary Secondary
TIME, min
Figure 1. Plot of product formation us. time for (CHaI) = 31.8 mm, (NO) == 318 mm, and I, = 4.1 p/min. CH30NO
ON02 is considerably longer. These effects were noticed in all runs where data were obtained. After the induction period, the products rise linearly with time. The linear portion of the NO2 curve has been extrapolated to zero and an induction time r ohas been found. After the lamp was turned off, at time r , the product curves usually continued to grow for some time before leveling off, after which the products were stable in the dark. The induction period and the leveling-off period arise from two causes. First, there is a lag due to diffusional mixing in our system. The products form in the stem of the T-shaped cell, but are not analyzed until they diffuse to the cross of the T. Separate experiments in our laboratory have shown the diffusional mixing time to be approximately 1 sec/mm of gas. Thus, at, high total pressures, most of the lag time is due to diffusional mixing. Second, one of the primary products of the reaction CH3N0 is unstable and slowly reacts to produce the secondary and tertiary products that we monitor. At low total pressures, this effect accounts for most of the delay. Based upon our time-history data and information previously obtained by others, the products can be classified as primary, secondary, or tertiary. The classification for each product is listed in Table I. It is well established that the primary products are I2 and
Classification
Secondary Secondary Secondary Tertiary
CHzO
Tertiary
HXOz
Tertiary
Comments
Not monitored
Not monitored Not monitored hfonitored continually during exposure RIonitored continually during exposure Final amount measured Final amount measured hlonitored continually during exposure Monitored continually during exposure Final amount measured
The detailed results of the photolyses are listed in Table 11. For those runs in which the induction period is not caused mainly by diffusional mixing, the NO2 concentration was plotted us. exposure time on a loglog plot, and the data points lay on straight lines of slope 2. From the intercepts of the plots, values of (N02)/t2 were found, and they are listed in Table 11; they rise with the absorbed intensity and the NO pressure. Quantum yields of X02 and CHsOSO formation were found from the linear portion of the product us. time curves. The values for CHSONO could be obtained for only a few runs because of the interference of an Nz03 band at the 5.88-p band of CH30?U'0. We have reported only @(CH30xO)for those cases where the N203correction based on its band at 7.65 I.( was less than about 30%. Both @(NOZ)and a(CH3OXO) rise with (ITO)/Ial'* to maximum values far Volume 70,Xumber 10 October 1966
TIMOTHY JOHNSTON AND JULIAN HEICKLEN
3092
in excess of unity. The quantum yields for Nz and NzO were obtained by dividing the amount of product formed by the absorbed intensity and the exposure time. Both quantum yields rise with (NO)/Ia"y initially, but then level off at about 100 for +(N,) and 1.0 for a(li20)at high values of (NO)/IS1". The (CH30N02), as well as (CH20) when possible, were plotted us. 1 - TO on log-log plots; in all cases, the plots had slopes of 2. The values for (CH30N02)/ ( t - T O ) , are listed in Table 11, and they rise with (NO) and I a . Absolute calibrations were not available for HN02 or CH20, but the ratios of their final optical densities are listed for the few runs where information was available. The ratio is reasonably constant, in view of the large errors introduced from the use of small peaks.
IV. Discussion The primary products are I, and CH3N0, and the primary reaction sequence is
+ h~ +CH3 + I 21 + ?tl+ I, + M CH3 + NO +CH3NO
CHJ b"'040f-
"?i24%" i
m
.*, .: .. .. .. .. .. .. . . . . . .
(4 (b) (c)
Because of its large concentration, the X O scavenges all the methyl radicals. The chain steps are
+ 2 N 0 +CH3 + Nz + NO3 NO3 + NO +2x02 NO3 + CH3I + NO2 + CH30 + I
CHINO
(d) (e) (f)
Step f is needed to account for the fact that the amounts of C H 3 0 N 0 and CH30N02 produced are very much greater than can be explained unless CH3I enters the chain decomposition. Further evidence is obtained from the ratio +(N02)/+(N2). If step f were unimportant, the ratio always would be 2. However, if step f were much more important than step e, then the ratio would only be unity. The functional relationship is
The values of +(N02)/+(N2) are listed in Table 11. They vary from 1 at high (CHJ)/(NO) ratios toward 2 at the lowest (CH31)/(NO) ratios. From the trend in the data, k f / k , can be estimated to be about 10. If the NO3 were symmetric, then it seems that CH3ON02 would be a likely product of reaction f. Yet our results clearly show that CH3ONO2is a tertiary product, and thus cannot come directly from f. There-
The Journal of Phyaieal Chemiatry
PHOTOLYSIS OF METHYL IODIDE IN THE PRESENCE OF NITRICOXIDE
fore, we believe that asymmetric NO3 is present, and that the Brown mechanism should be rewritten
I
3093
I
I
I l l
I
I
I
i l l
1
I
I
I
l_l
N=O
RNO
+ 2N0 +R-N /
+
\
0-N=O N
R--N
/\
0
I
I
0
0
\/ N
N R
+ Nz + U S ~ W L - N O ~R-N
-1
IO
//
103
IO2
(NO]* I,,, mm3/min
t-
I
0
0-0
Figure 2. Log-log plot of (N02)/t2us. (N0)21,. to total NO, including N~04and N ~ 0 3 . )
(NO, refers
\/ N
An attractive feature of this mechanism is that a sixmembered, rather than a five-membered, ring is the intermediate. At low total pressure, the induction period in SOz production results mainly from the fact that CHINO has not reached its steady-state value. Thus, the reactions tending to remove C&NO are not yet important. It also happens that for these conditions, reaction e is unimportant for our experiments. Thus, the mechanism requires that
(NOZ) ~22
k d l a (XO) 2 2
Figure 2 is a log-log plot of (KOJ/tz us. (NO)zI,. A reasonable straight line of unit slope can be drawn through the points. From the intercept, a value of k d = 47 1.2/molezsec is found, in good agreement with the room temperature value of 55 found by Christie, Frost, and Voisey.'s As the reaction proceeds, the steps responsible for CH3N0 removal become important and C H 3 S 0 reaches a steady state 2CH3NO + (CH3NO)z CH31VO NO +CH.30 NzO
+
+
(g) (h)
The methoxy yadical reacts with NO all the time to yield CH30NO NO +CH30NO CH30 6)
+
For three reasons we can ignore the possible competing step
+ NO +CHzO + HNO
CHBO
First, such a step would require CHzOto be a secondary product, when in fact our results indicate it to be a tertiary product. Secondly, Knight and Gunningz8 found that for the reaction of CH30 with S O , the important product was CHsOP\'O, and that the CHzO yield extrapolated linearly to zero for zero exposure time. Finally, i M ~ M i l l a nshowed ~~ that nitric oxide adds to isopropoxy radicals 6.6 times as fast as it abstracts hydrogen atoms. For methoxy, where the hydrogens are not nearly as labile, this ratio must be very much larger. The reaction mechanism predicts that under all conditions (3) Figure 3 is a log-log plot of @(Sz)/@(KzO) us. (KO). The data points are well fitted by a line of slope 1, and yield a value of 2800 ]./mole for kd/kh. Using our previously obtained value for k d , we find k h = 0.017 l./mole sec. Unfortunately, we cannot compare this value directly with that reported by Christie, (28) A. R. Knight and H. E. Gunning, Can. J . Chem., 39, 1231 (1961). (29) G. R. McMillan, J . Am. C h a . SOC.,83, 3018 (1961).
Volume '70, Number 10 October 1966
TIMOTHY JOHNSTON AND JULIAN HEICKLEN
3094
/
where R(g) and R(h) are the rates of reactions g and h, respectively. The values of @(N20) and @(IT2)/ (KO) are plotted us. (NO)/I,”* on log-log plots in Figure 4. Both plots are linear with a slope of 1 for low values of (NO)/Ial/’,ie., R(g) >> R(h), and constant for large values of (NO)/Ial’*, ie., R(g) R(h), we find
+
10-21 lo-‘
’
l4
’ ’’
I 10-3 x (NO) /
IO
I,1’2,
’
I
’
I ’ IO2
I
( mm-min I 1’2
Figure 4. Log-log plots of @(NzO) and +(N*)/(NO) vs. (NO)/Ial’z.
If R(g)
