Photochemistry of the pyruvate anion produces CO2, CO, CH3–, CH3, and a low energy electron

The photochemistry of pyruvic acid has attracted much scientific interest because it is believed to play critical roles in atmospheric chemistry. However, under most atmospherically relevant conditions, pyruvic acid deprotonates to form its conjugate base, the photochemistry of which is essentially unknown. Here, we present a detailed study of the photochemistry of the isolated pyruvate anion and uncover that it is extremely rich. Using photoelectron imaging and computational chemistry, we show that photoexcitation by UVA light leads to the formation of CO2, CO, and CH3−. The observation of the unusual methide anion formation and its subsequent decomposition into methyl radical and a free electron may hold important consequences for atmospheric chemistry. From a mechanistic perspective, the initial decarboxylation of pyruvate necessarily differs from that in pyruvic acid, due to the missing proton in the anion.

actinic regime, and watched the decay and dissociation channels via VMI photoelectron spectroscopy. Channels such as direct electron detachment, resonant processes that led loss of CO, CO2 and producing CH3-(methide) were detected, providing a rich and complex photochemistry for this important anion. Sophisticated theories were applied to support experimental observations. Overall this is a well executed work. The unraveled rich photochemistry should have important implications to understand the pyruvic acid transformation in the atmospheric chemistry setting. The discovery of producing methide and methyl radical is particularly worth noting. I would strongly recommend its publication.

I only have a couple of technique questions:
what is the temperature used in simulating direct detachment channel for CH3COCOO- (Fig 3 a) via NEA approach?
The weak spectral signature for CH3-is proposed via two-photon process. Was there any photonflux dependent study conducted?
Typo: top of page 4, Figure 1 ---> Figure 2 Reviewer #3 (Remarks to the Author): The communication describes a joint experimental and theoretical study of the title molecule, providing new information on it's possible photodissociation pathways. How pyruvate is decomposes plays a potential role in the cycles of atmospheric chemistry, and the fact that it may produce methide radicals is a novel, potentially important, observation. The paper also shows novelty in the good agreement between experiment and theory, supporting the role calculations have to play in unravelling the signals from time-resolved spectroscopy and demonstrating the accuracy of present state-of-the-art calculations.
Both of these novelties are of significance for the field of chemical dynamics, and the decomposition pathway of pyruvate of wider significance in physical and atmospheric chemistry. The paper is a sound and well argued piece of work, and as a result I support it's publication in Nature Communications. I have a few minor points that the authors should consider.
2. on p5 it is mentioned that the ground-state structure of pyruvate has a perpendicular carboxylate group, whereas pyruvic acid is planar. This is surprising as the electronic structure would not be expected to be very different so perhaps the authors could give a brief explanation as to why this is.
3. on p5 it is noted that the experiments show that the photoelectrons from pyruvate come from a non-bonding p-orbital. Does this tie in with the calculated electronic structure for the radical groundstate?
4. on p6 when discussing the "thermionic emission" signal, it is stated that this "... necessitates the absorption of a photon and the formation of a ground state anion". Does thermionic emission really require the absorption of a photon -it can occur if a molecule is heated to a high enough temperature? And does it really require that the electron is lost from the ground-state? Can it not occur for an electronic excited state as long as there is enough time for loss of coherence? 5. on p6 it is stated that "Enhanced thermionic emission was observed in the hv= 3.5 -3.7 eV spectra,...". I do not see this is in Figure 2. Furthermore, what is the relevance of the excited-state that may lie at 3.6eV? May this provide a resonant state that results in thermionic emission (which contadicts the statement discussed above in Q4). Or is this the pathway to fragmentation of the excited anion? 6. on p7 "a very weak feature at high eKE..." is mentioned and then discussed. This is not visible in the curves of Figure 2 and this should be made clear. We thank the Reviewers for their positive and constructive comments on our manuscript "Photochemistry of the pyruvate anion produces CO2, CO, and CH3".
We have updated the manuscript to address all the Reviewers' suggestions. We detail these changes and include responses to the Reviewers' questions below. We reproduce below the entire Reviewer comments in red for clarity, and additions/corrections to the text are denoted in blue. We also uploaded an annotated version of the revised manuscript, where all modifications are indicated in blue.
Reviewer #1 (Comments to the Author): The manuscript presents a combined experimental and theoretical study of photodecomposition of the pyruvate anion, which is a conjugate base of pyruvic acid. The authors employed the photoelectron imaging technique and DFT molecular dynamics simulations to qualitatively determine the photodissociation products. While the work is interesting and technically solid in general, I don't see it as sufficiently important to make a cut for publication in the Nature group of journals. The authors point at potential importance for atmospheric chemistry mentioning the relative abundance of pyruvic acid in the sea water, mostly in the form of its conjugate base. However, they argue themselves that "the vapour pressure of the anion is much lower than that of pyruvic acid and so, the role of the pyruvate anion as an isolated species may be less important than that of the acid." The study does not provide any kinetic data required for atmospheric kinetic models and without kinetic modeling it is impossible to say whether the photodissociation of the pyruvate anion can make any significant contribution to the atmospheric processes. Moreover, the authors single out the formation of methyl radical calling it "a highly reactive species that can contribute to the formation of secondary organic aerosols". In fact, at atmospheric temperatures CH3 can only rapidly recombine with other radicals but is virtually unreactive with closed shell species. Using MD simulations with ab initio (DFT) potentials is fancy but the results are only as good as the underlying potentials. DFT methods do not generally provide chemical accuracy comparable with, e.g. coupled cluster theory. I wonder if the less than spectacular agreement of the measured and calculated photoelectron spectra (Fig. 3) is due to the deficiency of the DFT energetics. Also, in my opinion, more accurate estimates of the lifetime of the CH3CO-anion can be obtained using statistical (RRKM) calculations of its decomposition rate constant using more accurate (CCSD(T)-like) energies of the pertinent local minima and transition states, instead of using DFT-AIMD. In any case, validation of the chosen density functional vs. more accurate electronic structure methods is required before the results of the AIMD calculations can be trusted.

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We thank the Reviewer for their suggestions and critical assessment of our work.
We start by addressing the Reviewer's comment on our calculations. We would like to stress that the calculated photoabsorption cross-sections obtained with the nuclear ensemble approach (NEA) are in excellent agreement with the experimental photoelectron spectra for pyruvate anion and CH3 -. The additional calculated spectrum for the acetyl anion (dashed line in Fig. 3b) does not correspond to any observed photoionization signal in this experiment and was included to highlight this. Our calculated photoelectron spectra of the acetyl and methide anions can be compared to their reference experimental spectrum (solid lines below, with computed as dashed), highlighting the agreement and the identification of the fragment observed in our experiment with the methide anion: Importantly, it is not only the energy of the band that is well reproduced for the photoelectron spectrum of the methide anion by our calculation, but also its overall shape. We believe that such excellent agreements between theory and experiment, and for the shape and energetic of the spectra, constitutes a strong validation of the theoretical protocol employed in this work.
For completeness, we have also performed the benchmark of our protocol proposed by the Reviewer, comparing the electron binding energy obtained with our (U)DFT/wB97X-D/aug-cc-pVDZ results with (U)CCSD(T)-F12/aug-cc-pVTZ-F12 for the three molecules of interest in our work (for each molecule, we employed the ground-state geometry obtained with DFT/wB97X-D/aug-cc-pVDZ). The results presented in the Table below shows the excellent agreement of our DFT binding energies with that predicted by CCSD(T)-F12. Taken together, all these results strongly validate our theoretical methodology for the quantities compared with the experiment and leading to the unambiguous identification of the methide anion. We have included this additional information in the main text and the SI: -Page 5: To support this assignment, we calculated the photoelectron spectra of the pyruvate anion using the nuclear ensemble approach (NEA) with (U)DFT/wB97X-D/aug-cc-pVDZ (see discussion below and further information on the calculations in the Methods section).
-Page 11 (Methods): For each geometry, the vertical ionization energy to D0 was calculated by a difference of electronic energy using DFT/wB97X-D/aug-cc-pVDZ for S0 and UDFT/wB97X-D/aug-cc-pVDZ for D0, and the corresponding intensity was approximated by using the norm of the Dyson orbital. The electron binding energy obtained with this level of theory for each molecule at its ground-state geometry (obtained with DFT/wB97X-D/aug-cc-pVDZ) is in excellent agreement with that obtained with CCSD(T)-F12 (Table 1). The CCSD(T)-F12 calculations were conducted with Molpro 2012.  The comment made by the Reviewer on our ab initio molecular dynamics (AIMD) made us realise that we should clarify further the use of these dynamics. These AIMD (described as 'exploratory' in our manuscript) have been used solely to understand which molecules could give the signal at low binding energies. They are not aimed to deduce any lifetime, as inferred by the Reviewer. The only information extracted from our AIMD is the suggestion that the acetyl anion could readily dissociate into CO and CH3at high internal energy. This then led us to compute the photoelectron spectrum for CH3which verified the origin of the experimental feature ( Fig. 3 in our manuscript and further validation above). We also note that we have now calculated the dissociation energy (De) of acetyl anion to CO and CH3and obtained a value of 113.4 kJ mol -1 with our DFT formalism and 93.1 kJ mol -1 with CCSD(T)-F12/augcc-pVTZ-F12. Both confirm that our exploratory AIMD calculations captured the view that the acetyl can dissociate.
To determine any kinetic information on this photodynamics process, one would need to perform excited-state dynamics to account accurately for the distribution of internal energy into the acetyl anion (see for example Ref. 37 cited in the manuscript about athermal dynamics). This is because both an AIMD or any RRKM calculations -as suggested by the Reviewer -would not be able to capture the non-statistical distribution of internal energy into the photoexcited pyruvate anion leading to the final formation of CO2, CO and CH3 -. We have specifically avoided discussing the details of the dynamics as we do not have sufficient information on this at present.
We have clarified our use of ab initio molecular dynamics in the text.
-Page 8: Specifically, during the AIMD conducted in the NVE ensemble, the ground-state acetyl anion appeared to be unstable with respect to CO loss already at an average temperature of 1400 K. Based on this observation, we calculated the photoelectron spectrum of CH3 − and compared it with the experimental spectrum at hn = 3.5 eV (Fig. 3b).
-Page 8: In our experiment, we do not observe the acetyl anion. This is likely because CH3CO − is formed with sufficient internal energy to lose CO and is therefore expected to act as a short-lived intermediate (< ns) en route to forming the methide anion.
-Page 13: The dissociation energy (De) of acetyl anion leading to the formation of CO and CH3 − is calculated to be 113.3 kJ mol -1 at the DFT/wB97X-D/aug-cc-pVDZ level of theory and 93.1 kJ mol -1 at the CCSD(T)-F12/aug-cc-pVTZ (using the same DFT optimised structures).
We now address the comments of the Reviewer regarding the atmospheric implication of our work.
The Reviewer questions the importance of pyruvate as an isolated species in the atmosphere. As cited by the Reviewer, we recognized this fact in our manuscript. However, pyruvate is far more abundant in 4/9 the presence of water molecules (droplets, microsolvation, ice surfaces), as noted (and supported through Refs 17 and 18). To place this in perspective, for a sea-spray droplet (pH 8.1), there will be 4 x 10 5 anions for every pyruvic acid molecule (and >2 x 10 7 at the surface)! Despite this abundance of pyruvate anion, it is surprising to realise that no information is available on its photochemistry, even in the gas phase. A first step in understanding the complex photolysis of pyruvate is therefore to investigate its photodecomposition as an isolated anion and understand which products are formed. We agree that our work highlights the photodissociation pathways that will take place in gas phase, but this raises many open (and potentially important) questions regarding their prevalence in more complex environment.
To address the comment of the Reviewer about the lack of kinetic data in our work, we would like to clarify that our main message is that the photodecomposition of pyruvate is likely to produce CO, CO2, CH3 -, CH3, and an electron. The possible presence of CH3as a photoproduct of the conjugate base of pyruvic acid is an unprecedented measurement and these species have not been considered to date in Earth's atmosphere. To the best of our knowledge, the methide anion has been discussed in the context of Titan's atmosphere only (see Refs. 32 and 40 in the revised version of the manuscript). More generally, the presence of unexpected photoproducts has been detected by employing a novel set of spectroscopic and theoretical tools for the field of atmospheric chemistry. Being able to study directly transient species with new spectroscopic tools has had profound impacts in atmospheric chemistry, as has been demonstrated by, for example, the elegant laser-induced fluorescence experiments on Criegee's intermediates.
Finally, the Reviewer questions the relevance of CH3 radical. its reaction with O2 forms the methylperoxy radical (see Ref. 43). While the main source of CH3 radical is the oxidation of methane by an OH radical, our findings show that a local formation of CH3 (and possibly methylperoxide radical) can be expected where pyruvate is present. This would strike us as an important consideration for atmospheric chemists and modelers, given the importance of pyruvic acid in aqueous photochemistry.
We have modified the text in the following way to account for the Reviewer's comments on the atmospheric implication of our work. We further stress the production of CH3and its decomposition into CH3 and a free electron. We also highlight the importance of considering the production of CH3 where pyruvate (and pyruvic acid) is present, in particular considering its reaction with O2 to form a methylperoxy radical.
-Title: Photochemistry of the pyruvate anion produces CO2, CO, CH3 -, CH3, and a low energy electron -Abstract: The observation of the unusual methide anion formation and its subsequent decomposition into methyl radical and a free electron may hold important consequences for atmospheric chemistry.
Page 1: We demonstrate that the pyruvate anion not only experiences decarboxylation, but also a subsequent unimolecular decay to form CO and a methide anion, CH3 -. The methide anion further decomposes into CH3 and a free electron.
-Page 9: Nevertheless, the study of the intrinsic dynamics offers important insight into the photoinduced decay pathways that are operable. The photochemical production of the methide anion -the simplest of carbanions -raises questions on its possible reactivity with other atmospheric compounds, which has not been considered previously, except in the atmosphere of Titan. 32,40 The methide anion can also decay into CH3 and a free electron. This low energy free (or partially solvated) electron can react with surrounding molecules. 41, 42 The CH3, which is normally formed from the reaction of methane with OH, reacts with O2 to form methyl peroxide. 43 Hence, the previously overlooked pyruvate anion in gas phase has the ability to produce a range of exotic and reactive species in the atmosphere following photo-absorption.

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----------------------Report of Reviewer #2 ----------------------Given the ubiquity of the pyruvic acid & pyruvate anion in solutions and at air/water interfaces, they play important roles in aerosol chemistry and atmospheric environments. Their photochemistry in actinic region is particularly relevant to their chemical transformations. Authors applied the state-of-theart ion spectroscopy and advanced theoretical methods to carry out a comprehensive probe of the pyruvate anion in its isolated form. Authors excited the anion to its first excited state in the actinic regime, and watched the decay and dissociation channels via VMI photoelectron spectroscopy. Channels such as direct electron detachment, resonant processes that led loss of CO, CO2 and producing CH3-(methide) were detected, providing a rich and complex photochemistry for this important anion. Sophisticated theories were applied to support experimental observations. Overall this is a well executed work. The unraveled rich photochemistry should have important implications to understand the pyruvic acid transformation in the atmospheric chemistry setting. The discovery of producing methide and methyl radical is particularly worth noting. I would strongly recommend its publication.
We thank the Reviewer for their very positive feedback.

I only have a couple of technique questions:
what is the temperature used in simulating direct detachment channel for CH3COCOO- (Fig 3 a) via NEA approach?
All the simulations employing the NEA are done at 0K, that is for a molecule in its ground vibrational state.
-Page 11: For each molecule, a Wigner distribution for uncoupled harmonic oscillators was constructed from the DFT ground-state geometry and corresponding vibrational frequencies, from which 500 geometries were randomly sampled assuming that the molecule is in its ground vibrational state (0K).
The weak spectral signature for CH3-is proposed via two-photon process. Was there any photon-flux dependent study conducted?
Yes, we did do what the Reviewer has suggested, and the results are now presented in the supplementary information. We prefer not to discuss it in the main manuscript simply because we feel it distracts from the flow and requires some space to explain the results. In essence, we observe that the CH3signal scales linearly with photon flux. This may at first glance appear to contradict a 2-phoron process (might expect quadratic behaviour), but the two separate photon absorptions have different cross sections (one for excitation of pyruvate and one for detachment from CH3 -). If either of these is significantly larger than the other -which is a reasonable expectation given their differing nature -then the one with the larger cross section will dominate and the photon flux dependence will appear as a one-photon process. We have added the following comment in the manuscript and discussed the details in the SI.
-Page 9: The photon flux dependence of the CH3 − peak is considered in Supplementary Data 1.