7360
J. Phys. Chem. 1995,99, 7360-7370
Solvent Effects on the Spectroscopy and Ultrafast Photochemistry of Chlorine Dioxide Robert C. Dunn,' Bret N. Flanders: and John D. Simon* Department of Chemistry and Biochemistry, University of California at San Diego,
La Jolla, Califomia 92093-0341 Received: July 28, 1994; In Final Form: October 5, I994@
Time-resolved spectroscopic techniques are used to examine the photochemistry of OClO in water, alcohol, acetonitrile, toluene, and sulfuric acid solutions. In all solvents, excitation of the near-UV ,A2 ,BI transition leads to two competing photochemical reactions: dissociation into OC1 0 and formation of C1 0 2 . Based on orbital correlations of the isolated molecule, C1 can be formed by two distinct mechanisms, (1) direct elimination of C1 from OC10, maintianing the GVsymmetry axis along the reaction coordinate to form C1+ 02('Ag), and (2) isomerization to C100, which thermally dissociates into C1+ 0 2 . Experimental evidence supporting both pathways for the condensed phase reactivity of OClO is presented. The quantum yield for direct elimination of C1 is solvent dependent. The role of ClOO in the excited state photochemistry of OClO is discussed in detail. In many of the solvents studied, a photogenerated transient forms within the instrument response that absorbs in the blue-green region of the optical spectrum. This species reveals identical kinetics to those exhibited by the UV absorption assigned to C100. These observations provide compelling evidence that ClOO has electronic excited state(s) similar in energy to the lowest energy states of OC10. The potential involvement of low-lying electronic states of ClOO in the photochemistry of OClO is discussed.
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Introduction Solvation effects on the reactivity of small molecules is a long-standing subject of both experimental aiid theoretical pursuits. Complex interactions between solute and solvent can alter the course of a reaction, generating reaction products which are different than those observed for the same system in the gas phase. To date, the majority of experimental and theoretical studies have focused on charge-transfer' and isomerization2 reactions. Many elegant studies have clearly demonstrated the underlying importance of solvent fluctuations in influencing the dynamics of such reactive processes. Solvent effects on molecular photodissociation processes, on the other hand, are not as well understood. The best-characterized reaction of this type is the photodissociation of Using a variety of timeresolved spectroscopic techniques, solvent effects on the photodissociation and subsequent recombination dynamics have been explored. Recent studies by Fleming and co-workers demonstrate that detailed information on the molecular dynamics can be learned despite the large bandwidth of the 12 absorption in s ~ l u t i o n .These ~ efforts have led to a basic understanding of many reactive processes in solution, including the dynamics of bond breakage, vibrational relaxation, and electronic-state surface crossing. However, to date, studies have been limited to reactants which undergo a single photochemical process from the populated excited electronic surface. There are many small molecular systems which can undergo competing photochemical reactions from a single electronic excited state. In such cases, understanding the role of the solvent in determining the partitioning between the reaction paths is a challenging and important problem. In addition to affecting the shape of excited-state potential energy surfaces
' Current address: Molecular Science Research Center, Pacific Northwest Laboratories, Battelle, Richland, WA 99352. Current address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104. * Author to whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, May 1, 1995. @
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(PES), viscous and dielectric friction effects can influence the chemistry by impeding (or accelerating) motion along a single PES. In the present paper, we examine solvent effects on the excited-state photoreactivity of the symmetric isomer of chlorine dioxide, OC10. From a combination of gas phase,5-" sohtion,12,' and matrix i ~ o l a t i o n ' ~ -experiments, '~ electronically excited OClO is believed to undergo four competitive reactions. These are given by eqs 1-4.
+ hv - ClOO - c1(2P,) + 0, O C ~ O+ hv - C~O(~II)+ o(~P,) OClO + hv - C1(2P,) + 0, ('A,)
OClO
OClO
(32,)
+ hv - c1(2P,) + 0,
(32;)
(1) (2) (3)
(4)
The relative importance of these competing photochemical pathways for gaseous, solvated, and matrix-isolated OClO is a topic of current research.'* This effort, in part, results from a desire to understand the chlorine-driven chemistry that occurs in the stratosphere, which to date has been mainly studied above the ant arc ti^.'^ Chlorine dioxides, ClOO in particular, are commonly invoked in mechanisms which lead to net ozone depletion. However, many other chlorine oxides, e.g., C10 and OC10, are observed in high mixing ratios. Concern over the photoreactivity of chlorine oxides other than ClOO has been a topic of intense discussion, in efforts to identify additional channels for stratospheric ozone loss. The majority of experimental studies reported on OClO to date examine the gas-phase molecular photoreactivity. Initially, electronically excited OClO was believed to only undergo asymmetric bond cleavage, eq 2.5 As a result, the photoreactivity of stratospheric OClO was not believed to be involved in ozone depletion chemistry. Detailed spectroscopic studies by Vaida and coworkers in the last couple of years clearly establish that chlorine atoms are also produced following photoexcitation of OC10.7 Recent translational spectroscopic measurements by
0022-365419512099-7360$09.0010 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 19, 1995 7361
Solvent Effects on Chlorine Dioxide I "
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a
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Figure 1. 'A2
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Wavelength (nm) 2B1 absorption spectrum of OClO in (a) the gas phase, (b) water solution, (c) 2-pantanol solution, and (d) sulfuric acid solution.
TABLE 1: Absorption Maxima for the 2A2 Transition of OClO in Various Solvents solvent formamide water acetonitrile methanol trifluoroethanol ethanol 2-pentanol
500
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dielectric constant, 111.0 78.4 37.5 32.7 26.1 24.5 13.9
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Davis and Lee suggest that a distribution between the 'Ag and '2; states of oxygen accompanies the generation of Cl.9 In a related study in water solution, we reported the observation of the '2; 'Ag emission of 0 2 following excitation of OC10.'2d Compared to the quantum yield for C1 production (lo%), the emission data showed that the ratio of '2; to 'A, production was greater than 20:l in aqueous solution. The relevant quantity for evaluating the importance of OClO to stratospheric ozone depletion is the quantum efficiency for C1 production. Several experimental results have been presented, ranging from 0.15 to 10-4,8-'0 thus the potential importance of this molecule in ozone depletion is still somewhat controversial. In the gas phase, mode-specific dissociation is observed. Davis and Lee demonstrated that excitation of the bending vibration enhanced the yield of the C1+ 0 2 channel.'O Bishenden and Donaldson showed enhancement of the C10 0 channel when the asymmetric stretch is excitedG8 The implications that heterogeneous chemistry plays a role in the atmospheric reactivity of chlorine dioxide (chemistry on the Polar Stratospheric Clouds) prompted our initial efforts to understand how the photoreactivity of OClO in solution relates to that observed in the gas phase. As discussed below, the solution photochemistry can also be used to unravel issues relevant to the gas phase and matrix-isolated system. In 1991, we reported the results of a comprehensive study examining the initial steps of the photochemistry of OClO in water.Iza In that study, time-resolved absorption data were
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7362 J. Phys. Chem., Vol. 99, No. 19, 1995 0.05 0.105
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Time (ps) Figure 3. Time-resolved absorption dynamics following the excitation of OClO in water at 355 nm. The data shown are representative of the types of dynamic behavior observed. Kinetic modeling of the experimental data using the absorption spectra given in Figures 1 and 2 indicates that 10 & 1% of the electronically excited molecules fragment into C1 0 2 (via isomerization to C100); the remaining molecules undergo dissociation into C10 and 0. Studies of singlet oxygen emission yields show that 0.5% of the C1 yield forms from a direct C2" dissociation of OC10, while the remaining 9.58 involves the asymmetric ClOO molecules as a reaction intermediate.
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recent calculations of the potential surfaces for the excited electronic states that give rise to the complex photoreactivity, we address how solvation influences the competition between the differing photochemical pathways.
Chemicals. The preparation of OClO followed the method reported by Bray.22 All solvents were spectral grade.
Experimental Section
Absorption Spectra of OClO in Solution. Figure 1 shows the 2A2 2 B absorption ~ spectrum of OClO in the gas phase and several solvents. Solvation of OClO gives rise to small shifts in the absorption maxima and varying degrees of the vibronic structure. A comparison of the vibronic structure in the gas- and solution-phase spectra indicate similar spacings, suggesting solvation has little effect on the shape of the excited *A2 potential energy surface. A small solvent effect on the absorption maximum, tabulated in Table 1, is observed. For solvents in which OClO shows extensive vibronic character, the absorption maximum was determined by fitting the peak intensities of the different vibronic transitions to a log-normal spectral distribution. Time-Resolved Absorption Studies. Time-resolved absorption studies were carried out in water, alcohol solutions (methanol, ethanol, 1-propanol, and 2-propanol), toluene, acetonitrile, and sulfuric acid. The sections presented below summarize the behavior of the transient signals. Water Solutions. The transient absorption dynamics following photoexcitation of OClO in water were recently described in detail. Prompted by observations in the other liquids studied, vide infra, the wavelength range examined was extended to include the green region of the spectrum. The absorption spectra of Cl(aq),23C10(aq,,23and C10024,25(gas phase) are given in Figure 2. Due to overlapping absorption in the region from 240 to 440 nm, the transient kinetics can be complicated if there is competition between the channels given by eqs 1-4. Figure 3 shows transient absorption data obtained at 250, 345, 385, and 420 nm following photolysis of OClO at 355 nm. These data, which have been previously reported and discussed,'2a
Transient Absorption Studies. The time-dependent absorption signals were collected by using a high repetition rate picosecond laser system which has been described in detail elsewhere.20 Our previous experience in studying the photochemistry of OClO showed the need to flow the solutions in a unidirectional manner to avoid signal artifacts due to the buildup of photoproducts. As a result, at each probe wavelength studied, a 4-L sample was prepared and flowed one way through a 2-mm quartz cell at a rate which guaranteed that fresh sample was available for each laser shot. For a 4-L solution, this limited the maximum data collection time to -6 min, which corresponded to a time delay of -2 ns. In all solvents, the concentration of OClO was adjusted to an optical density of 1 (in a 2-mm cell) at 355 nm. Oxygen Emission Measurements. The apparatus used to determine the quantum efficiencies for the 'Zi IAg and Z ' Cg IZg emissions of 0 2 have been previously described.21 Quantum yields of 02(l A,) were determined by extrapolation of the emission intensities to zero time, which were corrected to 100% absorption (from absorbances varying from 0.2 to 0.6). These were compared to the 02(lAg) quantum yields of the standard sensitizers meso-tetrakis(4-sulfonatopheny1)porphinein D20 or acridine in C6D6 and CC4. The detection limits of the apparatus enabled measuring quantum efficizncies of 0 2 ( Z ' ;) formation greater than 0.1%. All measurements were carried out under air-saturated conditions. Absorption Spectroscopy. Steady-state absorption spectra were collected on a Varian Cary-219. +-
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Results
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J. Phys. Chem., Vol. 99, No. 19, 1995 7363
Solvent Effects on Chlorine Dioxide 0.05
0.04 0.03
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Figure 4. Time-resolved absorption dynamics at 440 nm following the excitation of OClO in water at 355 nm. Examination of Figures l b and 2 indicates that OClO and the photoproducts are not expected to absorb at this wavelength. Upon excitation, a transient is observed that subsequently decays. The kinetics are identical to the fast component observed in the UV, which is attributed to the formation and decomposition of C100. These data supports the conclusion that there are low-lying excited states of ClOO in the vicinity of 2.5 eV.
serve as useful comparison for the transient absorption dynamics observed in other solvents. The evolution of the signals reflects the formation of the various intermediates shown in Figure 3. Figure 4 shows the transient absorption dynamics at 440 nm. This wavelength corresponds to the red edge of the OClO absorption (Figure 1). The information shown in Figure 2 also suggests that none of the reaction intermediates will absorb here. Thus, a small bleach corresponding to the depletion of the OClO concentration by photolysis is expected at this wavelength. However, the data indicate an instrument-limitedrise, followed by a decay in absorption to that expected for a small bleach. The rate of decay of this transient species quantitatively follows the fast component observed at 250 nm. The fast component at 250 nm was previously assigned to the thermal decomposition of photogenerated C100. This result suggests that previously unobserved electronic states of ClOO may exist in the region around 2.5 eV. The implications of this observation will be discussed in the final section of the discussion. Alcohols. All of the alcohol solvents studied (methanol, ethanol, I-propanol, and 2-propanol) exhibited similar kinetics throughout the wavelength range examined (250-440 nm). Figure 5 shows the results for the 355-nm photodissociation of OClO in 1-propanol. At a probe wavelength of 266 nm, an instrument-limited increase in absorbance is observed. This is followed by a small decay of the signal to a constant value at approximately 85% of its initial value by 400 ps. The signal recorded at 295 nm also exhibits an instrument-limited rise in absorbance. After the initial rise, the signal decays by -35%, leveling off to a constant value by 400 ps. The dynamics observed at these UV wavelengths are similar to that found in water. As the probe wavelength is tuned into the region where the ground state of OClO absorbs, the data do not reveal an immediate absorption bleach. At probe wavelengths of 330, 375, and 405 nm, a slow decrease in absorption is observed, which levels off to a constant value near 1 ns. At 440 nm, there is an initial instrument-limited rise in absorption that decays to a constant value with the same time constant as that observed at 330, 375, and 405 nm. In methanol, the dynamics probed at 405 nm were also recorded at several different temperatures. Analysis of the temperature-dependent absorption signals indicates that the chemical process responsible for the slow decrease in absorption found at these wavelengths has an activation barrier of -0.3 kcaVmol. Toluene. For toluene solutions of OC10, the absorption kinetics were recorded at probe wavelengths of 295, 330, 375, 405, 440, 532, and 595 nm. A selection of these data is shown in Figure 6. At 295 and 330 nm, photolysis at 355 nm causes an instantaneous absorbance increase at time zero, which does
not exhibit any appreciable change with delays up to 2 ns. At 375 nm the signal at zero time rises slightly then slowly bleaches, reaching a constant value for delay times greater than 500 ps. At probe wavelengths of 405 and 440 nm, an instrument-limited rise in absorption is observed. Following this initial rise, both signals decay with the same time constant as that observed for the bleach at 375 nm. At 405 nm, the signal decays -15% of the maximum by 500 ps. At 440 nm, the signal decays -35% by 300 ps. For both wavelengths, the absorption slowly decays with the same time constant. Lastly, the kinetics at both 532 and 595 nm demonstrate an instantaneous rise in absorption followed by a signal which remains constant out to the maximum delay of 1.5 ns. This transient is dominated by the charge-transfer absorption of the ground-state toluene-C10 charge-transfer complex.26 Sulfuric Acid. The time-dependent absorption signals of OClO dissolved in neat sulfuric acid were collected at probe wavelengths of 230,240,250,266,300,335, and 410 nm, some of which are shown in Figure 7. From 230 and 250 nm, an instrument-limited rise in absorption is observed. No decay is found for delays up to 2 ns. At 266 and 300 nm, an instrumentlimited rise in absorption is also observed. With increased delay, however, the signal slowly decays, dropping by over 75% of the initial value at 2 ns. The signals recorded at 335 and 410 nm show an instantaneous bleach at time zero, followed by a slow recovery. The time constant and amplitude of this recovery is identical to that exhibited by the absorption decay at 266 and 300 nm. Acetonitrile. The transient dynamics in acetonitrile were measure at the same probe wavelengths as in the alcohols. Qualitatively, the signals at 266 and 295 nm displayed kinetics similar to that shown for 1-propanol at 266 nm in Figure 5. After an instantaneousrise in the absorption signal at time zero, a steady decrease in signal occurs. However, the rate of decay is slower than that observed in either water or alcohol solutions. At the remaining wavelengths (330, 375,405, and 440 nm), an instrument-limited absorption bleach occurs at time zero. The signal remains constant out to the maximum delay of 1.5 ns. No kinetic components similar to the UV decays are observed. Wavelength-Dependent Studies in Water. The absorption dynamics at 266 nm were probed for a OClO water solution while varying the excitation wavelength from 355 to 430 nm. The dynamics observed were independent of excitation wavelength, suggesting that the partitioning between the C10 0 and C1+ 0 2 channels is not wavelength dependent in solution. Formation of Electronically Excited 0 2 . In solution, the formation of singlet 0 2 ('A,) can be conveniently detected by monitoring the 1270-nm emission accompanying the '2; 'A, 0 2 transition.2'a.b The emission exhibits solvent-dependent lifetimes with typical decays on the order of 150 ps. Emission signals at 1270 nm were detected following 355-nm excitation of OClO dissolved in D20, D2S04, CD3CN, C&, CD2C12, and CC4. This establishes the presence of 0 2 ('A,) in these solvents. The quantum yield of this channel was measured in D20, C6D6, and CC4 by comparison of the emission intensity following excitation of OClO with that of known sensitizers. The quantum yields were found to be sensitive to the solvent polarity; 0.005 in D20 (E~(30)= 63.1 kcaumol), 0.02 in C6D6 (E~(30)= 34.5 kcaVmol), and 0.07 in CC4 (E~(30)= 32.5 kcaVmo1). In solution, the formation of singlet 02('Ci) can be detected by monitoring the 5 192-cm-' emission characteristic of the 'A, '2;0, Emission signals at 5192 cm-' were looked for following 355-nm excitation of OClO dissolved CC14. CC4 was chosen as OClO exhibits the
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(a) 266 nm
