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Decomposition of Pyruvic Acid on the Ground State Potential Energy Surface Gabriel da Silva J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b10078 • Publication Date (Web): 20 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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The Journal of Physical Chemistry

Decomposition of Pyruvic Acid on the Ground State Potential Energy Surface

Gabriel da Silva*

Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010 Australia

*[email protected]

ABSTRACT A potential energy surface is reported for isomerization and decomposition of gas-phase pyruvic acid (CH3C(O)C(O)OH) in its ground electronic state. Consistent with previous works, the lowest energy pathway for pyruvic acid decomposition is identified as decarboxylation to produce hydroxymethylcarbene (CH3COH), with overall barrier of 43 kcal mol-1. This study discovers that pyruvic acid can also isomerize to the α-lactone form with a barrier of only 36 kcal mol-1, from which CO elimination can occur at 49 kcal mol-1 above pyruvic acid. An additional novel channel is identified for the tautomerisation of pyruvic acid to the enol form, via a double H-shift mechanism. The barrier for this process is 51 kcal mol-1, which is around 20 kcal mol-1 lower than the barrier for conventional keto-enol tautomerisation via a 1,3-H shift transition state. Rate coefficients are calculated for pyruvic acid decomposition through RRKM theory / master equation simulations at 800 – 2000 K and 1 atm, showing good agreement with the available experimental data. The dissociation of vibrationally excited pyruvic acid produced through photoexcitation and subsequent internal conversion to the ground state is also modelled under tropospheric conditions, and is seen to produce appreciable quantities of CO (~ 1 - 4 %) in addition to CH3COH via the dominant CO2 loss channel.

Keywords: Pyruvic acid; α-keto acids; α-lactones; tautomerization; thermal decomposition; photolysis

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Introduction Pyruvic acid (CH3C(O)C(O)OH) is a key atmospheric keto acid, encountered at significant levels in the atmosphere and in atmospheric aerosols,1 particularly in forested2,3 and urban4,5 environments. In the gas phase pyruvic acid is commonly detected at tens to hundreds of ppt, despite its short lifetime with respect to photolysis.6,7 Because of its high solubility in water it also comprises a significant fraction of particulate matter. Pyruvic acid is primarily formed in the photochemical oxidation of isoprene,2,3 although it also has primary sources from vehicular emissions.4,5 In addition to being an important trace atmospheric component in its own right, pyruvic acid also serves as one of the simplest model α-keto acids. Pyruvic acid is primarily removed from the atmosphere via photolysis,6,7 and this process has been extensively studied.8-15 Pyruvic acid absorbs actinic UV radiation in the range of around 380 – 300 nm,16-17 and in the gas phase it undergoes decarboxylation with high quantum yields. Excitation of O–H stretch overtones by near-IR (third νOH overtone, 12 800 cm-1) and visible (fourth νOH overtone, 16 000 cm-1) light may also lead to decarboxylation,1819 although absorption cross sections here are significantly lower. In both cases, however, reaction is thought to be dominated by unimolecular processes transpiring on the vibrationally excited ground electronic state surface, following internal conversion in the former case.20 Photolysis experiments, thermal decomposition studies,21-23 and theoretical investigations18,20,24,25 all support a mechanism in which decarboxylation occurs via concerted intramolecular H atom transfer and C–C bond dissociation to produce hydroxymethylcarbene (CH3COH), which can subsequently rearrange to acetaldehyde and vinyl alcohol if provided with sufficient energy.26 This work revisits the unimolecular rearrangement and dissociation of pyruvic acid on the ground state surface. Surprisingly we reveal a number of reaction pathways not previously characterized, proceeding at near to (and in one case even below) the threshold for forming CH3COH + CO2. Moreover, for the first time we perform energy grained master equation simulations of pyruvic acid decomposition, using a multiple-well multiple-channel reaction model. This allows us to calculate rate coefficients for thermal decomposition as well as to simulate the dissociation of vibrationally excited pyruvic acid produced in photolysis.

Methods Features of the C3O3H4 energy surface relevant to the isomerization and decomposition of pyruvic acid have been characterised using the multi-level G3X-K27 model chemistry. This method uses structures optimized at the M06-2X/6-31G(2df,p) level of theory in a sequence of single-point energy calculations from HF to CCSD(T) theory, with basis sets of incrementally decreasing size. These energies are combined with empirical scaling corrections in order to arrive at the final G3X-K energy. This method has been developed for 2 ACS Paragon Plus Environment

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the accurate treatment of thermochemical kinetics, with mean uncertainties in barrier heights expected to be around 0.6 kcal mol-1.27 All calculations were performed using the Gaussian 09 package.28 Intrinsic reaction coordinate scans were used to confirm transition state connectivity. Calculated structures and vibrational frequencies for C3O3H4 stationary points are provided as Supporting Information. Statistical reaction rate theory modeling of pyruvic acid decomposition has been conducted in the MultiWell-2012.1 suite of programs.29-31 Sums and densities of states are obtained from Stein-Rabinovitch-Beyer-Swinehart counts, using the rigid-rotor harmonic-oscillator approximation. Microscopic rate coefficients, k(E), are from RRKM theory, with quantum mechanical tunnelling corrections included for hydrogen shift reactions. The barrierless recombination of CH3COH and CO2 (and the reverse dissociation) is treated using the restricted Gorin model,32 as described in detail elsewhere.33,34 Collisional energy transfer is modeled using a single exponential-down model, with ∆Edown = 100 cm-1, and N2 as the bath gas. For C3O3H4 isomers the Lennard-Jones parameters were estimated to be σ = 5.46 Å and ε/k = 541 K from empirical equations based upon additive volume increments and estimated boiling points, respectively.35,36 Master equation simulations were performed using 2000 energy grains of 10 cm-1, with the quasi-continuum regime in the hybrid master equation formulation evaluated up to 200000 cm-1. For thermal decomposition simulations, rate coefficients were fitted using a similar method to ref. 37. These simulations consisted of 105 independent trials. Relaxation of vibrationally excited pyruvic acid was modeled in a similar fashion to ref. 38, using an offset thermal (300 K) energy distribution. These simulations featured 106 trials and typically required several thousand collisions in order to reach steady-state. The PPM code was used to assist in the post-processing of MultiWell data.39

Results and Discussion A theoretical potential energy surface for the rearrangement and decomposition of pyruvic acid is presented in Figure 1. Corresponding C3O3H4 minima and transition states are depicted in Figures 2 and 3, respectively. The lowest-energy channel for pyruvic acid decomposition corresponds to decarboxylation to produce the carbene CH3COH (which can subsequently rearrange to acetaldehyde and vinyl alcohol). This process has been identified in previous theoretical investigations and is consistent with a number of experimental studies into pyruvic acid photolysis and pyrolysis. Decarboxylation proceeds via C–C bond cleavage in concert with proton transfer from the carboxyl group to the α-keto moiety (TS4). The barrier for this process is 40.6 kcal mol-1, and it produces intermediate species 4, a post-reaction complex. This complex can subsequently dissociate to CH3COH and CO2, at 43.0. kcal mol-1 above pyruvic acid. So as to investigate this process further, the C3O3H4 energy surface has been mapped as a function of the C–CO2 and O–H bond interatomic distances in pyruvic acid (Figure 4), in a similar fashion to Chang 3 ACS Paragon Plus Environment


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et al.20 Included in Figure 4 is the minimum energy reaction pathway obtained from an intrinsic reaction coordinate scan of TS4. We see that decarboxylation initially involves C–C bond elongation to ca. 1.8 Å, following which proton transfer predominates. Subsequent to proton transfer the nascent CO2 is seen to depart on a relatively flat energy surface. Note that direct decarboxylation pathways are also available leading from pyruvic acid to acetaldehyde (TS8) and vinyl alcohol (TS7), although with substantially higher barriers. Pyruvic acid can tautomerize to its enol isomer, hydroxyacrylic acid (3), via two pathways. A conventional 1,3-H shift (not shown) requires a barrier of 70.5 kcal mol-1. Alternatively, a double H-shift can also transpire (TS3), with a substantially lower barrier of 50.7 kcal mol-1. In this process, the carboxyl group transfers a H atom to the α-keto functionality in concert with H transfer from the methyl group back to the carboxyl group (but on the other O atom). The transition state is product-like in structure, with O···H···O transfer completed early along the reaction coordinate. The substantial reduction in keto-enol tautomerisation barrier is consistent with other studies implicating this mechanism (both intramolecular40 and bimolecular41,42), and can be attributed to a loss of strain in the tight transition state. The ability for hydroxyl groups, and perhaps other functional groups, to reduce barriers to intramolecular keto-enol tautomerisation may be of relevance to other ketones and aldehydes where this mechanism could facilitate photoisomerization to enols.43,44 Following enol formation, decarboxylation pathways leading to vinyl alcohol (TS7) and CH3COH (TS5) were both identified, with the latter being substantially lower in energy, at 60.0 kcal mol-1 relative to pyruvic acid. However, both pathways remain uncompetitive with respect to the analogous processes in the pyruvic acid tautomer. Additionally, 3 can also undergo ring-closing to a four-membered ring compound, albeit with a prohibitively high barrier of 80 kcal mol-1 relative to pyruvic acid (not shown). Finally, we find that pyruvic acid can isomerize to the α-lactone form (2). Reaction proceeds via TS1, with barrier height of only 35.6 kcal mol-1 (albeit with a small barrier in the reverse direction). The transition state for lactone formation involves initial proton transfer from the carboxyl group to the α-keto group (as in TS3 and TS4), accompanied by ring closing at the other carboxyl O atom. The lactone isomer of pyruvic acid was proposed in a theoretical study by Norris et al.,45 and was also considered by Kakkar et al.,25 although no prior works appear to have examined the barrier for isomerization back to the acid form. The ability for hydroxy-α-lactones to readily isomerize with their more-stable carboxylic acid forms is of interest, however, as these compounds have been suggested as possible intermediates in the enzymatic hydrolysis of keto acids.45,46 The α-lactone 2 can decarboxylate to CH3COH, although this requires a very large barrier of 99 kcal mol-1 (not shown). Instead, CO elimination to produce acetic acid is much more favorable (TS2), with barrier height of 48.8 kcal mol-1 relative to pyruvic acid. This novel mechanism represents the second lowest decomposition pathway for pyruvic acid, behind 4 ACS Paragon Plus Environment

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direct decarboxylation to CH3COH, with the initial barrier for isomerization being considerably below that for decarboxylation. It is of interest to note here that α-lactones arise in a number of other atmospheric reaction mechanisms. The photolysis of 2-chloro and 2-alkoxy substituted propionic acids proceeds via HCl and alcohol elimination, respectively, to yield α-lactone intermediates that subsequently decompose to carbonyls and CO.47,48 Hydroxymethyl-methyl-α-lactone has been proposed as a product in the photochemical oxidation of methacrolein, where a similar decomposition reaction to lose CO was also identified.49 The decarbonylation of glyoxylic acid50 also shares some similarities with the mechanism described here for isomerization and subsequent decarbonylation of pyruvic acid, albeit via a concerted rather than a step-wise mechanism. A statistical kinetic model has been developed for the isomerization and decomposition of pyruvic acid on the ground state potential energy surface depicted in Figure 1. The primary decomposition channel has been treated using a two-transition-state model, where the impedance to reaction provided by TS4 and by barrierless dissociation of the post-reaction complex 4 are both incorporated. The restricted Gorin model32 was used to treat barrierless recombination of CH3COH and CO2, assumed to proceed at the Lennard Jones collision rate (approximately 6×10-11 cm3 molecule-1 s-1 at 300 – 2000 K). Master equation simulations were carried out between 800 and 2000 K at 1 atm pressure, in order to obtain rate coefficients and product branching fractions for pyruvic acid decomposition. As an example, results obtained at 1500 K are plotted in Figure 5, with calculated yields of important products (top panel) and the vibrational energy (Evib) of pyruvic acid (bottom panel) both shown as a function of time. We see in Figure 5 that pyruvic acid decomposition to CH3COH + CO2 dominates at 1500 K, and this is true of all temperatures studied. Minor CO loss to produce CH3C(O)OH is predicted, although at insignificant levels (e.g., 0.5 % at 2000 K and decreasing with decreasing temperature). From Figure 5 we also initially observe a brief induction period, required for the pyruvic acid population to reach a quasi-steady-state energy distribution in the stochastic master equation simulations. This coincides with an artificially high rate of decomposition, and this data must be excluded from rate coefficient determinations. After around 10-6 s (ca. 4000 collisions) Evib stabilizes and the decomposition of pyruvic acid obeys a good first order relationship (i.e., linear decay on a log scale). The slope of the decay plot in this regime gives the rate coefficient for total pyruvic acid loss at that temperature. Note that at late stages of reaction, as the pyruvic acid yield becomes small with respect to the total number of trajectories, calculated yields demonstrate increased noise and this data may also need to be excluded from rate coefficient fits (this effect is manifested in Evib from Figure 5 at the later stages of reaction). Calculated rate coefficients for pyruvic acid decomposition to CH3COH + CO2 are plotted in Figure 6. Under the conditions simulated, falloff effects are important, resulting in significant curvature on an inverse-log Arrhenius plot. A least-squares fit provides the three5 ACS Paragon Plus Environment


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parameter expression k[s-1] = 1.01×1063T[K]-15.3exp(-59200/T[K]); This expression is included in Figure 6, extrapolated down to 400 K. At temperatures of ca. 1000 K and below the calculated rate coefficient expression demonstrates relatively good linearity on the inverselog plot, suggesting that it is in the high-pressure limit. Within this regime the effective preexponential factor A is 3.6×1013 s-1 and the effective activation energy Ea is 40.8 kcal mol-1. Given the uncertainty in ∆Edown, the effect of varying this parameter over the range 75 – 125 cm-1 was investigated. At 1000 K, where falloff effects are yet to dominate, k was found to vary from 1.6×104 s-1 to 2.6×104 s-1 for 75-1 and 125 cm-1, respectively (where k = 2.1×104 s-1 for 100 cm-1 ). At 2000 K, where falloff effects are dramatic, increasing ∆Edown from 100 cm-1 to 125 cm-1 resulted in a more-dramatic increase in k from 1.7×106 s-1 to 2.6×104 s-1, whereas decreasing it to 75 cm-1 only reduced k to 1.2×106 s-1. This is consistent with a scheme in which collisional energy transfer via bath gas collisions has become rate-limiting. Calculated rate constants for pyruvic acid decomposition are compared with the three available experimental rate coefficient expressions in Figure 6. We observe reasonable agreement with the most recent measurements of Saito et al.,23 obtained in a shock tube apparatus at around atmospheric pressure, although they are somewhat over-predicted. This study reported Arrhenius parameters A = 2.88×1012 s-1 and Ea = 40.0 kcal mol-1, where the larger A value deduced here is compensated for by the increased activation energy. Our results are more consistent with those of Taylor,22 obtained in a static vessel at temperatures of around 550 – 600 K and pressures of 30 – 150 Torr. Note, however, that significant extrapolation is required here in order to facilitate a comparison. Taylor measured A to be 3.39×1013 s-1, with Ea = 41.25 kcal mol-1, in very close agreement with our extrapolated high pressure limit values. Finally, the earliest set of results from Yamamoto and Back21 diverges significantly from ours - and from the other two data sets - at moderate to high temperatures, consistent with an anomalously low pre-exponential factor (1.55×107 s-1) arising from that study (as identified by Yamamoto and Back themselves). The measured activation energy of 27.7 kcal mol-1 is also incompatible with the energetics of pyruvic acid decomposition. Accordingly, the Arrhenius expressions of Saito et al. or Taylor are recommended for use in kinetic modelling. Calculations have also been carried out in order to simulate the decomposition of vibrationally excited pyruvic acid under representative tropospheric conditions (300 K and 1 atm N2). This corresponds to photoexcitation of pyruvic acid at solar wavelengths (ca. 300 – 400 nm) followed by internal conversion back to the ground state with excess vibrational energy. Figure 7 depicts calculated yields and Evib as a function of time following vibrational excitation of pyruvic acid with energy equivalent to a 350 nm photon. We see that pyruvic acid is predicted to rapidly photodissociate to CH3COH + CO2 with a high quantum yield (~ 0.98), with lesser formation of CH3C(O)OH + CO (~ 0.02). Photoisomerization to the enol form (hydroxyacrylic acid, 3) is also predicted to take place, although with very low yields. Collisional deactivation of pyruvic acid is complete within around 100 ns (ca. 1000


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collisions), although it has been effectively quenched to below all relevant reaction thresholds much earlier, following around 200 bath gas collisions. Calculated yields as a function of excitation energy between 400 and 300 nm are plotted in Figure 8. Even at the longest wavelengths studied photodissociation is predicted to be efficient, with total quantum yield above 0.9. As one would expect, the higher-barrier process of CO loss increases in importance with increasing energy, from around 1 % of total product formation at 400 nm to about 4 % at 300 nm. Photoisomerization to produce hydroxyacrylic acid is predicted to be constant at around 0.1 % of the total products; this value increases somewhat as λ decreases, given more energy to surmount the reaction barrier, but begins to decrease again from around 340 nm onwards, due to increased backreaction to pyruvic acid. The above results suggest that small but quantifiable yields of acetic acid and CO should be produced in the atmospheric photolysis of pyruvic acid, if it is dominated by internal conversion back to the ground state as pyruvic acid. Chang et al.20 have studied the first excited state of pyruvic acid in detail, and found that internal conversion is mediated by excited state proton transfer leading to a conical intersection in the vicinity of TS4. Thus, some fraction of the excited pyruvic acid population may be able to preferentially bypass the CO elimination channel described here and directly decarboxylate. The present results should, therefore, also be of some use in pinpointing the extent to which this process can transpire. Finally, it is of interest to consider the photolysis of pyruvic acid initiated by excitation of the O–H stretch vibration (νOH) overtones. Indirect evidence has been presented for efficient decarboxylation following excitation of the third (781 nm) and fourth (625 nm) overtones in the gas phase, which are at around the threshold for CH3COH + CO2 formation. Through consideration of RRKM theory k(E) values and collisional deactivation rates Takahashi et al.18 suggested that statistical decomposition of pyruvic acid (i.e., decomposition following intramolecular vibrational relaxation) could not account for decarboxylation, and instead dynamical effects must be considered, involving coupling of the O–H stretch and the reaction coordinate. This can be confirmed here through our master equation model which combines RRKM theory k(E) values with a description of collisional energy transfer. At 625 nm the yield for decarboxylation is predicted to be 0.15 %, assuming rapid vibrational energy randomization, dropping to only 0.0001 % at 781 nm. Effectively, this provides us with lower estimates for overtone induced decarboxylation of pyruvic acid, where considerably higher quantum yields would need to be attributed to a non-RRKM process. It is also of interest to note that decarboxylation has been observed following excitation of the second νOH overtone of pyruvic acid between 1150 – 850 nm in aqueous media.13 These energies, corresponding to ca. 25 – 34 kcal mol-1, are well below the threshold for decarboxylation witnessed in the gas phase, and some alternate process has been implicated. One possibility arising from this work is isomerization to the α-lactone form, 7 ACS Paragon Plus Environment


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where the barrier is predicted to be 36 kcal mol-1 in the gas phase. Although this species would be short-lived, due to the small reverse barrier, it may be able to undergo bimolecular reactions such as protonation or hydrolysis when in the confines of a solvent cage.

Supporting Information Available: Optimized geometries and vibrational frequencies for C3O3H4 minima and transition states. Acknowledgements: This work was supported by the Australian Research Council (ARC) through the Discovery Projects (DP110103889) and Future Fellowship (FT130101304) schemes.


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23. Saito, K.; Sasaki, G.; Okada, K.; Tanaka, S. Unimolecular Decomposition of Pyruvic Acid: An Experimental and Theoretical Study. J. Phys. Chem. 1994, 98, 3756-3761. 24. Wu, C.-C.; Lien, M.-H. Ab Initio Study on the Substituent Effect in the Transition State of Keto-Enol Tautomerism of Acetyl Derivatices. J. Phys. Chem. 1996, 100, 594-600. 25. Kakkar, R.; Pathak, M.; Radhika, N. P. A DFT Study of the Structures of Pyruvic Acid Isomers and Their Decarboxylation. Org. Biomol. Chem. 2006, 4, 886-895. 26. Schreiner, P. R.; Reisenauer, H. P.; Ley, D.; Gerbig, D.; Wu, C.-H.; Allen, W. D. Methylhydroxycarbene: Tunneling Control of a Chemical Reaction. Science 2011, 332, 1300-1303. 27. da Silva, G. G3X-K Theory: A Composite Theoretical Method for Thermochemical Kinetics. Chem. Phys. Lett. 2013, 558, 109-113. 28. Gaussian 09, Revision B.01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al., Gaussian, Inc., Wallingford CT, 2010. 29. MultiWell-2012.1 Software, 2012, designed and maintained by John R. Barker with contributors Nicholas F. Ortiz, Jack M. Preses, Lawrence L. Lohr, Andrea Maranzana, Philip J. Stimac, T. Lam Nguyen, and T. J. Dhilip Kumar; University of Michigan, Ann Arbor, MI; http://aoss.engin.umich.edu/multiwell/. 30. Barker, J. R. Multiple-Well, Multiple-Path Unimolecular Reaction Systems. I. MultiWell Computer Program Suite. Int. J. Chem. Kinet. 2001, 33, 232-245. 31. Barker, J. R. Energy Transfer in Master Equation Simulations: A New Approach. Int. J. Chem. Kinet. 2009, 41, 748-763. 32. Smith, G. P.; Golden, D. M. Application of RRKM Theory to the Reactions OH + NO2 + N2 → HONO2 + N2 (1) and ClO + NO2 + N2 → ClONO2 + N2 (2); A Modified Gorin Model Transition State. Int. J. Chem. Kinet. 1978, 10, 489-501. 33. da Silva, G. Reaction of Methacrolein with the Hydroxyl Radical in Air: Incorporation of Secondary O2 Addition into the MACR + OH Master Equation. J. Phys. Chem. A 2012, 116, 5317-5324. 34. da Silva, G. Atmospheric Chemistry of 2-Aminoethanol (MEA): Reaction of the NH2CHCH2OH Radical with O2. J. Phys. Chem. A 2012, 116, 10980-10986. 35. Reid, R. C.; Sherwood, T. K. The Properties of Gases and Liquids: Their Estimation and Correlation; McGraw Hill: New York, 1958. 36. Joback, K. G.; Reid, R. C. Estimation of Pure-Component Properties from Group-Contributions. Chem. Eng. Commun. 1987, 57, 233-243. 37. da Silva, G.; Trevitt, A. J.; Steinbauer, M.; Hemberger, P. Pyrolysis of Fulvenallene (C7H6) and Fulvenallenyl (C7H5): Theoretical Kinetics and Experimental Product Detection. Chem. Phys. Lett. 2011, 517, 144-148. 38. Adamson, B. D.; Coughlan, N. J. A.; da Silva, G.; Bieske, E. J. Photoisomerization Action Spectroscopy of the Carbocyanine Dye DTC+ in the Gas Phase. J. Phys. Chem. A 2013, 117, 13319-13325. 39. Pinches, S. J.; da Silva, G. On the Separation of Timescales in Chemically Activated Reactions. Int. J. Chem. Kinet. 2013, 45, 387-396. 40. So, S.; WIlle, U.; da Silva, G. A Theoretical Study of the Photoisomerization of Glycolaldehyde and Subsequent OH Radical-Initiated Oxidation of 1,2-Ethenediol. J. Phys. Chem. A 2015, 119, 9812-9820. 41. da Silva, G. Carboxylic Acid Catalyzed Keto-Enol Tautomerizations in the Gas Phase. Angew. Chem. Int. Ed. 2010, 49, 7523-7525. 42. Karton, A. Inorganic Acid-Catalyzed Tautomerization of Vinyl Alcohol to Acetaldehyde. Chem. Phys. Lett. 2014, 592, 330-333. 43. Andrews, D. U.; Heazlewood, B. R.; Maccarone, A. T.; Conroy, T.; Payne, R. J.; Jordan, M. J. T.; Kable, S. H. Photo-Tautomerization of Acetaldehyde to Vinyl Alcohol: A Potential Route to Tropospheric Acids. Science 2012, 337, 1203-1206. 44. Clubb, A. E.; Jordan, M. J. T.; Kable, S. H.; Osborn, D. L. Phototautomerization of Acetaldehyde to Vinyl Alcohol: A Primary Process in UV-Irradiated Acetaldehyde from 295 to 335 nm. J. Phys. Chem. Lett. 2012, 3, 3522-3526. 45. Norris, K. E.; Bacskay, G. B.; Gready, J. E. Theoretical Study of “Protonated Pyruvate”: A Methylhydroxycarbene-Carbon Dioxide Complex – Implications for the Decarboxylation of Pyruvic Acid. J. Comp. Chem. 1993, 14, 699-714.


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46. Firth-Clark, S.; Rodriquez, C. F.; Williams, I. H. Hydroxyoxiranone: An Ab Initio MO Investigation of the Structure and Stability of a Model for a Possible α-Lactone Intermediate in Hydrolysis of Sialyl Glycosides. J. Chem. Soc. Perkin Trans. 2 1997, 1943-1948. 47. Rotinov, A.; Chuchani, G.; Andres, J.; Domingo, L. R.; Safont, V. S. A Combined Experimental abd Theoretical Study of the Unimolecular Elimination Kinetics of 2-Alkoxypropionic Acids in the Gas Phase. Chem. Phys. 1999, 246, 1-12. 48. Nishino, S.; Nakata, M. Photoreaction Mechanism of 2-Chloropropionic Acid in a Low-Temperature Argon Matrix. J. Mol. Struct. 2008, 875, 520-526. 49. Kjaergaard, H. G.; Knap, H. C.; Ornso, K. B.; Jorgensen, S.; Crounse, J. D.; Paulot, F.; Wennberg, P. O. Atmospheric Fate of Methacrolein. 2. Formation of Lactone and Implications for Organic Aerosol Production. J. Phys. Chem. A 2012, 116, 5763-5768. 50. Ding, W.-J.; Ni, L.-Y.; Fang, W.-H.; Yu, J.-G. Theoretical Study on the Unimolecular Reactions of Glyoxylic Acid. J. Theor. Comp. Chem. 2005, 4, 725-736.



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FIGURES

Figure 1. Energy diagram for isomerization and decomposition of pyruvic acid. Energies are 0 K enthalpies in kcal mol-1, calculated at the G3X-K level of theory.


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1

2

3

4

Figure 2. Optimized structures for minima on the C3O3H4 surface identified in Figure 1, calculated at the M06-2X/6-31G(2df,p) level of theory.



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TS1

TS2

TS3

TS4

TS5

TS6

TS7

TS8

Figure 3. Optimized structures for transition states on the C3O3H4 surface identified in Figure 1, calculated at the M06-2X/6-31G(2df,p) level of theory. Displacement vectors for imaginary frequencies are indicated.

14

ACS Paragon Plus Environment

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Figure 4. Potential energy surface for decarboxylation of pyruvic acid via concerted O–H and C–C bond cleavage (bonds identified in inset structure). Calculated at the M06-2X/631G(2df,p) level of theory, with all parameters except O–H and C–CO2 interatomic distances relaxed. Dashed line indicates the intrinsic reaction coordinate for TS4.



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Figure 5. Calculated product yields (top) and vibrational energy (bottom) as a function of time in the thermal decomposition of pyruvic acid at 1500 K and 1 atm, from energy grained master equation simulations.


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Figure 6. Arrhenius plot of rate coefficients (k) for pyruvic acid decomposition. Dashed line represents the three-parameter fit determined here from calculated k values at 800 – 2000 K and 1 atm, extrapolated to 400 K. Solid lines indicate literature Arrhenius expressions,21-23 plotted over the T ranges for which they were experimentally obtained.



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Figure 7. Calculated product yields (top) and vibrational energy (bottom) as a function of time following excitation of pyruvic acid with the energy of a 350 nm photon at 300 K and 1 atm, from energy grained master equation simulations.


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Figure 8. Steady-state product yields calculated following excitation of pyruvic acid with the equivalent energy of a photon of wavelength λ at 300 K and 1 atm.



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TOC Image


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Decomposition of Pyruvic Acid on the Ground ... - ACS Publications

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