Secondary Ozonide Formation from the Ozone Oxidation of

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J. Phys. Chem. C 2009, 113, 11060–11065

Secondary Ozonide Formation from the Ozone Oxidation of Unsaturated Self-Assembled Monolayers on Zinc Selenide Attenuated Total Reflectance Crystals Theresa M. McIntire, Olivia Ryder, and Barbara J. Finlayson-Pitts* Department of Chemistry, UniVersity of California, IrVine, IrVine, California 92697-2025 ReceiVed: February 19, 2009; ReVised Manuscript ReceiVed: April 21, 2009

The ozone oxidation of terminal unsaturated 7-octenyltrichlorosilane self-assembled monolayers (C8) SAMs) on ZnSe and on SiOx-coated ZnSe was followed as a function of time using Fourier transform infrared spectroscopy (FTIR). Zinc selenide substrates have a major advantage over silicon crystals used in previous studies in that they transmit to approximately 650 cm-1, well beyond the cutoff of ∼1500 cm-1 for silicon ATR crystals, and thus allow detection of additional product species. When uncoated ZnSe or SiOx-coated ZnSe ATR crystals with a C8) SAM are exposed to gaseous ozone at concentrations from 1013 to 1015 molecules cm-3 at 1 atm pressure in He at 296 K, peaks due to CdCH decrease and a strong product peak at 1110 cm-1 as well as a weaker peak at 1385 cm-1 increase, both of which are attributed to the formation of a stable secondary ozonide (1,2,4-trioxolane, SOZ). Peaks at 2860 and 2929 cm-1 are also formed, which we tentatively assign to the C-H stretch of the OC-H group in the SOZ. The magnitude of the 1110 cm-1 band relative to those due to carbonyl groups at ∼1722 cm-1 suggests that SOZ may, in fact, be the major reaction product. The SOZ has a sufficiently long lifetime when formed in the condensed phase that it is stabilized on the surface, indicating secondary ozonides may be formed during organic oxidations on surfaces in the atmosphere. I. Introduction Aerosol particles have well-documented effects on atmospheric processes and have numerous environmental consequences.1,2 Atmospheric particles affect visibility and light scattering, and they play an important role in the earth’s climate through direct and indirect radiative forcing.1-3 Particulate matter makes a significant contribution to the heterogeneous chemistry of the atmosphere1-3 and causes substantial health effects related to its size and chemical composition.4-6 A major area of concern, but also of large uncertainty, is the impact of these particles on global climate.3 A considerable part of this uncertainty is the lack of understanding of the nature of the organic component. This deficiency includes the chemical speciation and the distribution of the organics between the surface and the bulk of liquid particles, as well as changes during transport in the atmosphere. Dust storms provide large, episodic sources of particles that can be distributed globally.7 Airborne mineral dust and other environmental surfaces, i.e. aerosols and urban surfaces, such as glass, stone, and concrete, play a critical role in environmental chemistry and have been shown to be coated with films that are composed partially of adsorbed organic compounds8-20 and fatty acids of biological origin.10,11,19 These organic species may control the particle surface properties and their ability to act as cloud condensation nuclei (CCN) and ice nuclei (IN).21 It is generally accepted that when the organic films are first formed, these surfaces are typically hydrophobic. However, as these surfaces “age” in the environment, these organic films react with atmospheric trace gases8,9,22-25 to form polar compounds, potentially leading to changes in the hygroscopic nature of the surfaces which then take up increased amounts of water, improving their ability to act as CCN and IN. The increase in * Author to whom correspondence should be addressed. E-mail address: [email protected]. Telephone: (949) 824-7670. Fax: (949) 824 -2420.

particle size alters the associated light-scattering properties and, in addition, can impact the uptake and reaction of gases on the particles.9,25,26 Likewise, many environmental surfaces (walls of buildings, etc.) have coatings of water in sufficient amounts to promote heterogeneous reactions.15,18,27-29 Understanding the effect of a surface’s molecular and chemical structure on the interaction with or adsorption of species such as water is critical to our fundamental understanding of the subsequent properties and chemistry of such surfaces. There have been a number of laboratory studies of the oxidation of organics on solid surfaces, including proxies for mineral dust aerosols.13,14,22-36 Well-characterized organic selfassembled monolayers (SAMs) can be covalently attached to silica surfaces to form an ordered, relatively tightly packed molecular layer.35,37 These systems are a good model for organics adsorbed on mineral dust, since silica is a major soil component.1,23-26,30-32,35,38-40 Ozone oxidation of vinyl-terminated SAMs has been observed to generate gaseous products such as formaldehyde, CO, and CO2 as well as surface-associated carbonyl groups, at least part of which are carboxylic acids.25,31,32,35,41 Such products are consistent with the mechanism of gas phase ozone-alkene reactions. In addition, it was shown29,42 that their oxidation does not lead to increased water uptake on the surface, in contrast to the expectation for the formation of polar oxidation products. This has important implications for the cloud-nucleating properties of organiccoated airborne dust particles. Silicon attenuated total reflectance (ATR) crystals used in the previous studies have a spectral cutoff at ∼1500 cm-1. In order to determine if there are significant amounts of oxidation products formed that can only be seen at wavenumbers below 1500 cm-1, ZnSe and SiOx-coated ZnSe ATR elements which transmit in this region have been used in the present study. Relatively little attention has been given to the use of these substrates for self-assembled monolayers, since much less is

10.1021/jp901535t CCC: $40.75  2009 American Chemical Society Published on Web 05/28/2009

Secondary Ozonide Formation from Ozone Oxidation of SAMs known about how the trichlorosilyl precursor compounds bind to the ZnSe surfaces, unlike SiOx, where the formation of SAMs is reasonably well characterized.37,43 We report here evidence for the formation of a secondary ozonide (1,2,4-trioxolane, SOZ) as a major product of the ozonolysis of unsaturated alkene SAMs on uncoated ZnSe and SiOx-coated ZnSe ATR crystals. The implications for mechanisms of ozonolysis of unsaturated SAMs and of alkenes adsorbed on solid surfaces are discussed. II. Experimental Methods ATR Crystals. Zinc selenide attenuated total reflectance (ATR) crystals (Pike Technologies, 80 mm × 10 mm × 4 mm) having 10 reflections along the length of the crystal were used as received. A second set of ZnSe ATR crystals (Balboa Scientific) of the same dimensions were coated with a thin layer (∼20 nm) of SiOx using plasma enhanced chemical vapor deposition (PECVD). This coating was intended to provide similar SAM attachment chemistry to that on silicon ATR crystals while also providing the wider spectral range. Cleaning Procedure for ATR Crystal Surfaces. Attenuated total reflectance crystals were boiled in ethanol (Sigma Aldrich, 99.5%) and in dichloromethane (Sigma Aldrich, 99.9%). To remove remaining trace organics from the surface, the ATR crystals were placed in an argon plasma discharge (Plasma Cleaner/Sterilizer PDC-32G, Harrick Scientific Products, Inc., Ossining, New York) with a medium RF setting for ∼30 min. The crystals were rinsed with ultrapure water (Milli-Q, 18.2 MΩ cm, Millipore) and dried with nitrogen (UHP, Oxygen Services, 99.999%). Self-assembled Monolayer Coating Process. Self-assembled monolayers (SAMs) of 7-octenyltrichlorosilane (C8), Aldrich Chemical Corp., 97%) were generated on clean ZnSe and SiOxcoated ZnSe ATR substrates using silane coupling chemistry35,37,42 with a 2 mM solution of 7-octenyltrichlorosilane in hexadecane (Acros, 99%). The chemistry involved in the attachment of the SAM molecules to a ZnSe surface is discussed in more detail below. However, the native oxide layer either of zinc or selenium is expected to be hydrated when treated with water, leaving the surface with some hydroxyl termination that allows the trichlorosilyl precursor molecules to bind to the surface and cross-link with neighboring molecules.37,44-46 The top side of the ATR crystal was coated with the SAM solution using a custom-made Teflon holder for 30 min, and then the crystal was boiled in dichloromethane. The crystal was recoated again for 10 min using the original SAM solution, followed by boiling three times in dichloromethane to remove any surface aggregates, after which the ATR crystal was wiped to remove any remaining residues. The C8) SAM-coated ATR crystal was immediately placed in a flow-through horizontal ATR holder (Pike Technologies) with a flow of dry helium (UHP, Oxygen Services, 99.9995%) overnight in the sampling compartment of a Thermo Nicolet 6700 FTIR spectrometer (now Thermo Electron Corporation, Madison, Wisconsin). Atomic Force Microscopy (AFM) Imaging and Analysis. Specimens of ZnSe ATR crystals before and after coating with C8) SAM were imaged at ambient pressure and humidity using a Park Scientific Instruments AutoProbe CP Research (now Veeco Metrology Inc., Santa Barbara, CA) scanning probe microscope in intermittent contact mode. AFM images were acquired using highly doped silicon tips (BudgetSensors) with a force constant of 3 N/m in intermittent contact mode. The piezoelectric scanner was calibrated using a grating in the xy directions and in the z direction using several conventional height standards. Topographs were

J. Phys. Chem. C, Vol. 113, No. 25, 2009 11061 obtained as 256 × 256 pixels and were flattened line-byline and analyzed using AutoProbe image processing software supplied by the manufacturer of the AFM. The root-meansquare (rms) surface roughness over selected scanned areas was calculated from Rrms ) [ΣN(zn - jz)2/(N - 1)]1/2, where jz is the average z height, zn is the height at each point on the sample, and N is the number of points sampled. Ozone Oxidation of SAMs. The ozone flow system is the same as described elsewhere.35 Briefly, ozone was generated by flowing a mixture of O2 (UHP, Oxygen Services, 99.993%) and He (UHP, Oxygen Services, 99.9995%) through a custommade, stainless steel enclosure holding a low-pressure mercury lamp (Jelight Company Inc. or UVP, LLC), that emits light at 185 nm, causing dissociation of O2. The concentration of ozone was determined from the UV-vis (Hewlett-Packard 8452A, now Agilent Technologies) absorption at 254 nm and the flow rates of the O3/O2 from the lamp and of the He diluent gas introduced into a mixing bulb.35 At the start of an experiment, the ozone gas mixture was diverted to flow over the SAM-coated ATR crystal surface. Ozone concentrations ranged from ∼1 × 1013 to 1 × 1015 molecules cm-3. ATR-FTIR. Changes in the C8) alkene-terminated selfassembled monolayer coated ZnSe surfaces as a function of ozone exposure were monitored using ATR-FTIR spectroscopy.35 Infrared spectra were collected over the spectral range of interest, 4000 -650 cm-1, as single beam spectra at 4 cm-1 resolution using 128 scans over 1.3 min for both background and sample spectra. The reaction was initiated by diverting the ozone flow to the SAM-coated ATR crystal, and infrared scans were started simultaneously. Infrared absorbance spectra were obtained by ratioing the single beam spectra during the reaction to the background spectrum of the same helium-purged unreacted C8) SAM. SAM oxidation experiments were carried out using ZnSe ATR crystals both with and without a SiOx-coating. For the 4000-650 cm-1 region, the sampling depth in ATRFTIR is frequency dependent and the depth of penetration of the evanescent wave adjacent to the ZnSe ATR crystal was calculated to be 0.29-1.8 µm,47 much greater than the thickness of the SAM layer. The infrared beam thus interrogates the entire organic monolayer and generates a spectrum similar to a transmission spectrum.47 III. Results and Discussion Microscopic Characterization of Surfaces. Atomic force microscope (AFM) images were taken in intermittent contact mode to evaluate the topography of the uncoated ZnSe and SiOxcoated ZnSe surfaces with and without SAMs. Figure 1a shows an AFM image of an unused plasma-cleaned ZnSe surface, where the surface rms roughness was measured to be 26 Å (excluding the small particles). The small particles present are from the plasma cleaning process where small silica particles from the plasma cleaner inner glass tube get etched onto the substrate. Small particles could also be caused by “chipping” of the surface during handling. Figure 1b is that of a similar crystal coated with SiOx, where the root-mean square (rms) roughness is 73 Å. For comparison, a typical surface roughness of a clean oxidized surface of a silicon crystal is 3-5 Å.48 The large black or dark, pitted areas show that the SiOx coating is not uniform. The AFM image in Figure 1c shows that the SiOxcoated ZnSe after multiple SAM coating and oxidation experiments (with removal of the final coating) is smoother, with a rms roughness of 18 Å. This is likely due to the filling in of the holes seen in Figure 1b by the repetitive addition of SiOx from the trichlorosilyl end of each SAM coating.48 A plasma cleaned,

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Figure 1. Intermittent contact mode AFM images of (a) a plasma cleaned, uncoated, unused ZnSe ATR surface [rms roughness is 26 Å excluding particles]; (b) unused SiOx-coated ZnSe [rms roughness is 73 Å]; (c) SiOx-coated ZnSe used multiple times for SAM experiments after removal of the last SAM coating [rms roughness is 18 Å]; (d) C8) SAM-coated plasma cleaned, unused ZnSe ATR [rms roughness is 38 Å excluding particles]. Scale bar in all images is 500 nm.

new, unused ZnSe ATR coated with C8) SAM seen in Figure 1d is similar to that of the uncoated substrate shown in Figure 1a, with a rms roughness of 38 Å excluding particles. Ozone Oxidation of C8) SAM. Figure 2 shows typical spectra for a C8) SAM on ZnSe during reaction with 2.9 × 1013 O3 cm-3 at selected times. Time t ) 0 is defined as the beginning of the first scan, which therefore actually represents an average over the first 1.3 min of reaction; hence, small product peaks are seen in this first spectrum. The spectra show the formation of a new, strong absorption band at ∼1110 cm-1 that is characteristic of the peroxide C-O bond49-54 of the secondary ozonide ring (1,2,4-trioxolane, SOZ) known to be formed in ozone-alkene reactions.1,52,55-60 For example, Lai et al.53 isolated secondary ozonides from the ozonolysis of unsaturated phospholipids and the methyl esters of oleic and elaidic acids; they separated and analyzed the SOZ using HPLCUV, 1H NMR, and fast atom bombardment mass spectrometry, and all had a strong infrared peak at 1110 cm-1. A peak at ∼1385 cm-1, assigned to the O-C-H bending mode of the secondary ozonide,61 increases simultaneously with the peak due to the SOZ at 1110 cm-1. These peaks were not observed in previous studies because the silicon ATR crystals cut off below 1500 cm-1.35 It is noteworthy that the SOZ was stable over the course of several hours and did not show any significant decomposition upon exposure to ambient air after it had been formed in the reaction. This is consistent with studies of both gas- and condensed-phase secondary ozonides, which have been observed to be quite stable.59,62-64 Figure 2 also shows formation of a broad band centered at ∼1722 cm-1 assigned to a CdO stretch. Non-hydrogen-bonded carboxylic acids have a CdO stretch at ∼1740 cm-1, while hydrogen-bonded acids and aldehydes have a peak in the 1700-1720 cm-1 range.54 The broad peak in Figure 2 may have

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Figure 2. ATR-FTIR difference spectra of a C8) coated ZnSe ATR crystal as a function of reaction time during the reaction with 2.9 × 1013 molecules cm-3 O3. These are log S0/St, where S0 is the single beam spectrum of the SAM before reaction and St is the spectrum at the different reaction times shown.

contributions from all three, based on previous work with SAMs on silicon ATR crystals.35 (Note, however, that the inset to Figure 2 does not show bands in the O-H stretching region above 3000 cm-1 expected from carboxylic acids). Without quantitative absorption cross sections for the SOZ and carbonyl peaks, the relative contributions of the two types of products cannot be determined. However, as seen in the spectra in Figure 2, the peak height of the SOZ at 1110 cm-1 is about a factor of 4 larger than that at 1722 cm-1. Support for SOZ as the major product is also found in the work of DenBesten and Kinstle,65 who showed that ozonolysis of alkenes adsorbed on silica gel at low temperatures gave the SOZ in yields > 80%, whereas a more complex mixture of products is obtained for the reaction carried out in nonparticipating aprotic solvents.65 The formation of products was accompanied by decreases in peaks associated with the alkene group at 910, 1641, 3008, and 3080 cm-1. As seen in the inset in Figure 2, the -CH2asymmetric (νas) and symmetric (νs) stretches at 2926 and 2856 cm-1, respectively, also decrease upon reaction with ozone, indicating a loss of organic material from the SAM layer. However, overlapping peaks that are blue-shifted relative to those of the unreacted SAM are formed simultaneously, and their absorbance tracks SOZ formation. To probe the position of these new peaks in more detail, the spectrum of an unreacted C8) SAM was added back to a difference spectrum (Figure 2 inset) in order to cancel out the negative bands. Figure 3 shows the results for a typical experiment where the new overlapping bands are seen to be centered at ∼2929 and 2860 cm-1, respectively. Similar observations were made for the SAM ozonolysis on silicon ATR crystals. We tentatively assign these bands to a C-H stretch in the SOZ, since ethers are known to have two characteristic -CH2- bands in the 2970-2920 and 2880-2835 cm-1 regions,54 and bands in these regions have also been observed for SOZ.49-52

Secondary Ozonide Formation from Ozone Oxidation of SAMs

J. Phys. Chem. C, Vol. 113, No. 25, 2009 11063 SCHEME 1: Simplified Reaction Scheme for the Formation of SOZ

Figure 3. Corrected spectrum in the C-H stretching region after the O3 reaction with C8) SAM. The difference spectrum shown in the inset to Figure 2 was corrected here for the loss of the CH2 peaks at 2926 and 2856 cm-1 by adding in the spectrum of the unreacted C8) SAM.

Figure 4. Changes in ATR-FTIR spectra from ozone reactions in the 2000-870 cm-1 range. The y axis is log10(S1/S2), where S1 and S2 are defined for each spectrum. Black: reaction on a ZnSe surface for 10 min at 7.0 × 1014 O3 cm-3; S2 is the single beam spectrum of the oxidized ZnSe surface, and S1 is the single beam spectrum of a clean ZnSe surface. Red: reaction on a SiOx-ZnSe surface for 10 min at 8.0 × 1014 O3 cm-3; S2 is the single beam spectrum of the oxidized SiOxcoated ZnSe surface, and S1 is the single beam spectrum of a clean SiOx-coated ZnSe surface. Green: C8) SAM on ZnSe after ∼2 h of exposure to 4.2 × 1014 O3 cm-3; S2 is the single beam spectrum of the oxidized SAM, and S1 is the single beam spectrum of unoxidized SAM. Blue: C8) SAM on SiOx-coated ZnSe after ∼2 h of exposure to 7.4 × 1014 O3 cm-3; S2 is the single beam spectrum of the oxidized SAM, and S1 is the single beam spectrum prior to oxidation (dry He flowing).

Reactions of the C8) SAM on ZnSe ATR crystals showed very similar reaction products to those on silicon crystals above 1500 cm-1, indicating that the chemistry is similar on both substrates. The product peaks for the C8) oxidation on the SiOxcoated ZnSe substrate were also indistinguishable from those using ZnSe. Given the nature of the ZnSe and SiOx-coated ZnSe surfaces (Figure 1), a question is whether O3 can penetrate to the ZnSe and react with it. Figure 4 shows typical changes in spectra when these two substrates are exposed to O3 in the absence of SAM. The top two spectra represent upper limits to the contribution of changes in the substrate during exposure to ozone, since no reactive SAM coating is present. Also shown for comparison are typical changes in the spectra during the ozonolysis of C8) SAM on ZnSe and SiOx-ZnSe. Although some changes were observed on exposure of the uncoated ZnSe

and SiOx-coated ZnSe to ozone, comparison of these spectra to those from the SAM oxidations shows that the contribution from reaction of the underlying substrate must be small. A portion of the Criegee mechanism1 for ozone-alkene reactions is shown in Scheme 1. Ozone reacts with the terminal double bond of the C8) SAM molecules, producing a primary ozonide (1,2,3-trioxolane, POZ). The POZ can decompose to form a Criegee intermediate and aldehyde, which can react with each other to form a secondary ozonide. The formation of SOZ has also been observed recently in the ozonolysis of unsaturated phospholipid coatings on NaCl.60 Additional reaction paths are available for Criegee intermediates, including isomerization to the acid or cross-linking of the chains to form large aggregates.1,42 The decrease in the -CH2- group intensity during the reaction is likely due to the formation of gas phase products such as HCHO, CO, and CO2 from decomposition of the Criegee intermediates (Scheme 1).24,31,35,66 Aggregate formation seen in the past in this reaction42 may also contribute to this decrease, since they are sufficiently large that they may not be fully interrogated by the infrared beam. An advantage of ATR-FTIR measurements on surfaces is that reactions can be followed in real time.35 Figure 5 shows the increase in the SOZ band at 1110 cm-1 as a function of time for C8) on a ZnSe substrate for a range of O3 concentrations. Clearly, the final concentration of SOZ formed actually varies from experiment to experiment. This is not expected if the amount of organic coating is constant. Furthermore, the final concentration of SOZ formed does not show a consistent trend with the ozone concentration. This suggests either that the yield of SOZ, i.e. amount of SOZ formed per alkene reacted, is not

Figure 5. SOZ absorbance at 1110 cm-1 plotted versus time for oxidation of C8) SAM-coated ZnSe substrates at varying O3 concentrations. Green is for [O3] ) 2.9 × 1013 cm-3; pink, [O3] ) 2.8 × 1013 cm-3; black, [O3] ) 4.1 × 1012 cm-3; orange, [O3] ) 3.4 × 1012 cm-3; blue, [O3] ) 5.2 × 1013 cm-3.

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constant or that the amount of SAM initially on the ZnSe ATR crystals is variable, despite the use of a consistent method to coat the crystal. The ratio of the SOZ absorbance at 1110 cm-1 to that of a peak due to loss of CdCH2, e.g. at 3077, 1641, or 910 cm-1 respectively, is a measure of the SOZ yield. These ratios were constant within experimental error, ruling out varying yields as the source of different amounts of SOZ at the end of the reaction. Changes in the reactant SAM peaks upon ozonolysis indicate that the variability in the SAM coating on the uncoated ZnSe and SiOx-coated ZnSe crystals is actually responsible for the formation of different amounts of SOZ. (Measuring changes upon reaction is more reliable than using the absolute values of these peaks on the unreacted SAM because of changes in the baseline on repositioning the ATR crystal after coating and due to small and variable contributions from organic contaminants to the spectrum). The average change in the absorbance of the CdC band at 1641 cm-1 on SiOx-ZnSe due to oxidation is (6.4 ( 4.8) × 10-4 (2σ), and on ZnSe, it is (5.6 ( 1.7) × 10-4 (2σ). However, the average change in the same peak (data not shown) due to oxidation on a Si ATR is (2.2 ( 0.4) × 10-4 (2σ). Clearly, on Si the average amount of organic is smaller, and in addition, there is less variability from one coating to another. However, measuring the absolute amounts of SAMs on ZnSe and the thickness of each coating was not possible in these studies. There are relatively few studies in the literature regarding SAMs binding to ZnSe surfaces. Nuzzo and co-workers reported successful alkanethiol SAM formation on ZnSe surfaces,46 and a few studies reported the formation of SAMs on ZnSe using octadecyltrichlorosilane (C18) monolayers.45,67 However, there may be different attachment chemistry between the trichlorosilane SAM precursor molecules and the different ATR crystal surfaces. While one would expect the SiOx-ZnSe and silica to have similar density packing, since the surface layer on both is SiOx, the absorbance data cited above show this is not the case. This suggests the SiOx coating on the surface of the ZnSe crystal is not the same as the native oxide layer on the Si crystal and, thus, may not form SAMs that pack in a similar fashion. On the ZnSe surface, it is unclear whether zinc oxide, selenium oxide, or some combination of the two populates the crystal surface. The oxide species at the surface may influence the number of binding sites available for the SAM and will also influence its density and structure. Another potentially contributing factor to the increased organic signal on the ZnSe surfaces when compared to the Si surface is multilayer formation. Multilayer patches of C18 were reported on a 3-(mercaptopropyl)trimethoxysilane layer which binds to Ag surfaces and then allows for attachment of an added C18 to this mercapto film.68 A similar phenomenon may be occurring on the ZnSe surfaces where the formation of multilayers may cause more organic material to be present and hence produce a larger net absorbance for the reactant SAM peaks. This may also impact the kinetics, since some of the double bonds may not be accessible. In principle, the time dependence of the 1110 cm-1 peak could be used to extract reaction kinetics as was carried out earlier for SAM oxidation on a silicon ATR crystal using the CdO peak.35 Conversion of the absorbance at 1110 cm-1 to an SOZ surface density requires making a number of assumptions regarding the initial SAM surface density and the fraction that is converted to SOZ. However, the variability in the initial SAM coatings, the possibility of multilayer SAMs, and lack of quantitative absorption cross sections for the SOZ and carbonyl

McIntire et al. products discussed above preclude meaningful detailed kinetics analysis. It is noted that the initial rate of formation of the 1110 cm-1 peak increased with the O3 concentration out to ∼1 × 1014 cm-3, the highest concentration for which multiple data points could be collected before the SOZ plateaued. In a previous study,35 an inverse dependence of the rate of CdO formation on [O3] was observed above ∼1013 cm-3 for C3) and C8) terminal alkene SAMs on silicon ATR crystals, consistent with a Langmuir-Hinshelwood (L-H) mechanism. In the present study, there was no clear decrease in the rate of formation of the 1110 cm-1 peak at [O3] > 1 × 1013 cm-3, suggesting that a L-H mechanism may not apply to formation of the major SOZ product. The previously observed35 inverse dependence of the rate of CdO formation on [O3] above ∼1013 cm-3 may arise from mechanistic complexities in the formation of carbonyls rather than from adsorption of ozone on the surface prior to reaction, as assumed in the L-H mechanism. This remains to be explored in more detail. Stokes et al.36 used vibrational sum frequency generation spectroscopy (SFG) to follow the ozonolysis of 1-pentene and some cyclic alkenes attached to a silica surface via p-aniline silane linkers.40 They report formation of a new band at 2945 cm-1 during the reaction and assign this to a -CH3 group formed by decomposition of the surface Criegee intermediate, giving CO2 and a methyl-terminated surface organic. In the present studies, the corrected spectrum in this region (Figure 3) shows two bands at 2920 and 2860 cm-1, respectively. As discussed earlier, these peaks are consistent with OC-H stretches in ethers and ozonides, and they track the SOZ band at 1110 cm-1 during the reaction. For comparison, spectra of the saturated C8 or C18 SAMs on ZnSe ATR crystals in our system show a peak due to -CH3 at 2957 cm-1, significantly shifted from the new peaks formed during the C8) SAM oxidation. In the case of the terminal alkene C8) SAM studied here, decomposition of the surface-bound Criegee intermediate to form a methyl-terminated SAM does not appear to be a major reaction pathway. IV. Conclusions The use of ZnSe ATR crystals to follow the ozonolysis of C8) SAM has revealed the formation of a secondary ozonide as a major oxidation product that was undetectable in previous studies using a silicon crystal which does not transmit below 1500 cm-1. The formation of secondary ozonides along with carbonyls on airborne dust particles in the atmosphere is interesting because ozonides are known to cause oxidative damage to the body through production of cytotoxic free radicals.69-73 It is possible in dry polluted atmospheres that ozone-alkene reactions result in the formation of stable secondary ozonides which may be more widespread than previously assumed and that their chemistry, photochemistry, and health effects are potentially important. Acknowledgment. This work was funded by the National Science Foundation through an Environmental Molecular Science Institute Grant (CHE-0431312). The authors are grateful to the glassblower Mr. Jo¨rg C. Meyer, and the machinists Ron Hulme, Lee Moritz, and Chris Peterson in the Physical Sciences Machine shop for technical assistance. We thank Prof. Mark Bachman and his group at the UCI Integrated Nanosystems Research Facility (INRF) for assistance with coating the ATR crystals with SiOx. T.M.M. would like to thank Dr. Ronald L. Grimm for valuable discussions and Mr. Keith Matthews of Crystran Ltd., Dorset, U.K., for donation of some silicon ATR crystals used for some of this work.

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