J . Phys. Chem. 1990, 94, 6327-6331
6327
UV Spectrum and the Kinetics and Reaction Pathways of the Self-Reaction of CH2CH20HRadicals Christopher Anastasi,* Victoria Simpson, Department of Chemistry, University of York. Heslington, York YO1 5DD. UK
Jette Munk, and Palle Pagsberg Department of Chemistry, Riso National Laboratory, DK-4000 Roskilde, Denmark (Received: August 22, 1989; In Final Form: February 28, 1990)
CH2CH20Hradicals have been studied at room temperature by using a pulse radiolysis/kinetic absorption technique. The cm2 molecule-' ultraviolet spectrum lies in the range 210-265 nm with a maximum absorption cross section of 2.0 X at 210 nm. This technique has also been used to study the overall kinetics of the self-reaction of these radicals: 2CH2CH20H (+M) (CH2CH20H), (+M)(7a); 2CH2CH20H(+M) CH3CH0 + CH3CH20H(7b), while product studies using gas chromatography have yielded information on the reaction pathways. A value of k7 = 5.6 ( f l .O) X IO-" cm3 molecule-' s-I and a branching ratio of k7a/(k7a k7b) 3 0.88 have been measured at room temperature.
-
-
+
1. Introduction Hydroxyethyl radicals play an important role in the oxidation of ethanol in the atmosphere{ and in combustion processes.z There have been several recent studies concentrating on the kinetics of the a-hydroxyethyl radical, CH3CHOH.34 In contrast, only two studies of the 0-hydroxyethyl radical, CH2CHz0H,have been r e p ~ r t e d , and ~ . ~ in both cases kinetic measurements were made at low pressures (1 and, at times less than 600 s, >>1. To satisfy these observations C H 3 C H 0 must also be a primary product of the photolysis of iodoethanol:
t
ICH2CH20H + hv (254 nm)
0
250
750
1250 Time/s
Figure 3, Concentration-time profiles measured after the photolysis of 1.26 X 10I6molecules cm-3 ICH2CHzOHin 1.5 X I O l 9 molecules Nz. A, ICHzCHzOH; 0 , CH,CHO; 0,CH3CHzOH. The solid lines
represent simulated concentration-time profiles.
3. Reaction Pathways. The self-reaction of CH2CH20H radicals may proceed via combination or disproportionation: 2CH2CH20H(+M) (CH2CH20H)2(+M) (7a)
-
-+
CH3CHO CH3CHZOH (7b) Simple product studies using a gas chromatograph fitted with a flame ionization detector were used to measure the contributions k,a and kfb. CH2CH20Hradicals were generated in one of the dissociation pathways following the 254-nm photolysis of iodoethanol. The dominant products observed were acetaldehyde and ethanol; small quantities of methane and ethane were also observed, but no measurable quantities of formaldehyde and methyl iodide were observed nor of the dimer, (CH2CH20H)2,presumably due to its very low vapor pressure. The concentration-time profiles for the precursor and organic products are shown in Figure 3. Here the initial concentrations of ICH2CH20Hand the diluent gas, N2,were 1.3 X 10l6 and 1.5 x I 019molecules cmm3,respectively. Very similar profiles were obtained when the N2 concentration was reduced by a factor of approximately 8 to a final concentration of 1.8 X 10l8 molecules cm+. Inspection of the product concentration profiles indicates a marked increase in the rate of formation of CH3CH20H,C2H6 and CH4 after -600 s. The lack of a similar change in CH3CH0 suggests the latter is a primary product with secondary processes generating the other organic compounds. Also, if the photolysis of iodoethanol produces only CH2CH20H ICH2CH20H hu (254 nm) I CH2CH20H (loa)
+
-+
-+ I
H
+ CH3CH0
(lob)
In trying to model the experimental results a reaction scheme consisting of 43 reactions can be considered. However, a detailed variational analysis demonstrated that only 17 of these reactions are important in defining the products observed, and these constitute the mechanism shown in Table I1 together with the relevant rate constants. The photolysis rate of CHJHO was measured experimentally in separate experiments but in the same cell complex. The H atoms produced as a result of the primary photolysis step, reaction lob, react almost exclusively by abstraction of I atoms from the parent to produce HI: H
+ ICHzCH2OH
-
HI
+ CH2CH20H
(1 1)
Also, the dimerization of I atoms gives significant concentrations of 12. Both HI and I2 are radical traps, and as a result of this, the self-reaction of CH2CH20Hradicals plays a role only in the very early stage when the concentrations of these molecules are low. Under these conditions, the chemistry is accurately represented by reaction 10 followed by reactions 7 and 11. The concentration of ethanol is measurable after 120 s at 5 X molecule ~ m - This ~ . corresponds to the contribution of the disproportionation reaction 7b and is > [C2H6Im No explanation for this discrepancy is obvious at this time, but as the measured concentration accounts for less than 4% of the reactions products, the effect on the major conclusions is small. 4. Reacfionof F Afoms wifh Ethanol. The reaction of F a t o m with ethanol is a source of both a-and 8-hydroxyethyl radicals and ethoxy radicals: F
+ CH,CH,OH
--
-
CH,CHOH
+ HF CHlCH20 + H F
CH2CH,0H
+ HF
(23a) (23b)
The 2-Mev e- irradiation of Ar/SF6/CH3CH20Hmixtures using the PR/KA techniques yielded valuable spectral and kinetic information relevant to the study of hydroxyethyl radicals. Figure I b shows an example of a typical decay trace. The 'prompt" signal is due to the fast reaction of F atoms with ethanol, while the absorption is caused by the presence of both CH,CHOH and CH2CH20Hradicals. In the presence of a large excess of ethanol the F atoms are quantitatively converted to hydroxyethyl and ethoxy radicals. The maximum absorption observed in the range 230-300 nm is shown in Figure 4 and can be attributed entirely to the two hydroxyethyl species. Ethoxy radicals are expected to absorb only very weakly at wavelengths B 300 nm by analogy with the methoxy radicalx and will not contribute to the spectrum shown
Cmss-Radial Reactions
k/em' icsetion
--
ref
8 X ICr" 8 x IO-'' 8 X IO-"
this work. this work. this w o r r
.~..~
26 CH,CH20 + CH,CH,OH 21 CH,CH,O
molecule-' s-'
products products CH2CH,0H -products
+ CH,CHOH
28 CHICHOH
+
k, = k , = k..
in Figure 4. The UV absorption cross section of CH,CHOH in this spectral region has been measured recently by us.] Using these values, with the cross sections measured in the present work for CH,CH,OH and a total radical concentration of 7.8 X IO" molecules cm+, it is possible to adjust the contributions of CH,CH,OH and CHICHOH until a composite absorption spectrum coincides with the experimental observations. Figure 4 shows the most satisfactory results are achieved when the contribution of the radicals to the total are 29% 55%. and 16% for C H F H O H , CH2CH20H,and CH,CH20, respectively, Le., ku,:kZlb:k,,, = 0.290.55:0.16. The ethoxy radical contribution is calculated by difference, and the maximum deviation betwecn the composite and experimental spectra is 10% at 230 nm. The major uncertainty in the ratio is in the absorption cross sections for CH,CH,OH and CH3CHOH where errors of *25% and *12% apply, respectively. These errors also correspond to the uncertainty range applicable to the CHICH,O contribution. Several reactions involving all three radicals contribute to the signal decay shown in Figure Ib. A plot of the measured half life f1,, for each experimental trace at different wavelengths is shown in Figure 5. There appears to be little difference between those results obtained at A < 260 nm, where both CHICH20H and CH,CHOH absorb, and those at A > 260 nm, where only CH,CHOH is being monitored. Nevertheless, considering only thosedecaysatX> 260nmgivesaf,/20f I3.9(il.O) X 1O4s. Table 111 shows that in addition to the self-reactions of each radical, cross-radical reactions also play a role, for which there are no kinetic measurements. The rate constants were considered to be the same for each of the croscradical reactions and the value varied until the experimental fl,, at A > 260 nm results were accurately simulated. With the relative concentrations extracted from the spectral analysis, a value of 8.0 (i1.3) X IO-" cm3 molecule-' s-' gave the best results, the error reflecting the spread in the experimental fl/? Bearing in mind the similarity in the rate constants for the self-reactions of CH,CHOH and CH,CH,OH radicals, it is not surprising that the experimental f I values appear wavelength independent, as shown in Figure This is also predicted by computer simulations using the reaction scheme given in Table
4.,
111. (26) Wendt. H. R.; Hunriker, H. E. 3. Chem. Phw. 1979. 71, 5202. (27) Heieklen. J. Private mmmunicstion (review preprint). 1986.
Self-Reaction of CH2CH20HRadicals 4. Discussion 1. Spectrum of CH2Cff20H.The UV spectrum assigned to CH2CH20Hextends to -265 nm compared to that of CH3CHOH, which extends up to -300 nm. Also, the absorption cross sections for CH2CH20H are very much lower than those measured for CH3CHOH, and there is a possibility of fine structure for the latter. In explaining the differences in these spectra, the position of the lone-pair electrons in relation to the OH group may play a role. Of the two, the CH3CHOH spectrum, under higher resolution than has been achieved in a previous study, is more likely to resemble that of the CH20H radical.* CHzCH20H,on the other hand, with the OH group one carbon removed from the active center, has a spectrum more like that of alkyl radicals.2z 2. Kinetic Measurements. The rate constant for the self-reaction of CHzCHzOH radicals, k7 = 5.6 (k1.0) X cm3 molecule-l s-I, is essentially the same as that obtained for the self-reaction of CH3CHOH (k2: = 5.9 (f0.9) X lo-'' cm3 molecule-' s-' 3). However, any similarity may be coincidence since the product studies in this work suggest that combination of CH2CH20Hdominates with k7a/(k7a k"). 2 0.88, whereas there is evidence to suggest that disproportionation is more important for the CH3CHOHradicals.' The differences in reaction pathways is less surprising when the structures of the two radicals are considered. Disproportionation may occur in a single step for CH3CHOH, whereas the same process for CHzCH20Hradicals requires the formation of a four-membered ring followed by hydrogen transfer to produce the CH3CH0 product. The rate constants for the reaction CHzCH20H with HI and I, have not been measured before. The values of 2 X cm3 molecule-' s-I are very much lower than might be expected when compared to the equivalent alkyl radical reactions. However, recent work involving the reaction of CH20H with HI and HBr yielded rate constants of < I X and
