J . Phys. Chem. 1992, 96,982-986
982
influenced by interparticle interference effects either.
Conclusions The results of our work may be summarized as follows. Small-angle neutron-scattering investigations of the aqueous solutions of TMU as a function of concentration and temperature have furnished information on the interparticle interaction between the TMU molecules and on the perturbation of the solvent structure brought about by the solute molecule in its immediate neighbourhood. The second virial coefficient of tetramethylurea in water obtained from the concentration dependence of the forward scattering cross section (Figure 3) has a rather small negative value and decreases with increasing temperature. This observation is in amrdance with the view that the hydrophobic interaction becomes more attractive a t higher temperatures.2' The effective distance characterizing the intermolecular interaction, as found from the correlation between i(0) and R:, is very close to the dimensionsof a single TMU molecule. This seems to suggest that in the concentration range m = 0.7-2.0 contact pair formation sets in and eventually these pairs may become more stable than water separated pairs. The concentration dependence of the apparent radius of gyration of TMU (Figure 4) shows that at m 0.7 some structural changes occur in the solution similarly to those observed in aqueous solutions of other hydrophobic sol~tes.'A ~ ,quantitative ~~ interpretation of the low-concentration range ( m < 0.7) data would require more detailed experimental information. Experiments are planned which overcome the difficulties arising from the significant decrease in the scattering intensity with decreasing concentration. The contrast variation experiments ( m = 1.0) show that about two of the water molecules in the immediate neighborhood of a
TMU molecule are preferentially oriented with their hydrogen atoms toward the TMU molecule. This finding is in good agreement with the suggestion that two water molecules can form direct hydrogen bonds with the carbonyl group of tetramethylurea.24v25 In our previous neutron-scattering investigations of TMU solutions* the zero concentration value of REobtained by extrapolation of the data taken at aquamolalities of 0.5, 1.0, and 2.0 was reported as 3.9 A at room temperature; this is in good agreement with the present investigation. However, Figure 4 shows that the radius of gyration starts to decrease when m becomes less than 0.7 and the observed REvalues exhibit a remarkable temperature dependence in the concentration range m = 0.7-2.0. Both observations question the applicability of the earlier proposed model to the entire investigated concentration and temperature range. Finally, the results of measurements carried out with the incoherent mixtures ( m = 1.0) are in excellent agreement with the molecular structure data. This, on one hand, proves the applicability of SANS for investigating the solutions of such small particles as TMU molecules; on the other hand, it serves as a methodologically new way to carry out the absolute calibration of the small-angle scattering cross-section.
Acknowledgment. We are indebted to our colleagues N. I. Gorski and H. Pospisil (JINR, Dubna) for their valuable help in the neutron-scattering experiments. A number of the density measurements were can?& out by Sz. Vass (CRIP), and the major part of the tedious sample preparations by S.Milesz. Their efforts are gratefully acknowledged. Financial support from the Hungarian Research Fund under Grant OTKA-1846 is appreciated.
NO. TMU, 632-22-4.
R-try
Novel Method for the Measurement of Gas-Phase Peroxy Radlcal Absorption Spectra Timothy J. Wallington,* M. Matti Maricq, Research s t a n Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121 -2053
Thomas Ellen",
and Ole J. Nielsen*
Section for Chemical Reactivity, Environmental Science and Technology Department, Riscl National Laboratory, DK-4000 Roskilde, Denmark (Received: July 25, 1991; In Final Form: September 17, 1991)
The pulse radiolysis method has been used to generate methyl peroxy radicals in the presence of high concentrations of NO. The CH3O2 radicals are then rapidly converted to CH30N0by reaction with NO. Methyl nitrite absorbs strongly in the UV. Hence the initial absorption attributable to CH3O2 is rapidly replaced by absorption by CH30N0. By varying the monitoring wavelength one can readily determine the isosbestic point, where a(CH302) = a(CH30NO) u(NO2). For methyl peroxy radicals the isosbestic point was determined to lie in the wavelength range 228-230 nm, and hence a(CH302) = (3.9 f 0.4) X lo-'* cm2molecule-'. Quoted errors are the sum of statistical ( 2 4 and our estimate of potential systematic errors (5%). As part of the present work rate constants for reactions 11 and 14 at 298 K and 600 mbar total pressure of SF, were determined to be CH3 NO + M CH3NO + M F + 02 + M F02 + M (1 1) (14) cm3 molecule-' s-I at 298 K. Results are kll = (1.07 i 0.03) X lo-" cm3 molecule-1 s-l and kI4= (2.35 0.20)X discussed with respect to previous literature data.
+
+
+
Introduction Alkyl peroxy radicals are key reaction intermediates in the atmospheric oxidation and low-temperature combustion of every hydrocarbon. Recognition of the crucial role played by these species has led to a significant research effort to elucidate the kinetics and the mechanisms of the reactions of these species. Kinetic studies of these intermediates usually involve monitoring the change in concentration of these species using their strong Authors to whom correspondence should be addressed.
*
-
absorption features in the ultraviolet over the range 200-300 nm. Absolute absorption cross sections are then needed to determine the concentrations Of these peroxy Despite a large number of studies, there are significant uncertainties still associated with absolute values of the absorption cross sectionsOf ab'1 F o x y radicals* For the ab@on spectrum of CH3O2 radicals has been measured in 16 different studies.I-l6 While all studies are in good agreement concerning (1) Parka. D.A.; Paul, D.M.; Quinn, C.P.;Robinson, R.C. Chem. Phys. Lett. 1973, 23, 425.
0022-3654/92/2096-982%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 983
Gas-Phase Peroxy Radical Absorption Spectra the overall shape of the spectrum, there are significant differences in the reported absolute values of u(CH302). These differences reflect a fundamental experimental difficulty in the absorption cross section measurements, namely the absolute calibration of low concentrations of these reactive radicals. The conventional approach to this calibration problem is to conduct two experiments. First, the absorption of the peroxy radical in question is measured. Then, in a separate experiment, the yield of radicals in the system is determined either by following the loss of the photolytic precursor or by monitoring the appearance of a product. Here we describe a new variation in this approach in which measurement of the UV absorption of the peroxy radical and the absolute calibration are performed in one experiment. This is achieved by the stoichiometric conversion of the peroxy radical into an alkyl nitrite by reaction with NO. For example CH302+ NO CH30
-
+ NO + M
C H 3 0 + NO2
+
CH30NO
+M
(1) (2)
Alkyl nitrites absorb strongly in the UV. Hence, as peroxy radicals are converted into the nitrite, the initial UV absorption attributable to the peroxy radical is replaced by absorption by the nitrite. From a single experiment it is therefore possible to relate the absorption cross section of these two species. By varying the monitoring wavelength, one can readily determine the isashtic point, where the absorption cross section of the peroxy radical is equal to the sum of those of the nitrite and NO2. We demonstrate this method using two different experimental systems: a pulse radiolysis apparatus, which is described in this paper, and a laser photolysis apparatus, which is described in the companion paper following this one.l7
Experimental Section The pulse radiolysis transient UV absorption spectrometer and the experimental techniques used have been described in detail and are only briefly discussed here. Methyl peroxy
(2) Parkes, D. A. Inf.J . Chem. Kinef. 1977, 9,451. (3) Kan, C. S.; McQuigg, R. D.; Whitbeck, M. R.; Calvert, J. C. Int. J . Chem. Kinef. 1979, 11, 921. (4) Adachi, H.; Basco, N.; James, D. G. L. Inr. J. Chem. Kinef. 1980,12, 949. (5) Pilling, M. J.; Smith, M. J. C. J . Phys. Chem. 1985.89, 4713. (6) Hochanadel, C. J.; Ghormley, J. A.; Boyle, J. W.; Ogren, P. J. J. Phys. Chem. 1977,81, 3. (7) Cox, R. A.; Tyndall, G. S.J. Chem. Soc., Faraday Trans. 2 1980,76, 153. ( 8 ) Sander, S. P.; Watson, R. T. J . Phys. Chem. 1981, 85, 2960. (9) Jenkin, M. E.; Cox, R. A,; Hayman, G. D.; Whyte, L. J. J. Chem. Soc., Faraday Trans. 2 1988,84, 913. (10) Dagaut, P.; Kurylo, M. J. J . Phofochem. Photobiol. A: Chem. 1990, 51, 133. (1 1) Simon, F.; Schneider,W.; Moortgat, G. K. Int. J . Chem. Kinet. 1990, 22, 791. (12) Jenkin, M. E.; Cox, R. A. J . Phys. Chem. 1991, 95, 3229. (13) Moortgat, G . K.; Veyret, B.; Lesclaux, R. J . Phys. Chem. 1989,93, 2362. (14) McAdam, K.; Veyret, B.; Lesclaux, R. Chem. Phys. Lett. 1987,133, 39. (15) Kurylo, M. J.; Wallington, T. J.; Ouellette,P. A. J . Photochem. 1987, 39, 201. ( 1 6) Wallington, T. J.; Dagaut, P.; Kurylo, M. J. J. Photochem. Phofobiol. A: Chem. 1988, 42, 173. (17) Mariq, M. M.; Wallington, T. J. J . Phys. Chem., following paper in this issue. (18) Hansen, K. B.; Wilbrandt, R.; Pagsberg, P. Rev. Sci. Instrum. 1979, 50, 1532. (19) Nielsen, 0. J. Rim Natl. Lab., [Rep.] Rise-R 1984, Rim-R-480. (20) Wallington, T. J.; Nielsen, 0. J. Inr.J . Chem. Kinet. 1991, 23, 785. (21) Nielsen, 0.J.; Munk, J.; Locke, G.; Wallington, T. J. J . Phys. Chem., in press.
" t
n
0.4
v
I
z 0.0 0
I
I
I
200
400
600
800
1000
SF, ( m b a r )
Figure 1. Maximum transient absorption observed at 229 nm following the pulsed radiolysis of 10 mbar of CHI, 35 mbar of 02,and 103-955 mbar of SF6 as a function of the SF6 concentration.
radicals were generated by the irradiation of SF6/CH4/02mixtures with a 30-11s pulse of 2 MeV of electrons from a Febetron 705B field emission accelerator in a 1-L stainless steel cell. SF6 was always in excess and was used to generate fluorine atoms: SF6 + 6
+
SF6*
+ products F + CH4 H F + CH3 CH3 + 02 + M CH302 + M SF6*
+
F
+
(3)
(4) (5)
(6) The mechanism by which F atoms are generated following the pulsed radiolysis of SF6 has been explored recently in our laboratory.22 Production of F atoms is thought to be accompanied by the generation of SF4. Sulfur tetrafluoride has only a weak absorption in the wavelength region 220-300 nm, U ( S F ~=) ~ ~ ~ 3.3 X cm2m0lecule-',2~and it is not expected to react with any of the reactants used in the present work. Use of molecular oxygen in our experiments raises the potential problem of the formation of 0 atoms by the radiolysis pulse and their subsequent reaction with O2 to form ozone. To explore this effect, the transient absorption at 254 nm caused by ozone production was monitored following the pulsed radiolysis of lo00 mbar of oxygen. Using a value of u(O3)2S4",,,= 1.1 X IO-'' cm2 we calculate the production of an ozone concentration of 9.4 X lOI4 cm-3 (1.5 pM) from the full-dose radiolysis of 1000 mbar of 02. The majority of experiments in the present work were performed using [O,]= 35 mbar, where radiolysis of O2 will lead to the formation of an ozone concentration of 3.3 X 1013 cm-3 (approximately 2 orders of magnitude less than the concentration of the peroxy radical CH302). The generation of such small amounts of ozone is not expected to have any effect on the present measurements. Experiments were performed with, and without, added NO. When NO is present in the reaction cell, the CH3O2 radicals are rapidly converted into C H 3 0 N 0 via reactions 1 and 2. Changes in the concentrations of C H 3 0 2and C H 3 0 N 0were monitored using their UV absorption in the region 230-300 nm. The output of a pulsed 150-W xenon arc lamp was multipassed through the reaction cell using internal White cell optics. Path lengths of either 80 or 120 cm were used. The spectrometer was operated at a spectral resolution of 1 nm. Concentrations of reagents used were as follows: SF,, 105-955 mbar; 02,35-140 mbar; CHI, 10-40 mbar; and NO, 0-1 mbar. All experiments were performed at 298 K. Ultra high purity O2 was supplied by L'Air Liquide, SF6 (99.9%) and CHI (99%) were +
(22) Anastasi, C.; Muir, D. J.; Simpson, V. J.; Pagsberg, P. J. Phys. Chem. PT. ____. _ . _ ._ _ . (23) Modica, A. P. J . Phys. Chem. 1973, 77, 2713. 1991.
~
5791
(24) DeMore, W. B.; Molina, M. J.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara,A. R. JPL Publication 87-41, 1987.
Wallington et al.
984 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 TABLE I: Cbemicrl Mechanism reaction F+CHI-CHS+HF F + 0 2 --* F02 CH3 + 0 2 CH302 CH302 CH3C2 products C H 3 0 + NO2 CH3O2 + NO CH302+ NO2 CH302N02 CH30+ NO CH30N0 C H 3 0 + NO2 C H 3 0 N 0 2 C H 3 0 + O2 HCHO + O2 CH3 + CHI C2H6 CH3NO CH3 + N O CH3 CH3O2 2 C H 3 0
+
+
7 I I
rate constant" 8.0 X IO-" 2.4 x 10-13b 1.0 x 10-'2b 3.0 x 10-13 7.0 X 10-l2 6.5 X lo-'% 2.0 x 10-"b 1.5 X 10-'Ib 1.9 X 1.0 x 10-"b 1.1 x 10-"b 4.5 x lo-"
--.---
+
+
W'
Units are cm3 molecule-I s-l. bPseudo-second-orderrate appropriate for 600 mbar.
obtained from Gerling and Holz, NO (99.9%)was procured from Hede-Nielsen, and NO2 (99%)was obtained from Matheson Gas Products. Methyl nitrite was synthesized as described previously.2s All reactants were used as received.
Results The pulse radiolysis system used in the present series of experiments generates a high concentration of radicals (1-3 X lOIS ~ m - ~ When ). such high radical concentrations are used, care has to be taken in the choice of experimental conditions to avoid unwanted radical-radical reactions. For example, the following reactions are undesirable:
-
+ CH3 F + CH302 F
CH3
+ CH3O2
products products
CH30
+ CH30
(7) (8) (9)
To test for the presence of such unwanted radical-radical reactions, the transient absorption at 229 nm was monitored in experiments using [CH,] = 10 mbar and [O,] = 35 mbar with the SF, pressure varied over the range 103-955 mbar. Figure 1 shows the maximum of the transient absorption at 229 nm as a function of the SF6concentration. As seen from Figure 1, the maximum of the transient absorption increased linearly with the SF, concentration (and hence initial F atom concentration). This linearity suggests that, under our experimental conditions,reactions 7-9 are of negligible importance and that F atoms are converted stoichiometricallyinto C H 3 0 2radicals. Variation of the initial CH4 and O2 concentrations over the ranges 10-40 and 35-140 mbar, respectively, had no observable effect (
