Atmospheric heterogeneous stereochemistry

Article abstract of DOI:10.1021/ja904206t

While many biogenic and anthropogenic organic constituents in the atmosphere are surface-active and chiral, the role of stereochemistry in heterogeneous oxidation chemistry in the atmosphere has not yet been evaluated. Here, we present nonlinear vibration

Full text of DOI:10.1021/ja904206t

Published on Web 08/28/2009
Atmospheric Heterogeneous Stereochemistry
Grace Y. Stokes, Ehow H. Chen, Avram M. Buchbinder, Walter F. Paxton,
Alison Keeley, and Franz M. Geiger*
Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois 60208
Received May 30, 2009; E-mail: geigerf@chem.northwestern.edu
Abstract: While many biogenic and anthropogenic organic constituents in the atmosphere are surface-
active and chiral, the role of stereochemistry in heterogeneous oxidation chemistry in the atmosphere has
not yet been evaluated. Here, we present nonlinear vibrational surface spectra of fused silica substrates
functionalized with quinuclidine diastereomers during exposure to 1011 to 1013 molecules of ozone per cm3
in 1 atm helium to model ozone-limited and ozone-rich tropospheric conditions. Kinetic studies show that
diastereomers that orient their reactive CdC double bonds toward the gas phase exhibit heterogeneous
ozonolysis rate constants that are 2 times faster than diastereomers that orient their CdC double bonds
away from the gas phase. Insofar as our laboratory model studies are representative of real world
environments, our studies suggest that the propensity of aerosol particles coated with chiral semivolatile
organic compounds to react with ozone may depend on stereochemistry. We expect that the differences
in chemical accessibility will lead to the enrichment of one oxidation product stereoisomer over the other.
The oxidation products could be gaseous or surface-bound, indicating that kinetic resolution could lead to
the stereochemical enrichment of the gas phase or the aerosol, which may have also been important in
prebiotic chemistry. Implications of these results for chiral markers that would allow for source appointments
of anthropogenic versus biogenic carbon emissions are discussed.
Introduction
toward the gas phase should exhibit faster ozonolysis rate
constants than diastereomers that are adsorbed such that their
Organic compounds are ubiquitous in the atmosphere and play
major roles in the formation, chemical transformation, and
optical properties of aerosols.1,2 While the molecular level
understanding of atmospheric heterogeneous chemistry that has
emerged over the past 2 decades is very high,3-6 the role of
heterogeneous atmospheric stereochemistry has not yet been
evaluated. Ozone is achiral, and two mirror images of ozone
on an achiral surface are fully superimposable. This situation
may change for surfaces containing chiral olefins, and we
evaluate it here for a model system that is relevant for
understanding organic oxidation chemistry in the atmosphere.
Given the fact that many biogenic and anthropogenic organic
constituents in the atmosphere are chiral,7 it is the purpose of
this work to determine whether the heterogeneous oxidation of
CdC double bonds is controlled by stereochemistry when olefin
diastereomers are exposed to tropospherically relevant ozone
levels at a total pressure of 1 atm. Surface-bound diastereomers
adsorbed such that the reactive CdC double bonds are oriented
CdC double bonds are oriented away from the gas phase. Here,
we test this hypothesis by combining coherent interface-specific
vibrational probes that are uniquely sensitive to molecular
structure with synthetic methods that afford the preparation of
surfaces functionalized with selected diastereomers.
The laboratory model studies presented here focus on the
ozonolysis of CdC double bonds because they are commonly
found within biosurfactants, which are important in marine
aerosol chemistry,8 as well as terpenes,9 a key class of biogenic
atmospheric constituents that are readily rendered surface-active
via gas-phase oxidation.10 At aerosol surfaces, the CdC double
bonds are available for further heterogeneous reactions by
atmospheric oxidants such as ozone. Laboratory model studies
show that the impact of heterogeneous CdC double-bond
oxidation processes on atmospheric chemistry includes changes
in its chemical composition and in the propensity of aerosols
to act as cloud condensation nuclei.8,10-36 Specifically, atmo-
(1) Finlayson-Pitts, B. J., Jr. Chemistry of the Upper and Lower Atmo-
sphere; Academic Press: New York, 2000.
(2) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics;
John Wiley & Sons: New York, 1998.
(8) Karagulian, F.; Lea, A. S.; Dilbeck, C. W.; Finlayson-Pitts, B. J. Phys.
Chem. Chem. Phys. 2008, 10, 528–541.
(9) Shaw, S. L.; Chisholm, S. W.; Prinn, R. G. Marine Chem. 2003, 80,
227–245.
(3) Molina, M. J.; Tso, T. L.; Molina, L. T.; Wang, F. C. Y. Science
1987, 238, 1253–1257.
(10) Ziemann, P. J. J. Phys. Chem. A 2003, 107, 2048–2060.
(11) Elisason, T. L.; Aloisio, A.; Donaldson, D. J.; Cziczo, D. J.; Vaida,
V. Atmos. EnViron. 2003, 37, 2207–2219.
(4) Tolbert, M. A.; Rossi, M. J.; Golden, D. M. Science 1988, 240, 1018–
1021.
(12) Northway, M. J.; de Gouw, J. A.; Fahey, D. W.; Gao, R. S.; Warneke,
C.; Roberts, J. M.; Flocke, F. Atmos. EnViron. 2004, 38, 6017–6028.
(13) Eliason, T. L.; Gilman, J. B.; Vaida, V. Atmos. EnViron. 2004, 38,
1367–1378.
(5) Laskin, A.; Gaspar, D. J.; Wang, W. H.; Hunt, S. W.; Cowin, J. P.;
Colson, S. D.; Finlayson-Pitts, B. J. Science 2003, 301, 340–344.
(6) Rudich, Y.; Donahue, N. M.; Mentel, T. F. Annu. ReV. Phys. Chem.
2007, 58, 321–352.
(14) Fiegland, L. R.; McCorn Saint Fleur, M.; Morris, J. R. Langmuir 2005,
21, 2660–2661.
(7) Atkinson, R.; Arey, J. Chem. ReV. 2003, 103, 4605–4638.
9
10.1021/ja904206t CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 13733–13737 13733

A R T I C L E S
Stokes et al.
spherically important gas-phase species such as formaldehyde
and carbon dioxide have been detected as product species.
Furthermore, the heterogeneous ozonolysis of CdC double
bonds has been reported to follow a Langmuir-Hinshelwood
mechanism.8,37 Finally, depending on the ozone partial pressure,
more or fewer polar surface-bound product species have been
reported to form, which results in the formation of more or fewer
hydrophobic ozone-processed surfaces.38
Given the atmospheric importance of heterogeneous organic
oxidation processes, and given the ubiquity of chirality in nature,
we determined the ozonolysis rates of two olefin diastereomers,
each containing terminal CdC double bonds which are steered
toward and away from the gas phase by the use of a quinuclidine
group covalently bound to a fused silica surface (Figure 1A).
A mere displacement of the CdC double bond from a terminal
position toward an embedded position within the alkyl chain
would not achieve the same effect as the one made possible by
using the quinuclidine ring because terminal CdC double bonds
are vastly more reactive than nonterminal double bonds.7
Experimental Approach
The surfaces are prepared by covalently linking (2S,4S,5R)-5-
vinyl-2-quinuclidinecarboxaldehyde(CAS#66140-82-7)and(2R,4S,5R)-
(15) Lai, C. C.; Yang, S. H.; Finlayson-Pitts, B. J. Langmuir 1994, 10,
4637–4644.
(16) Wadia, Y.; Tobias, D. J.; Stafford, R.; Finlayson-Pitts, B. J. Langmuir
2000, 16, 9321–9330.
(17) Dubowski, Y.; Vieceli, J.; Tobias, D. J.; Gomez, A.; Lin, A.;
Nizkorodov, S. A.; McIntire, T. M.; Finlayson-Pitts, B. J. J. Phys.
Chem. A 2004, 108, 10473–10485.
(18) Usher, C. R.; Michel, A. E.; Grassian, V. H. Chem. ReV. 2003, 103,
4883–4940.
(19) Moise, T.; Rudich, Y. J. Geophys. Res.-Atmos. 2000, 105, 14667–
14676.
(20) Thomas, E. R.; Frost, G. J.; Rudich, Y. J. Geophys. Res.-Atmos. 2001,
106, 3045–3056.
(21) Poschl, U.; Letzel, T.; Schauer, C.; Niessner, R. J. Phys. Chem. A
2001, 105, 4029–4041.
(22) Mmereki, B. T.; Donaldson, D. J. J. Phys. Chem. A 2003, 107, 11038–
11042.
(23) Katrib, Y.; Martin, S. T.; Hung, H. M.; Rudich, Y.; Zhang, H. Z.;
Slowik, J. G.; Davidovits, P.; Jayne, J. T.; Worsnop, D. R. J. Phys.
Chem. A 2004, 108, 6686–6695.
(24) Smith, G. D.; Woods, E.; DeForest, C. L.; Baer, T.; Miller, R. E. J.
Phys. Chem. A 2002, 106, 8085–8095.
(25) Thornberry, T.; Abbatt, J. P. D. Phys. Chem. Chem. Phys. 2004, 6,
84–93.
(26) Hearn, J. D.; Lovett, A. J.; Smith, G. D. Phys. Chem. Chem. Phys.
2005, 7, 501–511.
Figure 1. (A) Coupling of (2S,4S,5R)-5-vinyl-2-quinuclidine carboxalde-
hyde and (2R,4S,5R)-5-vinyl-2-quinuclidine carboxaldehyde to aniline
silane-functionalized fused silica substrates. (B) N(1s) XPS spectra of fused
silica substrates functionalized with the 2S- and the 2R-diastereomers (top
and bottom, respectively). Gray areas and thin lines indicate the N(1s) XPS
response of the unreacted aniline, the amide, and the quinuclidine groups.
See text for details.
(27) Morris, J. W.; Davidovits, P.; Jayne, J. T.; Jimenez, J. L.; Shi, Q.;
Kolb, C. E.; Worsnop, D. R.; Barney, W. S.; Cass, G. Geophys. Res.
Lett. 2002, 29.
5-vinyl-2-quinuclidine carboxaldehyde (CAS# 66140-84-9) to
aniline-functionalized fused silica substrates (see Supporting In-
formation). After rinsing and sonication in absolute ethanol and
then methylene chloride (Mallinckrodt) for 5 min, the substrates
are dried under nitrogen. They were then studied using N(1s) XPS
measurements (please see Supporting Information) to determine the
relative surface coverages for the 2R- and 2S-diastereomers, which
are found to be 51 ( 5% and 36 ( 5%, respectively (Figure 1B).
Using a previously described39,40 laser system suitable for
carrying out broadband vibrational sum frequency generation (SFG)
studies, we identified the surface-bound diastereomers (Figure 2A,B)
in the CH stretching region. The olefinic CH stretches of the
terminal CdC double bond are clearly observable above 3000 cm-1
in both diastereomers when we interrogate the surface with a
polarization combination that probes vibrational transitions oriented
(28) Jang, M. S.; Carroll, B.; Chandramouli, B.; Kamens, R. M. EnViron.
Sci. Technol. 2003, 37, 3828–3837.
(29) McIntire, T. M.; Lea, S. A.; Gaspar, D. J.; Jaitly, N.; Dubowski, Y.;
Li, Q.; Finlayson-Pitts, B. J. Phys. Chem. Chem. Phys. 2006, 7, 3605–
3609.
(30) Clifford, D.; Donaldson, D. J.; Brigante, M.; D’Anna, B.; George, C.
EnViron. Sci. Technol. 2008, 42, 1138–1143.
(31) Kwamena, N.-O. A.; Staikova, M. G.; Donaldson, D. J.; George, I. J.;
Abbatt, J. P. D. J. Phys. Chem. A 2007, 111, 11050–11058.
(32) Donaldson, D. J.; Vaida, V. Chem. ReV. 2006, 106, 1445–1461.
(33) Park, J.; Gomez, A. L.; Walser, M. L.; Lin, A.; Nizkorodov, S. A.
Phys. Chem. Chem. Phys. 2006, 8, 2506–2512.
(34) Hung, H.-M.; Katrib, Y.; Martin, S. T. J. Phys. Chem. A 2005, 109,
4517–4530.
(35) Presto, A. A.; Donahue, N. M. EnViron. Sci. Technol. 2006, 40, 3536–
3543.
(36) Gross, S.; Bertram, A. K. J. Phys. Chem. A 2008, 112, 3104–3113.
(37) McCabe, D. C.; Gierczak, T.; Talukdar, R. K.; Ravishankara, A. R.
Geophys. Res. Lett. 2001, 28, 3135–3138.
(39) Stokes, G. Y.; Buchbinder, A. M.; Gibbs-Davis, J. M.; Scheidt, K. A.;
Geiger, F. M. J. Phys. Chem. A 2008, 112, 11688–11698.
(40) Stokes, G. Y.; Buchbinder, A. M.; Gibbs-Davis, J. M.; Scheidt, K. A.;
Geiger, F. M. Vib. Spectrosc. 2009, 50, 86–98.
(38) Stokes, G. Y.; Chen, E. H.; Walter, S.; Geiger, F. M. J. Phys. Chem.
A 2009, 113, 8985-8993.
9
13734 J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009

Atmospheric Heterogeneous Stereochemistry
A R T I C L E S
Figure 3. Contact angle measurement histograms of over 50 measurements
each for fused silica substrates functionalized with the 2S- and the 2R-
diastereomers before [blue and red histograms in (A) and (B), respectively]
and after exposure to 350 ppb of ozone (empty boxes).
orientation has a dramatic impact on the vibrational spectra,
emphasizing the exquisite sensitivity of nonlinear vibrational
spectroscopy to molecular structure. After characterizing the
surfaces under investigation, we exposed the two surfaces to 500
ppb ozone by using a previously described Teflon gas flow
chamber.40,52
Results and Discussion
Upon exposure to 500 ppb of ozone in 1 atm helium, the
SFG spectra show a marked decrease in the SFG signal intensity
attributed to the vinylic CH stretch at 3050 cm-1 (black traces
in Figure 2A,B), indicating that ozonolysis has taken place.
Simultaneous changes in the symmetric methylene CH stretch
signatures of the ethyl bridge at 2850 cm-1 are quite pronounced
in the 2R-diastereomer, which is consistent with the notion that
its ethyl bridge reorients while ozone interacts with the
quinuclidine ring to reach the buried CdC double bond. In
contrast, the SFG spectra of the 2S-diastereomer exhibit little
changes in the aliphatic CH stretch region, which suggests that
little molecular rearrangement of the quinuclidine ring occurs
upon ozone interaction. This interpretation supports the hypoth-
esis that the CdC double bond of the 2S-diastereomer is readily
accessible to gas-phase ozone.
Contact angle histograms (Figure 3A,B) show that exposing
the 2S-diastereomer to 350 ppb of ozone results in 29 of 56
measurements with lower contact angles than those before ozone
exposure, and that exposing the 2R-diastereomer to 350 ppb of
ozone results in 15 of 52 measurements with lower contact
angles than those before ozone exposure. These results are
consistent with the formation of polar ozonolysis products at
this ozone level. Similar to the finding by McArthur et al. that
saturated siloxanes chemisorbed to silica are not degraded by
ozone unless oxygen and UV light are present simultaneously,51
we find that cyclohexane-functionalized surfaces yield no
significant changes in the SFG signal intensities upon exposure
to up to 1 ppm of ozone (see Supporting Information). This
also indicates that the imine linkers are resilient to ozonolysis
when using the low ozone partial pressures that are applied in
this work.
Figure 2. (A) ssp-Polarized vibrational sum frequency generation spectra
of fused silica substrates functionalized with the 2R-diastereomer and the
2S-diastereomer before [red line (A); blue line (B)] and after [black line,
(A) and (B)] exposure to 500 ppb of ozone in 1 atm helium. Please see text
for details.
predominantly perpendicular to the surface.41-45 In accordance with
literature data,46-50 the vibrational modes below 3000 cm-1 are
assigned to the methylene asymmetric and symmetric CH stretches
of the quinuclidine ring.
Parts (A) and (B) of Figure 2 probe the same number of
vibrational modes because the SFG spectra are acquired using a
polarization combination that probes achiral vibrational signatures
and because the surface-bound diastereomers both have the same
number of CH oscillators. However, (A) and (B) in Figure 2 also
show that the SFG intensity attributed to the symmetric methylene
stretch modes below 2900 cm-1 is much stronger in the 2R- than
in the 2S-configured diastereomer, as indicated by the asterisks.
This finding suggests that the ethyl bridge orientation in the 2R-
diastereomer is orthogonal to the one in the 2S-diastereomer, as
indicated by the thick single bonds in the Chemdraw structures.
These results thus show how a subtle change in molecular
(41) Richmond, G. L. Annu. ReV. Phys. Chem. 2001, 52, 357–389.
(42) Esenturk, O.; Walker, R. A. J. Phys. Chem. B 2004, 108, 10631–
10635.
(43) Shen, Y. R. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 12104–12111.
(44) Eisenthal, K. B. Chem. ReV. 1996, 96, 1343–1360.
(45) Geiger, F. M. Annu. ReV. Phys. Chem. 2009, 60, 61–83.
(46) Bruesch, P. Spectrochim. Acta 1966, 22, 867–875.
(47) Chu, W.; LeBlanc, R. J.; Williams, C. T.; Kubota, J.; Zaera, F. J.
Phys. Chem. B 2003, 107, 14365–14373.
After characterizing the olefin-functionalized surfaces before
and after ozonolysis, we carried out kinetic measurements in
which we tracked the SFG signal intensity from the olefinic
CH stretches as a function of time while exposing the surfaces
to 400 ppb of ozone (Figure 4A). These ozone-rich conditions
are representative of highly polluted urban environments.1,2 The
(48) Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Appl. Spectrosc. ReV.
2005, 40, 103–145.
(49) Liu, Y.; Wolf, L. K.; Messmer, M. C. Langmuir 2001, 17, 4329–
4335.
(50) Xu, M.; Liu, D.; Allen, H. C. EnViron. Sci. Technol. 2006, 40, 1566–
1572.
(51) McArthur, E. A.; Ye, T.; Cross, J. P.; Petoud, S.; Borguet, E. J. Am.
Chem. Soc. 2004, 126, 2260–2261.
9
J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009 13735

A R T I C L E S
Stokes et al.
Figure 4. (A) SFG E-field vs time traces recorded at 3050 cm-1 of the
2S- and 2R-diastereomers upon exposure to 400 ppb of ozone (top and
bottom, respectively). The white lines indicate the best fit to a first-order
kinetic rate law, and the vertical arrows indicate the corresponding half-
times. (B) Heterogeneous ozonolysis rates for the 2S-diastereomer plotted
against that of the 2R-diastereomer. The dashed line indicates the result
expected if there were no rate difference.
Figure 5. Reactive uptake coefficients (γ values) as a function of ozone
concentration for the 2R- and 2S-diastereomers, indicated by the red and
blue filled circles, respectively. Uncertainties in the x values are due to
error (signal/noise and drift) in the UV-vis spectrometer, and uncertainties
in the y values come from propagation of error of the first-order exponential
fit of the decay in ESFG.
change in the SFG E-field versus time trace for the 2R-
diastereomer was increased by 13% and then renormalized to
the t ) 0 level to account for its larger surface coverage when
compared to the 2S-diastereomer.
that ozone adsorbs more strongly to the diastereomer-function-
alized surfaces and may have a longer residence time there than
for any of the other substrates. Given the similarity of the inverse
Langmuir binding constants, we conclude that gas-phase ozone
molecules reside in the olefin adlayer for similar residence time
durations for both the 2R- and 2S-diastereomers. Thus, the faster
reactivity we observe for the S diastereomer is most likely due
to the faster reaction rate constant as opposed to the nonreactive
interactions between ozone and the surface.
Figure 4A shows that the 2S-diastereomer reacts faster than
the 2R-diastereomer. Fitting the kinetic traces to a first-order
rate law yields ozonolysis rates of 0.118 ( 0.004 min-1 and
0.063 ( 0.003 min-1 for the 2S-diastereomer and 2R-diastere-
omer, respectively. Repeating our kinetic studies for various
ozone levels ranging from 10 ppb to 1 ppm (Figure 4B), we
find that the ozonolysis rate of the 2S-diastereomer is consis-
tently twice as high as the ozonolysis rate of the 2R-diastere-
omer. It is important to note that the SFG E-field does not only
depend on the number density of oscillators located at the
interface but also on their molecular orientation. Yet, on the
basis of detailed polarization-resolved SFG studies for related
olefin species before and after ozonolysis,1 we attribute the SFG
E-field change to CdC double-bond ozonolysis.
Summary and Outlook
Given that many of the partially oxidized gas-phase products
originating from terpene ozonolysis contain stereocenters, and
given the fact that many uncatalyzed reactions common in
organic CdC double bond oxidation produce diastereomers with
high stereoselectivity,53 atmospheric heterogeneous stereochem-
istry may be one of the keys necessary for understanding the
product formation and branching ratios of organic oxidations.
This is especially important when considering the cis and trans
ylides formed during ozonolysis.54 If the ozonolysis of terpene
diastereomers were modulatedsor even controlledsby the
chemistry occurring at the stereogenic carbon centers, it would
be conceivable that stereochemistry also plays a key role in
heterogeneous ozonolysis. In this work, we have demonstrated
that heterogeneous ozonolysis reaction parameters depend on
stereochemistry when quinuclidine-based olefin diastereomers
covalently coupled to fused silica substrates are exposed to
tropospherically relevant ozone levels. For ozone-limited and
ozone-rich conditions that mimic ambient ozone concentrations
in remote continental environments and highly polluted met-
ropolitan areas, which typically range from 1011 to 1013
molecules of ozone per cm3, the Langmuir binding constants
were found to be within experimental error for both diastere-
omers, indicating that the nonreactive interactions between the
two diastereomers and ozone are very similar. In contrast, the
heterogeneous ozonolysis rate constant for the (2S,4S,5R)-5-
vinyl-2-quinuclidine-functionalized surface, which contains a
Following Finlayson-Pitts and co-workers,17 Thornberry and
Abbatt,25 and our earlier work,39,52 we calculated reactive uptake
coefficients (γ values) for the heterogeneous ozonolysis reac-
tions. Figure 5 shows higher γ values for the 2S-diastereomer
than for the 2R-diastereomer at comparable ozone concentrations
between 10 ppb and 1 ppm. By fitting the γ values to an
expression that assumes a Langmuir-Hinshelwood mecha-
nism,17 we obtain second-order reaction rate constants that differ
by a factor of 2 for the two diastereomers (6.4(2) × 10-18 cm2
molecule-1 s-1 for the 2R-diastereomer and 2.9(3) × 10-18 cm2
molecule-1 s-1 for the 2S-diastereomer).
The reaction rate constants reported here are somewhat
smaller than that reported for terminal alkene-functionalized
glass surfaces (2 × 10-17 cm2 molecule-1 s-1).17 We also
quantify the ratio of the ozone desorption to adsorption rate
constants, which is the inverse of the Langmuir binding constant,
and find it to be identical within error for both diastereomers
(4 ( 3 × 1011 molecules cm-3 and 1.3 ( 4 × 1011 molecules
cm-3). These values are about 10 times smaller than those
reported for benzo-[a]pyrene (BaP)-coated soot,21 100 times
smaller than the one reported for terminal alkene-functionalized
glass surfaces,17 and about 1000 to 10,000 times smaller than
those reported for anthracene adsorbed on water,22 indicating
(53) Smith, M. B.; March, J. March’s AdVanced Organic Chemistry:
Reactions, Mechanisms, and Structure, 5th ed.; John Wiley & Sons:
New York, 2001.
(54) Fenske, J. D.; Kuwata, K. T.; Houk, K. N.; Paulson, S. E. J. Phys.
Chem. A 2000, 104, 7246–7254.
(52) Voges, A. B.; Stokes, G. Y.; Gibbs-Davis, J. M.; Lettan, R. B.; Bertin,
P. A.; Pike, R. C.; Nguyen, S. T.; Scheidt, K. A.; Geiger, F. M. J.
Phys. Chem. C 2007, 111, 1567-1578.
9
13736 J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009

Atmospheric Heterogeneous Stereochemistry
A R T I C L E S
CdC double bond that is directed toward the gas phase, is 2
times faster than that for the opposite diastereomer, which
contains a CdC double bond that is directed away from the
gas phase. Reactive uptake coefficients, which are commonly
used in global circulation models, mirror these differences.
Insofar as our laboratory model studies are representative of
real world environments, one could speculate that heterogeneous
stereochemistry may be important for understanding the chem-
istry of the troposphere. Our results suggest that the propensity
of an aerosol particle coated with a chiral semivolatile organic
compound to react with ozone may depend on surface orienta-
tion, which could be governed by stereochemistry. We expect
that the differences in chemical accessibility will lead to the
enrichment of one oxidation product stereoisomer over the other.
The oxidation products could be gaseous or surface-bound,
indicating that kinetic resolution55 could lead to the stereo-
chemical enrichment of the gas phase or the aerosol.
The results presented here suggest the presence of a stere-
ochemically specific set of reactivity parameters that may
improve the quantification and prediction of organic oxidation
processes in atmospheric chemistry models. The finding that
the first model system examined exhibits a clear reactivity
difference when two opposite diastereomers are studied encour-
ages us to pursue this work with chiral tropospheric olefins such
as the important classes of diterpenes and oligoterpenes. Given
that 1S,5S R-pinene is more common in European pine trees
than in North American pine trees, where the opposite 1R,5R
R-pinene diastereomer is more common,56 it may be conceivable
that atmospheric heterogeneous stereochemistry could play a
key role in the formation of clouds over Europe as opposed to
clouds over North America. Field measurements focusing on
atmospheric stereochemistry are vitally needed for quantitatively
assessing this role.
guished from the volatile chiral oligomers released during the
combustion of commodity chemicals with predetermined ster-
eochemistry, such as syndiotactic polypropylene. Such a use
of “chiral atmospheric markers” could complement the use of
isotopic and other markers to aid regulatory agencies and policy
makers in environmental stewardship. Our results are also
applicable toward the investigations of prebiotic chemistry and
the origin of life, which has been hypothesized to involve
heterogeneous aerosol reactions.57-59 Heterogeneous stereo-
chemistry occurring at prebiotic aerosol surfaces could have led
to the production of a chiral excess of water-soluble product
species, each containing the same stereogenic carbon atoms. It
would be instructive to study whether continuing reactions inside
the bulk aerosol could have led to homochiral building blocks
of life and resulted in the fact that all amino acids are left-
handed.60
Acknowledgment. G.Y.S. gratefully acknowledges support by
NASA Headquarters under the NASA Earth and Space Science
Fellowship program (Grant 08-Earth08R-0049). Support for this
project is provided by the National Science Foundation Atmospheric
Chemistry Division (Grant NSF-ATM 0533634). Support from the
DOE-funded Northwestern University Institute for Catalysis in
Energy Processes (ICEP) (Grant DE-FG02-03-ER15457/A004) and
the Northwestern University International Institute for Nanotech-
nology (IIN) is greatly appreciated. F.M.G. is a Sloan Foundation
Fellow. The authors also acknowledge donations, equipment loans,
and the technical support of Spectra Physics, a Division of Newport
Corporation.
Supporting Information Available: Information on the syn-
thesis and surface functionalization methods used in this work
and the analysis of the XPS measurements. This material is
available free of charge via the Internet at http://pubs.acs.org.
Building upon this work is the potential use of chiral organic
molecules as atmospheric markers for source appointment and
distinguishing between anthropogenic and natural sources of
organic carbon. For example, the enantiomerically pure partially
oxidized terpenes released during forest fires may be distin-
JA904206T
(57) Oberbeck, V. R.; Marshall, J.; Shen, T. J. Mol. EVol. 1991, 32, 296–
303.
(58) Donaldson, D. J.; Tuck, A. F.; Vaida, V. Phys. Chem. Chem. Phys.
2001, 3, 5270–5273.
(55) Eliel, E. L. Asymmetric Reactions and Processes in Chemistry;
American Chemical Society: Washington, DC, 1982.
(56) Simonsen, J. L. The Terpenes; Cambridge University Press: Cambridge,
1957; Vol. 2, pp 105-191.
(59) Dobson, D. M.; Ellison, G. B.; Tuck, A. F.; Vaida, V. Proc. Natl.
Acad. Sci. 2000, 97, 11864–11868.
(60) Voet, D.; Voet, J. G. Biochemistry, 3rd ed.; Wiley Text Books: New
York, 2004.
9
J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009 13737