Volume 63. number 3 CHEMICAL PHYSICS LETTERS 1 June 1979

KINETIC SPECfROSCOPY STUDY OF THE REACTION OF CHgO, WITH NO

Hiroyuki ADACHI and Norman BASCO

Thr ovrmfl i-ate constant for the rczction bctwwn mrthyIprroxy mdicaIs and nitric oxide has bwn dirccrly mr~sured by fi.t~h photo!ysi?, and kinetic ~ppectro~copy. At roam temperature. k~ = cl.8 5 0.1) X 10' E mole-' <" and independent evi- deuce indiates that this mJy be identitied with the reaction CI130= + X0 + CI130 t NO?.

I_ Introduction

The reaxion of alLyI perory radiczds with nitric oxide is the crucial step in the chaiin reaction beliebed

to be respousib~e for the conversion of XD to ND7 in

poifuted atmospheres_

R + O2 (*&I) --)r RO, 6+&l), (I)

RO, f SO + RO + M,_ (3

RO + O2 -)r R’R”C0 f H02. (3)

HO2 -k so-f Oki + NO,* (-0

OH + Rkt -+ Hz0 i- R. (5)

The tlrst laboratory study of the rextion of the Rlethy~perOxy raiicaf with NO ill indicated, however, thst the reaction

f’ti3t3, * KO + CH30 + x0, (h)

was unimportant (< 2%). since no CH30N0 was de- tected as ;t prOduct of the pbotooxiddation of ;Izomethme in the presence ofir;O. Instead, the presence of CI i,OW3, as a major product. together with HCOOH, appeared to show that the predominant reactions were

CII,O, f ;?;O - CH&xO, (bb)

CH,O, + NO - CIi,O +- HONO_ (W

In 3: Iater study, Simonaitis and Heicklen [21 suc- ceeded in showing that CH~O~O was a major product

of the photoiysis of mixtures of N,O + CH, + 0, + NO_ They concluded that 85 * 15% of the reaction of CH,02 with NO proceeds through (6a), whiIe Pate et aI_ [3] have presented evidence for reaction (64 being

the on@ path_ Cox et al_ [4] studied the photolysis of HONO in

the presence of CH4 f NO f 820, + irf, + O-, mi..tures_

They found that the CH, was converted almost quan- titatively to CHZO and that this was accompanied by the oxidation of two molecules of NO to NO?. These observations were interpreted in terms of the chain mechanism, reactions (l)-(S) with R = CH, and R’ = R” = H. They concluded that 3 90% of the CH307 LI- diczds produced reacted with KO to give CH,O radicals. The rate constant ks could not be measured; but, using P numerical simulation programme involving 46 reactions, they were able to show that 3 good fit of their data re- quired that k,, Z 3k, _

XH;O, + products_ (7)

Thus, with the experimental v&e f5] k, = 2.4 X lOa P mofe-t s-t. ks, > 72 X 108 P mole-t s-t _ They point out that this resuIt is idential to the value kG = 7.2 X IO* P m01e-~ s-* derived for the analozons reaction (4) by a computer simulation of the results obtained by Cox and Derwent [6] _ However, although a similar value for 4 was obtained by Simonaitis and Heicklen [?I, values approximately three times lower were reported by Payne et al. [8] and by Hack et al. [9] _ Moreover, a direct measureinent by Howard and Evenson [lo] esing laser- magnetic resonance detection in a discharge flow system gave k4 = 49 X IO9 Q mole-t s-t.

490

Volume 63, number 3 CHEMICAL PHYSICS LETTERS 1 funt 1979

An unsuccessful attempt to measure the rate con- stant kG has been reported by Anastasi et nl- [f I] . They Rash-photolysed azomethane in the presence of oxygen and followed the methylperoxy radical through its absorption near 240 nm. However, no absorption due to CH302 could be detected at 150 its after the flash (the shortest delay at which absorption mtasure- ments were possible) in the presence of even the smnll- est concentration of NO (= 1.7 X iOm6 mole E-l). Ac- cordingly, they deduce that k, 2 6 X lo8 P mole-* s-t.

This letter reports the direct measurement of the mte constant for the reaction of CH,O, radicals with

NO 6y ffash photolysis and kinetic spe&roscopy. With the apparatus described below, it ~3s possible to follow the decay of CH30, in the presence of a wide range of MI concentratio& over a period from zero to > 1 ms delay after the photoflash.

2, Experiment38

hfethyiperoxy wdicals were generated by the fkssh photolysis ofazomethane (92 X lWs-l -7 X IO-- mole Vi) in the presence of oxygen (12 X fOd-- 1.4 X IO-3 mole E-1) snd argon (%4 X 10-Z mole Q-1) with it f&h energy of I _I kf. The reactions of these rddicais was followed by recording the temporal vnria- tion of the absorbawe dt 3-40 run or 250 nm by a photo- electric technique_

The anaIytica1 light source w.\lits a 150 W xenon lamp intensified by ;Ipplying an approximately square have voitsge pulse lasting for up to 10 ms_ The collimated output of this lamp was spIit and the resulting parailel beans passed through two identica1 reaction vessels and were then focussed on the slit of 3 I III focal length spectrometer..

The cylindrical reaction vessels were 9 14 mm long and mzde of Pyres with~uprasil end windows. The use of Pyrex sened the double purpose of restricting the photoiytic radiation to wavelengths longer than

about 280 nm and ofvirtudlly eliminating scattered !ight from the photoflash in the wavelength region used

for monitoring the CH$O, radical. Furthemlore, J double gas filter containing chlorine and bromine was placed before the spectrometer slit to remove radiation between 280 nm and 520 nm which could hake been scattered within the spectrometer. These precautions made it possible to make measurements even during the

photoflash.

The two beams emerging from the spectrometer were directed to b&need photomultipliers and the

output from these. lo and I were analysed in tx\o ways. The first used a Tektronix model 7704 oscilloscope with 7Ai2 and 7A13 plug-in units to display IO_ I and I0 - 1 and these were recorded 01% polarold tilm for

measurement. The second used a digital storsge osctl- ioscope (Ncoiet 1090A) interfaced to 3 czkulrttor (HP 9825A). Up to 2045 readings of logifo/I). taken at intervak of 0.5 ,us or longer, \\ere stored on magnetic t3pe for subsequent muraiysis.

3. Results and discussion

The r&e constant X-, wx mewred front d series of 34 experiments in which tfte NO concentmtion was varied over the range 2.8 X 10V7 -3.6 X 10-6-moIe 4-’ _ The large mcreJse in the rate of decay of CH;O, c3used

by even the s@Iest [NO] showed that reaction- was overwhelmingly predominant in this system under these conditions. Furthermcre, over most of this r.mge. the NO concentration is sufficiently high to allow the

results to be anrtiysed by pseudo-firsr order kinetics. Accordingly. the first order rate constzmts. X- = II-, [NO]. obtained from plots of in(ribsorbance) against rime. were themselves plotted against [NO] . This plot w;1s

linear with a sldpe. X-, = (I -76 t 0 07) X IO9 P m01c-~ s-l_ The small intercept of (0.10 f: 0.1 I) X 103 s-1

confim:s that the removal ofCHj02 by other reactions

is unimportsnt. Nevertheless. the correction due to tile mutual combination reaction ~3s calcufat+d xing the v&es ofk, and E(CH,O?) measured by us [I21 which are close to those reporte’-i by P&es [13] _ It amounted to not more than 10% for 2.3 experiments and exceeded 1% only once. The o\eralI effect on the plot of IrI ~gktls? [MO] was quite negligible with ks = (I -77 r O-07) X IO9 G moie-1 s-l and an cssentiaiiy zero intercept of -(O-O.02 f 0.1) X 103 s-t. The plot is shown in fip. I_

The xcurxy of the pseudo-tirst order approsimation was also tested by doing a rigorous second order analysis of the data. For this, the initral CH,O, concentmtion w;Ls messured just after the phorotldsh - at which time no ;Ippreciabie depletion of NO had occurred_ Reaction (66) was xsumed to be the only reaction in which hO was removed, i-e_ neglecting the reactions

CH,O + NO + products,

Volume 63. number 3 CHJX!CAL PHYSICS LETTERS

by Howard and Evenson [lo] rather than the much lower values reported by other authors [6--83 - since it seems unlikely that “6 exceeds 4 _ However, the measurement of% was done at a total pressure of between 140 and 220 Pa (1 -1-l-7 Torr) compared to IO- 13 kPa (i’S- 100 Torr) in our work. if, after all, the relative importance of reactions (6a), (6b). and (SC) proves to be pressure dependent, the similarity between the rate constants&t and may be fortuitous_

1 June 1979

I(

5 I 2 3

~06fNOha;e L-*

Fig.. 1. Variation of the fust order rate constant for the dray ofCLf,O, %\ith NO concentrztion_

CH,O + O2 * CH,O + HO,. (9)

HO,_ -t- NO + OH f NO,, (4)

*as weli as

CH,O, + NO2 4 products. (10)

On this basis, and including a correction for reaction (7), we obtain ks = (1.76 + 0.07) X lo9 P moIe-1 s-l_

Thus, we conclude that the accuracy of the v&e for & obtained is only very slightly dependent on the

-assumptions made in its calculation_ This conclusion is supported by the results obtained in other series of experiments in which the effect ofvarying the oxygen concentration over a ten-fold range or of adding small concentrations of NO, were studied.

Finally, the vahre for 4 obtained here tends to sup port the value I$ = 4.9 X IO9 g mole-l s-l obtained

Acknowledgement

We thank the National Research Council of Canada for the financia1 support of this work.

References

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492