Article pubs.acs.org/JPCA
Near Infrared Photochemistry of Pyruvic Acid in Aqueous Solution Molly C. Larsen and Veronica Vaida* Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: Recent experimental and theoretical results have suggested that organic acids such as pyruvic acid, can be photolyzed in the ground electronic state by the excitation of the OH stretch vibrational overtone. These overtones absorb in the near-infrared and visible regions of the spectrum where the solar photons are plentiful and could provide a reaction pathway for the organic acids and alcohols that are abundant in the earth’s atmosphere. In this paper the overtone initiated photochemistry of aqueous pyruvic acid is investigated by monitoring the evolution of carbon dioxide. In these experiments CO2 is being produced by excitation in the nearinfrared, between 850 nm and ∼1150 nm (11 765−8696 cm−1), where the second OH vibrational overtone (Δν = 3) of pyruvic acid is expected to absorb. These findings show not only that the overtone initiated photochemical decarboxylation reaction occurs but also that in the aqueous phase it occurs at a lower energy than was predicted for the overtone initiated reaction of pyruvic acid in the gas phase (13 380 cm−1). A quantum yield of (3.5 ± 1.0) × 10−4 is estimated, suggesting that although this process does occur, it does so with a very low efficiency.
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INTRODUCTION Most photochemical reactions considered in atmospheric chemistry have focused on reactions initiated by high energy ultraviolet photons that typically excite molecular electronic transitions,1−3 giving the molecule ample energy to break covalent molecular bonds. An alternative mechanism for inducing photochemical reaction is excitation of a molecule with visible and near-infrared light to a reactive state along the ground-state potential energy surface, through the excitation of vibrational overtones of the molecule.4−30 This type of reaction is more difficult to initiate because the forbidden overtone transitions have small cross sections and the energy supplied in these excitations is much less than for excitation in the ultraviolet.5,6,8,29,30 However, OH stretch overtones occur at high frequencies with some intensity so exciting only a few quanta of this overtone can provide enough energy for a molecule to overcome a reaction threshold.4,8,17−22,24,25,29,31−44 Overtone initiated photochemistry has recently been shown to be important in explaining discrepancies between atmospheric measurements and models.5,6,8,9,11−13,15,16,30 This type of photochemistry is expected to be particularly important for molecules with UV absorption spectra that do not overlap with the solar spectrum or at high solar zenith angles, when the UV light has been attenuated significantly.4−6,8,9,12,13,15,27,29,30,45 This overtone initiated photochemistry has been predicted for carboxylic acids and alcohols.5,7,8,10,13−15,26,28,29,36,46−48 In pyruvic acid7,8 excitation of the carboxylic acid’s OH vibrational overtone is expected to lead to decarboxylation and evolution of CO2. In this paper experiments are presented where the OH overtones of pyruvic acid in aqueous solutions are excited and the evolution of the CO2 photoproduct is monitored. © 2012 American Chemical Society
Pyruvic acid is abundant in the atmosphere both in the gas phase and in aerosols as it is an oxidative product of isoprene.49−52 Pyruvic acid has been used as a proxy for modeling the behavior of other atmospheric α-dicarbonyls.53,54 Solutions of pyruvic acid have been used as a model for organic aerosol matter.55 Pyruvic acid can be decarboxylated with UV excitation,52,53,55−70 thermally,55,56,65,71,72 and through infrared multiphoton pyrolysis.73−75 Recent gas-phase experimental and theoretical results suggest that pyruvic acid should be the ideal model system to use for the experimental observation of the vibrational overtone initiated photochemical reaction of αdicarbonyls.7,8,54 In gas-phase pyruvic acid experiments, significant spectral broadening was observed for the excitation of the third (ΔνOH = 4) and fourth (ΔνOH = 5) overtone of the lowest energy conformer of pyruvic acid.7,8,54 Theoretical results helped assign this broadening as being due the initiation of a concerted reaction of pyruvic acid, where two covalent bonds are broken simultaneously at energies below the bond dissociation energy.7,8,29 The predicted photoproducts are CO2 and methylhydroxycarbene.7,8 However, due to the low density of pyruvic acid molecules in the gas phase and the low absorption cross sections of the overtones, the formation of CO2 and other organic photoproducts were not experimentally observed. Special Issue: A. R. Ravishankara Festschrift Received: September 12, 2011 Revised: January 9, 2012 Published: January 10, 2012 5840
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two cylindrical arms that were sealed with calcium fluoride windows to allow for the FTIR monitoring of the air above the photolyzed liquid. The cell was placed in a cooled water bath in a Styrofoam box. The cell was surrounded by copper tubing that was chilled using a Neslab RTE4 refrigerated circulating bath to keep the sample’s water bath at ∼2−5 °C during the photolysis. Some heating of the sample was observed, but as described in the Supporting Information, this heating did not affect the photolysis results. The freeze/pump/thaw preparation of the samples was done the day before the photolysis experiments and the sample was kept chilled in the water bath overnight. Photolysis was done using a Newport 450 W xenon arclamp. For all experiments a 435 nm long pass filter (Edmund Optics) was placed on top of the cell in the beam’s path to avoid exciting the UV band of pyruvic acid, which extends out to ∼400 nm.56,76 Only the beam that went through the filter was allowed to enter the cell; all other stray light was blocked. In experiments where other filters were used, the second filter was placed on top of the 435 nm filter so that the beam went through both filters before illuminating the sample. All the filters, except for the 1100 nm short pass filter, were 2 in. square. The 1100 nm short pass filter (Edmund Optics) was 25 mm in diameter. The lamp intensity was measured by using a International Light RSP900 wide band spectroradiometer. Because the area of the detector is smaller than the beam diameter, the beam profile was divided into 16 regions with each region being measured separately. Only the visible portion of the beam intensity is measured and the infrared intensity is determined by the known spectral shape of the xenon lamp as obtained from Newport. The absorption spectrum of the 1100 nm short pass filter was taken using a Shimadzu UV-3600 UV−vis−NIR spectrophotometer. The Shimadzu spectrometer was also used to take an absorption spectrum of a 4.26 M sample of aqueous pyruvic acid and the spectrum of a distilled 98% pyruvic acid sample for comparison. Spectra of the samples were taken in a 1 cm quartz cell with the empty quartz cell used as a reference. Scans to monitor the rate of CO2 production were taken at a 0.5 cm−1 resolution using a Fourier transform infrared spectrometer (Bruker IFS 66v/s). Data were taken using the external mode of the spectrometer with a glowbar source and a mercury cadmium telluride detector. Plastic bags were used to encase the detector and to isolate the beam path from the environment to prevent CO2 contamination. These plastic bags were continuously purged with air that was purified using a Balston FTIR purge gas generator to eliminate contamination from ambient fluctuations of CO2. In each experiment the background levels of CO2 were monitored before the lamp was turned on to make sure the bags and the cell were tightly sealed. Once the light was turned on, scans to monitor the production of CO2 were performed every ∼2−10 min. The only gas-phase product observed, other than CO2, was water that likely came from the small amount of heating of the sample (see Supporting Information). For all the CO2 kinetic data the integrated absorption of CO2 was determined using the Bruker integration software. Peaks were integrated from 2250 to 2400 cm−1 with the baseline being approximated as a linear function between the two end points. The integrated absorption was converted to a concentration using the 2008 HITRAN values for the absorption of CO2 in the integrated region,77 the volume of
In this paper results on the evolution of CO2 due to the photoexcitation in the near-infrared of pyruvic acid in the aqueous solution phase are presented. By studying the overtone initiated reaction in solution, we detemined the effect of water on the reaction. Clear evidence that low-energy photons cause the production of CO2 is observed, suggesting that an overtone initiated reaction does occur in pyruvic acid.7,8 Photoreaction in aqueous solution occurs at energies below the predicted gasphase reaction threshold, and below the expected reaction energy to form CO2 and the methylhydroxycarbene. This suggests that in water the reaction proceeds by a different mechanism, likely forming different products, than was predicted in the gas phase.8,9,29,47
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EXPERIMENTAL SECTION A schematic of the experimental setup is shown in Figure 1. Solutions were made using pyruvic acid, (Sigma-Aldrich, 98%)
Figure 1. Schematic of the photochemical experiment. Filtered radiation from a xenon lamp is used to excite pyruvic acid samples from the top, and the CO2 produced is monitored using FTIR spectroscopy. The sample cell volume is 632 mL. Plastic bags purged with filtered air were used to keep ambient CO2 contamination out of the beam path between the FTIR and the detector. The sample was kept in a chilled water bath.
which was double distilled under reduced pressure, and Nanopure water (NREL reagent grade, 18 MΩ). Solutions were analyzed before photolysis using electrospray ionization mass spectrometric detection (ESI-MS(−)) to check for the presence of impurities. Only pyruvic acid and the pyruvic acid dimer were observed in significant quantities. Photolysis was done on solutions of 4.26 M (mole fraction ∼0.10) and 7.10 M (mole fraction ∼0.15) pyruvic acid. These large concentrations were necessary to observe the produced CO2 because the absorption cross section of the overtone is very small and because (as shown below) the quantum yield for this process is very low; therefore, very little CO2 is produced upon illumination. The pH of the 4.26 M solution was measured to be 18 868 cm−1) was inserted into the beam (Figure 3, orange triangles). This filter decreased the rate of production of CO2 to (3.4 ± 2) × 10−8 M/min. This confirms that it was not the excitation of the low energy edge of the electronic absorption that caused the CO2 production.79 The filter with λ < 530 nm was then replaced by a HA 50 filter (HOYA) that has a broad cutoff and blocks light with λ > ∼800−1000 nm (ν < ∼12 500−10 000 cm−1) (Figure 3, blue squares). This filter caused the CO2 production to decrease to (9 ± 9) × 10−9 M/min. This is strong evidence that it was light in the near-infrared, with λ > ∼800−1000 nm, which caused the production of CO2. Finally, the HA 50 filter was replaced by a filter that blocks light with λ < 850 nm (ν > 11 765 cm−1) (Edmund) (Figure 3, black crosses). This filter caused the rate of production of CO2 to increase to (3.0 ± 0.9) × 10−8 M/min, nearly the rate of production of CO2 when only the 435 nm filter was in the beam. This indicates that it is infrared or near-infrared light (with λ > 850 nm, ν < 11 765 cm−1) that caused the production of CO2. Because this energy is far from any electronic absorption of pyruvic acid, this result provides strong evidence that what was observed was a vibrational overtone initiated reaction. To further narrow the range of possible excitation energies that could have caused the production of CO2, experiments were performed where the volume of the 4.26 M solution was doubled to 98 mL. After the initial photolysis of this sample, a 1100 nm short pass filter was inserted into the beam, causing the rate of CO2 production to decrease but not disappear. The results of this experiment suggest that only a narrow energy window, between 850 nm and ∼1150 nm (11 765−8696 cm−1), was responsible for the production of CO2. The results for this experiment can be seen in Figure 4. The rate of production of CO2 when the sample volume was doubled to 98 mL is slightly more than double the rate of production of CO2 in the experiment where the 49 mL sample was used (Figure 2); i.e., the rate of CO2 per volume of solution did not significantly change when the volume was doubled and is measured to be (5.54 ± 0.09) × 10−8 M/min. Because water has a very strong and broad absorption throughout the infrared that starts at ∼8696 cm−1 (∼1050 nm) and extends to lower energies,80 the
Figure 2. CO2 produced as a function of time upon photoexcitation of 49 mL of 4.26 M (blue diamonds) and 7.10 M (purple squares) samples of pyruvic acid in water. The lamp was turned on at 0 min. CO2 is given as moles of CO2 produced per liter of pyruvic acid sample. A filter that blocks light with λ < 435 nm (22 989 cm−1) has been placed in the beam. For the 4.26 M sample the lamp was entirely blocked from entering the sample 315 min after the lamp was turned on (green diamonds).
produced as a function of time when 49 mL of a 4.26 M pyruvic acid sample and 49 mL of a 7.10 M pyruvic acid sample were excited with a xenon lamp where all the light with λ < 435 nm (ν > 22989 cm−1) was blocked. As can be seen from this figure, CO2 was clearly produced, indicating that visible or infrared excitation of pyruvic acid does induce photolysis. For the 4.26 M sample the beam was blocked from entering the sample 315 min after the lamp was turned on to show that blocking the light turns off the production of CO2.78 The rate of production of CO2 is (4.73 ± 0.11) × 10−8 M/min for the 4.26 M sample and (7.7 ± 0.2) × 10−8 M/min for the 7.10 M sample; i.e., the rate approximately decreased by a factor of 0.6 when the concentration was decreased by a factor of 0.6. This could suggest that the mechanism of CO2 production is first order with respect to pyruvic acid or it could suggest that, in this very high-concentration limit, a pseudo-first-order reaction is what was observed. A series of glass filters were subsequently used to determine that excitation in the infrared, with λ > 850 nm (ν < 11 765 cm−1), is responsible for the production of CO2. The results of this experiment are shown in Figure 3. In this experiment, a filter that blocks light with λ < 435 nm (ν > 22 989 cm−1) was 5842
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occurs at about 1100 nm (9091 cm−1) in a region where ∼70% of the band is under the 1100 nm filter’s absorption band. This provides strict restrictions on the energies that can be responsible for photolysis, even without having to infer anything about the energy or the intensity of the overtone transition being excited. The low energy likely corresponds to a vibrational overtone transition and lies at significantly lower energy than the electronic transition of pyruvic acid (λ < ∼400 nm56,76). Quantum Yield of Overtone Initiated Reaction. Previous gas-phase and theoretical results suggest that the overtone initiated reaction of pyruvic acid should occur by excitation of pyruvic acid’s acidic OH stretching overtone.7,8 Furthermore, the OH stretching overtone is expected to have a relatively large cross section when compared to the overtones of other X−H bonds, making it a likely candidate for being the absorbing chromophore.8,25,29,34−36,82,83 To determine the absorption of the OH overtone of aqueous pyruvic acid that is responsible for the CO2 production, the absorption spectrum of pyruvic acid in the near IR is examined. In Figure 5 the absorption spectrum of a 4.26 M pyruvic acid sample in the region from 850 to 2000 nm (11 765−5000 cm−1) is presented. In this figure the absorption spectrum of distilled 98% pyruvic acid is also shown. From Figure 5 it can be seen that the absorption spectrum of aqueous pyruvic acid is very complicated in the region where the photochemistry occurs, due to the strong overlapping absorption of water80 and several conformers of pyruvic acid.84 Because the overtone cross section is expected to be very small,7,8,54 it is difficult to assign the overtone absorption peaks underneith all the other absorption transitions in the spectral region where the photochemistry occurs. Thus the gas-phase overtone spectrum and the photochemistry results are used to approximate both the position and the intensity of the reactive overtone of pyruvic acid. The position of the solution-phase OH overtone is estimated by using what is known about solution-phase overtones of other molecules with OH functional groups. Typical solutionphase vibrational OH stretch absorption bands consist of a sharp free OH peak, near where the gas-phase OH stretch absorbs, and a very broad red-shifted hydrogen bonded OH peak. This is true for both the OH stretch fundamental and the overtone transitions.34,35,82,85−88 The gas-phase fundamental of the most abundant Tc form of pyruvic acid absorbs at 3467 cm−1 (2884 nm).7,54 The overtone transitions absorb at 6696 cm−1 (1493 nm) for Δν = 2, 9973 cm−1 (1003 nm) for Δν = 3, and 12 920 cm−1 (774 nm) for Δν = 4.7,54 The absorption responsible for the photolysis of aqueous pyruvic acid is in the ∼1100 nm region, close to, but lower in energy than, the gasphase second overtone, Δν = 3, transition. We therefore assign this absorption as being due to the second overtone in the aqueous solution. For further details on how the position of this absorption band was estimated see the Supporting Information. To determine the efficiency of CO2 production from the overtone photolysis of pyruvic acid, the quantum yield is calculated. To get an estimate of the quantum yield, the firstorder rate constant is experimentally determined allowing the quantum yield to be calculated using the equation3
Figure 4. Amount of CO2 produced as a function of time upon photoexcitation of 98 mL of a 4.26 M sample of aqueous pyruvic acid. The lamp was turned on at 0 min with a filter in the beam that blocks light with λ < 435 nm (22 989 cm−1). The concentration of CO2 is given as moles of CO2 produced per liter of pyruvic acid sample. Approximately 220 min after the lamp is turned on, the 1100 nm filter (Figure 5) is inserted into the excitation beam.
fact that doubling the sample did not change the rate of production of CO2 (per volume of solution) puts very strict restrictions on the energy region that could be responsible for the pyruvic acid photochemistry, because excitation cannot occur in any region where water has a strong absorption. If it did, exciting the larger volume would not produce more CO2, because the water in the sample would effectively be acting as a filter, not allowing the light to penetrate through the sample. This indicates that the absorption must be coming from light between 850 nm (11 765 cm−1) and ∼1150 nm (∼8696 cm−1), the edge of water’s strong absorption band. After illuminating the larger volume sample with light, where only λ < ∼435 nm has been blocked for ∼220 min, an 1100 nm short pass filter was inserted into the beam (Figure 4, blue triangles). An absorption spectrum of the 1100 nm filter, which blocks light with λ < ∼460 nm (ν > 21 739 cm−1) and light between ∼1125 nm < λ < 1440 nm (∼8889 cm−1 > ν > 6944 cm−1), is shown in Figure 5. As can be seen from Figure 4, the
Figure 5. Near infrared absorption spectrum of a 4.26 M aqueous pyruvic acid sample (red line), a 98% pyruvic acid sample (green line), and the 1100 nm filter (black line).
1100 nm filter caused a significant decrease in, but did not entirely eliminate, the rate of CO2 production. The rate of CO2 production after the 1100 nm filter was inserted was (1.72 ± 0.18) × 10−8 M/min, ∼0.3 times the rate of CO2 production before the filter was inserted, indicating that the filter blocked ∼70% of the light available for the photolysis of pyruvic acid. This further narrows the spectral region that could be responsible for the CO2 production.81 The results of this experiment indicate that the pyruvic acid overtone absorption
j=
∫ F(λ) σ(λ) φ(λ) dλ
(1)
where j (s−1) is the first-order rate constant, F(λ) is the lamp intensity, σ(λ) is the absorption cross section, and ϕ(λ) is the 5843
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partially, as dimers.92,105,106 However, the extent of these dimers in aqueous pyruvic acid solutions has not been determined. Because the reaction occurs with a very low quantum yield, the identity of the solution-phase product could not be determined. However, even without knowing the solution-phase photoproducts, the energetics of these experiments make it clear that low energy excitation of pyruvic acid produces CO2 but that it does so through a different mechanism than was predicted for the gas phase.7,8,54 Finally, although pyruvic acid does react to form CO2, the estimated quantum yield is very small, ∼10−4. This is much smaller than the quantum yield of 0.39 that was observed for UV excitation of pyruvic acid,64 suggesting that this process is unlikely to be an important reaction pathway for pyruvic acid in the atmosphere. There are several possibilities for this observed low quantum yield. One reason could be that the conformer responsible for the reaction in the aqueous phase could be in low abundance. Another possible reason for the low quantum yield could be that other processes,11,107−109 such as vibrational relaxation or recombination, are competing with the reaction to produce CO2. For example, a low quantum yield was observed theoretically for the overtone initiated HF elimination in fluoromethanol, because of a competing reaction to dehydrate the fluoromethanol hydrate.108 In these calculations,108 the quantum yield was found to be very low for the elimination reaction near the reaction barrier, even though the reaction barrier was lowered by the presence of water. Similarly, competing processes could lower the quantum yield observed here for the OH vibrational overtone pumping of aqueous pyruvic acid. OH vibrational relaxation is expected to occur on an approximately picosecond time scale in the solution phase.110−112 If the only process competing with the overtone initiated reaction is vibrational relaxation, then for the observed quantum yield we expect a reaction rate to be occurring on a approximately nanosecond time scale. However, this would be a lower limit for the estimated reaction rate from the quantum yield because it is likely that other processes, such as molecular recombination, are also competing with the overtone initiated reaction.
quantum yield. The first-order rate constant used is the average of the slopes from the data in Figure 2 (blue diamonds), Figure 3 (yellow squares), and Figure 4 (yellow squares) divided by the concentration of pyruvic acid (4.26 M). This gives the firstorder rate constant (1.8 ± 0.5) × 10−10 s−1.89 The quantum yield and the absorption cross section are assumed to be independent of wavelength over the wavelength range used. The absorption cross section is estimated by using the oscillator strength from theoretical results,54 assuming that it does not change when the sample is in the solution phase. Using these approximations, a quantum yield of (3.5 ± 1.0) × 10−4 is obtained for the OH vibrational overtone excited pyruvic acid in aqueous solution. In the Supporting Information further information on how the quantum yield was calculated is provided.
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DISCUSSION The experiments described show that excitation of aqueous pyruvic acid between 850 nm and ∼1150 nm (11 765−8696 cm−1) produces CO2. This energy is much lower than the pyruvic acid electronic transition, which extends out to ∼400 nm,56,76 suggesting that the reaction proceeds along the ground state potential energy surface and is likely an overtone initiated reaction. That pyruvic acid would undergo an overtone initiated photoreaction to produce CO2 was predicted from the spectral width analysis of the OH overtone transitions of pyruvic acid in the gas phase.7,8,54 In gas-phase pyruvic acid, theoretical results were used to help assign the observed OH overtone spectral broadening to energy leaving the OH overtone stretch and coupling to a hydrogen atom chattering mode, where the hydrogen atom translates quickly back and forth between the acidic and the carbonyl oxygens, followed by large distensions and breaking of the C−C bond to form CO2 and the methylhydroxycarbene.7,8,90 The calculated energy change for this reaction is 14 200 cm−1.7 This suggests that even at the high energy edge of the absorption band measured in these experiments there is not enough energy to form the methylhydroxycarbene photoproduct7,91 and that the reaction leading to CO2 formation in aqueous solution proceeds by a different mechanism than was predicted in the gas phase. Although the Tc form of pyruvic acid was predicted to be responsible for the formation of (methylhydroxy)carbene and CO2 in the gas phase, the fact that the (methylhydroxy)carbene is unlikely to be the photoproduct in the solution phase following overtone excitation suggests another conformer of pyruvic acid could be responsible for the photochemistry. In the aqueous phase, pyruvic acid is known to have many conformations,84,92,93 in addition to the Tc and Tt forms observed in the gas phase, all of which could also be responsible for the observed photochemical reaction. In the restricted water−carbontetrachloride matrix experiments, even on samples that have the same water/pyruvic acid ratios as the samples used in this experiment, the Tc form was the most abundant, but three stable keto conformations and one stable enol form have also been observed.84,94 These enol forms are likely to be more prevalent at the low pH conditions of this experiment.95 Aqueous pyruvic acid, like many other aldehydes and ketones, is also known to be present in the diol form, where the nonacidic carbonyl reacts with water and is replaced by two hydroxy groups.10,84,92,96−103 At the high pyruvic acid concentrations used in this experiment the diol was estimated to be ∼52% of the sample.84,104 At higher concentrations, it has also been suggested that pyruvic acid could exist, at least
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CONCLUSIONS The photochemical study reported finds that aqueous pyruvic acid reacts to form CO2 at energies between 8696 and 11 765 cm−1, much lower than any electronic excitation energies of this molecule. This reaction is assigned as being initiated by OH vibrational overtone transitions in the ground electronic state. As far as we know, this is the first time CO2 production has been directly observed following vibrational overtone excitation of a carboxylic acid, even though the chemistry has been predicted for this7,8 and other10,14,28 systems. This reaction occurs at an energy below the expected energy required to form CO2 and the methylhydroxycarbene, suggesting that a different reaction than was predicted in the gas phase is being observed. A quantum yield for the overtone initiated photochemical reaction of (3.5 ± 1.0) × 10−4 is estimated. This quantum yield is too low to expect that this reaction pathway would be able to compete with the UV initiated decarboxylation of pyruvic acid52,53,55−70 or that this reaction pathway would have significant atmospheric implications. However, the fact that this reaction is observed for pyruvic acid would suggest that similar low energy photochemical reactions could also be possible for other carboxylic acids5,7,8,10,13−15,26,28,29,46 and alcohols,8,29,36,47,48 which do not have the same UV absorption 5844
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that pyruvic acid does. Further experiments need to be done to determine the solution-phase mechanism and products for the overtone initiated reaction of pyruvic acid and to look at other carboxylic acids that could also undergo this overtone initiated photochemistry.
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ASSOCIATED CONTENT
* Supporting Information S
Details of the effect of temperature and details of how the quantum yield was calculated. This information is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ACKNOWLEDGMENTS This work was funded by the Cooperative Institute for Research in Environmental Sciences visiting fellowship and the National Science Foundation under grant number CHE1011770. We thank Dr. Andrew Ferguson and Dr. Ross Larsen for taking the UV−vis absorption spectra at the National Renewable Energy Lab, Meghan Dunn, Dr. Katy Plath, and Jessica Axson for help with experiments, Dr. Marta Maron for providing information on her NMR results, and Dr. Shuji Kato for doing the electrospray ionization mass spectrometry. We also thank Dr. Rex T. Skodje, Zebuliah C. Kramer, and Dr. Kaito Takahashi for helpful discussions regarding the theory of aqueous pyruvic acid.
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REFERENCES
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