Insights into Heterogeneous Atmospheric Oxidation Chemistry

J. Phys. Chem. C 2007, 111, 1567-1578

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FEATURE ARTICLE Insights into Heterogeneous Atmospheric Oxidation Chemistry: Development of a Tailor-Made Synthetic Model for Studying Tropospheric Surface Chemistry Andrea B. Voges, Grace Y. Stokes, Julianne M. Gibbs-Davis, Robert B. Lettan II, Paul A. Bertin, Rachel C. Pike, SonBinh T. Nguyen, Karl A. Scheidt, and Franz M. Geiger* Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 ReceiVed: August 15, 2006; In Final Form: NoVember 1, 2006

The synthesis, characterization, and oxidation reaction of a tropospherically relevant terpene bound to a glass surface are reported. Vibrational broadband sum frequency generation (SFG) is used to characterize the various terpene-modified glass surfaces and track their interaction with ozone. SFG spectra indicate that, although orientations of the surface-bound terpenes depend on the linker strategies employed, the CdC double bond is accessible to gas-phase ozone regardless of the strategy applied. Exposure of the terpene-functionalized surface to ppm levels of ozone at 1 atm and 300 K yields an initial reaction probability of approximately 1 × 10-5 per surface collision, which is significantly higher than the corresponding gas-phase reaction involving 1-methyl-1-cyclohexene (5 × 10-7 from gas-phase collision theory). The interaction of ozone with a saturated octyl silane-functionalized glass surface leads to a slight molecular reorientation, or tilting, of the terminal CH3 groups on a much slower time scale. Our work demonstrates that SFG spectroscopy can be used to determine reaction probabilities of heterogeneous atmospheric reactions and bridges the gap between atmospheric chemistry and surface functionalization.

I. Introduction Continental biogenic emissions of volatile organic compounds are estimated to exceed those due to anthropogenic activities by a factor of 5.1 The largest class of nonanthropogenic biogenic emissions consists of terpenes,1 a diverse group of compounds produced by plants. As terpenes contain isoprene units and are often doubly unsaturated,2 their gas-phase oxidation by tropospheric ozone3 can lead to partially oxidized unsaturated hydrocarbons. The resulting partially oxidized molecules contain polar functional groups that can interact with the surfaces of atmospheric particulate matter, such as aqueous and mineral dust aerosols. Therefore, it is not surprising that the chemical analysis of such aerosols reveals the presence of partially oxidized organic compounds.4,5 Hydrophobic portions of these surface-bound organic species, including their remaining CdC double bonds, would be expected to orient toward the gas phase during the formation of the organic adlayer. Gill et al.6 and Ellison et al.7 proposed that reactions can occur on aerosol surfaces that involve the heterogeneous oxidation of CdC double bonds by gas-phase oxidants such as ozone or OH radicals to form polar products on the surface. The resulting polarity switch is important in cloud microphysics, as it can lead to the formation of cloud condensation nuclei and control the radiative properties of atmospheric particulate matter, including the extent to which they scatter light.8 In addition, the chemical composition of the troposphere may be altered as a result of the gas-phase species formed during heterogeneous * To whom correspondence should be addressed. E-mail: geigerf@ chem.northwestern.edu.

oxidation processes. Quantifying such chemical changes in the troposphere and its radiative budget is essential for accurately assessing and evaluating climate change.9 Due to the importance of ozone oxidation reactions in the gas phase,3,10-13 heterogeneous reactions between ozone and unsaturated organic compounds have been proposed to play a major role in tropospheric chemistry and are now the subject of intense research.6,7,14-32 A common goal is to obtain reaction probabilities,33 which are unitless values that indicate the fraction of reactant collisions that result in a reaction. These parameters can then be incorporated in climate models, the majority of which currently consider only gas-phase reactions. 34 For reference, reaction probabilities above 0.1 have been reported for highly reactive species, such as the interaction of OH radical with vinyl-terminated surface species,35 whereas reaction probabilities around 10-8 to 10-9 reflect very slow reaction kinetics.36,37 Oleic acid and related compounds have commonly been used in laboratory studies that model the heterogeneous oxidation of alkenes by ozone in the atmosphere.24-30 Using a coated flow reactor coupled to a chemical ionization mass spectrometer, Moise and Rudich found a reactive uptake coefficient of (8.3 ( 0.2) × 10-4 for liquid oleic acid.24 When the acid was frozen, limiting any potential reaction to the surface layer, the reactive uptake coefficient decreased to (5.2 ( 0.1) × 10-5. Based on aerosol mass spectrometry experiments, Morris et al. determined that the half-life of submicron oleic acid aerosol particles should be on the order of minutes in environments containing 100 ppb of ozone, which is in contrast to field measurement that suggest a half-life on the order of days.30 Finlayson-Pitts and co-workers

10.1021/jp065277l CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007

1568 J. Phys. Chem. C, Vol. 111, No. 4, 2007 Andrea B. Voges is a native of Detroit, MI, and received her Chemistry B.S. from Valparaiso University, IN. She joined Franz Geiger’s research group in 2001 and won a NASA Earth System Science Predoctoral Fellowship in 2002. After defending her Ph.D. in chemistry in 2006, Andrea joined the Rohm and Haas Chemical Company as Senior Scientist near Philadelphia, PA. Her research interests include atmospheric chemistry as well as the search for practical solutions to a wide variety of environmental problems. Grace Y. Stokes was born in Taiwan and obtained her Chemistry B.S. from Stanford University after which she joined Franz Geiger’s research group in 2004. Grace received a NASA Earth System Science Predoctoral Fellowship in 2006 and expects to complete her Ph.D. in 2009. Her current research involves studying both biological and environmental interfaces using nonlinear optics. Julianne M. Gibbs-Davis grew up in Flagstaff, AZ, and received her Chemistry B.A. from Arizona State University, AZ. Juli received her Chemistry Ph.D. with SonBinh T. Nguyen at Northwestern University in 2005 for her work on polymer-DNA hybrid materials and biomolecule detection. In 2006, she joined the Geiger group as a Camille and Henry Dreyfus Environmental Chemistry Postdoctoral Fellow. Robert B. Lettan is a native of Rochester, NY, and received his Chemistry B.S. from Otterbein College, Columbus, OH, after which he began his Ph.D. work with Karl Scheidt at Northwestern University. Rob is working on the Lewis base activation of silylalkynes and is developing novel synthetic methods using acylsilanes. In 2006, he was awarded the Edmund W. Gelewitz award for research and service associated with the Northwestern University Chemistry Department and the surrounding community.

studied the reaction of phosphocholines, a class of unsaturated compounds that are important in biological systems and oceanatmosphere coupling, with ozone (0.3 and 30 ppm) at macroscopically flat air-water interfaces. The reaction probability was estimated to be approximately 4 × 10-6.32 Silane chemistry has also been utilized to model the heterogeneous oxidation of alkenes. Finlayson-Pitts and co-workers studied the oxidation of silicon-supported CdC double bondterminated silanes by ozone using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) at 296 K.17 Similar to the reflection-absorption infrared spectroscopy studies by Fiegland et al.,18 infrared signatures of the terminal vinyl groups decreased upon exposure to ozone, and carbonyl signatures appeared. Lifetimes for the surface-bound organic reactants were found to be approximately 20-30 times shorter than those of the corresponding gas-phase olefins, and the authors identified that the reaction followed a LangmuirHinshelwood mechanism. In addition, reaction probabilities varied between 10-6 and 10-5 for ozone concentrations below approximately 4 ppm. Using a similar model system, Moise and Rudich monitored the ozonation of surfaces functionalized with aliphatic and vinylic groups.20 At ozone concentrations between 0.04 and 4 ppb, vinylic adlayers were reported to display reactive uptake coefficients between 1 × 10-4 and 3 × 10-4, whereas uncoated glass slides and glass slides functionalized with aliphatic compounds were found to be much less reactive with coefficients less than 3 × 10-6. Initial reaction probabilities of 7 × 10-5 were reported by Grassian and coworkers, who used a Knudsen cell to monitor the reaction of ozone with SiO2 particles functionalized with either octenyltrichlorosilane or octyltrichlorosilane.19 Although solid-state 13C NMR showed a decrease in the number density of unsaturated surface species, oxygenated product species were not observed. Further work by Thomas et al. identified CH2O, CO, and CO2 as gas-phase products,21 and ATR-FTIR studies showed an intensity decrease for the vinylic vibrational modes. Following

Voges et al. Paul A. Bertin was born in Youngstown, OH, and earned his B.A. and B.S. degrees at Miami University, OH. He received his Chemistry Ph.D. with SonBinh T. Nguyen at Northwestern University in 2005 for his work on the development of polymer-based nanostructures for the targeted delivery of bioactive agents from ring-opening metathesis polymerization. Paul is currently a Principal Scientist at Ohmx Corporation with broad interests in hybrid organic/inorganic materials and electron transfer processes.

Rachel C. Pike is a native of San Francisco, CA, and received her chemistry B.S. from Northwestern University, where she worked in Franz Geiger’s laboratory. Rachel is currently a Gates Cambridge Scholar and a Ph.D. candidate at the University of Cambridge Centre for Atmospheric Science. She is working with Professor John Pyle to examine the effects of biofuel use on atmospheric chemistry and climate. Karl A. Scheidt received his undergraduate degree from the University of Notre Dame where he began his research career in the laboratories of Professor Marvin J. Miller. His graduate studies were conducted at Indiana University and the University of Michigan under the direction of Professor William R. Roush. He was a National Institutes of Health Postdoctoral Fellow in the laboratory of Professor David Evans at Harvard University and started his independent career at Northwestern in 2002. His research interests include catalysis using organic molecules, total synthesis of bioactive compounds, and the integration of organic chemistry with biology and materials science. Karl is the recipient of a National Science Foundation CAREER Award, new investigator awards from 3M, Abbott, and Boehringer-Ingelheim, and a Northwestern University Distinguished Teaching Award. SonBinh T. Nguyen grew up in Vietnam and graduated in 1990 with dual bachelor degrees in Chemistry and Physics from Penn State University. He carried out undergraduate research with Gregory L. Geoffroy at Penn State as well as in Henry Bryndza and Steve Ittel’s groups at duPont Central Research. SonBinh received a doctoral degree in Chemistry in 1994 under Profs. Robert H. Grubbs and Nathan S. Lewis at Caltech. In 1996, after an NSF postdoctoral fellowship with Prof. K. Barry Sharpless at The Scripps Research Institute, SonBinh began his independent career at Northwestern University. He is now the Director of the Integrated Science Program and the McCormick Professor of Teaching Excellence. Franz Geiger is a native of Berlin, Germany, who received his Vordiplom in Physical Chemistry from the Technische Universita¨t Berlin. After his Ph.D. with Janice Hicks at Georgetown University, where he was a NASA Predoctoral Fellow in Earth Systems Sciences, and a NOAA Climate and Global Change Postdoctoral Fellowship at MIT with Mario Molina, he joined the faculty of Northwestern University in 2001 and was promoted to Associate Professor with tenure in 2006. He currently holds the Dow Research Chair in Northwestern Department of Chemistry.

oxidation, the spectra showed product species containing terminal methyl, carbonyl, and hydrogen-bonded carbonyl groups. Although the aforementioned studies show efficient interactions between ozone and atmospherically relevant unsaturated compounds, they are limited to a group of commercially available molecules that do not reflect the wide range of compounds and functionalities comprising continental biogenic emissions. In this work, we expand the scope of tropospheric heterogeneous organic oxidation models to include the surface chemistry of tailor-made unsaturated substrates. Specifically, we focus on the synthesis, characterization, and interaction of ozone with glass slides that have been functionalized with tropospherically relevant terpenes. Glass is used to model SiO2, a common component in mineral dust.19 Because the Kelvin effect1,38 (i.e., the increase of surface tension with decreasing particle radius) is negligible for particle sizes above several tens of nanometers, macroscopically flat substrates are expected to

Feature Article SCHEME 1: Surface Functionalization with Atmospherically Relevant Molecules

be suitable for studying tropospheric chemistry. We avoid the competition between the ozonation reactions and the desorption processes involving the surface bound reactants, which have appreciable vapor pressures, by chemically attaching menthenol, an atmospherically relevant natural cyclic terpene,1,39 to the glass surfaces via robust silane linker chemistry. Furthermore, the ozonolysis of a cyclic terpene should result in ring cleavage and thus avoid the loss of product species into the gas phase, facilitating chemical balancing. In this work, we employed both electrophilic and nucleophilic linker chemistries to afford a broad reaction scope in the attachment step. The chemical versatility made possible through these methodologies enables us to study the heterogeneous oxidation of a plethora of organic molecules that closely mimic those present in the troposphere, thus extending the scope of tropospheric heterogeneous organic oxidation models studied by others. Our tailor-made surfaces are characterized using vibrational sum frequency generation spectroscopy. We track the interaction of surface-bound species with ozone in real time, determine reaction probabilities, and address the molecular origin of the enhanced surface reactivity observed in heterogeneous organic oxidation reactions that involve ozone. II. Experimental Section A. Synthetic Strategies. We aim to provide versatile methods for linking a wide variety of atmospherically relevant compounds to SiO2-based substrates (Scheme 1). To this end, we pursued two strategies using (1) an electrophilic linker adlayer that can be attacked by a nucleophile and (2) a nucleophilic linker adlayer that can then attack solution-phase electrophiles. The first strategy was accomplished by functionalizing the glass surfaces using a succinimidyl ester-terminated silane that was then coupled to menthenol. This approach was compared to the synthesis of menthenol-functionalized trichlorosilane, followed by direct surface deposition without coupling to a linker molecule. For the second strategy, we functionalized the glass surfaces using amine-terminated silanes that were then linked to menthenoyl chloride. The Supporting Information contains descriptions of the synthesis and surface functionalization methods, schematics visually summarizing the synthetic pathways, and results from complementary ellipsometry, XPS, SHG, and contact angle measurements indicating high surface coverages and conversion efficiencies. B. Ozone Generation. For the oxidation studies, ozone was generated online by flowing oxygen from a medical grade oxygen tank (Airgas) through a Gilmont analog flow meter (range 0.5 to 22 L/min) and then past an Hg pen ray lamp (VWR, #74670-008) housed in a custom-built glass casing. The resulting ozone/oxygen mixture was then directed toward the substrate surface via a Teflon directed doser. Ozone exited the chamber and then passed through a custom-built 10 cm quartz gas flow cell. This allowed for the direct measurement of the gas-phase ozone concentrations by monitoring the absorbance at 254 nm with a UV-vis spectrometer (Ocean

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1569 Optics, 185-850 nm). Ozone concentrations up to 35 ppm were employed in this work. C. Sum Frequency Generation. Detailed descriptions of the theoretical and experimental aspects of sum frequency generation (SFG) are available elsewhere;40-50 therefore, the following is meant to serve only as a brief summary of the most significant points that are relevant to this work. Two optical fields of sufficient intensity can be coupled in a noncentrosymmetric medium, such as an interface, in order to produce a new optical field oscillating at the sum of the two incident light field frequencies. This sum frequency signal is specific to the noncentrosymmetric environment and consequently can yield surface information without contributions from centrosymmetric bulk phases, such as air and glass. Resonantly enhanced SFG can be used in the infrared frequency region to probe vibrational modes of interfaces and their adsorbates, lending molecular specificity to this inherently surface selective technique. The intensity of the SFG signal, ISFG, is directly proportional to the square modulus of the second-order susceptibility of the interface χ(2), and the intensity of the input visible and IR electric fields, IVis and IIR, respectively, as shown in eq 1

ISFG ∝ |χ(2)|2IVisIIR

(1)

χ(2) is the sum of a nonresonant contribution and a resonant (2) contribution, χ(2) NR and χRv. For hydrocarbons on insulator surfaces, the nonresonant contributions to the SFG signal are generally small.51 For a given vibrational mode, χ(2) Rv is proportional to the number of molecules at the interface, Nads, and a resonance term that is orientationally averaged

χ(2) Rv ∝ Nads

〈

Aν,ijMν,k ωIR - ων + iΓν

〉

(2)

For a given vibrational mode ν, Aν is the Raman transition probability, Mν is the infrared transition dipole moment, i, j, and k refer to the surface coordinate system, ων is the infrared frequency of the vibrational mode in the surface-bound species, ωIR is the frequency of the infrared probe light, and Γν is the damping coefficient that avoids singularities in eq 2 and can be used to describe the natural line width of the infrared resonance. χ(2) Rv increases when the frequency of the incoming IR beam matches a vibrational transition of the adsorbate or interface, thus leading to resonant enhancement of the SFG signal. IR fields that are broad in the frequency domain allow for the collection of SFG spectra within a single laser pulse. Referred to as broadband sum frequency generation,52,53 this technique is used in the current work, can lead to shorter collection times compared to traditional scanning SFG experiments and can allow for real time monitoring of multiple vibrational modes. The SFG electromagnetic field, ESFG, is directly proportional to the number of adsorbates

ESFG ∝ xISFG ∝ Nads

(3)

Thus eq 3 can be used to provide a real time measure of the relative surface population of a given vibrational mode at a particular orientation, allowing reaction kinetics to be determined. D. SFG Experiments. The optical setup has been described previously.54 To summarize, the current studies were carried out using an 800 nm, 120 fs regeneratively amplified Ti:sapphire system (Hurricane, Spectra Physics/Newport Corp.). The Hur-

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Figure 1. Diagram of optical setup and ozone flow chamber.

ricane system pumps an optical parametric amplifier (OPA800CF, Spectra Physics/Newport Corp.) to produce IR light around 3.4 µm with a bandwidth (full width at half-maximum) of 140 cm-1. The energies of the incident infrared and visible light fields were measured using an energy meter (EPM10000110L99, Molectron) and ranged between 10 and 20 µJ for the infrared and 25 and 35 µJ for the visible light fields before spectral narrowing. Both beams were focused onto a sample stage after which the SFG signal was collected for detection with a 0.5-m spectrograph (Acton Research) and CCD camera (Roper Scientific). The polarization combinations used in this work were ssp and sps (polarization of the SFG, visible, and IR light fields respectively). By definition, s polarization refers to light polarized in the surface plane and p polarization refers to light polarized perpendicular to the surface plane. The ssp polarization combination probes modes with IR transition moment components perpendicular to the surface plane, whereas the sps combination probes modes with IR transition moment components in the plane of the interface.46 The spectrum of the directdeposited ester terpene and all of the sps SFG spectra were collected with a slightly different optical alignment that included an IR waveplate (L/2 CdGaS4 3.1 µm CA 10 mm, Altechna Co. Ltd.). Despite these changes, the SFG spectra of the compounds studied in this work were consistent. Other additions to the previously published setup include a custom-built Teflon chamber encasing the sample stage that allowed for gas flow studies at room temperature and ambient pressure conditions (Figure 1). The chamber was built to include the IR and visible input lens and recollimating output lens as chamber windows to minimize the use of optics and thus optimize delivery of the input light fields and outgoing SFG signal. The lenses were attached to the chamber via custom-designed chemically inert Hypalon bellows (A&A Manufacturing Co. Inc.), allowing for focus adjustment. In order to access the entire spectral region of interest (32002800 cm-1) a hybrid tuning-scanning broadband method was adopted. Following the work of Esenturk and Walker,51 broadband SFG spectra were recorded with four different input IR center frequencies. This ensured that all vibrational modes in this frequency region received approximately the same infrared power when considering the final spectrum. The center frequencies were approximately ∼100 cm-1 apart, ranging from 2850 to 3150 cm-1. Raw and background spectra were acquired at each center frequency, with acquisition times of 1-2 min

Voges et al. per spectrum. Background spectra were acquired by blocking the IR input, thus accounting for optical scatter from the 800 nm upconverter. Averaged SFG spectra were obtained by first subtracting one background spectrum from one raw spectrum and then averaging between five and ten of these processed spectra. After normalizing to the energy of the incident visible and infrared light fields, the individual broadband SFG spectra were summed directly to yield a final SFG spectrum from 2800 to 3200 cm-1. An example of this approach is demonstrated in Figure 2, in which the four individual spectra (A) and the summed spectrum (B) of an eight-carbon alkane bound to a glass substrate are shown. The alkane organic adlayer was produced using octyltricholorsilane (OTS) by following previously published surface functionalization techniques.54 For comparison, the SFG spectrum of a non-functionalized glass slide is shown as well. The dark black lines shown in Figure 2B are the fit result of a custom-written Igor Pro software procedure that convolutes Lorentzians in accordance with eq 2. This procedure allows for vibrational modes to be either in or out of phase, i.e., 0° or 180°, and includes a term to account for the relatively small nonresonant contribution. As in earlier work,54 all spectra are referenced to the 2955 cm-1 C-H symmetric stretch of the methoxy groups in PMMA. In general, spectral features between 2850 and 2900 cm-1 are associated with symmetric stretches of CH3 and CH2 groups of straight chain aliphatic compounds.52,54-70 Spectral features between 2900 and 3000 cm-1 are associated with asymmetric and Fermi resonance modes.52,54-70 Spectra in this work were assigned in accordance with literature values and take into account that the asymmetric stretches are out of phase with respect to the symmetric stretches and Fermi resonances.63 Using spectral fitting procedures and by comparison to literature values, we have made the following assignments for the OTS-functionalized surface. The modes at 2860 and 2881 cm-1 are assigned to the CH2 symmetric stretch and CH3 symmetric stretch, respectively. Phase relations indicate the peak at 2909 cm-1 results from the CH2 Fermi resonance. The mode at 2953 cm-1 is assigned to the CH3 asymmetric stretch. Last, the feature at 3070 cm-1 is assigned to the beginning of the OH stretching band, indicating the presence of small amounts of water adsorbed at the air/glass interface. Atmospheric water vapor in our unpurged SFG line absorbs the incident IR light field beginning near 3100 cm-1 and limits our ability to fully observe OH stretching modes at this time. These assignments differ slightly from those published in our earlier work,54 reflecting our system improvements that allow for better resolution. The SFG spectrum of a plasma-cleaned bare glass slide with no silane present (Figure 2, bottom trace) also displays a broad feature centered at 3067 cm-1, indicating again the presence of small amounts of adsorbed water. This broad spectral feature above 3000 cm-1 was weakly present in many of the spectra and in general is not discussed in the remainder of this work but included in fits when present in the spectra. III. Results A. Succinimide Linker. N-hydroxy succinimidyl esters are excellent acylating agents71 and have been used in the synthesis of highly functional electrophilic surfaces.72,73 Using SFG, the surface vibrational spectra of glass slides functionalized with the succinimide linker were recorded first and analyzed as follows. Fitting the uncoupled succinimide linker ssp SFG spectrum (Figure 3A) indicates the presence of three vibrational modes. The peak at 2861 cm-1 is assigned to a CH2 symmetric

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Figure 2. (A) Individual SFG spectra of eight carbon adlayer. IR center frequencies were determined from gold SFG spectra and are listed on the left. (B) Summed ssp SFG spectrum of eight-carbon adlayer (top trace) with spectral fit shown as a black line through the data. Individual modes are shown as gray subpeaks with out-of-phase modes with respect to the nonresonant background are represented with a dashed line. This holds for Figures 3-8. Bottom trace is the summed ssp SFG spectrum of an unfunctionalized glass slide.

Figure 3. SFG spectrum of succinimidyl linker with ssp (A) and sps (B) polarization combinations.

stretch. The higher energy feature is fit with two vibrational modes that are assigned to a CH2 Fermi resonance at 2930 cm-1 and a CH2 asymmetric stretch at 2958 cm-1 (out of phase with respect to the symmetric and Fermi resonance modes). The energy of this latter mode is very high for methylene asymmetric stretch modes of straight-chain hydrocarbons but is consistent with the ethylene moiety of ethylene glycol adsorbed to silica.62 Furthermore, this high frequency reflects the mixing of sp2 character into the sp3 hybridization state of the carbon atoms in the succinimide linker, suggesting that the 2958 cm-1 mode originates predominantly from the cis-configured ethylene moiety within the succinimide group. The presence of the symmetric and the asymmetric CH2 stretching modes from the succinimide moiety in the ssp spectrum suggests a significant tilt of the succinimide moiety away from the surface normal. In general, SFG spectra recorded with the sps polarization combination display a far lower intensity when compared to the ones recorded with the ssp polarization combination.64 In addition, symmetric CH2 stretch modes have been proposed to be absent in sps spectra of straight-chain alcohols, but it was noted that rotational barriers around the methylene C2 symmetry axis may lead to small SFG signal intensities from symmetric CH2 stretching modes in the sps spectra.64 When probing the succinimide linker with the sps polarization combination (Figure 3B), the SFG spectrum displays peaks at 2842 and 2896 cm-1,

which are assigned to the CH2 symmetric stretch mode and the CH2 Fermi resonance. In order to ensure that the succinimide moiety does not convolute information from the terpene-functionalized surfaces, the final synthetic step for formation of the terpene adlayers is hydrolysis. This should remove any remaining succinimide groups. Upon hydrolysis of the succinimide linker itself (Figure 4), the ssp SFG spectrum of the adlayer shows a CH2 symmetric stretch mode at 2859 cm-1. Both the ssp and the sps SFG spectra display a CH2 Fermi resonance of the alkyl chain at 2920 cm-1. A CH2 Fermi resonance generally suggests the presence of an active CH2 symmetric stretching mode, but its presence cannot be resolved with the current signal-to-noise level of our sps spectrum. These assignments, along with the assignments of the other compounds under investigation in this work, are listed in Table 1. B. Ester-Linked Terpene. After coupling menthenol to the N-hydroxysuccinimidyl ester-functionalized substrates, the ssp SFG spectrum (Figure 5A) displays a new peak at 2880 cm-1, which is characteristic of CH3 symmetric stretching modes. The CH2 symmetric stretching mode is observed as a distinct peak at 2850 cm-1. The symmetric stretch modes assigned in this work are in agreement with those reported by Belkin et al. for neat limonene.65 In our ester-linked terpene, which contains one additional CH2 and CH group as compared to limonene, we

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Figure 4. SFG spectrum of hydrolyzed succinimide linker with ssp (A) and sps (B) polarization combinations.

TABLE 1: Spectral Assignments of Terpene Adlayersa,b

Entries given in wavenumbers (cm-1). b Entries in italics from sps spectra, nonitalic entries from ssp spectra. † Assigned to the terpene moiety and the linker backbone, respectively. a

use phase relations to assign the spectral feature at 2929 cm-1 to the CH2 Fermi resonance. The sps spectrum (Figure 5B) displays three spectral features in the CH stretching region. Two of these features can be assigned to the CH2 symmetric stretch (2840 cm-1) and to the CH2 Fermi resonance (2892 cm-1). These two resonances are similar to those identified as the ethylene moiety in the succinimide ring (vide supra) and are consistent with the terpene having a strained ring system. The third mode, located at 2951 cm-1, is assigned to the collective asymmetric stretch of the two CH3 groups in the terpene. C. Direct-Deposited Terpene Ester. For comparison with the two-step method of synthesizing the ester-linked terpenemodified surface via the N-hydroxysuccinimidyl ester linker, a menthenyl ester bearing a trichlorosilyl group that would be directly reactive with glass was synthesized (see the Supporting Information). For this synthesis, selective hydrosilylation conditions had to be determined that would not react with the substituted double bond within the terpene ring. Chloroplatinic acid, the commonly used catalyst for hydrosilylations, reacted with both CdC moieties on the menthenyl undecenoate. However, a heterogeneous Pt/C catalyst led to selective hydrosilylation of the terminal double bond. After deposition on

glass, this directly deposited terpene ester and the ester-linked terpene (generated using the succinimidyl linker) are chemically identical and thus suitable for comparison. Overall, the challenge of selective hydrosilylation of a doubly unsaturated compound emphasized the need for a more versatile linker strategy for generating a wide variety of functional adlayers. The ssp SFG spectrum (Figure 6A) results in assignments for the directly deposited terpene ester that are consistent with those of the ester-linked terpene. The feature at 2852 cm-1 is assigned to the CH-2 symmetric stretch with a weak shoulder at 2883 cm-1 corresponding to the CH3 symmetric stretch. The mode at 2922 cm-1 is identified as the CH2 Fermi resonance and an out of phase CH3 asymmetric mode at 2950 cm-1 is assigned using spectral fitting. The sps SFG spectrum (Figure 6B) shows a CH2 symmetric stretching mode at 2845 cm-1 and a CH2 Fermi resonance at 2897 cm-1. D. Amine Linker. Aminopropyltriethoxysilane was used to synthesize the nucleophilic linker adlayer for the second linkerbased surface functionalization strategy. The ssp SFG spectrum of the amine-functionalized glass substrates prior to terpene functionalization (Figure 7A) shows two distinct features at 2882 and 2929 cm-1. SFG spectra of diamines have been reported to display a set of CH2 symmetric stretching modes at approximately 2850 and 2880 cm-1.61 In contrast to these ethylene diamines, the amine-functionalized surfaces studied in this work possess only one amino group and a propyl instead of an ethyl chain. In addition, they are chemically tethered to the surface using silane chemistry. Spectral fitting leads us to assign the 2882 cm-1 mode of our amine-functionalized substrates to a CH2 symmetric stretch and the 2929 cm-1 mode to a CH2 Fermi resonance. The higher energy feature could be modulated by spectral interference from higher energy vibrations of coadsorbed water or the amine symmetric stretch, but spectral fitting solutions including these possibilities resulted in unreasonably high mode amplitudes and were thus rejected. The sps SFG spectrum (Figure 7B) of the amine adlayer shows the appearance of a broad and weak CH2 symmetric stretching mode centered around 2881 cm-1. Broad spectral features are consistent with the presence of disorder within the adlayer, which is attributed to a high likelihood of gauche defects along the short carbon chain (only three methylene groups).74,75 As a consequence, other possible modes are not well resolved in the spectrum. It should be noted that the spectral feature above 3000 cm-1 could contain contributions from the NH stretching

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Figure 5. SFG spectrum of ester-linked terpene with ssp (A) and sps (B) polarization combinations. Shading denotes CH3 modes.

Figure 6. SFG spectrum of the directly deposited terpene ester with ssp (A) and sps (B) polarization combinations.

Figure 7. SFG spectrum of amine linker with ssp (A) and sps (B) polarization combinations.

mode in addition to the OH stretching modes from surface adsorbed water molecules. E. Amide-Linked Terpene. The coupling of acid chlorides to amine-terminated surfaces has been shown to be a successful strategy for functionalizing surfaces.76 On the basis of this approach, we synthesized menthenoyl chloride, an acid chloride derivative of menthenol, and reacted it with the nucleophilic amine-functionalized adlayer to yield a surface modified with amide-linked terpenes. Spectral fitting of the ssp SFG spectrum of the amide-linked terpene (Figure 8A) confirms the presence of a new a spectral feature near 2961 cm-1, which indicates

the presence of the CH3 asymmetric stretch upon linking the terpene to the amine adlayer. In addition, the intensity of the SFG signal recorded in the NH and OH stretching frequency (>3000 cm-1) appears higher than in the SFG spectra of the other surfaces presented in this work. This increased signal intensity could be indicative of a lateral hydrogen bond network among amide groups, similar to what is found in peptide sheets.77 The remainder of the spectrum displays vibrational modes similar to those of the amine linker, namely a 2928 cm-1 CH2 Fermi resonance and a 2884 cm-1 CH2 symmetric

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Figure 8. SFG spectrum of amide-linked terpene with ssp (A) and sps (B) polarization combinations. Shading denotes CH3 modes.

stretching mode, which can be attributed to the propyl chain linking the amide to the glass substrate. The sps SFG spectrum (Figure 8B) displays four features in the CH stretching region: a CH2 symmetric stretching mode at 2848 cm-1, a CH2 symmetric stretching mode at 2874 cm-1, a CH2 Fermi resonance located at 2899 cm-1, and a CH3 asymmetric stretching mode at 2962 cm-1. The peak at 2874 cm-1 is high enough in frequency such that it could also indicate the presence of a symmetric CH3 stretching mode. However, this would indicate an unlikely disappearance of the propyl linker CH2 symmetric stretch. It is also possible to fit the spectrum with a single mode under the lower energy feature, namely a CH3 symmetric stretching mode at 2863 cm-1. However, a CH2 Fermi resonance generally suggests the presence of an active CH2 symmetric stretching mode, leading us to fit the broad lower energy feature with two peaks at 2848 and 2874 cm-1. A definitive assignment is limited by the low signal-to-noise. IV. Discussion The two linker strategies employed in this work allow for control of the terpene alignment as evidenced by the differences in the SFG spectra recorded for the ester-linked and the amidelinked terpenes. The differences in molecular orientation are explored in the following section. A detailed orientation analysis is the subject of future studies, and will include the analysis of various orientation distribution functions. The terpene studied in this work contains two adjacent methyne groups on sp3-hybridized carbon atoms (at the base of the ring and at the tertiary carbon atom in R position) that should adopt a low-energy configuration with a 180° dihedral angle for H-C-C-H. This particular configuration would result in the cancellation of their infrared CH stretching modes. It is thus not entirely surprising that methyne CH stretches, which have been observed near 2900 cm-1,65 are absent in the SFG spectra. The vinylic CH stretch, which would be expected between 3025 and 3070 cm-1,78,79 could be present within the broad spectral feature attributed to the NH and OH stretches of the amide groups and surface-bound water molecules. On the basis of the molecular orientations discussed in the following sections, the vinylic CH stretch would be expected to appear in the ssp spectra of the ester-linked terpene and the directly deposited terpene ester, whereas the amide-linked terpene would be expected to display a contribution of this stretch in the sps spectra. A. Ester Terpenes. In the ssp spectrum of the ester-linked terpene (Figure 5A), we observe a well-resolved CH3 symmetric

Figure 9. Vector addition of the IR transition moments of the two CH3 symmetric stretches (A). The resultant IR transition moment is shown for the ester-linked terpene (B) and amide-linked terpene (C).

contribution. Vector addition of the two CH3 symmetric stretch transition moments, as shown in Figure 9A, allows us to approximate the terpene orientation. In order to give rise to a symmetric methyl stretch in the ssp SFG spectrum, the resultant CH3 symmetric stretch transition moment should contain a significant component perpendicular to the surface. In molecular orientations that would give rise to such a perpendicular component, the terpene ring is significantly tilted away from the surface normal (Figure 9B). Because the transition moments for the asymmetric and the symmetric stretch modes are perpendicular to one another, the absence of asymmetric CH3 stretches in our ssp spectrum is consistent with this molecular orientation. The sps spectrum provides a probe for the presence of infrared transition moments aligned parallel to the interface. The presence of CH3 asymmetric stretches at 2951 cm-1 in the sps spectrum of the ester-linked terpene (Figure 5B) is consistent with the molecular orientation shown in Figure 9B. The spectral features observed for the direct-deposited terpene ester are broader than those observed in the ssp SFG spectrum of the ester-linked terpene. This broadening suggests a larger distribution of molecular orientations for the direct-deposited terpene ester when compared to the ester-linked terpene. This larger distribution can be attributed to increased disorder in the terpene adlayer resulting from steric hindrance from the bulky cyclohexene moiety during adlayer formation. B. Amide-Linked Terpene. In contrast to the ester-linked terpene, a strong CH3 asymmetric stretching mode is observed in the ssp SFG spectrum of the amide-linked terpene (Figure 8A). The presence of this mode indicates that the resultant infrared transition moment for the asymmetric stretches of the two CH3 groups in the terpene contains a significant component parallel to the surface. The sps SFG spectrum (Figure 8B) also shows the CH3 asymmetric stretching mode. This observation, combined with the absence of the CH3 symmetric mode in the

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Figure 10. Addition of ozone to the amide-linked terpene, formation of the cyclic intermediate, and one possible end stage oxidation product.

ssp spectrum, suggests that the resultant asymmetric CH3 transition moment forms an acute angle with respect to the surface normal, whereas the resultant symmetric CH3 transition moment is aligned parallel to the interface. The presence of a CH3 asymmetric stretching mode in both the sps and the ssp spectra could also be due to a broad orientation distribution of the terpene at the interface, which would be likely considering the disorder of the amine linker. The ssp spectrum of the amidelinked terpene could be fit with a second CH2 symmetric stretch at 2836 cm-1. This second CH2 symmetric stretch would display an amplitude that is about 20 times smaller than of that of the visible CH2 symmetric stretch (2880 cm-1). The alignment of the amide-linked terpene that is most consistent with these observations is shown in Figure 9C, in which the cyclohexene ring is tilted by approximately 90° from the orientation it assumes in the ester-linked terpene. V. Ozone Exposure and Atmospheric Implications The bulky, nonpolar nature of the terpene suggests that with either functionalization strategy the cyclohexene moiety of the terpene is aligned such that the CdC double bond is directed toward the gas phase. It is important to note, however, that without an absolute phase measurement47 the SFG measurements only give the magnitude of the projection of the CH3 stretch onto the surface normal. This means that the CdC double bond could in principle be pointed down if the flexibility of the aliphatic chain and packing density allowed for this scenario. Exposure of the surface-bound species to ozone is expected to result in the addition of ozone to the CdC double bond,80 leading to the formation of an ozonide, which changes the hybridization states of the two carbon atoms involved from sp2 to sp.3 This change in hybridization state is expected to reorient the adjacent methyl group and therefore lead to changes in the SFG intensity of the CH3 stretch mode (Figure 10). Ultimately, reducing agents can decompose the cyclic ozonide and result in the formation of polar carbonyl groups.2,80 In contrast to the terpene, octyl silane-functionalized surfaces do not possess Cd C double bonds and display negligibly low gas-phase reactivities toward ozone, and the interaction of ozone with this saturated organic surface is expected to be governed by van-der-Waals interactions at the low ozone partial pressures employed here.3,17 Using our Teflon reaction chamber at room temperature and 1 atm, we exposed the amide-linked terpene-functionalized surface to ozone concentrations up to 35 ppm. According to eqs 1-3, the SFG E-field, ESFG, at a given vibrational frequency can be directly related to the number of oscillators on the surface that have a given average orientation. Figure 11 shows that the net SFG E-field, ∆ESFG (where ∆ESFG ) ∆ESFG,t - ∆ESFG,t)0), generated by the CH3 asymmetric stretch mode, decays to a constant level within 20 min upon ozone exposure. This decay

Figure 11. (A) Gas-phase ozone concentration (absorbance at 254 nm vs time) and (B) ∆ESFG of the CH3 asymmetric stretch of the amidelinked terpene as a function of time. Data is fit with a single exponential (dark line). Inset: First derivative of the fit function.

indicates a change in the orientation of the vinylic CH3 group that occurs on a time scale of about 20 min. It is important to note that the time-dependent changes in the ∆Esfg are solely due to CH3 reorientation since the methyl group is tethered to the surface via one half of the cyclic structure and therefore cannot be lost to the gas phase. Assuming that the reorientation process involving the CH3 group adjacent to the CdC double bond in the amide-linked terpene follows first-order kinetics, we can fit a single exponential of the form ∆ESFG ) a + b e-kt to the ∆ESFG vs time traces (Figure 11). Here, k is the decay rate constant and a and b are fit parameters indicating the initial and final levels of ∆ESFG. The rate constant yields a half-life of approximately 5 min (∼10% error). The rates calculated from the first derivative in the fit are shown in the inset of Figure 11. These rates indicate a fast reorientation process at early times and show completion of the surface reaction after about 50 min. In contrast to the terpene-functionalized surface, the octyl silane-functionalized surface shows no dramatic change in the ESFG of the CH3 asymmetric stretch (Figure 12B). However, the ESFG of the CH2 symmetric stretch mode decreases in a linear fashion over the duration of ozone exposure (about 2 h). The ratio of the SFG intensities recorded for the CH3 and the CH2 symmetric stretches yields information on the molecular align-

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Figure 13. Solid line: Heterogeneous reaction probabilities for the first 5 min of the interaction between the amide-linked terpene and ozone. Dashed line: gas-phase reaction probability for 1-methyl-1cylcohexene with ozone calculated from Atkinson.3

Figure 12. (A) Gas-phase ozone concentration (absorbance at 254 nm vs time) and (B) ESFG of the CH3 asymmetric stretch and CH2 symmetric stretch for an octyl silane adlayer as a function of time. (C) Ratio of signal intensity of the CH3 symmetric stretch to the CH2 symmetric stretch (top trace) and of the CH3 symmetric stretch to the CH3 asymmetric stretch (bottom trace). Dark lines are linear fits to the data.

ment along the carbon chain.51 Figure 12C shows no change in the molecular alignment of the alkyl chain during ozone exposure. However, the CH3 groups appear to tilt away from the surface normal during ozone exposure, as indicated by a slight decrease in the ratio of the CH3 symmetric and asymmetric stretch intensities. The slight reorientation of the terminal CH3 groups could be a consequence of their hydrophobic interactions with ozone. This would be consistent with the enrichment of ozone at the terminal CH3 section of the interface recently suggested in molecular dynamic simulations by Tobias and coworkers that were carried out on similar systems.81 Following Dubowski et al.,17 we calculated the heterogeneous reaction probabilities for the interaction of ozone with the amidelinked terpenes for the first 5 min of the reaction, when the reaction is less than 50% complete. The heterogeneous reaction probabilities were calculated using the following expression:17

γ)

d[∆ESFG] [terpene]0,surface |t)tr×n dt ∆ESFG,t)∞[O3]xRT/2πM

(4)

Here, the first factor is the reaction rate obtained from the ∆ESFG vs time traces; the initial surface coverage of the terpene, [terpene]0,surface, is assumed to be 3 × 1014 cm-2;17 ∆ESFG,t)∞ is the change of the SFG electric field at the end of the reaction; [O3]g is the ozone concentration in the gas phase, R is the universal gas constant, T is the temperature (300 K), and M is the molar mass of ozone (0.048 kg/mol). It is important to note that the initial terpene coverage estimate is assumed to be similar to that of the alkenes studied by Dubowski et al.17 and could be smaller by a factor of up to three given the increased size of the molecules studied in this work. The gas-phase ozone concentration at each point in time is taken from the UV-vis absorbance vs time trace recorded simultaneously with the SFG measurements. As seen in Figure 13, the heterogeneous reaction probability for ozone and the surface bound terpene starts above 1 × 10-5 and decreases to 1 × 10-7 within the first 5 min of reaction. Our value for the heterogeneous reaction probability between ozone and the surface-bound terpene can be compared to the gas-phase reaction probability of ozone and 1-methyl-1-cyclohexene. As an estimate, the interaction between these two gasphase species is assumed to be barrierless (thus giving a diffusion-controlled rate constant), the molecular radii are assumed to be 2 Å for ozone and 3 Å for 1-methyl-1cyclohexene, and the steric factor is assumed to be unity. Using basic collision theory and assuming each collision results in reaction, the rate constant for ozone and 1-methyl-1-cyclohexene is calculated to be about 3.5 × 10-10 cm3 molec-1 s-1.1,38 When divided by the experimentally determined rate constant, 1.6 × 10-16 cm3 molec-1 s-1,3 the reaction probability (298 K) is estimated to be 5 × 10-7. This translates to one successful reaction event in two million gas-phase collisions between 1-methyl-1-cyclohexene and ozone. This value is about 20 times smaller than the one obtained from our surface reaction, clearly suggesting that the interfacial environment plays an important role in the reaction. Figure 13 shows that the initial reaction probability for ozone interacting with the amide-linked terpene is quite high (γ > 1 × 10-5) and that 2 in two million collisions of ozone with the surface still result in reaction after 1 min of reaction (γ ∼ 1 × 10-6). The initial reaction probability is significantly higher than the corresponding gas-phase reaction with 1-methyl-1-cyclohexene. After 2-3 min, the heterogeneous reaction probability decreases below that of the gas-phase reaction. The initial

Feature Article reaction probabilities reported here are in good agreement with those reported for neat linear alkenes at solid and aqueous interfaces.18,19,32 They are, however, smaller than those reported for liquid oleic acid studied by Moise and Rudich.20 The larger initial reaction probability for the heterogeneous, as compared to homogeneous, organic oxidation reaction can be reconciled with a physical picture that invokes the CdC double bond accessibility to gas-phase ozone molecules and the initial CdC double bond density at the surface. In the gas phase, all of the carbon atoms in 1-methyl-1-cyclohexene are accessible; however, collisions of ozone with the five saturated carbon atoms do not result in a reaction. In contrast, most of the saturated carbon atoms in the surface-bound amide-linked terpene are not accessible from the gas phase, and thus there is a larger percentage of reaction-favored collisions (i.e., encounters between ozone molecules and the CdC double bond). Additionally, during the initial stages of the reaction, each incoming ozone molecule is likely to interact with multiple, and possibly adjacent, CdC double bonds from neighboring terpenes before scattering off the surface, which would further increase the reaction probability. This effect is reminiscent of multivalent interactions in biological systems, where the high local concentration of ligand molecules increases the likelihood of a binding event.82 In the dilute gas phase, these multivalent interactions are much less likely to occur, resulting in a lower reaction probability for the gas-phase process. This scenario is consistent with computer simulations by Tobias and co-workers, who reported increased ozone residence times at CdC double bond-terminated surfaces. As the reaction progresses and the CdC double bond surface density decreases, the reaction probability decreases, as shown in Figure 13. To identify the various intermediates and product species involved, future work will include surface characterization in the carbonyl-stretching region using SFG and SHG studies on the ester-linked terpene. The surface functionalization methods described here open up possibilities to investigate stereospecific heterogeneous reactions of atmospheric relevance. As evidenced by the high specificity and stereoselectivity in many olefin oxidation reactions,83 the sterics and the stereochemistry of surface-bound alkenes can significantly impact product formation and branching ratios of atmospheric heterogeneous organic oxidation reactions. Kinetic resolution84 could lead to stereochemical enrichment in the gas phase, the aerosol, or both. Investigating stereoselectivity in heterogeneous atmospheric reactions is important because of the potential to develop chiral atmospheric markers for differentiating between anthropogenic and biogenic carbon species, as well as possible implications concerning prebiotic chemistry.85,86 VI. Conclusion In conclusion, we have developed several versatile methods for linking natural terpenes to glass and demonstrate our ability to generate tailor-made organic surfaces that can serve as models for the complicated organic coatings found on mineral dust. Terpenes are common tropospheric constituents whose gas-phase oxidation by ozone forms partially oxidized species that are expected to adsorb to the surfaces of mineral dust particles. To avoid the competition between the ozonation reactions and the desorption processes involving the surface bound reactants, intermediates, and products, we have chemically attached cyclic terpene derivatives to the glass surfaces via robust silane linker chemistry. We presented two general strategies for surface functionalization: (1) an electrophilic ester linker adlayer that can be attacked by nucleophiles such as alcohols and (2) a

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1577 nucleophilic amine linker adlayer that can attack solution-phase electrophiles such as acid chlorides. The surfaces are characterized using vibrational sum frequency generation spectroscopy, which allows us to study their interaction with ozone. The two linker strategies yield glass slides functionalized with ester- and amide-linked terpenes whose SFG spectra indicate two different orientations of the surface-bound terpenes. On the basis of the methyl symmetric stretch signatures found in the SFG spectra, the cyclohexene moiety in the ester-linked terpene is tilted away from the surface normal, whereas it is oriented more along the surface normal in the amide-linked terpene. In both compounds, the CdC double bond is available for reaction with atmospheric oxidants like ozone. Exposure of the amide-linked terpene to ppm levels of ozone at 1 atm and 300 K demonstrates that SFG can be used to track the interaction of ozone with tethered cyclic unsaturated atmospheric compounds at gas-solid interfaces. These measurements allow for the determination of reaction probabilities important in climate models. The initial reaction probability for ozone interacting with the cyclohexene moiety of the surfacebound terpene is greatly enhanced over the corresponding gasphase reaction involving 1-methyl-1-cyclohexene. The higher reaction probability of the heterogeneous process is consistent with the notion that most of the saturated carbon atoms in the surface-bound terpene are not accessible from the gas phase. This leads to a larger percentage of collisions between ozone molecules and the terpene CdC double bond for the heterogeneous process as opposed to the gas phase interaction of ozone with 1-methyl-1-cyclohexene. The increased heterogeneous reaction probability can also be explained by a high local initial terpene surface coverage as opposed to the dilute gas-phase that increases the likelihood of individual ozone molecules interacting with CdC double bonds. The interaction of ozone with a saturated octyl silane-functionalized glass surface occurs on a time scale that is significantly slower and is consistent with a slight reorientation of the terminal CH3 groups. This work allows us to address multivalent interactions in heterogeneous atmospheric chemistry as well as atmospheric stereoselectivity. Acknowledgment. The authors gratefully acknowledge donations, equipment loans, and the technical support of Spectra Physics, a division of Newport Corporation, and CVI Laser LCC. A.B.V. and G.Y.S. gratefully acknowledge NASA Graduate Student Fellowships in Earth System Science. J.M.G.-D. gratefully acknowledges a fellowship from the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry. Support for this project is provided by the NSF Atmospheric Chemistry division, the ACS Petroleum Research Fund, the Northwestern University Institute for Environmental Catalysis, and a Dow Chemical Company Professorship to F.M.G. Supporting Information Available: Descriptions of the synthesis and surface functionalization methods, schematics visually summarizing the synthetic pathways, and results from complementary ellipsometry, XPS, SHG, and contact angle measurements indicating high surface coverages and conversion efficiencies. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Chemistry of the Upper and Lower Atmosphere; Academic Press: New York, 2000. (2) Ege, S. N. Organic Chemistry: Structure and ReactiVity, 4th ed.; Houghton Mifflin Company: Boston, 1999. (3) Atkinson, R.; Arey, J. Chem. ReV. 2003, 103, 4605.

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