J. Phys. Chem. 1992, 96, 986-992
986
TABLE II: UV Absorption Cross Sections Measured in This Work u x 1020, u x 1020, wavelength, cmz molecule-' wavelength, cm2 molecule-' nm CH302 nm CH302 229 389 270 202 280 116 240 440 290 60 250 390 3 00 21 260 312
of SF6/CH4/Ozmixtures with [SF,] = 575 mbar. Initial absorptions were then scaled to that at 229 nm, and our measured absorption cross section at 229 nm was used to convert the initial absorption into an absorption cross section. Values obtained are listed in Table I1 and are shown in Figure 6. An additional absolute absorption cross section measurement for CH302radicals can be derived from the data shown in Figure 1 given that the yield of F atoms in the pulse radiolysis system Linear is 2.8 X lOI5cm-3 at full dose and 1000 mbar of SF6.20,z1 least-squares analysis of the data in Figure 1 yields a slope of (3.83 f 0.41) x lo4, which in turn yields u22qnm(CH30z) = (3.94 0.42) X lo-'*cmzmolecule-'. Quoted errors are 2u. We estimate that, in addition, there is 15% uncertainty in the absolute calibration of the F atom yield. Combining the statistical and p i b l e cm2 molesystematic errors, we arrive at (3.94 1.00) X cule-'. Thii value is in excellent agreement with the value derived above from measurements on the isosbestic point.
*
Discussion
The absorption cross sections for CH302radicals measured in this work are compared to the spectrum recommended by Wallington et al?q (based upon a fifth-order regression fit to data in refs 5, 7, 9, 10, 1 1 , and 13) in Figure 6. The agreement is excellent. However, the purpose of the present work is not to provide another determination of the UV absorption spectrum of methyl peroxy radicals but to demonstrate a novel method for measuring gas-phase peroxy radical absorption spectra with high accuracy. This method relies on the conversion of the peroxy radical into the corresponding alkyl nitrite and NO2 and the determination of the isosbestic point. The advantage of this new method is that, once the isosbestic point has been established, the problem of measuring the absorption cross section of the peroxy radical is simplified to (29) Wallington, T. J.; Dagaut, P.; Kurylo, M. J. Chem. Rev., submitted for publication.
measuring that of the nitrite and NOz at that wavelength. Alkyl nitrites are, in general, stable compounds under ambient conditions and are readily synthesized in high purity. Measurement of the UV spectra of these compounds can be performed with great accuracy using conventional UV spectrometers. This latter factor largely explains the reduced uncertainties associated with the alkyl nitrite measurement over the conventional approach. Providing that conditions can be chosen to ensure stoichiometric conversion of peroxy radicals into the nitrite, this approach offers an additional advantage over conventional methods in that the nitrite acts as an internal standard and removes the need for a separate experiment to determine the absolute radical yield. The pulse radiolysis system used in the present work generates a large initial concentration of radicals (1-3 X 1015~ m - ~ )With . such high radical concentrations the resulting optical densities are sufficiently large that no signal averaging needs to be employed and experimental traces are then derived from a single shot. With the formation of methyl nitrite acting as an internal standard, UV absorption cross sections are then measured in a single experiment consisting of a single shot. The present determination of u(CH302) is therefore free of any experimental uncertainties associated with drifts in reagent concentrations, radiolytic pulse intensity, and analysis beam intensity over time scales longer than used in a single experiment (100-1000 ps). The disadvantages of the present technique for determining a(R02) are twofold; first, a calibrated UV spectrum of the alkyl nitrite is needed and, second, experimental conditions must be chosen to ensure stoichiometricconversion of the peroxy radical into the nitrite. Clearly, these two requirements will not be satisfied for all peroxy radicals. Nevertheless, the present technique is a useful addition to our methods for the determination of the UV spectra of alkyl peroxy radicals with a high degree of accuracy. Finally, as illustrated in the companion paper following in this issue, use of this method is not restricted to the pulse radiolysis system, nor is the determination of the isosbestic point the only way to achieve an absolute calibration.
Acknowledgment. O.J.N. thanks the Commission of the European Communities for financial support. We thank Jette Munk (Rim, Denmark) and Elzbieta Bartkiewicz (Agricultural and Teachers University, Siedlce, Poland) for technical assistance. R-by NO. SF,, 2551-62-4; CH4, 74-82-8; 02, 7782-44-7; NO, 10102-43-9; CH302,2143-58-0; CHIONO, 624-91-9; CH3,2229-07-4;
F, 14762-94-8.
Absolute Ultraviolet Cross Sections of Methyl and Ethyl Peroxy Radicals M. Matti Maricq* and Timothy J. Wallington Research Staff. Ford Motor Company, P.O. Box 2053, Drop 3083, Dearborn, Michigan 481 21 (Received: July 25, 1991; In Final Form: September 17, 1991)
-
The method of transient ultraviolet spectral photography has been used to follow the photolytically initiated reaction CH302 + 2 N 0 CH30N0+ NO2and the analogous reaction between C2H5O2 and NO. A comparison of the reaction mixture absorbance to reference UV absorption spectra of CH30N0, C2H50N0,and NO2allows a determination of the product concentrations as a function of time. The final product concentrations,along with mass balance and the initial peroxy radical absorbance, then yield absolute CH3O2 and C2H5O2 absorption cross sections.
I. Introduction Peroxy radicals exhibit a rich and varied chemistry which is of importance both in atmospheric and in combustion processes. An important consequence of the relative stability of peroxy
* Author to whom correspondence should be addressed. 0022-3654/92/2096-986$03.00/0
radicals to reactions with closed shell species is that peroxy radical-radical reactions become a major removal mechanism for of insight has been achieved these Species. A considerable "nt into peroxy radical kinetics via techniques such as flash photolysis and molecular modulation. These techniques rely on spectroscopic detection of peroxy radicals via their UV absorption in the 2000 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96,No. 2, 1992 987
UV Cross Sections of Peroxy Radicals
S+mdH-d method
uv
1
I
lamp
U
monochromator
cell
electronics E x i m e r laser
\
wauelensth
diode arraM
Xe or 02 lamp
,
, I
I
L
/ ',
.
monochromator
Figure 1. Block diagram of the flash photolysis-transient UV spectral photography apparatus and a comparison to a conventional flash photolysis experiment.
300-nm range. Because of the second-order nature of radicalradical reactions, however, the relative variation in radical concentration as a function of time is insufficient to determine a reaction rate constant; instead, absolute concentrations and, consequently, absolute absorption cross sections are needed. Methyl peroxy radicals, generated by photooxidation of methane, are of special interest in atmospheric chemistry, and consequently it is of little surprise that much attention has been paid both to measurement of its absorption cross section and to its reaction kinetics.'-16 However, as recently as in the past yearI5 earlier agreement about the rate constant for the methyl peroxy disproportionationreaction has been termed fortuitous largely on the grounds that various researchers have not employed the same optical cross sections in ascertaining their methyl peroxy concentrations. Simon et al.I5 gives a good account of the discrepancies both in the methyl peroxy absorption spectrum and in the rate constant for its self-reaction, as does the somewhat earlier work of Kurylo et al." The purpose of the present paper is twofold: First, we describe a transient UV spectroscopy apparatus newly set up at the Ford (1) Parkes, D. A.; Paul, D. M.; Quinn, C. P.; Robinson, R. C. Chem. Phys. Lett. 1913, 23, 425.
(2) Parkes, D. A. Inr. J. Chem. Kinet. 1977, 9, 451. (3) Hochanadel. C. J.: Ghormlev. P. J. J. Phvs. .. J. A.:. Bode. - . J. W.:. Omen. Chem. 1977. 81. 3: (4) Kan, C.S.; McQuigg, R. D.; Whitbeck, M. R.; Calvert, J. C. Int. J. Chem. Kinet.1979, 11, 921. (5) Cox, R. A.; Tyndall, G. S. Chem. Phys. Lett. 1979,65, 357. (6) Cox, R. A.; Tyndall, G. S.J. Chem. SOC.,Faraday Tram. 2 1980,76,
Scientific Research Laboratory for the purpose of studying the kinetics of atmospheric reactions. Second, we demonstrate the usefulness of the apparatus by measuring absolute UV cross sections for CH302and C2H502,where the conversion from absorbance to cross section is made by following the reaction of these species with NO to form their corresponding nitrites plus NO2 and comparing the latter to reference spectra. The same calibration procedure is implicitly used by Wallington et ale1'in the companion study (hereafter referred to as paper 1). Those experiments rely on finding a wavelength at which the absorption of the CH3O2 + NO reaction mixture does not change as a function of time. By implication, the methyl peroxy cross section equals the methyl nitrite plus NO2 cross section at this wavelength and, thus, affords a scaling factor for converting methyl peroxy absorbance to cross section. The relatively high accuracy of the calibration procedure and the agreement between this work and paper 1 should resolve any remaining discrepancies in the CH3O2 spectrum. The outline of the paper is as follows: Section I1 provides a description of the experimental apparatus and the conditions used in our measurements. Section I11 provides the experimental results and a discussion of the errors. We compare our methyl and ethyl peroxy absorption spectra in section IV to previously published work.
1
*1 J.? J.
(7) Adachi, H.; B a w , N.; James, D. G. L. Int. J . Chem. Kinet. 1980, Z2, 949. (8) Sander, S. P.; Watson, R. T. J . Phys. Chem. 1981, 85, 2960. (9) Pilling, M. J.; Smith, M. J. C. J. Phys. Chem. 1985, 89, 4713. (10) McAdam, K.; Veyret, B.; Lesclaux, R. Chem. Phys. Lett. 1987,133, 39. (1 1) Kurylo, M. J.; Wallington, T. J.; Ouellette,P. A. J. Phorochem. 1987, 39, 201. (12) Jenkin, M. E.; Cox, R. A.; Hayman, G. D.; Whyte, L. J. J . Chem. Soc., Faraday Tram. 2 1988,84, 913. (13) Moortgat, G. K.; Veyret, B.; Lesclaux, R. J . Phys. Chem. 1989, 93, 2362. (14) Dagaut, P.; Kurylo, M. J. J . Phorochem. Photobiol. A: Chem. 1990, 51, 133. (15) Simon, F.; Schneider, W.; Moortgat, G. K. Inr. J . Chem. Kinet.1990, 22, 79 1 . (16) Jenkin, M. E.; Cox, R. A. J . Phys. Chem., in press.
11. Experimental Section
The experiments were carried out using a laser photolysistransient UV spectral photography apparatus that has been newly assembled at the Ford Scientific Research Laboratory. A schematic diagram of this apparatus is compared in Figure 1 to a conventional flash photolysis experiment, such as the one described in paper 1. In the conventional experiment, a reaction is initiated at time r = 0, and the absorbance of the reaction mixture is monitored at wavelength X as a function of time. The principal difference in the present system is that it incorporates a gated array detector which records the transmitted light intensity as a function of wavelength at a given time delay, Ar, after initiation of the reaction by the photolysis laser. The reactant methyl peroxy radicals were generated by 193-nm laser photolysis of methyl chloride in a flowing mixture of CH3Cl, (17) Wallington, T. J.; Maricq, M. M.; Nielsen, 0. J. J. Phys. Chem., preceding paper in this issue.
988 The Journal of Physical Chemistry, Vol. 96,No. 2, 1992
CHI, O,,and NO using a Lambda Physik LPX 300 excimer laser. The gases composing this mixture, of >99% purity, were obtained from Matheson and Airco and used without further purification. The 193-nm excitation falls in the longer wavelength portion of the diffuse band assigned to the X'AI-IE transition in CH3Cl. It excites a chlorine nonbonding electron into a u* antibonding molecular orbital, which correlates with the dissociative channel leading to CH3(X2A") and Cl('P312). Recent work by Matsumi et a1.18 shows significant curve crossing to produce 23% C1(2PI/2). Excited chlorine atoms formed in our system will be rapidly relaxed to the ground state by collisions with the diluent gases N2 and 02.
The methyl chloride photolysis products react rapidly (on the order of 1 ps under our conditions) to form methyl peroxy radicals via the reactions, CH3Cl + hv CH3 + C1 CH3
+ O2 + M
C1+ CH4
-
--c
+
CH302 M
+
(1)
+ HCl
(2)
CH3
Two different reaction mixtures were employed: the first, hereafter referred to as experiment 1, was composed of 172 Torr of air, 210 Torr of CH4, 3.1 Torr of CH3Cl, and 0.7 Torr of NO, whereas the second, experiment 2, contained 306 Torr of air, 289 Torr of CH4, 3.6 Torr of CH3C1,and 0.5 Torr of NO. A large concentration of methane was used to overcome the relatively slow rate constant for reaction 2, k2 = 1.0 X 10-13cm3 s-l so as to limit the amount of CH2C102that is formed from the reaction of C1 with CH3Cl to a maximum of about 3% of the CH302concentration. The [NO]/[02] ratio was determined from a compromise between two conflicting constraints (vide infra). Too high a ratio may lead to systematic errors owing to the formation of CH3N0 from the reaction of CH3 with NO and the formation of NO2 from the reaction between NO and 02.Too low a ratio may yield misleading results due to the formation of C H 3 0 N 0 2and CH302N02via reactions competing with alkyl nitrite formation. The two gas flow mixtures given above were used to ensure that the measured UV cross sections are independent of the flow composition. Ethyl peroxy radicals are formed via the analogues of reactions 1 and 2, except that ethyl compounds replace the respective methyl species in these reactions. The flow mixture used for the crosssection measurements included 183 Torr of air, 5.7 Torr of C2H6, 3.1 Torr of C2H5Cl,and 0.3 Torr of NO. Relatively much less ethane than methane is required because of the much larger rate con~tant,'~ k = 5.7 X lo-" cm3 s-' molecule-', for the reaction of Cl with C2Hs than for reaction 2. The gas mixture was flowed through a cylindrical quartz cell (3.2 cm in diameter by 51 cm long). The component gas ratios were kept constant using Tylan FC260 flow controllers, with the total pressure in the cell set by a throttle valve between the cell outlet and the exhaust pump. The pressure in the cell was determined from the average reading of Raritron capacitance manometers at the gas inlet and outlet. It is important to set the flow rates sufficiently high to ensure that the gas mixture in the cell is replenished between successive photolysis laser pulses. This was accomplished by verifying that the absorbance measurements were independent of photolysis laser repetition rate. The excimer laser radiation, which is divergent and has a beam size of roughly 3 X 1 an,was collimated using quartz cylindrical lenses to a beam approximately 1 X 1 cm in dimensions. As shown in Figure 1, the laser radiation traversed the cell longitudinally in a single pass, with narrow band 193-nm dielectric mirrors (on suprasil substrates) used to introduce and remove the beam from the cell axis. This geometry allows collinear propagation of the probe UV light through the photolyzed gas mixture. The laser pulses were of 204s duration. The pulse energy delivered to the (18) Matsumi, Y.; Das, P. K.; Kawasaki, M. J . Chem. Phys. 1990, 92, 1696. (19) Atkinson, R.; Baulch, D. L.; Cox,R. A.; Hampson, R. E., Jr.; Kerr, J. A,; Troe, J. J . Phys. Chem. Ref. Dara 1989, 18, 881.
Maricq and Wallington flow cell varied between 100 and 200 mJ/pulse, as estimated from the measured laser power before and after transmission through the cell. Either Xe arc or deuterium lamps were used as probe UV light sources. After passing through the cell, the probe light was dispersed by an Instruments SA HR320 0.32-m monochromator and detected by an intensified, gated diode array detector (Princeton Instruments IPDA-700SB detector and STlOOO controller). Spectra were obtained at a resolution of about 2 nm and calibrated against a low-pressure mercury lamp. The single-pass counterpropagating geometry was used to minimize saturation of the detector by the photolysis laser pulse. Triggering of the photolysis laser and diode array was accomplished using a home-built pulse generator. It triggered the excimer laser at a repetition rate of 0.5-1.0 Hz, opened the mechanical shutter in front of the monochromator for approximately 10 ms, and triggered the high-voltage gate of the intensified diode array. For experiments designed to follow the peroxy radical kinetics, pulses of 2-3-ps duration and delayed by 1-100 ps with respect to the photolysis laser pulse were used to gate the diode array. In experiments comparing product absorbances with and without the NO reactant, 10-15-ps gating times were employed to enhance signal to noise ratios. Typically 500 photolysis pulses were averaged to obtain an absorbance measurement. The mechanical shutter was used to reduce to a negligible amount the UV probe light that leaks through the intensifier while it is not gated and is integrated by the diode array during the time between photolysis laser pulses. Normally in a flash photolysis experiment, the absorbance is computed via the Beer-Lambert law, where Zo is measured under the same conditions as I, except that the photolysis laser pulse is blocked. Under the present experimental conditions, the measurement of Io in this fashion could lead to systematicerrors in the absorption cross sections on account of the ozone generated both outside the cell by the 193-nm dissociation of 0,and inside the cell by the dissociation of NO and 0,.Absorption measurements in the absence of the CH3Cl, or C2H5C1,precursor revealed approximate levels of [O,] = 4-5 X 1013cm-3 and [O,] for the ozone generated, respectively, with and = 5-6 X lOI3 without NO, under typical experimental conditions. These levels are too low to significantly affect the chemistry; however, owing to the strong UV absorption by 03,they do affect the measured absorbance. Two methods were used to overcome this problem: in experiment 1, Io was measured with the photolysis laser irradiating the cell but with the concentration of CH3C1(C2H5Cl) in the gas mixture reduced to zero. This in turn necessitated a correction for the absorption of UV probe light by the CH3Cl (C2H5Cl)found in the cell during the measurement of I. Experiment 2 was performed in two ways: first as just described and second with Io measured with the photolysis laser off and the correction for ozone formation applied by subtracting the ozone absorbance measured with [CH3Cl] = 0. Both procedures yielded the same results. In addition to 03,the dissociation of O2and NO by the photolysis pulse can lead to NO2 generation via the termolecular reaction of 0 NO M. The effect of this additional NO2 on the peroxy radical plus NO product spectrum is removed by the same methods as used above to eliminate the interfering effects of O3generation. A second potential complication is that [N0210 is not exactly zero as assumed in the chemical model. In experiments with [CH3ClIo= 0, we were unable to observe directly the formation of NO,. Because this could be due to its relatively weak absorption, the initial NO2 concentration was estimated from the observed O3production by accounting for the fact that k ( 0 + NO)/k(O + 0,)5 150, whereas [N0]/[02] 5 0.02, to be Changing [NO,], in the kinetic model from zero about lOI4 to this value had no discernible effect on the predicted CH3O2 and C H 3 0 N 0 concentration profiles. Two other corrections were found necessary when the absorbance was calculated: one was important when using the Xe arc lamp, the other when using the deuterium lamp. The Xe arc lamp output decreases rapidly with decreasing wavelength in the 200-
+
+
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 989
UV Cross Sections of Peroxy Radicals
C2H5O2 + NO
-.-- 1 h
9 B
T!
0
I\
___-----
I
_,_.-.-.-
1
0.06 0.04 0.0s
- C,%% - . - C240NO
0.02
6
0.01
200
226
276
260
so0
0
10
Wavelength (nm)
300-nm range of interest in the present experiments. As a result, scattered light reaching the detector pases an increasing problem at the shorter wavelengths. To correct for this, the scattering background was determined from the measured light intensity below 190 nm, under the assumption that only a negligible amount of actual light at these wavelengths would be transmitted by the 193-nm reflectors at either end of the cell, and was subtracted from the raw I and Io measurements.20 In contrast to the Xe arc lamp, the deuterium lamp intensity falls off with increasing wavelength. It is sufficiently weak in the 350-500-nm range that laser-induced fluorescence from the quartz windows and cell contributes a measurable amount of light to the I and Io intensities. More importantly, this contribution depends on the time delay between photolysis laser pulse and detector gate. It is most important in the 0-10-ecs range and becomes negligible at longer delay times. To correct for this, fluorescence intensities were measured under conditions identical to those used to determine I and Io, except that the probe UV light was blocked. Finally, the absorbance was determined from
Io - Fo(At) - SB I(At) - F(At) - SB
1
- A(CH3Cl)
44
time @a)
Figure 2. Time dependence of the UV absorption of a C2HSCI,C2H6, 02,and NO mixture following flash photolysis at 193 nm showing the disappearance of C2HS02and the growth of C2HJON0absorption.
A(At) = In
so
20
(3)
where I and I, are the transmitted probe light intensities with and without the alkyl chloride precursor, F and Foare the fluorescence intensities with and without the alkyl chloride, and SB represents the scattering background. Under the conditions of experiment 2, the absorbance of O3 is subtracted in lieu of CH3Cl. Samples of C H 3 0 N 0 and C 2 H 5 0 N 0were freshly prepared by slowly adding HZSO4 to a saturated solution of N a N 0 2 in MeOH (EtOH) and purified by bulk to bulb distillation.21 Purities of 195% were ascertained from FTIR spectra of the samples. Reference spectra of C H 3 0 N 0 , CzH50N0,and NOz were obtained with the same apparatus used for the transient measurements, except that the 193-nm reflectors were removed and a static gas cell, 5.36 cm in length, was substituted for the flow cell. The absolute reference spectra obtained with and without 1 atm of N2 buffer were similar. They were found to be essentially independent of alkyl nitrite pressure over the range 0.5-8 Torr; a small increase of approximately 9% in absorption with increasing pressure was noted between reference spectra taken in the range 0.5-2 Torr and attributed to a systematic error in the pressure measurements at the low pressures. Measurements above 2 Torr of alkyl nitrite were judged to be of decreasing reliability because of their increasing optical thickness. Thus, the 2-Torr mea(20) This procedure introduces a maximum 15% correction at 200 nm; it is 4% at 220 nm and is negligibleabove this wavelength. Thus, in essence the correction assumes that the scattering background measured at 190 nm remains contant to 220 nm. (21) Adickes, F. J. Prakr. Chem. 1943, 161, 271. Wallington, T. J.; Andino, J. M.; Skewes, L. M.; Siegl, W. 0.;Japar, S.M. Inr. J. Chem. K i m . 1989, 21, 993.
Figure 3. Time vs concentration profiles of C2HS02,C 2 H 5 0 N 0 ,and NO2, as predicted from the kinetic model of eqs 1, 2, and 4-11.
surements were chosen as reference spectra. The accuracy of these reference cross sections is judged to be about f5%.
III. Results Figure 2 shows the variation in time of the UV absorption spectrum for the reaction of C2H502with NO. A similar time dependence is obtained for the methyl peroxy reaction. The spectra show a decrease in absorption with time at wavelengths longer than 230 nm, a region characteristicof alkyl peroxy radicals, and an increase with time of the absorption at wavelengths below 230 nm, at which alkyl nitrites absorb strongly. This variation is consistent with the accepted mechanism
+ NO C2H50+ NO
C2H502
+
C2H50 + NO2 (k = 8.9 X
--*
(4)
C2HSONO (k = 3.0 X lo-")
(5)
for the reaction of alkyl peroxy radicals with nitric oxide (rate constants in this paper are obtained from ref 19 and have units cm3 s-I molecule-' unless otherwise specified). A more complete mechanism includes the following additional reactions: the formation of chloromethyl peroxy radicals via
+ C2HSClC2H4CI + 0 2 + M C1
+
C2HdCl + HC1 (k = 8.0 X lo-'')
(6)
C2H4C102 + M (k
(7)
-
5.0
lo-'')
X
the reaction of alkyl radicals with NO C2Hs
+ NO + M
C2HsNO
+ M (k = 1.1 X
(8)
the self-reaction of ethyl peroxy radicals
+
+ +
C2H502 C2H502 2CzH50 O2 (k = 6.0 X -m CH3CHO CZHSOH 0
+
(9)
(k = 3.0 x 10-14) 2
and the reactions
+ NO2 C2H50 + NO2
C2H5O2
---+
+
CzH502N02 (k = 5.0
X
10-l2) (10)
C2H5ON02 (k = 3.0 X lo-'')
(1 1)
which compete with the formation of ethyl nitrite. The above model, with the initial concentrations [C2HS02]0 = [Cl], = 2.7 X lOI5 molecule/cm3, predicts the time evolution of the C2H502, C2HSON0,and NO2 concentrations illustrated in Figure 3.23 Thus, the peak in the ethyl peroxy formation occurs -2 ps after initiation by the photolysis pulse, approximately half of the initial peroxy radical has reacted by about 10 bs, and the reaction is essentially complete by 50 p, exactly as observed spectroscopically in Figure 2. It is important to point out that, under the conditions of our experiments, the predictions of C2HSO2, CzHsONO,and 522) In the absence of a literature value, we assume here a rate constant which is twice that of the CH, + NO reaction (ref 17), this being the same ratio as for the ethyl vs methyl radical plus oxygen reaction. (23) Acuchem kinetic modeling program. Braun, W.; Herron, J. T.; Kahaner, D. K. Int. J . Chem. Kinet. 1988, 20, 51.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
990
0.08
-
0.06-
8.40 10" 6.36 cm path
- CHsONO - C2HgONO
0.03 -
E
B
-
0.02 :
4-
+ NO
4
'.... NO2 a. ' ! .
176
i
: 0.02 ._,.... -.
....'. .*... 226
276
326
0.04
.d
\
U
0.00
C2H5O2
1 0.06.
0.04-
m
Maricq and Wallington
376
wavelength (nm) Figure 4. Absolute reference spectra for CH30N0, C2H50N0,and NO2. These were obtained using the same UV light source and detection equipment as the transient radical spectra, except that the 193-nm reflectors were removed. The NO2 spectrum is uncorrected for N20,; however, it is obtained with 0.04 Torr of NO2 in 400 Torr of air and a path length of 50.8 cm, which essentially reproduce the conditions under which NO2 is formed by the alkyl peroxy plus NO reaction.
NO, concentrations as a function of time obtained using the full reaction model (reactions 1,2, and 4-1 1) agree within 10% with predictions from the much simpler model which includes only reactions 1,2,4, and 5. Therefore, the corrections to be discussed below, which compensate for systematic errors due to the "interfering" reactions 6-1 1, are relatively small; in fact, they nearly cancel. A special feature to note from Figure 2 is the constancy in time of the absorption cross section at 232 f 2 nm. A time-independent cross section is also observed at approximately 232 f 2 nm in the reaction of CH302and N O and confirms the results presented in paper 1. This feature occurs at wavelengths for which a(reactants) = u(products)
As mentioned in section 11, the absorption spectra reported herein were obtained under conditions that minimized possible interfering effects from reactions 6-1 1; thus, to a good approximation ~(CzH502)= u(CZHSONO) + ~ ( N 0 z ) (12) at 232 nm. Because the alkyl nitrite and NO, products are stable species, their absolute UV cross sections are readily obtained; examples are shown in Figure 4. The utility of finding a wavelength which exhibits a constant absorption, therefore, is that it provides a scaling factor by which to convert the measured alkyl peroxy absorbance into an absolute cross section. While the method of transient UV spectral photography can easily locate a wavelength at which reactant and product absorption cross sections become equal, such a feature is not necessary for assigning absolute values to the peroxy radical absorption. An alternative procedure is as follows. One chooses initial reactant concentrations that assure quantitative conversion of the peroxy radical into the corresponding alkyl nitrite. Fitting the absorbance measured at the completion of the reaction to the sum of the alkyl nitrite and NO2 spectra (shown in Figure 4) yields the concentrations of the products and, by implication, the initial concentration of peroxy radical. The experiment is repeated under identical conditions, except that [NO], = 0, thereby generating a known concentration of peroxy radicals and, thus, permitting an assignment of absolute values to the measured absorption spectrum. The absolute cross sections (uncorrected for the systematic errors to be discussed below) obtained in this manner for UV absorption by CzH5O2 and CH302are shown in Figures 5 and 6, respectively. The scale factors for converting R 0 , absorbance to cross section were obtained by a linear least-squares fit over the range 210-285 nm of the alkyl nitrite plus NO2 absorbance, produced by reaction of the alkyl peroxy radical with NO, to the reference alkyl nitrite spectrum plus NOz, Le., from a(RON0 + NO2) = mA(RON0 + NOz) + b The value of m thus obtained had a standard deviation of 1%, while b was not statistically different from zero.
u
C2%0NO
+ NO2
0.01
0.00
200
260
300
360
400
wavelength (nm) Figure 5. Spectra of C2H502and its decomposition products after reaction with NO. The latter has been fitted to a reference C2HSON0 NO2 spectrum.
+
CH3O2
+ NO
%ON0
200
260
300
360
+ NO2 400
wavelength (nm) Figure 6. Spectra of CH3O2 and its decomposition products after reaction with NO. The latter has been fitted to a reference CH30N0 NO2 spectrum.
+
The principal advantage of the procedure just described over that described in paper 1 is that it does not rely on the absorption cross section at a single wavelength; rather, spectral information over a wide range of wavelengths contributes to evaluation of the absolute cross sections. In addition to increased accuracy, the present procedure allows corrections to be made, such as compensating for scattered light reaching the detector or for absorption by species not relevant to the desired measurement. The advantage of comparing spectra taken at long times with and without addition of NO, as opposed to following the spectra in time, is that it permits the use of longer gating times and consequently yields spectra having superior signal to noise characteristics. In concluding this section, it is pertinent to discuss the errors involved in the cross-section measurements given in Figures 5 and 6. Random errors arise primarily from the noise associated with the absorption measurement, in this case 1-5% of the peak absorption depending on the amount of smoothing one applies to the spectrum. Systematic errors arise (a) from determination of cross sections for the reference spectra and (b) from the interfering reactions 6-1 1. As discussed in section I1 a 5% error is estimated for the reference spectra. Errors arising from the interfering reactions are small and tend to cancel. They are determined from kinetics simulations using the reaction model discussed above. Under the gas flow conditions listed in Figure 5 , the "initial reactant" distribution, after the very rapid chlorine plus alkyl radical reactions, is calculated to be 91% C2HSO2,4% C2H4C102,and 5% CzHSNO,when N O is present in the flow mixture. Of the ethyl peroxy radicals formed, 96% are converted to C2H50N0. When N O is not added to the flow mixture, the initial C2HS02concentration is about 4% higher (no C2H5N0is formed), but this is partially canceled by a loss of 2% of the radicals due to self-reaction in the intervening 100 ps between photolysis pulse and absorption measurement. Therefore, a s a net correction, we need to reduce the C2H502cross section
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 991
UV Cross Sections of Peroxy Radicals
CH302
TABLE I 0.08
wavelength,
a(CH302)7
nm
expt 1
310.9 308.4 305.8 303.2 300.6 298.1 295.5 292.9 290.4 287.8 285.2 282.6 280.1 277.5 274.9 272.3 269.8. 267.2 264.6 262.1 259.5 256.9 254.3 25 1.8 249.2 246.6 244.1 241.5 238.9 236.3 233.8 231.2 228.6 226.0 223.5 220.9 218.3 215.8 213.2 210.6 208.0 205.5 202.9 200.3
0.20 0.24 0.26 0.29 0.33 0.41 0.44 0.53 0.65 0.74 0.86 1.02 1.14 1.29 1.45 1.70 1.88 2.12 2.34 2.58 2.88 3.09 3.27 3.53 3.67 3.86 4.05 4.06 4.12 4.09 4.06 4.01 3.87 3.74 3.53 3.33 3.12 2.90 2.62 2.44 2.33 2.07 1.70 1.29
cm2 expt 2 -0.03 0.11 0.12 0.18 0.17 0.23 0.38 0.55 0.59 0.68 0.79 0.99 1.11 1.32 1.60 1.72 1.94 2.16 2.42 2.64 2.88 3.23 3.42 3.69 3.92 4.08 4.27 4.33 4.37 4.39 4.46 4.30 4.19 3.92 3.66 3.59 3.45 3.11 2.96 2.72 2.40 2.40 1.91 1.41
o(C2~502), IO-'* cm2 0.24 0.29 0.34 0.48 0.44 0.53 0.61 0.68 0.87 1.02 1.19 1.36 1.55 1.64 1.83 2.07 2.22 2.48 2.75 2.98 3.29 3.61 3.87 4.02 4.20 4.31 4.53 4.58 4.63 4.54 4.44 4.34 4.14 3.99 3.70 3.31 3.1 1 2.76 2.45 1.99 1.74 1.48 1.27 1.11
by approximately 6%, a correction that is only slightly larger than the signal to noise ratio inherent in the absorption measurement. The above correction is made under the assumption that errors from the interfering reactions enter cross-section measurements only through the concentrations used to convert absorbance to cross section. Another possibility is that one of these minor species, such as C2H4C102or C2H50N02,has an aberrantly large absorption cross section and, hence, introduces a significant error into the absorbance itself. However, the very good fit between the absorbance of the C2H5O2 + NO reaction products and the summed C2H50N0and NO2reference spectra suggests that this possibility is unlikely. Therefore, after making the 6% correction discussed above, we suggest an estimated residual error, due to noise and inaccuracies in the systematic corrections, of approximately 5% of the peak absorption for the ethyl peroxy radical cross sections. Table I lists numerical values for the corrected cross sections. The discussion of errors with respect to the CH302cross-section measurements parallels the one given above. With the concentrations of experiment 1, there is a 93%conversion of CH3O2 into methyl nitrite. Because of the relatively faster self-reaction of CH302vs CZH502,there is a larger, approximately IO%, loss of methyl, as compared to ethyl, peroxy radicals in the 100 ps intervening the photolysis pulse and the absorbance measurement. However, this is mostly offset by the predicted initial CH3O2 concentration being 7% larger when NO is absent from the flow mixture. As a result, the net correction to the CH3O2 cross sections is a reduction by about 4% from the values shown in Figure 6.
^.
3 B
3I
!
u
] a.
0.051
4
e
e
-
d
0.04
0.03
0*02
0.01 0.00 200
220
240
280
280
300
wavelength (nm) Figure 7. Comparison of CH3O2 cross sections, after correction for systematic errors (see text), to literature values.
Under the conditions of experiment 2, there is an 88% conversion to C H 3 0 N 0 , but this is offset by a 12%loss of CH302 due to self-reaction. The net correction is a reduction of the cross sections by 2%owing to the slightly higher initial concentration of CH3O2 in the absence of NO. Again, these systematic corrections of 2-4% are essentially within the signal to noise ratio of the absorbance measurements. The overall error in the reported cross sections is estimated to be approximately 10%. It is somewhat larger than the error for the ethyl peroxy radical owing to the slightly poorer fit of the reference spectrum in the CH3O2 as opposed to C2HS02experiment and to the larger, albeit nearly canceling, systematic corrections. The corrected CH302cross sections from experiments 1 and 2 are provided in Table I. Perhaps a more realistic view of the errors is provided by a comparison (in Table I or Figure 7) of the CH302cross sections obtained under two rather different sets of conditions, namely from experiments 1 and 2. Besides the different flow mixtures, experiment 1 employed a Xe arc lamp, whereas experiment 2 used a D2 lamp as a UV probe light source. These two results agree very well, the difference between them being within the error bounds. The cross sections obtained from experiment 2 appear to be systematically larger than those from experiment 1 below 240 nm, although this difference is not much larger than the noise in the spectra. A possible explanation is that the deuterium lamp has a higher intensity at the shorter wavelengths than does the Xe lamp. It is, therefore, less susceptible to scattered light which tends to reduce the cross sections from their true values. Our recommendation is that the average of the measurements from experiments 1 and 2 be used for the CH302cross sections.
IV. Discussion The absorption of UV light by alkyl and other substituted peroxy radicals has proved advantageous in studying the chemical kinetics of these species. If one knows the optical cross sections of the absorbing species in a reaction mixture, then it becomes possible to extract the concentrations of these species as a function of time and thereby deduce reaction rates. Using the standard method, as illustrated in Figure 1, kinetics experiments have relied on monitoring the UV absorbance as a function of time at one or at a few wavelengths. Previous reports of CH3O2 and C2HS02 absorption cross sections have involved 1-20 individual measurements in the region 200-300 nm, with the exception of some recent molecular modulation experiments. The advent of fast gated array detectors has made possible transient measurements of broadband UV-vis spectra (Figure 1). To take full advantage of this technique, a library of reference spectra is needed to convert absorbance spectra at time At into concentrations vs At. This provided one impetus for the absorption measurements reported herein, the other being the improved accuracy made possible by determining the initial alkyl peroxy concentration via its chemical conversion to the corresponding nitrite. In Figure 7 we compare our CH3Oz cross sections with those from previous studies and find that they fall in the lower middle of the range of previous values, except below 230 nm, for which
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
992
C2HS02 0.08 h
3 fi 3P 1
ue,
0.06 0.04 0.09 0.02 0.01
o.0ol. . 200
.
.
I
220
. . . .
I
UO
. . . . . . . . . . 260 280 I
I
.4
SO0
wavelength (nm) Figure 8. Comparison of C2HSO2cross sections, after correction for systematic errors (see text), to literature values.
they are at the higher end of the range. This figure displays only the more recent, and presumably more accurate, results;earlier work, as discussed by Kurylo et al." and by Simon et al.,I5 shows an even larger variation. Previously reported cross sections for methyl peroxy radicals differ by as much as 50%. Two different techniques have been used to measure the cross sections: Simon et a1.,I5Jenkin and Cox,I6 Parkes et a1.,I2 and Cox and TyndaIl5s6 employed molecular modulation; Adachi et al.,' McAdam et a1.,I0 Kurylo et al.," Hochanadel et al.,3 Kan et a1.,4 Sander and Watson: filling and Smith? Moortgat et a1.,l3 and Dagaut and KuryloI4 used flash photolysis. However, there is no apparent correlation between technique and reported cross-section values. Similarly, there seems to be no systematic dependence of the measured cross section on the method used to generate CH30z; all of the studies use the dissociation of Clz in the presence of CHI and Oz or the dissociation of (CH3)2NZin O2 to generate the peroxy radicals, except for Jenkin and Cox,16who dissociated CHJ in O2 Figure 8 presents a comparison of our CzHsO2cross sections to previously reported literature ~alues.2"~~ As with the methyl peroxy cross sections, there is a roughly 50% variation in the previous results. Our values are higher than those of Adachi et al.,24 significantly lower than those of Munk et a1.,26and very close to the recent measurements of Bauer et al.?O who also employed an array detector in conjunction with their molecular modulation apparatus. Three methods have been used to measure ethyl peroxy absorption cross sections: molecular modulation by Anastasi et al.,z5Cattell et al.?7 and Bauer et flash photolysis by Adachi et alaz4and Wallington et al.;z9 and pulsed radiolysis by Munk et a1.26 The previous studies of ethyl peroxy cross sections by Adachi et al.," Anastasi et al.,zs and Cattell et al?7 used photolysis of diazoethane, whereas Munk et a1.26employed the reaction of H atoms with ethene, and Wallington et al?9 used the photolysis of ClZ/C2H,mixtures to generate ethyl radicals and subsequently ethyl peroxy radicals by reaction with Oz. As with the methyl (24) Adachi, H.; Basco, N.; James, D. G. L. In$.J . Chem. Kiner. 1979,
11. 1211. --. (25) Anastasi, C.; Waddington, D. J.; Woolley, A. J . Chem. Soc., Faraday
Trans. 1 1983, 79, 505. (26) Munk, J.; Pagsberg, P.; Ratajczak, E.; Sillesen, A. J . Phys. Chem. 19116. --, 90.2752. (27) Cattell, F. C.; Cavanagh, J.; Cox, R. A.; Jenkin, M. E. J . Chem. Soc., Faraday Trans. 2 1986,82, 1999. (28) Anastasi, C.; Brown, M. J.; Smith, D. 9.; Waddington, D. J. Presented at the Joint Meeting of the French and Italian Sections of the Combustion Institute, Amalfi,-June, 1987. (29) Wdlington, T. J.; Dagaut, P.; Kurylo, M. J. J. Phorochem. Phorobiol. A: Chem. 1988,42, 173. (30) Bauer, D.; Crowley, J. N.; Moortgat, G. K.J. Phorochem. Photobiol., submitted for publication.
----.
Maricq and Wallington peroxy radicals, there is no significant correlation between technique and reported absorption cross sections. There is, however, an important distinction between the present method (also that of paper 1) for determining alkyl peroxy cross sections and the previously reported studies. In all experiments, the raw data are in the form of absorbances and require concentrations for conversion to cross sections. To accomplish this, most of the previous studies have relied on measuring the loss of precursor, e.g., Clz, and equating this loss to the initial peroxy radical concentration formed via reactions 1 and 2. Obtaining accuracy with this procedure is, in general, difficult as it depends on detecting a small change, perhaps a few tenths of a percent, in the otherwise large precursor absorption. In contrast,the present method relies on converting the initially unknown concentration of peroxy radicals into an equal, but still unknown, concentration of stable nitrite molecules, none of which are initially present in the reaction mixture. The concentration of alkyl nitrite molecules and, by inference, the peroxy radical concentration can then be obtained from comparison to a reference alkyl nitrite spectrum. A few studies have avoided the problems associated with measuring the amount of precursor consumed but at the expense of introducing a separate calibration experiment under conditions different from the peroxy radical measurements. For example, Adachi et al? and Kan et a1.4 used the yield of Nz from the flash photolysis of azomethane to calibrate the methyl proxy radical concentration; however, their radical concentrations were sufficiently high to throw into question the stoichiometric production of methyl peroxy radicals. Similarly, the calibration procedure of converting C1 radicals to NOC1, used by McAdam et a1.,I0 also involved a separate set of experimental conditions. In contrast, measurements of the ROz and RONO absorbances in the present work were made under identical conditions, thus minimizing systematic errors. In principle, each photolysis pulse can simultaneously provide absorbances for both ROz and RONO, by recording the UV absorption at t = 0 and t = Q),respectively. The accuracy afforded by the present procedure for finding the ROz concentration is attested to by the close agreement obtained between cross sections measured by such disparate techniques as the flash photolysis of CH3Cl used in this work and the pulsed radiolysis of SF6 described in paper 1.
V. Conclusion The method of transient UV spectral photography promises to enhance the study of radical reaction kinetics via flash photolysis by providing a means of recording the timedependent change in the UV spectrum of a reaction mixture as opposed to its absorption at a single or a few discrete wavelengths. We have employed a new apparatus of this kind to measure UV absorption spectra of methyl and ethyl peroxy radicals. Monitoring the conversion of the alkyl peroxy radical to its respective nitrite plus NOz, via reaction with NO, and comparing the resultant UV spectrum to the appropriate reference alkyl nitrite plus NOz spectrum provide an accurate means for establishing absolute absorption cross sections for the peroxy radicals. The cross sections thus obtained are consistent with previous work and are in very good agreement with the results reported in paper 1. Acknowledgment We thank John Beers (Ford) for construction of the flow cell, Chaitanya Narula (Ford) for help with the MeONO and EtONO synthesis, Ishai Nir (Princeton Instruments) for useful discussions regarding the use of diode array detectors, Joe Szente (Ford) for designing the gating electronics, and Ole J. Nielsen (Risca National Laboratory, Denmark) for helpful discussions.
"R NO. CHS02, 2143-58-0; C2H~02,3170-61-4; NO, 1010243-9; CHICI, 74-87-3; CHI, 74-82-8; 02,778244.7; C2H6, 7 4 - 8 4 4 CZHsCl, 75-00-3; CHIONO,624-91-9; CZHSONO, 109-95-5.
