COMMUNICATIONS TO THE EDITOR
3640
mechanism of reaction remaining virtually independent of the n-hexane concentration. One description of the results is in terms of molecular aggregates of ethanol molecules trapping and stabilizing the electrons. An infrared spectroscopic study undertaken in conjunction with this work indicates that G varies with the concentration of ethanol tetramer and possibly higher multimers.10 The presence of these as existing traps is presumably necessary for solvation stabilization of electrons. Acknowledgment. We wish to thank the Australian Institute of Nuclear Science and Engineering for supporting this work. I
500
I
1
600
700 800 WAVELENGTH (nm)
I
900
Figure 1. Comparative absorption spectra of (e-,olV)in ethanol-n-hexane solutions at 22" : A, pure ethanol ( D , = 26.7); B, 60 mole % ( D , = 8.5); C, 31 mole % (D,= 3.2); D, 5 mole % ( D , = 1.78).
(9) T. J. Kemp, G. A. Salmon, and P. Wardman in "Pulse Radiolysis," M.Ebert, J. P. Keene, A. J. Swallow, and J. H. Baxendale, Ed., Academic Press, London, 1965, pp 247-257. (10) W. C. Coburn, Jr., and E. Grunwald, J . Amer. Chem. Soc., 80, 1318 (1958).
DEPARTWENT OF NUCLEAR AND R a D I A T I O N CHEMISTRY
BRUCEJ. BROWN NORMAN T. BARKER
NEW SOUTH W.4LES SYDNEY, AUSTRALIA UXIVERSITY OF
DAVIDF. SANOSTER* AUSTRALIANATOMICENERGY COMMISSION RESEARCH ESTABLISHMENT SUTHERLAND, N.S.W. AUSTRALIA RECEIVED JULY 9, 1971
Reactivity of Hydroxyl Radicals w i t h Olefins Publication costs assisted by the Ford Motor Company STATIC
DIELECTRIC CONSTANT
Figure 2. G for solvated electrons as a function of D, for ethanol-n-hexane solutions: 0 , values found under steady-state irradiations; A, values estimated spectrophotometrically by pulse radiolysis ( n / k € i o d ) , where A is the spectral absorbance by the electron a t 585 nm, k is a proportionality factor, and Eiod is the molar extinction coefficient of iodide in the same mixture; 0, values for a number of pure 1iquids;a - - -, theoretical behavior for pure liquid^.^
The decrease in C: with D,shown in Figure 2 differs from the theoretical trend calculated by Freeman and Fayadh3 for pure liquids. Electron solvation in ethanol-n-hexane mixtures therefore appears not to be simply a function of the static dielectric constant. Kemp, Salmon, and Wardmang found the lifetime of the solvated electron was unchanged in pure methanol, 6?% methanol in tetrahydrofuran, and 4% methanol in cyclohexane. We have found a constant halflife of 3.2 & 0.3 Fsec for the decay over the initial 4 mec after pulse shut-off for the range of coricentrations 2-100 mol % ethanol. This, together with there being no shift in the position of the absorption spectrum maximum, is interpreted to suggest that the electron is trapped in basically the same type of potential well, the The Journal of Physical Chemistry, Vol. r5, hTo.33, 1971
Sir: The rate of conversion of NO to NOz in smog chambers has been shown to depend markedly on the particular olefin present.'S2 The reaction of hydroxyl radicals with olefin has been suggested as an important step in this photooxidation.3 However, Greiner4 has suggested the rate of hydroxyl attack on all monoolefins should be approximately constant and equal to the OHethylene rate which he has determined. In a recent paper me have shown the rate constant for the reaction of OH with propylene (k = 1.7 X lo-" cm3 molecule-' sec-') to be ten times faster than that for ethylene (IC = 1.8 X 10-12).6 In the present work this series is extended to a number of higher olefins. The reaction rates were determined in a discharge flow system with mass spectrometric detection as described previo~sly.~The concentration of OH and (1) W.A. Glasson and C. S. Tuesday, Environ. Sci. Technol., 4, 916 (1970). (2) A. P. Altshuller and I. R. Cohen, Int. J . Air Water Pollut., 7, 787 (1963). (3) D. H. Stedman, E . D. Morris, Jr., E. E. Daby, H. Niki, and B. Weinstock, 160th National ,Meeting of the American Chemical Society, Chicago, Ill., 1970, Abstract No. WATR 26. (4) N. R. Greiner, J . Chem. Phys., 53, 1384 (1970). (5) E. D. Morris, Jr., and H. Niki, J . Amer. Chem. Soc., 93, 3570 (1971).
CO~IMUNICATIONS TO THE EDITOR
3641
olefin was measured simultaneously by monitoring the corresponding parent peaks as a function of the distance between the position of the movable inlet and the sampling pinhole of the mass spectrometer. Hydroxyl radicals were produced by the reaction H NO2 OH NO. The olefin and NOz were both introduced through the movable inlet to a stream of approximately l O l 3 molecules/cm3 of H atoms. All of the H atoms were converted to OH in the mixing distance allowed before kinetic measurements were begun. Typical flow conditions were 30 m/sec at 1 Torr pressure of helium diluent at 25". In order to keep the ratio of OH to olefin as large as possible, the minimum concentration of olefin was used that could be detected reliably. The exponential decay of each olefin was compared to that of propylene under identical experimental conditions. The relative rate constants were determined from the equation
+
~hole
(~H)C,H,
- In [(Ole)d(O1e) t l l
J C C ~ H ~ In
+
+-
[(CBHB)I~/(CIHB)Z~] (OH)oi,
(1)
The OH concentration was averaged over the reaction time ( t z - t l ) , to correct for decay caused by reaction with olefin and by wall recombination. It should be noted that eq 1 requires the knowledge of only relative concentrations of both OH and olefin. The relative rates determined by this procedure are summarized in Table I. These values represent an average of four independent runs and are reproducible to within 10%. According to eq 1, the rate ratios should be independent of OH concentration. HouTever, because of the extremely Table I: Comparison of 0 and OH Rate Constants to Photochemical Reactivity (Normalized to Propylene) OH rate
Olefins 0.1 1.OC 1.1
(CHa)zC=CHCH3 (CHa)zC=C(CHs)z HCHO CHaCHO CHaCHzCHO CHaCHzCHzCHa Xylened
2.4 2.5 3.6 4.2 3.8 5.3 5.3 7 9
0 atom ratea
Reactivityb
0.17 1.o
0.4 1.0
1.0
0.9 0.6 2 3 1
...
...
4.1 4.9 4.3
...
3.9 13.7 17.7
Other Compounds 0.9 0.01 0.9 0.01 1.8 ... 0.24 0.008 1.1
