Comment on acp-2021-424 Anonymous Referee # 1 Referee comment on " Photo-initiated ground state chemistry : How important is it in the atmosphere ?

Rowell and co-authors present a theoretical assessment of photo-initiated ground-state chemistry in carbonyls, and discuss the potential importance of various reaction pathways for tropospheric chemistry. Some of these pathways are not generally considered in current models and could be of chemical relevance. The topic is certainly suitable for ACP. Since I am not a theoretical chemist I can offer no perspective on the reliability of their approach or the level of theory employed -I leave that to other reviewers to assess. With that caveat, I found the paper of high quality, thorough, and a pleasure to read. I expect the results to be a useful step towards improved resolution of this class of reactions in atmospheric models. I recommend publication and have only a few minor suggestions for the authors’ consideration.


Introduction
Carbonyls are a class of volatile organic compounds (VOCs) central to atmospheric chemistry. They arise, in large quantities, from primary anthropogenic and biogenic emissions and via secondary atmospheric processes (Kesselmeier and Staudt, 1999; Table 1. Lowest energy zero-point vibrational energy corrected B2GP-PLYP-D3/def2-TZVP S0 thresholds (kJ/mol) for the indicated unimolecular reactions of the carbonyls in the dataset (see text).
All calculated decarbonylation thresholds are accessible at the maximum tropospheric photon energy of 400 kJ/mol although, with the exception of formaldehyde (Moortgat et al., 1983), CO-loss QYs are low. For example, they are measured to be ∼0.02 in acetaldehyde (Blacet et al., 1942) and ∼0.01 for butanal (Blacet and Calvert, 1951).  and branching can be rationalised in terms of the alkene product formed: the relative stability of the alkene increases with increasing substitution about the double bond (Whangbo and Stewart, 1982).

Concerted 4-centre H 2 -loss
H 2 can be formed on S 0 via a 4-centre TS where adjacent hydrogen atoms form an H-H bond and dissociate as H 2 , leaving an 235 unsaturated carbonyl product. In aldehydes these adjacent hydrogens can be the formyl and α hydrogens, but as the alkyl chain lengthens possibilities include hydrogens in the: α+β, β+γ, etc., positions. These H 2 -loss mechanisms can be distinguished by the point of unsaturation in the co-product.
The S 0 reaction thresholds for the possible H 2 -loss channels are given in Table 1    with H 2 -loss thresholds from the β and γ positions higher still. For a given carbonyl, the highest H 2 -loss threshold is predicted for removal of a hydrogen from the terminal carbon. Like the NTIII reaction, the TSs for 4-centre H 2 -loss are 'late', and the 245 threshold energies are related to the stability of the forming alkene product. Increasing substitution around the double-bond increases alkene stability (Whangbo and Stewart, 1982), so products with a terminal C=C bond are comparatively less stable than products from H 2 -loss at other sites. For example, there is an approximately 20 kJ/mol decrease in threshold for formyl + α H 2 -loss from acetaldehyde to propanal. Like NTIII, there is little effect on the formyl and α H 2 -loss threshold upon further chain extension. For example, the formyl and α H 2 -loss thresholds for propanal, butanal and pentanal are all ∼315 kJ/mol.

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Branching at the α position also has little effect on the formyl and α H 2 -loss threshold (cf. propanal and 2-methylpropanal).
Similar trends are seen for H 2 -loss from the α and β and β and γ positions.
The results in Table 1 and Fig. 7 reinforce, for multiple carbonyl species, that only H 2 -loss from the formyl and α positions is energetically accessible in the actinic energy range. Indeed, the 4-centre H 2 -loss channel in acetaldehyde has recently been observed experimentally under tropospherically relevant conditions (Harrison et al., 2019). In the absence of a formyl hydrogen, 255 none of the ketone H 2 -loss channels are accessible in the troposphere. Similarly, H 2 -loss is not accessible in species lacking an α-hydrogen, like 2,2-dimethylbutanal and methacrolein.
The thresholds for H 2 -loss from the formyl and α positions in acrolein and crotonaldehyde are also in the actinic range, at 373 and 386 kJ/mol, respectively. These thresholds are significantly higher than those for the saturated aldehydes because of the high energy of the product propadienone and 1,2-butadienone species and are close to the maximum actinic energy, 260 suggesting S 0 H 2 -loss is unlikely to be important in α,β-unsaturated aldehydes.
Glycolaldehyde is calculated to have the lowest H 2 -loss threshold, 292 kJ/mol, for loss of the formyl and α hydrogens. This can be rationalised in terms of the electron withdrawing nature of the OH stabilising the 4-centre TS and the hydroxyketene product. Glyolaldehdye is also the only carbonyl with an energetically accessible β-H loss channel. Loss of the OH and β hydrogen forms glyoxal and H 2 with a 384 kJ/mol threshold, close to the actinic maximum energy.

Alkane/alkene elimination (AE)
In ketones, migration of an α-H atom to the 'other' α carbon via a 4-centre TS can form an alkane and a ketene. For example, acetone can dissociate to methane and ketene. In methyl vinyl ketone, the unsaturation leads to alkene elimination and formation of ethene and ketene.
The S 0 reaction thresholds for the lowest energy AE channels are given in Table 1 and are shown in Fig. 8. In asymmetric  In linear unsaturated ketones, the AE TSs are 'late', with C-C breaking bond lengths over 1.8 Å. Naïvely, we would therefore infer AE thresholds for these species will reflect the relative stability of the forming products. From tabulated 0 K enthalpies 275 of formation (Ruscic et al., 2005;Ruscic, B. and Bross, D. H., 2020), although methyl ketene is 9 kJ/mol more stable than ketene, butanone is 17 kJ/mol more stable than acetone. This would imply a lower AE threshold in acetone. The AE threshold in butanone, however, is calculated to be 12 kJ/mol lower than that for acetone. The TS for AE in butanone ( Fig. S6b) is marginally earlier than in acetone (Fig. S6a), suggesting electronic effects are responsible for the lower threshold. Only a 4 kJ/mol decrease in threshold, to 347 kJ/mol, is seen for production of ethylketene in pentan-2-one, indicating further chain 280 lengthening has little effect. This threshold is the same as that calculated for pentan-3-one forming ethane and methylketene, that is, there is a small reduction in threshold energy on formation of a larger alkane. This is also seen in the alternate AE pathways (Sect. S6). As shown in Fig. S6, in linear unsaturated ketones, thresholds for producing ketene (via the alternate AE channels) are ∼360 kJ/mol and thresholds for producing methylketene are ∼350 kJ/mol. We expect these thresholds to be generalisable to larger linear unsaturated ketones.

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The lowest AE thresholds are for methyl vinyl ketone and methyl isopropenyl ketone (MIPK), both of which yield ketene.
The AE TSs in these molecules are much 'tighter' than in the linear unsaturated ketones (Fig. S6). Here there is resonance stabilisation of the saddle points, leading to thresholds of 329 and 334 kJ/mol, respectively.

Keto-enol tautomerisation
Carbonyls can exist in two tautomeric forms: a keto form (encompassing, here, both ketones and aldehydes), and an enol form 290 where an H atom has transferred to the carbonyl oxygen, forming an OH substituent and a point of unsaturation. Keto-enol tautomerisation is known to occur as a dynamic equilibrium in S 0 carbonyls in aqueous solution at room temperature, although the keto tautomer is thermodynamically favoured (Keeffe et al., 1988). Keto-enol tautomerisation has been observed in gas phase photolysis experiments on acetaldehyde (Clubb et al., 2012;Shaw et al., 2018) and the authors suggest it may occur in many other carbonyls.

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The lowest energy calculated keto-enol tautomerisation thresholds for the relevant carbonyls in the dataset are given in Table   1 and shown in Fig. 9. A keto-enol TS involving an α hydrogen, that is, a [1,2]-H atom shift, was found in all of these species.
In crotonaldehyde, however, the lowest energy threshold, 133 kJ/mol, was for a [1,5]-H atom shift involving a γ hydrogen and this is less than half the 294 kJ/mol threshold we predict for a [1,2]-H atom shift. This mechanism is analogous to the [1,5]-H atom shift in the Norrish Type II reaction. Here, however, because it occurs in an enal, conjugation prevents the bond between 300 the α-and β-carbons from breaking and, instead, but-1,3-diene-1-ol, is formed.
There may be multiple possible [1,2]-H atom shift keto-enol tautomerisation pathways for a given carbonyl. These correspond to formation of geometric isomers (e.g. cis-or trans-enols) or, in asymmetric ketones, tautomerisation involving hydrogens from either alkyl substituent. These additional keto-enol tautomerisation thresholds are reported in Table S7  Sect. S7, due to steric factors, thresholds for tautomerisation to trans-enols are typically ∼20 kJ/mol lower than those to the corresponding cis-enol.
As shown in Fig. 9, the keto-enol tautomerisation thresholds for linear aldehydes lie in a narrow energy range (278-281 kJ/mol), indicating chain extension has no effect on threshold as long as the bulky alkyl group can be oriented trans to the enol OH group. This is not the case for the α-branched 2-methylpropanal, and the steric penalty leads to the highest keto-enol 310 tautomerisation threshold calculated here (296 kJ/mol).
The tautomerisation thresholds for ketones are also in a narrow range (270-276 kJ/mol), ∼5 kJ/mol lower than the corresponding aldehydes. As shown in Table S7 this extends to the alternate pathway in asymmetrically substituted ketones, which have thresholds for formation of the alternate trans-enol within ∼1 kJ/mol of those shown in Fig. 9.
In α,β-unsaturated carbonyls, the keto-enol tautomerisation thresholds are low when tautomerisation involves a H-atom 315 from an aliphatic group, for example, the CH 3 group in methyl vinyl ketone and crotonaldehyde. Tautomerisation thresholds involving olefinc H-atoms, however, are significantly higher, for example 292 kJ/mol in acrolein, reflecting the relatively unstable propadienol product.
Glycolaldehyde has an -OH electron withdrawing functional group. For this molecule the tautomerisation threshold is predicted to be lowered by 8 kJ/mol compared to acetaldehyde, that is, the electron withdrawing group stabilises the TS to the 320 forming enol. Notably, all calculated keto-enol tautomerisation thresholds are significantly below the maximum actinic photon energy. We expect this pathway to be energetically accessible in all carbonyls with an α hydrogen and appropriate unsaturated species with a γ hydrogen. Moreover, all linear aldehydes and ketones are calculated to have keto-enol tautomerisation thresholds close to, or below, that of acetaldehyde. Given the experimental observation of keto-enol tautomerisation in acetaldehyde (Clubb et al.,

Enal-ketene tautomerisation
There has been recent interest in the formation of ketenes as atypical and relatively uncharacterised products of carbonyl photolysis (Harrison et al., 2019;Toulson et al., 2018). As seen above, the formyl + α H 2 -loss mechanism forms ketenes in 330 aldehydes and the AE mechanism forms ketenes in ketones. In enals there is also an S 0 tautomerisation mechanism involving a [1,3]-H shift of the formyl-hydrogen to the β-carbon that can form ketenes. First order saddle points have been optimised for enal-ketene tautomerisation in acrolein, crotonaldehyde and methacrolein. These are shown in Fig. 10, together with the calculated B2GP-PLYP-D3 threshold energies. The enal-ketene tautomerisation threshold in methacrolein, 285 kJ/mol, is ∼13 kJ/mol lower in energy than the 298.7 kJ/mol G3X-K//M06-2X/6-31G(2df,p) threshold previously calculated by So et al.  atmospheric pressure decarbonylation QY for acetaldehyde is ∼0.5% (Moortgat et al., 2010;Warneck and Moortgat, 2012) and ∼0.2% for 2-methylbutanal (Gruver and Calvert, 1958). For larger saturated aldehydes, we predict other S 0 pathways will be favoured over decarbonylation, which will be at most a minor channel.
Our calculations predict keto-enol tautomerisation to be the lowest energy pathway on S 0 for all saturated aldehydes, lying

Saturated ketones
Following excitation by an actinic photon, the relevant S 0 chemistry of ketones is much simpler than that of aldehydes. No TS for direct decarbonylation was found and the absence of a formyl H-atom in ketones removes the TF and formyl + α H 2 -loss pathways. The only S 0 dissociation pathways in ketones are NTIII, AE and NTI. The thresholds for AE and NTI are similar 390 in the ketones considered, at ∼340 kJ/mol. Given NTI is barrierless, it is likely to dominate over AE. We therefore expect NTI to be the dominant dissociation mechanism in acetone, butanone and pentan-2-one, where it has a lower threshold than NTIII, although NTIII may be important in pentan-2-one and in larger saturated ketones, where there is alkyl chain extension past the β-hydrogen involved in the reaction. For example, the production of acetaldehyde from 3-methylbutan-2-one is one of the few experimental examples of the NTIII mechanism in the literature (Zahra and Noyes, 1965).

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The keto-enol tautomerisation thresholds for saturated ketones are slightly lower than in saturated aldehydes. This, combined with the absence of low energy S 0 dissociation pathways and the experimental observation of keto-enol tautomerisation in acetone (Couch et al., 2021), suggests photo-initiated tautomerisation may be important in ketones.

α,β-Unsaturated carbonyls
In general, because of resonance stabilisation of the bond between α-and formyl-carbons, we calculate higher S 0 thresholds in 400 α,β-unsaturated carbonyls than in equivalent saturated carbonyls. However, excited state NTIa thresholds are also elevated and all excited state NTI thresholds are close to, or above, the maximum available actinic energy . Thus any photolysis must occur on S 0 . Further, unlike many other carbonyls, the electronic structure of the α,β-unsaturated carbonyls promotes rapid electronic relaxation from S 1 to S 0 (Lee et al., 2007;Schalk et al., 2014;Cao and Xie, 2016), suggesting high S 0 internal energies and therefore relatively high probability of S 0 reaction.

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Triple fragmentation is the lowest energy S 0 dissociation pathway for the unsaturated aldehydes in the dataset. The TF thresholds are ∼40 kJ/mol higher than in saturated aldehydes and are slightly lower than the S 0 NTIb asymptotic energies and decarbonylation thresholds. The other possible S 0 dissociation reactions (H 2 -loss, NTIII and S 0 NTIa) have thresholds >370 kJ/mol and are unlikely to be significant under tropospheric conditions. NTIb possible. In methyl vinyl ketone, AE has the lowest threshold, at 329 kJ/mol, 9 kJ/mol lower than the NTIb threshold.
NTIb dissociation, however, is barrierless. The 334 kJ/mol threshold for AE is the lowest energy S 0 pathway in methyl isopropenyl ketone, 10 kJ/mol lower than the threshold for NTIb dissociation. Although NTIb will dominate at higher energies, there may be a small energy window where AE may be important, although it has not been experimentally observed. Similarly, there may be an energy window for AE in methyl vinyl ketone. The other possible dissociations in methyl vinyl ketone, NTIa 415 and NTIII, have thresholds >375 kJ/mol and are unlikely to be important under tropospheric conditions.
Given the lack of low energy ground or excited state dissociation pathways, we expect photolysis QYs for α,β-unsaturated carbonyls to be small. Indeed the total photolysis QY of methacrolein is around 1% at atmospheric pressure (Raber and Moortgat, 1996). Photo-induced keto-enol and enal-ketene tautomerisations, however, have thresholds under 300 kJ/mol, with crotonaldhyde having a lowest energy keto-enol tautomerisation threshold of 133 kJ/mol. Tautomerisation has been observed 420 in methyl vinyl ketone (Couch et al., 2021), and it may therefore be competitive for appropriate α,β-unsaturated species (So et al., 2018). The higher energy enol and ketene isomers may be collisionally stabilised under tropospheric conditions, although they are significantly more reactive and will be difficult to experimentally isolate.

Other carbonyls
The α-dicarbonyls have low energy π * + excited states (Arnett et al., 1974;Dykstra and Schaefer, 1976), red-shifted absorption 425 spectra compared to other carbonyls ( Fig. 1) and weakened α-C-C bonds due to the electron withdrawing nature of the two C=O chromophores. As a result, the excited state NTIa thresholds in dicarbonyls are lowered to ∼390 kJ/mol on S 1 and ∼300 kJ/mol on T 1 , with both accessible in the troposphere. The S 0 asymptotic energies for NTIa are slightly lower than the triplet thresholds, giving an energetic window for S 0 radical dissociation. Indeed, in glyoxal, two distinct, wavelength-dependent mechanisms of HCO • formation have been observed and attributed to dissociation on two electronic 430 states (Chen and Zhu, 2003;Kao et al., 2004;Salter et al., 2013). Decarbonylation to form H 2 CO + CO and TF to form H 2 + 2CO have the lowest energy thresholds in glyoxal, at 225 and 247 kJ/mol, respectively. Indeed, CO is known to form following irradiation of glyoxal at energies below 272 kJ/mol (Loge and Parmenter, 1981;Hepburn et al., 1983;Burak et al., 1987;Dobeck et al., 1999). Decarbonylation in methylglyoxal also has a low threshold energy, 235 kJ/mol, and we expect it to be the dominant S 0 dissociation mechanism. In diacetyl, without a formyl hydrogen, NTIa has the lowest dissociation threshold. 435 We therefore predict, for larger α-dicarbonyls, there will be an actinic window from ∼300-266 kJ/mol with no accessible dissociation pathways. In this case, collisional cooling and thermalisation to the parent carbonyl is the likely fate.
The remaining carbonyl in the dataset is glycolaldehyde, with an actinic range between 352 and 400 kJ/mol (Bacher et al., 2001). The NTIa thresholds on S 1 and T 1 for glycolaldehyde are 379 and 334 kJ/mol, respectively , and this channel dominates at all actinic energies (Bacher et al., 2001;Zhu and Zhu, 2010), with cleavage of the C-OH bond to 440 form • OH also reported (Zhu and Zhu, 2010). Thus, despite having the lowest calculated TF and H 2 -loss thresholds, these reactions have not been experimentally observed in glycolaldehyde. Keto-enol tautomerisation to 1,2-ethenediol, for which we calculate a threshold of 272 kJ/mol, has been postulated as an atmospheric route to formation of HO • 2 and formic acid (So et al., 2015), although it has not been experimentally observed.
5 Tropospheric relevance of S 0 reactions 445 A few guiding principles can be used to determine which of the possible S 0 pathways may be tropospherically relevant in a given carbonyl following absorption of an actinic photon.
• Reactions with thresholds greater than the actinic maximum energy of 400 kJ/mol are inaccessible at tropospheric photon energies. This rules out most S 1 reactions, with exceptions of NTIa dissociation in glycolaldehyde and NTIb in methyl vinyl ketone and methyl isopropenyl ketone (Haas, 2004;Rowell et al., 2019). T 1 NTI dissociations are accessible. T 1

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• For photon energies above their thresholds, T 1 reactions are fast and dominate photolysis (Kirkbride and Norrish, 1931;Zhu et al., 2009). If the photon energy is near threshold, non-radiative transitions and S 0 reaction may be competitive with excited state reaction. For photon energies (or collisional cooling on S 1 or T 1 ) below any excited state threshold, any photolysis must occur on S 0 . There is significant overlap of the absorption spectrum of most carbonyls with these lower energy photons (Fig. 1) and S 0 reactions have been observed in saturated carbonyls under these conditions (Heazlewood 460 et al., 2009;Amaral et al., 2010;Andrews et al., 2012;Tsai et al., 2015;Quinn et al., 2017;Toulson et al., 2018).
• For an S 0 process to be tropospherically important, its rate must be competitive with collisional cooling and thermal equilibrium. Only S 0 dissociations with thresholds <350 kJ/mol have been experimentally observed following photoexcitation of aldehydes in 1 atm of N 2 (Clubb et al., 2012;Shaw et al., 2018;Harrison et al., 2019). This suggests the most important S 0 dissociations are those with the lowest thresholds, that is, TF if available, NTI in ketones and selected AE, 465 NTIII and formyl+α H 2 -loss reactions.
• Although S 0 keto-enol and enal-ketene tautomerisations have the lowest calculated thresholds, these are reversible reactions where collisional stabilisation of the tautomer competes with isomerisation back to and dissociation of the parent carbonyl. Tautomerisation is therefore unlikely to be important if there are low energy S 0 dissociation pathways.
It will be important in the absence of such a channel, for example, in acetaldehyde (Clubb et al., 2012;Shaw et al., 2018), 470 some ketones and some α,β-unsaturated carbonyls.
The atmospheric importance of photo-initiated S 0 reactions is as yet unknown, however, S 0 reactions are likely to be broadly accessible under tropospheric conditions for all carbonyls. They are therefore likely in regions of the troposphere with high et Ehhalt and Rohrer, 2009;Yashiro et al., 2011;Patterson et al., 2020). Notably, in 'top-down' models, Rhee et al. (2006) and Xiao et al. (2007) propose increased H 2 production from photolysis of oxidized non-methane VOCs.

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The tautomerisation of acetaldehyde to vinyl alcohol, addition of atmospheric • OH and subsequent oxidation has been modelled and shown to produce significant global tropospheric formic acid and was found to be the dominant mechanism for formic acid formation in the marine boundary layer (Shaw et al., 2018). This one reaction, however, is not sufficient to explain the factor of two discrepancy between modelled and measured global organic acid concentrations (Shaw et al., 2018). Ketoenol isomerisation is present in almost all carbonyls we have considered, having amongst the lowest S 0 reaction thresholds.

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Similar reactions involving other unsaturated products, for example alkenes and ketenes (Atkinson et al., 2006;Kahan et al., 2013), will also lead to organic acid formation. We expect these reactions to be energetically accessible in most atmospheric carbonyls and their cumulative effect may address the modelled deficit (Shaw et al., 2018;So et al., 2018). Understanding the mechanism of formation of organic acids in the troposphere has further application to secondary aerosol formation; the higher oxygen content of organic acids reduces their volatility and, as proton donors, they are believed to be key species in the 520 nucleation and growth of atmospheric particles Bianchi et al., 2016;Tröstl et al., 2016;Liu et al., 2021).
Molecular hydrogen is an important atmospheric reducing agent and is an indirect greenhouse gas because it increases the atmospheric lifetime of CH 4 (Ehhalt and Rohrer, 2009). Current understanding indicates the major photochemical source of H 2 in the atmosphere is photolysis of formaldehyde, which accounts for at least half of the photochemically generated H 2 (Hauglustaine, 2002). The mechanisms generating the other half are unknown (Grant et al., 2010). The energetically accessible 525 S 0 TF and/or H 2 -loss reactions present in all aldehydes in our dataset, and expected in all atmospheric aldehydes, provide primary photolysis routes to H 2 that have not previously been considered. By better understanding the current atmospheric H 2 budget, we are better placed to model any future increase in atmospheric H 2 , for example, due to leakage of H 2 in any transition to a hydrogen economy. approximately 1 × 10 7 , 1 × 10 6 , 2 × 10 6 , and 2 × 10 7 s −1 , respectively. There is experimental evidence for all of these S 0 reac-550 tions under tropospheric conditions (Heazlewood et al., 2009;Horowitz and Calvert, 1982;Heazlewood et al., 2008;Moortgat et al., 2010;Harrison et al., 2019;Clubb et al., 2012;Shaw et al., 2018), although decarbonylation and formyl+α H 2 -loss are minor channels. Many of the S 0 thresholds in Table 1 are lower than those for acetaldehyde, implying higher reaction rate coefficients, although these are yet to be calculated, Meeting these challenges will enable the tropospheric importance of photo-initiated S 0 reactions in individual carbonyls 555 to be determined. Even if these individual reactions have small QY, their presence in all atmospheric carbonyls may lead to cumulative products that may be atmospherically significant.

Conclusions
We have calculated S 0 reaction thresholds for nine reaction types in seven classes of carbonyl within a 'small' carbonyl dataset.
In general, the S 0 transition states are 'late' and resemble the products. Reaction threshold energies typically correlate with the 560 stability of the product molecules, enabling our results to be generalised to larger carbonyls.
In the smallest carbonyls and dicarbonyls, formaldehyde, glyoxal and methylglyoxal, the lowest energy threshold is for direct decarbonylation and this mechanism will compete with NTI dissociation. In larger aldehydes direct decarbonylation via a TS mechanism has a threshold of ∼350 kJ/mol in saturated species and ∼360-375 in α,β-unsaturated species. This reaction is therefore likely unimportant in larger aldehydes. However, alternate roaming pathways may be viable if there are low energy 565 barrierless S 0 NTI pathways.
In larger carbonyls, the lowest energy S 0 dissociation thresholds are for triple fragmentation (TF) of both saturated (∼300 kJ/mol) and α,β-unsaturated (∼340 kJ/mol) aldehydes, with negligible impact from extension of the main alkyl chain. Branching at the α-position only increases TF thresholds by ∼5 kJ/mol, but increases the reaction path degeneracy and hence, likely, the QY.

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The only 4-centre H 2 -loss mechanism relevant at actinic energies involves the formyl and α hydrogens, and so need only be considered for aldehydes. This threshold is highest, at 337 kJ/mol, for acetaldehyde, and we expect it to be ∼315 in other aldehydes since a terminal double-bond is not being formed.
The NTIII β-hydrogen transfer reaction is energetically accessible in saturated carbonyls, but largely inaccessible for α,βunsaturated species. This pathway is often overlooked in the interpretation of photolysis experiments but should be considered 575 when the production of alkenes and aldehydes shows little-to-no pressure dependence.
We expect alkane elimination to have thresholds of ∼350 kJ/mol in linear unsaturated ketones. Whilst energetically accessible under tropospheric conditions, these thresholds are above those for NTI dissociation and we do not expect alkane elimination to be significant. The AE threshold, however, is reduced to ∼330 kJ/mol by branching or unsaturation at the α position and alkene elimination may be important in these species.

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Finally, where present, tautomerisation pathways have the overall lowest S 0 thresholds and yield highly reactive unsaturated species: enols or ketenes. The keto-enol tautomerisation of acetaldehyde to vinyl alcohol leads to significant formation of formic acid in the troposphere. This process may be relevant to other atmospheric carbonyls. In particular, the α,β-unsaturated carbonyls have UV absorption spectra that extend to low energies, high excited state reaction thresholds and relatively high S 0 dissociation thresholds.

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The calculations in this paper demonstrate a range of ground state reactions are accessible within the tropospheric "photochemistry" of carbonyls. The energetic thresholds for these S 0 reactions are some of the lowest calculated for any carbonyl reaction, on any electronic state. Many of the TF reactions as well as the keto-enol and enal-ketene tautomerisation are predicted to have reaction thresholds 300 kJ/mol. These reactions are likely to be important following photoexcitation at energies below any excited state reaction threshold or following collisional cooling of excited state carbonyl molecules below 590 such thresholds. It may be that the QY for an individual carbonyl is relatively small. The fact that one or more of these reactions are expected to occur in all atmospheric carbonyls suggests that, cumulatively, they may be significant in the troposphere and may have atmospheric consequences.
Supplementary Material: The supplement related to this article contains a figure and discussion of the energetic thresholds for relevant S 0 reactions in butanal; For the reaction classes considered, excepting enal-ketene tautomerisation, a review of the 595 previous computational literature, additional higher energy thresholds, as applicable, and representations of the optimised S 0 first-order saddle points (excepting those for H 2 -loss); Cartesian coordinates for all optimised saddle points are provided as text filed in a ZIP folder. https://doi.org/10.5194/acp-0-1-2021-supplement