Photo-tautomerization of acetaldehyde to vinyl alcohol: A potential route to tropospheric acids

Article abstract of DOI:10.1126/science.1220712

Current atmospheric models underestimate the production of organic acids in the troposphere. We report a detailed kinetic model of the photochemistry of acetaldehyde (ethanal) under tropospheric conditions. The rate constants are benchmarked to collision-free experiments, where extensive photo-isomerization is observed upon irradiation with actinic ultraviolet radiation (310 to 330 nanometers). The model quantitatively reproduces the experiments and shows unequivocally that keto-enol photo-tautomerization, forming vinyl alcohol (ethenol), is the crucial first step. When collisions at atmospheric pressure are included, the model quantitatively reproduces previously reported quantum yields for photodissociation at all pressures and wavelengths. The model also predicts that 21 ± 4% of the initially excited acetaldehyde forms stable vinyl alcohol, a known precursor to organic acid formation, which may help to account for the production of organic acids in the troposphere.

Full text of DOI:10.1126/science.1220712

Photo-Tautomerization of Acetaldehyde to Vinyl Alcohol: A Potential
Route to Tropospheric Acids
Duncan U. Andrews et al.
Science 337, 1203 (2012);
DOI: 10.1126/science.1220712
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your
colleagues, clients, or customers by clicking here.
Permission to republish or repurpose articles or portions of articles can be obtained by
following the guidelines here.
The following resources related to this article are available online at
www.sciencemag.org (this information is current as of June 23, 2014 ):
Updated information and services, including high-resolution figures, can be found in the online
version of this article at:
http://www.sciencemag.org/content/337/6099/1203.full.html
Supporting Online Material can be found at:
http://www.sciencemag.org/content/suppl/2012/08/15/science.1220712.DC1.html
This article appears in the following subject collections:
Chemistry
http://www.sciencemag.org/cgi/collection/chemistry
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the
American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright
2012 by the American Association for the Advancement of Science; all rights reserved. The title Science is a
registered trademark of AAAS.

REPORTS
6. S. V. Yanina, K. M. Rosso, Science 320, 218 (2008).
7. P. A. Cox, Transition Metal Oxides: An Introduction
to Their Electronic Structure and Properties (Oxford Univ.
Press, New York, 2010).
31. A. L. Ankudinov, B. Ravel, J. J. Rehr, S. D. Conradson,
The ability to probe electron small polarons
is a first step toward experimentally distinguish-
ing the multiple steps that comprise redox trans-
formations of iron (oxyhydr)oxides and measuring
their rates. The small-polaron hopping rate is a
fundamental mineral phase–dependent limit
on the kinetics of many iron redox reactions. As
shown in Fig. 4B, we additionally observed iron(II)
in the nanoparticles at microsecond time scales,
representing itinerant electrons that were trapped
within the iron oxides before leaving either by
dissolution or recombination. Complementing the
present measurements of hopping rates with rates
of additional steps, such as bond breaking or the
nucleation of new mineral phase, will enable more
detailed mechanistic descriptions of the reductive
dissolution of iron(III) (oxyhydr)oxides and other
important environmental reactions. We anticipate
that the combination of transient-absorption spec-
troscopy, conventional kinetics measurements,
and ab initio and kinetic modeling offers a frame-
work for this program.
Phys. Rev. B 58, 7565 (1998).
32. B. Gilbert et al., J. Phys. Chem. A 107, 2839 (2003).
33. N. Iordanova, M. Dupuis, K. M. Rosso, J. Chem. Phys.
122, 144305 (2005).
34. A. Manceau, Clay Miner. 44, 19 (2009).
35. D. G. Rancourt, J. F. Meunier, Am. Mineral. 93, 1412
(2008).
8. T. Holstein, Ann. Phys. 8, 343 (1959).
9. K. M. Rosso, M. Dupuis, J. Chem. Phys. 120, 7050 (2004).
10. J. Ederth, A. Hoel, G. A. Niklasson, C. G. Granqvist,
J. Appl. Phys. 96, 5722 (2004).
11. A. S. Alexandrov, N. F. Mott, Polarons and Bipolarons
(World Scientific, Singapore, 1996).
12. I. Djerdj et al., J. Am. Chem. Soc. 130, 11364 (2008).
13. N. Mannella et al., Phys. Rev. Lett. 92, 166401 (2004).
14. J. E. Katz et al., J. Phys. Chem. Lett. 1, 1372 (2010).
15. Ch. Bressler et al., Science 323, 489 (2009).
16. G. A. Waychunas, M. J. Apted, G. E. Brown Jr.,
Phys. Chem. Miner. 10, 1 (1983).
36. W. Xu et al., Am. Mineral. 96, 513 (2011).
Acknowledgments: Time-resolved x-ray transient spectroscopy
was performed at beamline 11-ID-D at the Advanced Photon
Source (APS). Laser facilities at 11-ID-D were provided by the
Solar Energy Conversion group of Chemical Sciences and
Engineering Division of Argonne National Laboratory, which is
funded through New Facility and Mid-scale Instrumentation
grants to L. X. Chen et al. We thank L. X. Chen, G. Jennings,
and C. Kurtz. PDF analysis was performed at beamline 11-ID-B
at the APS. Transient absorption spectroscopy was performed
at the Argonne Center for Nanoscale Materials (CNM), and we
thank G. Wiederrecht and D. Gosztola. This work was supported by
the Chemical Imaging program of the U.S. Department of
Energy, Office of Science, and Office of Basic Energy Sciences
(DOE-BES), under contract DE-AC02-05CH11231. K.M.R. and
P.Z. gratefully acknowledge support from DOE-BES Geosciences
program to PNNL. C.F. acknowledges support from the Danish
Council for Independent Research. Use of the APS and the CNM is
supported by DOE-BES under contract DE-AC02-06CH11357.
17. M. Grätzel, Nature 414, 338 (2001).
18. C. Bressler, M. Chergui, Chem. Rev. 104, 1781 (2004).
19. L. X. Chen, Annu. Rev. Phys. Chem. 56, 221 (2005).
20. A. Cannizzo et al., Coord. Chem. Rev. 254, 2677
(2010).
21. J. Chen et al., J. Phys. Chem. A 111, 9326 (2007).
22. F. M. Michel et al., Science 316, 1726 (2007).
23. J. M. Cowley, D. E. Janney, R. C. Gerkin, P. R. Buseck,
J. Struct. Biol. 131, 210 (2000).
24. N. J. Cherepy, D. B. Liston, J. A. Lovejoy, H. Deng,
J. Z. Zhang, J. Phys. Chem. B 102, 770 (1998).
25. G. Benkö, M. Hilgendorff, A. P. Yartsev, V. Sundström,
J. Phys. Chem. B 105, 967 (2001).
26. M. Hilgendorff, V. Sundström, J. Phys. Chem. B 102,
10505 (1998).
27. M. Wilke, F. Farges, P.-E. Petit, G. E. Brown Jr., F. Martin,
Am. Mineral. 86, 714 (2001).
28. U. Colombo, G. Fagheraz, F. Gazzarrini, G. Lanzavecchia,
G. Sironi, Nature 219, 1036 (1967).
29. E. Tronc, P. Belleville, J. P. Jolivet, J. Livage, Langmuir 8,
313 (1992).
References and Notes
1. W. Stumm, B. Sulzberger, Geochim. Cosmochim. Acta 56,
3233 (1992).
2. H. D. Pedersen, D. Postma, R. Jakobsen, Geochim.
Cosmochim. Acta 70, 4116 (2006).
3. O. Larsen, D. Postma, Geochim. Cosmochim. Acta 65,
1367 (2001).
4. C. M. Hansel, S. G. Benner, S. Fendorf, Environ. Sci.
Technol. 39, 7147 (2005).
Supplementary Materials
www.sciencemag.org/cgi/content/full/337/6099/1200/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S16
Table S1
References (37–50)
5. H. D. Pedersen, D. Postma, R. Jakobsen, O. Larsen,
Geochim. Cosmochim. Acta 69, 3967 (2005).
30. J. Tang, M. Myers, K. A. Bosnik, L. E. Brus, J. Phys. Chem. B
107, 7501 (2003).
20 April 2012; accepted 11 July 2012
10.1126/science.1223598
keto-enol tautomerization occurs under atmo-
spheric conditions and provides such a source.
Carbonyl compounds feature centrally in all
atmospheric oxidation pathways (6). Their role
toward the end of the oxidation pathway is also
clear, with measurable concentrations of acetalde-
hyde and acetone present in smog chambers (7).
The atmospheric photochemistry of carbonyls
seems uncontroversial. In the case of acetalde-
hyde, there are only two reactions that are impor-
tant in the troposphere, where photochemistry
is limited to wavelengths l > 295 nm (8).
Photo-Tautomerization of Acetaldehyde
to Vinyl Alcohol: A Potential Route
to Tropospheric Acids
Duncan U. Andrews,* Brianna R. Heazlewood,*† Alan T. Maccarone,‡ Trent Conroy,
Richard J. Payne, Meredith J. T. Jordan,§ Scott H. Kable§
Current atmospheric models underestimate the production of organic acids in the troposphere.
We report a detailed kinetic model of the photochemistry of acetaldehyde (ethanal) under
tropospheric conditions. The rate constants are benchmarked to collision-free experiments,
where extensive photo-isomerization is observed upon irradiation with actinic ultraviolet radiation
(310 to 330 nanometers). The model quantitatively reproduces the experiments and shows
unequivocally that keto-enol photo-tautomerization, forming vinyl alcohol (ethenol), is the
crucial first step. When collisions at atmospheric pressure are included, the model quantitatively
reproduces previously reported quantum yields for photodissociation at all pressures and
wavelengths. The model also predicts that 21 T 4% of the initially excited acetaldehyde forms
stable vinyl alcohol, a known precursor to organic acid formation, which may help to account
for the production of organic acids in the troposphere.
CH3CHO þ hn → CH3 þ HCO
→ CH4 þ CO
ð1Þ
ð2Þ
Under atmospheric conditions, the photolysis
quantum yield of acetaldehyde via reactions 1 and
2 is fph = 14% (vide infra). Atmospheric models
assume implicitly that the other 86% of mole-
cules collisionally cool, reverting to thermalized
acetaldehyde. We demonstrate experimentally
rganic acids are important trace compo- been interest in enols as efficient precursors for
nents of the troposphere, with estimates the formation of organic acids (4, 5). Archibald
ranging from 60 to 120 megatonnes et al. (4) modeled the atmospheric fate of enols
School of Chemistry, University of Sydney, NSW, 2006, Australia.
*These authors contributed equally to this work.
†Present address: Department of Chemistry, University of
Oxford, Oxford, OX1 3QZ, UK.
‡Present address: Department of Chemistry, University of
Wollongong, NSW, 2522, Australia.
§To whom correspondence should be addressed. E-mail:
scott.kable@sydney.edu.au (S.H.K.); m.jordan@chem.usyd.
O
formed per year (1, 2). However, the mechanism and concluded that the production of formic acid
responsible for producing these acids is incom- (HCOOH) was significantly enhanced by their
plete (2–5); current models underpredict their inclusion into the model. However, a viable at-
production by about a factor of two (1), implying mospheric source of enols has not been sug-
a missing acid precursor (2). Recently, there has gested. We present evidence that photo-induced edu.au (M.J.T.J.)
www.sciencemag.org SCIENCE VOL 337 7 SEPTEMBER 2012
1203

REPORTS
and theoretically that photo-induced keto-enol lution of population with time for photolysis at f1 and f2, and direct versus indirect products, f1a
tautomerization of acetaldehyde to vinyl alcohol 320 nm. When the reaction is complete, the main and f1b, validating the critical aspects of the ki-
(VA) is significant under collision-free condi- products are CO + CH3D (f2 = 15%) and formyl + netic model.
tions. Under atmospheric conditions, we predict methyl radicals (f1 = 85%) partitioned as direct
that VAwill constitute a significant fraction of the products (f1a = 70%) and indirect products (f1b
The pressure dependence of acetaldehyde
photolysis was modeled using a one-dimensional
=
photochemical product yield and propose that 15%). Experimentally, at 320 nm, f1/f2 = 6 when ME simulation, as implemented in the MultiWell
photo-tautomerization might be responsible for a extrapolated to zero pressure (12), in excellent program package (13–15), employing an exponen-
significant concentration of organic acids un- agreement with the 85/15 ratio above. The calcu- tial down collisional energy-transfer model and
accounted for in current models.
lations reveal that the dominant mechanism for Lennard-Jones collision frequencies with N2 at
A detailed description of the experiments can reaction 1b, with >90% of the flux, is the isomer- 300 K. The average downward collisional energy
be found in the supplementary materials. In brief, ization pathway shown in Fig. 2.
CH3CDO, cooled in a supersonic expansion of
transfer is taken to be hDEdowni = 150 cm–1, as
The calculated yields f1a and f1b at zero pres- described in the supplementary materials and as
helium, was photolysed at a variety of wave- sure are benchmarked by our experimental data, used previously (16). We assume that vibration to
lengths using a laser. The production of formyl also under conditions of few collisions. The solid rotational/translational (V-R,T) energy transfer
fragments was measured by laser-induced fluo- line in Fig. 1B shows the calculated ratio of f1b/f1 dominates because N2 has a high vibrational fre-
rescence. Isotopic labeling allows a more thor- as a function of wavelength. Triplet reaction is quency. We also assume hDEdowni is independent
ough investigation of the reaction mechanism. not included in the model, so it cannot reproduce of energy; we are only interested in collisional
Specifically, direct photolysis products can be the rapid fall-off observed for l < 320 nm. The deactivation above the keto-enol tautomerization
distinguished from indirect products that require model slightly overestimates f1b; however, the barrier (>282 kJ/mol) where the density of vi-
isomerization to CH2DCHO; for example, reac- wavelength dependence is reproduced. f1b is brational states is large. The rate of energy transfer
tion 1 manifests direct and indirect channels 1a and very sensitive to the height of the enol-carbene below this barrier only affects how quickly the
1b, respectively.
isomerization barrier shown in Fig. 2. If this stable products are thermalized, not the final
barrier is raised by only 2 kJ/mol, f1b is reduced, distribution of products.
as shown by the dotted curve in Fig. 1B, which
The ME model is benchmarked to experi-
CH3CDO þ hn → CH3 þ DCO
CH2DCHO → CH2D þ HCO
ð1aÞ
ð1bÞ
provides an excellent fit to the experiments at all ments of Horowitz and Calvert (HC) (12) and
energies below the T1 threshold. A correction of Moortgat et al. (MMW) (17). The wavelength
2 kJ/mol is well within the uncertainty in the cal- dependence of fph is shown in Fig. 3A, which
Figure 1A shows two action spectra of the S1 ← culated barrier height, estimated to be T5 kJ/mol, also shows that the selectedhDEdowni = 150 cm−1
S0 (n,p*) electronic transition of CH3CDO that based on the level of theory of the calculation. provides the best fit to experiment. For l > 300 nm,
were obtained by monitoring the production of Our model for the collision-free decomposition the model reproduces the experiments well, but
DCO or HCO photoproducts while scanning the of CH3CDO now reproduces quantitatively the it overestimates fph at l < 300 nm. At higher
photolysis wavelength. The vibrational structure relative quantum yield of both chemical channels, photolysis energies, quenching on T1 should be
is the same; however, the intensities are clearly
different, reflecting the changing quantum yield
Fig. 1. (A) CH3CDO action spectra
monitoring HCO and DCO (displaced
vertically). HCO intensity has been
increased by a factor of 5 with re-
spect to DCO. (B) The percentage of
HCO to total formyl radical produc-
tion measured at each dominant peak
in the spectra. Representative error
bars are provided. The results of a
ME model are shown as a solid line.
The effect of raising the rate-limiting
transition state (TS) energy by 2 kJ/mol
is shown by a dotted line.
for direct (f1a) versus indirect (f1b) products as a
function of wavelength. The ratio of indirect to
total formyl yields, f1b/f1, derived from these in-
tensities is plotted in Fig. 1B, as explained in the
supplementary materials.
At l > 320 nm, photolysis can only occur on
the ground S0 state, following internal conver-
sion from S1, and here f1b/f1 falls slowly from 20%
at 330 nm to 12% at 320 nm. At 320 nm, there is a
sudden decrease in f1b, indicated by an exponen-
tial function in the figure. This arises because of
intersystem crossing from S1 to T1. On the triplet
surface reaction 1b cannot occur, whereas reaction
1a becomes increasingly dominant at higher en-
ergy (9–11), with f1b/f1 falling to ≈0 by 312 nm.
The production of HCO and DCO on S0 was
modeled by calculating transition state theory
(TST) or variational TST rate constants for every
isomerization and dissociation process. These cal-
culations mirror those that we published (9) for
another isotopolog of acetaldehyde, CD3CHO,
and so an extensive description of the theoretical
methods is left to the supplementary materials.
Table S1 lists the critical energies, and table S2
presents a set of rate constants at a typical energy.
A collision-free master equation (ME) analy-
sis was then used to calculate the population of
each species as a function of time, with final
populations numerically converged to <1%. More
details of the ME modeling are provided as sup-
plementary materials. Figure S2 shows the evo-
1204
7 SEPTEMBER 2012 VOL 337 SCIENCE www.sciencemag.org

REPORTS
included (with a different quenching rate). How- explore the effect of collisional quenching under
The consequences of keto-enol tautomerization
ever, the solar flux at l < 300 nm is small, and the atmospheric conditions on keto-enol tautomeri- for the atmospheric photochemistry of acetalde-
discrepancies between our ME model and ex- zation. Figure 4A shows the predicted quantum hyde can now be evaluated. The rate k of a photo-
perimental results will be unimportant for at- yield of stable enol, fenol, as a function of wave- chemical reaction in the troposphere is the product
mospheric chemistry.
length, for hDEdowni = 150 cm–1 and 1 atm pres- of the actinic flux J(l), the absorption cross section
HC and MMW also report the dependence of sure. The yield is a maximum at ~315 nm; at s(l), and the quantum yield f(l) for the specific
fph on N2 or CO2 pressure at specific wave- shorter wavelengths, the increasing fph causes a photochemical process, integrated over wavelength.
lengths (12, 17). Figure 3B shows their experi- decrease in fenol, whereas at longer wavelengths
mental data as a Stern-Volmer (SV) plot of 1/fph the molecule cools before keto-enol tautomeriza-
versus pressure, P (dashed lines). The experi- tion reaches equilibrium. The most sensitive pa-
k ¼ ∫JðlÞsðlÞfðlÞdl
ð3Þ
mental data at 331.4 nm have considerable un- rameter in the model is the height of the keto-enol Figure 4B shows s(l) (19) and J(l) under typical
certainty, and the zero pressure intercept is reported barrier. Figure 4A also shows the results of the conditions (albedo = 0.3 and overhead sun) (6) in
as 1/fph = 18 (fph = 0.06) (12). However, at zero ME model when the barrier is raised and lowered the spectral region where both are nonzero.
pressure, fph must be ≈1 because the radiative by 5 kJ/mol, an estimate of the theoretical un- Figure 4C shows the right-hand side of Eq. 3,
quantum yields are very small [< 0.01 at all certainty. The predicted fenol changes slightly; integrated over 1-nm intervals, for absorption of
wavelengths here (18)]. We have replotted all the the maximum quantum yield increases or de- light (i.e., f = 1) and for photolysis and photo-
experimental SV plots in Fig. 3B, assuming a creases by about 4%, giving an estimate of the tautomerization using fph from MMW (17) in
zero pressure intercept of 1, and compared them computational uncertainty.
Fig. 3A and fenol from Fig. 4A. The overall pho-
to our ME model results (solid lines). The ME The collision-free ME model has only been tolysis quantum yield is 14%, in agreement with
model provides a good representation of the ex- strictly validated for l > 320 nm (Fig. 1B). experiments (12, 17). The worst-case scenario for
perimental data, with the largest variation occur- However, the collisional ME model appears valid enol production, if the T1 reaction follows the
ring at 331.4 nm, where experimental uncertainties at atmospheric pressure for l > 300 nm (Fig. 3A) collision-free quantum yield (Fig. 4A), is 13%,
are largest. Indeed our ME model predictions sit because quenching on S1 and T1 competes with effectively the same as the photolysis yield. Our
between the two independent sets of experimen- reaction on T1. As a worst case, we might assume best estimate of fenol is 21 T 4%.
tal data at l = 313 nm.
We now consider the fate of excited acetal- applies at P= 1 atm. The result of this assumption pound that can undergo keto-enol tautomeriza-
dehyde molecules that do not photodissociate and is shown by the dashed curve in Fig. 4A.
tion, and its photochemical properties are typical
that the sharp decrease in f1b/f1 in Fig. 1B also
Acetaldehyde is the simplest carbonyl com-
Fig. 2. A schematic showing energies (kJ/mol) and important structures on the free conditions. Under atmospheric conditions, we predict that excited acetaldehyde
CH3CDO S0 potential energy surface for the production of DCO and HCO. The green collisionally relaxes into both keto and enol forms, as shown by the blue curvy
arrows indicate the experimental yields of HCO and DCO at 320 nm under collision- arrows. All energies are from this work, as defined in the supplementary materials.
www.sciencemag.org SCIENCE VOL 337 7 SEPTEMBER 2012
1205

REPORTS
of smaller carbonyls. The (n,p*) transition gen- vinyl alcohol is formic acid (4, 5). Despite this,
It is interesting to speculate about photo-
erally lies between 280 and 330 nm, with similar they have not been included in current atmo- tautomerization for larger carbonyls. For linear
absorption cross sections throughout the family spheric models for two reasons. First, enols have aldehydes with ≥4 C atoms, the rate of a Norrish
(8, 19). The barrier to tautomerization is also sim- not been observed in the atmosphere. As reactive type II mechanism dominates even C-C bond
ilar (20). Therefore, we expect photo-tautomerization intermediates, their concentration will be very cleavage (21), so we expect photo-tautomerization
of small aldehydes and ketones to be a general low. Second, there is no sufficiently large atmo- to be less important for these compounds. When
characteristic of their fate in the atmosphere.
spheric source of enols. On the basis of this work, chain flexibility is hindered by unsaturation, branch-
Enols have a different atmospheric chemistry we propose that photo-tautomerization of car- ing, or cyclization, the Norrish type II mechanism
from carbonyls; indeed, the atmospheric fate of bonyls to enols may provide such a source.
will likely turn off again, whereas tautomeriza-
tion will always be present. Evaluation of the
importance of this new mechanism for enol
formation under the complex chemistry of the
troposphere requires its inclusion in the rele-
vant atmospheric models. However, good ex-
perimental data are first required for a range
of prototypical carbonyls under atmospheric
conditions.
Fig. 3. Comparison of photo-
chemical quantum yields from the
ME model and available experiment
data. (A) fph as a function of wave-
length for P = 1 atm. (B) Pressure
dependence shown as Stern-Volmer
plots. The dashed lines are derived
from reported SV fits to experimental
data. The ME model used 〈DEdown〉 =
150 cm−1. In both panels: HC (12),
MMW (17).
Wavelength (nm)
300
310
320
330
0.5
0.4
0.3
0.2
0.1
0.0
12
10
8
Exp’t
∆Edown (cm−1
)
<
>
HC
MMW
100
150
200
References and Notes
1. A. Ito, S. Sillman, J. E. Penner, J. Geophys. Res.
112, D06309 (2007).
2. F. Paulot et al., Atmos. Chem. Phys. 11, 1989
(2011).
3. A. H. Goldstein, I. E. Galbally, Environ. Sci. Technol.
A
B
Model
HC
MMW
41, 1514 (2007).
6
4. A. T. Archibald, M. R. McGillen, C. A. Taatjes,
C. J. Percival, D. E. Shallcross, Geophys. Res. Lett.
34, L21801 (2007).
5. G. da Silva, Angew. Chem. Int. Ed. 49, 7523 (2010).
6. B. J. Finlayson-Pitts, J. N. Pitts Jr., Chemistry of
the Upper and Lower Atmosphere (Academic Press,
San Diego, CA, 2000).
7. A. Lee et al., J. Geophys. Res. 111, D17305 (2006).
8. R. Atkinson et al.; IUPAC Subcommittee, Atmos. Chem.
Phys. 6, 3625 (2006).
4
2
304 nm
700
0
0
100
200
300
400
500
600
800
Pressure (torr)
9. B. R. Heazlewood et al., Nat. Chem. 3, 443 (2011).
10. B. R. Heazlewood, S. J. Rowling, A. T. Maccarone,
M. J. T. Jordan, S. H. Kable, J. Chem. Phys. 130, 054310
(2009).
11. A. C. West et al., J. Phys. Chem. A 113, 12663 (2009).
12. A. Horowitz, J. G. Calvert, J. Phys. Chem. 86, 3105
(1982).
13. MultiWell-2012.1 Software, 2012, designed and
maintained by J. R. Barker with contributors N. F. Ortiz et al.;
University of Michigan, Ann Arbor, MI; http://aoss.engin.
umich.edu/multiwell.
14. J. R. Barker, Int. J. Chem. Kinet. 33, 232 (2001).
15. J. R. Barker, Int. J. Chem. Kinet. 41, 748 (2009).
16. L. B. Harding, Y. Georgievskii, S. J. Klippenstein,
J. Phys. Chem. A 114, 765 (2010).
Fig. 4. (A) Quantum
yield for enol formation
according to the ME mod-
el for three keto-enol bar-
rier heights. (B) Solar flux,
J, and acetaldehyde ab-
sorption cross section, s.
(C) Absorption and pho-
tochemistry rates. The area
surrounding the photo-
tautomerization line repre-
sents the computational
uncertainty. The dashed
lines in (A) and (C) repre-
sent a worst-case correc-
tion for triplet reaction.
17. G. K. Moortgat, H. Meyrahn, P. Warneck, ChemPhysChem
11, 3896 (2010).
18. D. A. Hansen, E. K. C. Lee, J. Chem. Phys. 63, 3272
(1975).
19. R. D. Martinez, A. A. Buitrago, N. W. Howell, C. H. Hearn,
J. A. Joens, Atmos. Environ. 26A, 785 (1992).
20. P. Pérez, A. Toro-Labbe, J. Phys. Chem. A 104, 1557 (2000).
21. J. Tadic, I. Juranic, G. K. Moortgat, J. Photochem.
Photobiol. A 143, 169 (2001).
Acknowledgments: This work was funded by the Australian
Research Council (DP1094559) and National Computational
Infrastructure Facility. We thank G. Da Silva (University of
Melbourne) for discussions about acids in the atmosphere.
Supplementary Materials
www.sciencemag.org/cgi/content/full/science.1220712/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S3
Tables S1 to S3
References (22–35)
17 February 2012; accepted 1 August 2012
Published online 16 August 2012;
10.1126/science.1220712
1206
7 SEPTEMBER 2012 VOL 337 SCIENCE www.sciencemag.org