J. Phys. Chem. 1996, 100, 5729-5736
5729
UV Spectrum and Kinetics of Hydroxycyclohexadienyl Radicals Erling Bjergbakke, Alfred Sillesen, and Palle Pagsberg* EnVironmental Science and Technology Department, Risø National Laboratory, DK-4000 Roskilde, Denmark ReceiVed: June 9, 1995; In Final Form: NoVember 30, 1995X
High yields of H-atoms and OH-radicals were produced by pulse radiolysis of Ar/H2O mixtures, and in the presence of C6H6, the addition reactions (1a) OH + C6H6 f HO-C6H6 and (2) H + C6H6 f H-C6H6 have been studied by transient ultraviolet absorption spectroscopy of the short-lived radicals OH, HO-C6H6, and H-C6H6. The ultraviolet absorption spectra of HO-C6H6 and H-C6H6 have been recorded in the range 250-350 nm with a maximum value of σ(HO-C6H6) ) (4.6 ( 0.7) × 10-18 cm2 molecule-1 observed at 280 nm. Studies of OH-decay and the simultaneous formation of HO-C6H6 have been used to determine an overall rate constant of k(OH + C6H6) ) (1.2 ( 0.2) × 10-12 cm3 molecule-1 s-1 at T ) 298 K. Phenol was identified as a primary product with a relative yield of 25 ( 5% derived from the amplitude of the strong absorption band at 275 nm. Different reaction mechanisms have been considered, but only the direct displacement reaction (1b) OH + C6H6 f H + C6H5OH seems to account for the experimental results. The kinetics of the reaction (3) HO-C6H6 + NO2 f products has been studied, and a rate constant of (1.1 ( 0.2) × 10-11 cm3 molecule-1 s-1 has been determined at T ) 338 K. Attempts to identify the reaction products by UV spectroscopy were unsuccessful. Transient absorption signals observed in the presence of oxygen have been assigned to peroxy radicals produced in the reaction HO-C6H6 + O2 f HO-C6H6-O2. A rate constant of (5.0 ( 1.0) × 10-13 cm3 molecule-1 s-1 has been derived from the observed formation kinetics.
1. Introduction Large amounts of aromatic hydrocarbons are released into the atmosphere through automobile exhaust, and these compounds are reactive in photochemical smog formation in urban areas. The tropospheric oxidation of aromatic hydrocarbons is initiated by reactions with hydroxyl radicals. In the case of benzene, the hydroxyl radical reacts by an addition reaction, producing the hydroxycyclohexadienyl radical (1a), while the abstraction reaction (1c) to form phenyl radicals is unimportant under tropospheric conditions.
OH + C6H6 a HO-C6H6
(1a)
OH + C6H6 f H + C6H5OH
(1b)
OH + C6H6 f H2O + C6H5
(1c)
The rate constant for reaction 1 has been the subject of numerous experimental investigations including direct measurements of the decay rate of hydroxyl radicals1-12 as well as relative rate measurements.13,14 Thus, the reversible addition reaction (1a) is well established, and the available rate constant data have been reviewed by Atkinson.15 In the most recent evaluation a value of k1a ) 1.23 × 10-12 cm3 molecule-1 s-1 with an estimated uncertainty of (30% has been recommended for the forward reaction at 298 K. The reverse reaction, i.e., the unimolecular decomposition of the OH-adduct, has also been investigated, and the rate constant has been derived by numerical analysis of the biexponential decay of OH.3,6,10,12 On the basis of studies of the temperature dependence of the equilibrium constant, it has been possible to determine the reaction enthalpy for reaction 1a, which leads to an estimate of the bond energy D(HO-C6H6) ) (19.9 ( 1.2) kcal mol-1. The fate of the adduct under tropospheric conditions is not well established. Although phenol is known to be the major ringretaining product with a yield of about 24%, the mechanism X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5729$12.00/0
for the formation remains unknown.14 The direct displacement reaction (1b) was considered to be of minor importance under tropospheric conditions. Reactions of the adduct with O2, NO, and NO2 have been investigated, and the reaction with O2 has been identified as the main reaction channel in the troposphere. However, a detailed reaction mechanism has not yet been established. The chemical reactivity of alkyl-substituted benzenes is different in terms of the simultaneous formation of OH-adducts and radicals produced by abstraction of a hydrogen atom, e.g., in the case of toluene we have observed the addition reaction OH + C6H5CH3 a HO-C6H5CH3 as well as the abstraction reaction OH + C6H5CH3 f H2O + C6H5CH2, which is a minor channel at room temperature. The benzyl radical was easily identified by UV spectroscopy because of the characteristic sharp bands at 253 and 305.3 nm.16-18 In addition to the benzyl radical we also observed the absorption spectrum of the OHadduct, which is produced with a branching ratio of 0.89 at 388 K.19 At room temperature a branching ratio of 0.93 has been reported.14 In the present investigation we have recorded the ultraviolet spectrum of HO-C6H6 and studied the kinetics of this radical in the presence NO2 and O2. 2. Experimental Section Our experimental setup for pulse radiolysis, combined with time-resolved ultraviolet spectroscopy, has previously been described in detail.20,21 Briefly, high yields of free radicals are obtained by irradiation of a gas mixture with a short pulse (50 ns) of 2 MeV electrons from a Febetron 705B field emission accelerator. High yields of hydroxyl radicals are obtained by the source reaction (S1) initiated by pulse radiolysis of SF6/ H2O mixtures:
F + H2O f HF + OH
(S1)
Another source reaction is observed by radiolysis of Ar/H2O mixtures. This reaction proceeds via the formation of excited © 1996 American Chemical Society
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Figure 1. Rotational fine structure within the X2Πi f A2Σ+ transition of OH. The spectra were recorded with an optical path length of L ) 120 cm and a varying spectral band-pass at ∆λ ) 0.2, 0.4, and 0.8 Å.
metastable Ar-atoms followed by an energy transfer reaction producing both H and OH:
Ar* + H2O f Ar + (H2O)* f Ar + H + OH
(S2)
The hydroxyl radicals are monitored by time-resolved ultraviolet absorption spectroscopy. Figure 1 shows an example of the UV spectrum of hydroxyl radicals obtained with an optical path length of 120 cm. The spectrum was recorded with a photomultiplier at the exit slit of a 1 m grating monochromator with a 1200 lines/mm grating corresponding to a reciprocal dispersion of 8 Å/mm. In order to observe individual rotational lines, we had to employ a spectral band-pass of 0.2 Å. For kinetic studies we have used a band-pass of about 1 Å in order to obtain a better signal-tonoise ratio at the expense of spectral resolution. Under these conditions we employ a modified expression, A ) (σLC)n, which has been tested in previous experimental studies.21,22 With a band-pass of 0.8 Å a value of n ) 0.72 was derived from a linear plot of log A vs log L with optical path lengths of L ) 40, 80, and 120 cm. The yield of OH and the apparent absorption cross section at 309 nm were determined by studying the source reaction (S1) in comparison to F + CH4 f HF + CH3 initiated by radiolysis of SF6/CH4 mixtures. Under experimental conditions where all F-atoms were converted quantitatively into CH3, the absolute radical yield was derived from the maximum absorption at 216.4 nm using the wellestablished value of σ(CH3) ) (4.12 ( 0.24) × 10-17 cm2.23,24 With SF6 as the bath gas we have obtained yields of [F]o ) [OH]max ) (1.7 ( 0.2) × 1015 molecules/cm3. With Ar as the bath gas the yield is lower, i.e., [OH]max ) (5.5 ( 0.5) × 1014 molecules/cm3. Although reaction S1 provides a clean source of OH radicals, this reaction could not be employed in studies of the addition reaction OH + C6H6 f HO-C6H6 because of competition between the reactions (S1) and F + C6H6 f F-C6H6 giving rise to strong transient absorption signals around 300 nm assigned to the adduct F-C6H6. Thus, the experimental studies presented below are based on radiolysis of Ar/H2O/C6H6 mixtures. However, since OH and H are produced in equal amounts in accordance with S2, the reactions of H-atoms had to be taken into account as well as the energy transfer reaction Ar* + C6H6 f Ar + H + C6H5. The latter reaction could be
almost completely suppressed by reaction S2 using an initial concentration ratio of [H2O]o/[C6H6]o > 20. The best experimental conditions were obtained at T ) 340 K using partial pressures of p(H2O) ) 80-100 mbar backed up with Ar to a total pressure of 1 atm. The yield of OH-radicals produced by radiolysis of Ar/H2O mixtures was also determined by addition of NO and by monitoring UV absorption at 354.2 nm due to nitrous acid produced in the titration reaction OH + NO + M f HONO + M. Likewise, by radiolysis of Ar/H2O/NO2 mixtures, we have studied the consumption of NO2 via the reactions H + NO2 f OH + NO and OH + NO2 + M f HNO3 + M. Thus, from the loss of NO2 monitored at 400 nm, we calculated the initial yields of H-atoms and OH-radicals, ∆[NO2] ) 2[H]o + [OH]o. The yield of OH-radicals determined by these titration reactions was found to be in agreement with the value presented above to within (10%. By radiolysis of Ar/H2O/C6H6 mixtures, we have observed phenol as a primary reaction product that has been identified by UV spectroscopy. Although the UV absorption spectra of phenol in water and organic solvents have been reported, we have not been able to find current literature on the gas-phase spectrum. Thus, in order to determine the yield of phenol, we studied the spectrum of phenol vapor in thermal equilibrium with the crystals to obtain a reliable value of the absorption cross section of the strong band at 275 nm. From a spectrum recorded with an optical path length of 10 cm and a spectral band-pass of 0.2 nm, we obtained a value of A(275 nm) ) 0.48. The vapor pressure of phenol at T ) 300 K was calculated from the expression log p(mm) ) 11.421 - 3540/T ) 0.418 Torr obtained from sublimation data for organic compounds.25 Combining these values, we obtain σ(275 nm) ) 7.9 × 10-18 cm2 molecule-1 with an estimated uncertainty of (20%. This value is close to the value reported for phenol dissolved in light petroleum.26 3. Experimental Results and Interpretation 3.1. Kinetics of OH-Radicals. OH-radicals were produced by pulse radiolysis of gas mixtures containing 15 mbar H2O backed up with Ar to a total pressure of 1 atm at 298 K. The decay rate of hydroxyl radicals was studied by monitoring the transient absorption signals at 3090.4 Å using an optical path length of L ) 120 cm and a spectral band-pass of 0.8 Å. In the absence of benzene the observed second-order kinetics is governed by the radical-radical reactions between OH-radicals and H-atoms, which are produced in equal amounts.
OH + OH + M f H2O2 + M H + H + M f H2 + M H + OH + M f H2O2 + M Examples of the kinetic features are shown in Figure 2. The absorption signals were treated in accordance with the modified Lambert-Beer expression as described in the Experimental Section. In the absence of benzene the observed decay halflife was about 72 µs. When small amounts of benzene were added to the Ar/H2O mixtures we observed simple exponential decay curves, i.e., [OH] ) [OH]o exp(-k*t) in accordance with reaction 1 proceeding under pseudo-first-order conditions. Curve b in Figure 2 was obtained with a partial pressure of p(C6H6) ) 4.0 mbar, and the decay half-life is reduced to 11.6 µs. The observed first-order rate constants were analyzed by linear regression expressed by k* ) ko + k1[C6H6]. The intercept ko corresponds to the decay rate of OH via the mixed radical-radical reactions that control the decay rate in the
Hydroxycyclohexadienyl Radicals
Figure 2. Kinetics of OH radicals monitored at 309 nm with a bandpass of 0.8 Å: (a) the absence of benzene and (b) p(C6H6) ) 4 mbar at T ) 298 K.
absence of benzene. On the basis of these results, we have determined a value of k1 ) k(OH + C6H6) ) (1.2 ( 0.2) × 10-12 cm3 molecule-1 s-1 at T ) 298 K and a total pressure of p(Ar) ) 1 atm. This value is in good agreement with the recommended value obtained in the most recent evaluation of previous experimental studies.10 The decay of OH is accompanied by the simultaneous formation of the adduct HOC6H6. 3.2. UV Spectrum and Kinetics of Hydroxycyclohexadienyl Radicals. By pulse radiolysis of Ar/H2O mixtures, and in the presence of small amounts of benzene, we observe transient absorption signals in the range 250-350 nm, which we assign to hydroxycyclohexadienyl radicals. However, we have to consider also the fate of H-atoms that are produced along with the OH-radicals. The reaction H + C6H6 f H-C6H6 has been studied previously by pulse radiolysis of Ar/H2/C6H6 mixtures, and transient absorption signals observed around 302 nm were assigned to the cyclohexadienyl radical.27 The UV spectrum and the formation and decay kinetics of H-C6H6 have been reinvestigated in our laboratory. The formation kinetics was studied as a function of the benzene concentration to obtain a value of k(H + C6H6) ) (6.2 ( 0.5) × 10-14 cm3 molecule-1 s-1 in good agreement with previous studies.27 Thus, the formation of H-C6H6 is a factor of 20 slower than the formation of HO-C6H6 when both reactions are initiated by pulse radiolysis of Ar/H2O/C6H6 mixtures, and at short time scales the transient absorption signals observed around 300 nm are primarily due to HO-C6H6 while the contribution from H-C6H6 becomes important at longer time scales. The ultraviolet spectra of HO-C6H6 and H-C6H6 are shown in Figure 3. The absorption cross section of HO-C6H6 was derived from the maximum of the transient absorption signals observed under experimental conditions where all OH-radicals were captured in the reaction with benzene, i.e., σ(HO-C6H6) ) A(HOC6H6)max/L[OH]o using the experimental yield of [OH]o ) (5.5 ( 0.4) × 1014 molecules/cm3 determined by the titration reactions described in the Experimental Section. Thus, for the absorption maximum at 280 nm we have determined a value of σ(HO-C6H6) ) 4.55 × 10-18 cm2 molecule-1 with an estimated uncertainty of (15%. The spectrum of HO-C6H6 shown in Figure 3 was recorded with a spectral resolution of 0.8 nm and with a spacing of 2 nm between adjacent points. The effect of spectral resolution on the value of the absorption
J. Phys. Chem., Vol. 100, No. 14, 1996 5731
Figure 3. Absorption spectra of HO-C6H6 and H-C6H6 observed by pulse radiolysis of Ar/H2O/C6H6 and H2/C6H6 mixtures. The reference spectrum of phenol vapor was recorded as described in the Experimental Section.
cross section has not yet been investigated. However, a value of σ(HO-C6H6) ) (9.2 ( 2) × 10-18 cm2 molecule-1 at 308.3 nm has been obtained by flash photolysis combined with detection of OH and HO-C6H6 with a frequency-doubled ring dye laser with a spectral resolution on the order of 10-6 nm.28 Thus, although the low-resolution spectrum of HO-C6H6 shown in Figure 3 does not show any significant fine structure, it seems likely that a high-resolution spectrum may exhibit a dense vibronic structure at wavelengths above 300 nm. Our experimental results on OH-kinetics also show that σ(HOC6H6) , σ(OH) at (309.04 ( 0.08) nm. However, preliminary attempts to observe the vibronic structure of HO-C6H6 using a diode array detector have not been successful because of insufficient spectral resolution. The UV spectrum of phenol is also included in Figure 3 to show the overlap between the spectra of the radicals and phenol, which has been identified as a primary reaction product. Because of the spectral overlap, the kinetics of HO-C6H6 can only be studied without interference from H-C6H6 and phenol at wavelengths above 340 nm. However, at these wavelengths the transient absorption signals could not be monitored with a good signal-to-noise ratio because of the low absorption cross section. The best performance was obtained in the range 280-320 nm where the contributions from H-C6H6 and phenol must be taken into account. Examples of the formation and decay kinetics of HO-C6H6 observed at 300 nm are shown in Figure 4. The transient absorption reaches a maximum value occurring about 15 µs after the irradiation pulse. Employing a simple pseudo-first-order fit to the experimental formation curve, i.e., A ) Amax(1 - exp(-k1[C6H6]*t), we obtained a value k1 ) 1.1 × 10-12 cm3 molecule-1 s-1 in good agreement with the value derived from the observed decay rate of OH under the same experimental conditions. As shown in Figure 4b, the subsequent decay takes place on a time scale of several hundred microseconds where the contribution from H-C6H6 must be taken into account. So far, it has not been possible to assign individual rate constants for the mixed radical-radical reactions of HO-C6H6 and H-C6H6. However, these reactions are not important in the tropospheric oxidation of benzene. At the higher wavelengths, i.e., λ ) 280 nm and above, the transient absorption signals decayed toward zero on a time scale of about 10-3 s. At
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Bjergbakke et al.
Figure 5. Kinetics of HOC6H6, H-C6H6 and C6H5OH mixtures monitored at (a) 275 nm, (b) 292 nm, (c) 302 nm, and (d) 309 nm.
Figure 4. (a) Formation kinetics of HO-C6H6 monitored at 300 nm. The reaction was initiated by pulse radiolysis of 10 mbar C6H6 and 90 mbar H2O backed up with Ar to 1 atm at T ) 338 K. (b) Decay kinetics monitored at 300 nm where both HO-C6H6 and H-C6H6 contribute to the transient absorption signal.
wavelengths below 280 nm we observed residual absorption due to stable products. Figure 5 shows an example of transient absorption signals observed at different wavelengths in the range 275-309 nm. At 275 nm the observed residual absorption has been assigned to phenol. The assignment is confirmed by the product spectrum obtained with a diode-array detector showing the characteristic band of phenol at 275 nm. The product spectrum shown in Figure 6 was observed after pulse radiolysis of Ar/H2O/C6H6 in comparison to a reference spectrum of pure phenol vapor. On the basis of the residual absorption observed at 275 nm as shown in Figure 5, we have estimated a relative yield of phenol, ∆[C6H5OH]/[OH]o ) 0.25 ( 0.05. Thus, phenol is formed with a relative yield of about 25% of the OH-radicals consumed, and we have considered various reaction mechanisms that may explain the formation of this product. By pulse radiolysis of Ar/H2O/C6H6 mixtures, we observe the formation of OH-radicals as well as phenylradicals produced in the reactions Ar* + H2O f Ar + H + OH and Ar* + C6H6 f Ar + H + C6H5. By pulse radiolysis of Ar/C6H6, we obtained high yields of phenyl radicals, and the UV spectrum was recorded in the range 230-300 nm. To confirm the assignment, we have also studied the formation of
Figure 6. UV spectra of phenol recorded with a gated diode array detector with a delay of 200 µs after the electron pulse. The product spectrum (a) was observed after pulse radiolysis of a gas mixture containing 2 mbar C6H6 + 15 mbar H2O backed up with Ar to 1 atm at T ) 300 K. The reference spectrum (b) was recorded by admitting phenol vapor to the gas cell after careful evacuation.
nitrosobenzene produced in the reaction C6H5 + NO + M f C6H5NO + M by monitoring the buildup of the characteristic broad absorption band with a maximum at 270 nm. Subsequently, we studied the relative yield of phenyl radicals as a function of the C6H6/H2O mixing ratio. Using equal amounts of H2O and C6H6, we observed both OH and C6H5 as well as phenol produced in the consecutive reaction OH + C6H5 + M f C6H5OH + M. However, by the employment of an initial ratio of [H2O]o/[C6H6]o > 20, the relative yield of phenyl radicals could be suppressed to less than 5%. Under these conditions we still observe the formation of phenol with a yield of about 25%. Another reaction of potential interest is the radical-radical cross reaction OH + HO-C6H6 f H2O + C6H5OH. This reaction proceeds in competition with the adduct formation (1a), which becomes the predominant reaction at high benzene concentrations. Since the yield of phenol was found to remain virtually independent of the benzene concentration, it seems justified to rule out the radical-radical reaction as a source of phenol under the experimental conditions employed. Finally, we have considered the direct displacement reaction (1b), which has been proposed to be a minor channel at high temperatures.29 The formation of HO-C6H6 proceeds via a “hot adduct”, which is cooled down by collisions with bath gas
Hydroxycyclohexadienyl Radicals
J. Phys. Chem., Vol. 100, No. 14, 1996 5733
molecules. However, considering the thermochemistry of reaction 1b, it seems possible that the hot adduct may also split off a hydrogen atom whereby phenol is formed directly. OH + C6H6 + M
(HO-C6H5-H)* + M
H + C6H5OH + M; (1a,b)
HO-C6H6 + M
The reverse reaction, H + C6H5OH f OH + C6H6 is slightly endothermic, and the displacement reaction was identified by the formation of benzene.29 However, the proposed displacement reaction (1b) deserves further experimental studies to verify the occurrence at temperatures in the range 300-340 K, which was used in the present investigation. On the basis of the absorption spectrum of HO-C6H6, it is now possible to carry out direct kinetic studies of the reactions of the adduct with various additives that are considered of importance in the tropospheric oxidation of benzene. Previously, this technique has been employed by Zellner et al. using a frequency-doubled ring dye laser for the detection of OH at 308.419 nm as well as HO-C6H6 at 308.3 nm.28 Recently, this technique has also been employed in studies of the reaction HO-C6H6 + O2 f products.30 3.3. Kinetics of the Reaction HO-C6H6 + NO2 f Products. The rate constant for this reaction has been determined in previous experimental studies.10,11,30 However, the products of this reaction have not yet been identified. We have considered the following series of reactions of which some have been proposed previously.
HO-C6H6 + NO2 f C6H5OH + HONO
(3a)
HO-C6H6 + NO2 f C6H5NO2 + H2O
(3b)
HO-C6H6 + NO2 f HO-C6H6-NO2
(3c)
HO-C6H6 + NO2 f HO-C6H6-O + NO
(3d)
Figure 7. HO-C6H6 kinetics monitored at 300 nm (a) in the absence of NO2, (b) in p(NO2) ) 0.20 mbar and (c) in p(NO2) ) 0.34 mbar at 338 K. Curve a has been multiplied by a factor of 2.0 to avoid overlap between the kinetic curves. Curve b was fitted with a double exponential formation and decay curve m.
HO-C6H6 + NO2 f C6H6 + HONO2 ∆H ) -29.6 kcal/mol (3e) Reactions 3a, 3b, and 3c have been discussed by Atkinson et al.,14 while reaction 3d has been proposed by Zellner et al.30 However, reaction 3e has not been considered so far although it seems likely that this exothermic reaction may also occur. In the present investigation we have studied the decay rate of HO-C6H6 in the presence of NO2 by monitoring the transient absorption of HO-C6H6 at 300 nm. The reactions were initiated by pulse radiolysis of gas mixtures containing 5 mbar C6H6 + 90 mbar H2O backed up with Ar to 1 atm at 338 K. In the absence of NO2 the decay of HO-C6H6 proceeded with a halflife of 260 µs. In the presence of small amounts of NO2 the decay rate increased as shown in Figure 7. The residual absorption shown in Figure 7b must be due to a stable reaction product, which was taken into account in the analysis of the kinetic curves using the expression A ) Amax exp(-k*t) + Ap, where Ap represents the absorption of the stable product. From a plot of the observed pseudo-first-order rate constants vs NO2 concentration, i.e., k* ) ko + k3[NO2], a value of k3 ) (1.1 ( 0.2) × 10-11 cm3 molecule-1 s-1 was obtained at 338 K. The yield of HO-C6H6 was found to be strongly dependent on the added amount of NO2. The observed increase at low NO2 concentrations is ascribed to the conversion of H-atoms via the reaction H + NO2 f OH + NO. The yield went through a maximum at p(NO2) ) 0.2 mbar, where the yield was almost a factor of 2 higher than that in the absence of NO2. The
Figure 8. (a) Kinetics of HO-C6H6 + NO2 monitored at 275 nm showing the residual absorption of phenol after decay of the OH-adduct. (b) Decay of the OH-adduct monitored at 300 nm.
decrease in the yield at higher concentrations is explained in terms of competition between adduct formation and the fast reaction OH + NO2 + M f HNO3 + M. Despite these competing reactions, the variation in yields and decay rate of HO-C6H6 could be accounted for by detailed computer simulations using the value of k3 determined by the simple firstorder analysis described above. Considering the reaction mechanism, the products of reaction 3a can be studied directly by UV spectroscopy because of the characteristic spectra of phenol and nitrous acid. Figure 8 shows a comparison of the kinetic features monitored at 300 and 275 nm. In both cases the decay is exponential with the same time constant, but the residual absorption is much stronger at 275 nm where the absorption is a sum of contributions from HO-C6H6 and phenol. From the residual absorption at 275 nm we have estimated a yield of [C6H5OH] ) 1.6 × 1014 molecules/cm3 compared to [OH]o ) 5.5 × 1014 molecules/cm3. Based on the low yield of phenol, which is nearly the same as that observed in the absence of NO2, we conclude that reaction 3a is unimportant. Attempts to observe the formation of HONO in accordance with reaction 3a were also unsuccessful. Calibration experiments were carried out by pulse radiolysis of
5734 J. Phys. Chem., Vol. 100, No. 14, 1996
Figure 9. Kinetics of HO-C6H6 and HO-C6H6-O2 observed by pulse radiolysis of Ar/H2O/O2 mixtures at 320 K: (a) p(O2) ) 0; (b) p(O2) ) 20 mbar.
Ar/H2O/NO mixtures to initiate the addition reaction OH + NO + M f HONO + M, and the formation of HONO was observed at 354.2 nm, which is the strongest vibronic band within the A1A′′ r X1A′ transition.31 However, by pulse radiolysis of Ar/H2O/C6H6/NO2 mixtures, we did not observe the absorption of HONO, even after repetitive pulse irradiations of the same gas mixture. Thus, the low yield of phenol and the absence of HONO demonstrate that reaction 3a must be relatively unimportant and has an estimated branching ratio of less than 5%. Attempts to observe the formation of nitrobenzene in accordance with reaction 3b were also unsuccessful. Kinetic studies were carried out by monitoring the transient absorption signals at selected wavelengths within the strong K-band of nitrobenzene centered at 252 nm. On a time scale of 100 µs the absorption was observed to build up toward a constant value because of a stable product. However, the product absorption was found to be independent of the amount of added NO2. Other experiments were carried out using the diode-array detector to record the product spectra in comparison to the reference spectra of benzene and nitrobenzene. By comparing the reference spectra, we conclude that the yield of nitrobenzene produced by radiolysis of Ar/H2O/C6H6/NO2 mixtures is low with an estimated yield of less than 5%. Transient absorption signals that might be assigned to radicals produced in the reactions 3c and 3d have not been observed in our spectrokinetic investigations covering the range 220-350 nm. Thus, although we cannot rule out the possible occurrence of these reactions, it seemed worthwhile to also consider reaction 3e, which was estimated to be strongly exothermic. Detection of HNO3 by UV spectroscopy was not feasible in the present investigation because of the fairly weak absorption of HNO3 in the range 200-220 nm, which is covered by the much stronger absorption of benzene. In order to study the formation of HNO3, we have employed a new experimental technique based on pulse radiolysis combined with tunable infrared diode laser spectroscopy.32 By pulse radiolysis of a gas mixture containing 0.1 mbar NO2 and 15 mbar H2O backed up with Ar to a total pressure of 100 mbar, we observed the formation of HNO3 by monitoring the absorption of a strong transition near 1327.8 cm-1. In the absence of benzene we observed simple exponential formation kinetics in accordance with the reaction HO + NO2 + M f HNO3 + M. By addition of increasing amounts of benzene, the yield of HNO3 decreased because of the competing reaction HO + C6H6 f HO-C6H6. We did not observe a delayed formation of HNO3 that might be ascribed to reaction 3e.
Bjergbakke et al. Despite our systematic experimental efforts to identify the reaction products by UV and IR spectroscopy, we have not been able to verify any of the proposed reactions. However, our results show that reactions 3a, 3b, and 3e must be regarded as unimportant in the tropospheric oxidation of benzene. 3.4. Reactions of HO-C6H6 in the Presence of O2. The reaction of hydroxycyclohexadienyl radicals with oxygen has been proposed to be the dominant channel under tropospheric conditions. Previously, the reaction has been studied indirectly by monitoring the decay rate of OH in equilibrium with HOC6H6 as well as directly by monitoring the decay rate of HOC6H6 using a UV laser long-path absorption technique.10 The reported value of k(HO-C6H6 + O2) ) 1.8 × 10-16 cm3 molecule-1 s-1 at 298 K is very low compared to that for the reaction with NO2. Despite the low value of the rate constant, the reaction with O2 is thought to be the most important reaction of HO-C6H6 in the troposphere. The products of this reaction have not been identified in the gas phase, but it seems possible that both abstraction and addition reactions occur.
HO-C6H6 + O2 f HO2 + C6H5OH
(4a)
HO-C6H6 + O2 a HO-C6H6-O2
(4b)
Reaction 4a has been proposed as a major channel in the gas phase, which could give rise to a chain reaction in the presence of NO by the reaction HO2 + NO f OH + NO2. The addition reaction (4b) has been observed by pulse radiolysis of benzene in aqueous solutions where a strong absorption band at 310 nm has been assigned to HO-C6H6, while a broad continuum in the range 260-320 nm observed in the presence of oxygen was assigned to the adduct HO-C6H6-O2.33 The rate constant k4b ) 5.2 × 10-13 cm3 molecule-1 s-1 in aqueous solution is more than 3 orders of magnitude higher than the reported gas-phase value. In the present investigation we have studied the effect of O2 on the transient absorption signals in the range 265-330 nm. The reactions were initiated by pulse radiolysis of gas mixtures containing 5 mbar C6H6 and 90 mbar H2O backed up with Ar to a total pressure of 1 atm at 328 K. In the absence of oxygen we observe the transient absorption signals due to HO-C6H6 and H-C6H6 produced in reactions 1a and 2.
OH + C6H6 a HO-C6H6 λmax ) 280 nm
(1a)
H + C6H6 f H-C6H6 λmax ) 302 nm
(2)
In the presence of oxygen the yield of H-C6H6 decreases because of the competing reaction 5:
H + O2 + M f HO2 + M λmax ) 210 nm
(5)
Although no information on the reaction HO2 + C6H6 could be found in the literature, we have considered reactions 6a and 6b:
HO2 + C6H6 a HO-O-C6H6 HO2 + C6H6 f OH + O-C6H6 λmax ) 275 nm
(6a) (6b)
Since H-atoms, F-atoms, O-atoms, and OH-radicals react with benzene to form adducts with characteristic UV spectra in the range 265-330 nm, it seems likely that HO2 also reacts with benzene in a similar way. The UV spectrum of O-C6H6 around 275 nm has previously been investigated by pulse radiolysis.34
Hydroxycyclohexadienyl Radicals Thus, in the range 265-330 nm we may expect to observe contributions from several transient species in addition to HOC6H6. Figure 9 shows a comparison of the transient absorption signals observed at 280 nm in the the absence of oxygen (a) and in the presence of 20 mbar O2 (b). The large increase in the the transient absorption signal in the presence of oxygen must be due to the formation of a radical that absorbs more strongly than HO-C6H6. A residual product absorption as shown in Figure 9b was also observed at other wavelengths in the range 265-300 nm. The strong transient absorption signals observed in the presence of oxygen may be assigned to radicals produced in reactions 4b, 6a, or 6b. In order to investigate a possible contribution from the HO2-adduct produced via reaction 6a, we have studied the kinetics of H-C6H6 produced by radiolysis of H2/C6H6/O2 mixtures. The transient absorption due to H-C6H6 was found to decrease with increasing oxygen concentrations in accordance with the competition between reactions 2 and 5. In the range 270-310 nm we found no evidence for transient absorption signals that could be assigned to products of reactions 6a or 6b. Considering the direct reaction 4a we have studied the yield of phenol, which was found to remain constant within (25 ( 5)% and to be independent of the oxygen concentration. Thus, under the experimental conditions employed, we find no evidence for the formation of phenol via the abstraction reaction 4a. Reaction 4b may explain the increase in amplitude of the transient absorption signal shown in Figure 9b. The spectrum of the transient species observed in the presence of oxygen is similar to the spectrum of HO-C6H6-O2 observed in aqueous solution. Also, the value of k4b ) (5.0 ( 1.0) × 10-13 cm3 molecule-1 s-1 derived from the observed formation kinetics is very close to the value obtained from studies of the reaction in aqueous solution.33 4. Conclusions The observation of the ultraviolet absorption spectrum of hydroxycyclohexadienyl radicals has allowed us to carry out direct spectrokinetic studies of this important intermediate in the absence and presence of additives. A summary of the experimental results is shown in Table 1. In addition to the kinetic studies we have attempted to identify various reaction products by ultraviolet spectroscopy, i.e., phenol and nitrobenzene. The strong B-band of phenol at 275 nm has been employed to determine the yield of this product under varying experimental conditions. Much to our surprise, we found that phenol is formed even in the absence of additives like NO2 and O2. Furthermore, the formation of phenol was found to take place on the same time scale as the formation of HO-C6H6. These findings lead us to consider a direct displacement reaction (1b) OH + C6H6 f (HO-C6H6)* f HOC6H5 + H. On the basis of the experimental yield of phenol we have estimated the branching ratio of reaction 1b and a value of k1b ) 2.8 × 10-13 cm3 molecule-1 s-1 at 298 K. However, this value is almost 6 orders of magnitude higher than the value extrapolated from experiments carried out at temperatures in the range 1000-1150 K.19 Thus, the proposed displacement reaction certainly needs further experimental studies in order to verify its occurrence at room temperature. The increase in the decay rate of HO-C6H6 observed in the presence of NO2 was used to determine a value for the overall rate constant that is in reasonable agreement with the results of previous studies as shown in Table 1. On the basis of product studies, we conclude that reactions 3a, 3b, and 3e can be ruled out as major reaction channels.
J. Phys. Chem., Vol. 100, No. 14, 1996 5735 TABLE 1: Comparison of Rate Constants Obtained in the Present and Previous Investigations k, cm3 molecule-1 s-1
methoda
refs
OH + C6H6 f HO-C6H6
1.2 × 10-12 1.2 × 10-12 1.2 × 10-12 1.0 × 10-12 1.0 × 10-12 1.2 × 10-12 1.2 × 10-12
FP/RF 299 K FP/RF 298 K FP/RF 298 K FP/RF 296 K DF/RF 297 K PR/UV 298 K eval 298 K
1 2 4 5 11 present 15
OH + C6H6 f H + C6H5OH
2.8 × 10-13 2.5 × 10-19
PR/UV 298 K present estimate 298 K 28
H + C6H6 f H-C6H6
6.1 × 10-14 6.2 × 10-14
PR/UV 298 K PR/UV 298 K
34 present
HO-C6H6 + NO2 f products
8.5 × 10-12 2.8 × 10-11 4.0 × 10-11 1.1 × 10-11
FP/UV 298 K FP/RF 305 K DF/RF 353 K PR/UV 338 K
30 10 11 present
HO-C6H6 + O2 f products
1.8 × 10-16 5.0 × 10-13
FP/UV 298 K PR/UV 338 K
10 present
reaction
a Methods: flash photolysis (FP), pulse radiolysis (PR), discharge flow (DF), resonance fluorescence (RF), and UV absorption (UV).
Transient absorption signals observed in the presence of oxygen have been assigned to the peroxy radical produced in the addition reaction (4b) HO-C6H6 + O2 a HO-C6H6-O2. A value of k4b ) (5.0 ( 1.0) × 10-13 cm3 molecule-1 s-1 has been estimated based on the observed fast formation kinetics. The same reaction has been observed by pulse radiolysis of benzene in aqueous solution, and the reported rate constant is very close to the value obtained in the present gas-phase study. However, this value of the rate constant is more than 3 orders of magnitude higher than the value obtained in a recent gasphase study where the rate constant was derived from the kinetics of OH and HO-C6H6 observed at longer time scales.10 Although the formation of peroxy radicals may be studied at short time scales without interference from competing reactions, the subsequent decay takes place via a complex reaction mechanism involving OH, HO-C6H6, and HO-C6H6-O2 with concentratios ratios controlled by the reversible reactions 1a and 4b. Thus, interpretation of the observed decay rates requires a detailed analysis including the forward and reverse rate constants for the reversible reaction 4b. Further experimental studies of reaction 4b are required in order to determine the equilibrium constant and the rate constant of the reverse reaction. Although extensive experimental studies have been carried out during the past 15 years, it appears that the fate of hydroxycyclohexadienyl radicals under tropospheric conditions cannot be accounted for in terms of detailed chemical reaction mechanisms. In the case of benzene the most important oxidation products have been identified. Phenol, which is the major ring-retaining product, accounts for 25%, while glyoxal is produced via unidentified ring-opening reactions with a yield of about 20%. The mechanim of ring-opening reactions presents a great challenge for future experimental studies, and we intend to employ pulse radiolysis combined with tunable infrared diode laser spectroscopy, which allows us to monitor time profiles of short-lived intermediates as well as stable products. Acknowledgment. The experimantal studies have been carried out as a contribution to the european research programs CEC (STEP 0007C MB) and CEC (EV5V-CT93-0309) supported by the Commission of the European Communities. The authors thank the project coordinator, professor Cornelius Zetzsch for his leadership, and all members of these European joint projects for stimulating discussions.
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