The tropospheric fate of an illustrative carbonyl, acetaldehyde (CH3CHO), is shown schematically in Fig. . A near-UV photon excites the S1←S0 (n,π*) transition. The actinic absorption window, indicated in Fig.  by violet shading, is bounded by the lowest energy excitation to S1 and the highest energy photon available in the troposphere. For most carbonyls the S1 state is bound at actinic energies, as shown in Fig. , and S1 reactions are inaccessible . Intersystem crossing (ISC) from S1 to the lowest energy triplet state, T1, is formally spin-forbidden; however, as shown schematically in Fig. , carbonyl S1 and T1 minimum energy geometries are almost identical, and the energetic separation between them is small. This allows relatively fast S1→T1 ISC , indicated in Fig.  by a thick ISC arrow. On T1, NTI α-bond cleavage is the most common photolysis reaction , and we define NTIa and NTIb as cleavage of the α-bond to the larger and smaller substituents, respectively. In CH3CHO NTIa results in the formation of ⚫CH3 + ⚫HCO and NTIb yields CH3CO⚫+⚫H. On T1 both NTIa and NTIb involve an energetic barrier, shown in orange in Fig. .

For larger carbonyls, the Norrish type II (NTII) intramolecular hydrogen-transfer reaction, forming an alkene and an enol, also occurs and is initiated in an electronic excited state . Although NTI and NTII are typically the major contributors to the photolysis quantum yield (QY) of carbonyls , if excitation is to energies below excited state photolysis thresholds, or if the electronically excited carbonyl is collisionally cooled below these thresholds, non-radiative relaxation to S0 must occur; fluorescence and phosphorescence QYs in carbonyls are typically <1 % . The large energy separation between S1 and S0 means S1→S0 internal conversion (IC) will be slow and less important than ISC . Indeed, rapid IC is usually associated with conical intersections: geometric configurations in which two electronic surfaces are degenerate . In the main, these lie above the actinic energy range in carbonyls .

The carbonyl RC=O group is planar in the S0 minimum energy geometry but pyramidalised in the S1 and T1 minima . The change in electron spin, geometric dissimilarity and energetic separation between the T1 and S0 minima suggest T1→S0 ISC may be slow. However, the non-bonding oxygen lone pair orbital, n, in C=O is approximately orthogonal to the anti-bonding π* excited state orbital. Thus the change in electron spin angular momentum in T1→S0 relaxation is accompanied by a change in electron orbital angular momentum. This results in conservation of total electron angular momentum and a high T1→S0 ISC rate, as dictated by El-Sayed's rule . Indeed, ISC to S0 can be kinetically competitive even when NTI reactions are accessible .

The most likely carbonyl relaxation route to S0 is therefore via two sequential ISC steps . This generates vibrationally “hot” photoexcited S0 molecules that retain much of the initial UV photon energy as internal energy. This “hot” carbonyl can collide with other atmospheric molecules, and collisional energy transfer (CET) removes excess vibrational energy, returning the molecule to thermal equilibrium. This is indicated in Fig.  by angular green arrows in the S0 well, although CET will also occur in S1, T1 and isomer wells . The photoexcited S0 carbonyls are conformationally flexible, and S0 dissociation and isomerisation will compete with CET.

This paper explores, using computational chemistry, the possible S0 dissociation and isomerisation pathways and how they depend on the structural class of the carbonyl.

There are a vast number of carbonyls in the atmosphere with little experimental data in comparison to this diversity . Here we consider a test set of 20 carbonyls, which includes the 12 species that have explicit photolysis reactions within the Master Chemical Mechanism and 5 species in the GEOS-Chem atmospheric chemistry models. We also include carbonyls with atmospherically representative structural features based on bond order or substitution around the C=O chromophore. The 20 carbonyls in the test set are shown in Fig.  and were previously used in a study of excited state NTI photolysis . They are colour-coded into seven structurally distinct classes, which determine their absorption spectra (Fig. ) and chemical behaviour .

Quantum chemistry methods are used to calculate reaction thresholds for up to nine S0 unimolecular reactions that may be accessible under tropospheric conditions. In our quantum chemistry calculations, the threshold is defined as the limiting zero-point-corrected electronic energy for the reaction to occur. Experimental thresholds are the lowest energies for which reaction has been observed to occur and hence include tunnelling through any barrier to reaction. In the figures below, the nine possible unimolecular reactions are colour-coded according to Fig. . Explicitly, the reactions are as follows:

decarbonylation (CO loss)

Decarbonylation, the unimolecular loss of a CO molecule, is uncommon in gas-phase ketones. It has been observed in acetone, although the mechanism is unclear; postulated a direct mechanism, presumably via a TS, and roaming reactions have also been suggested in acetone . Here, like previous studies , we find no computational evidence for direct decarbonylation TSs in ketones.

In aldehydes, the formyl hydrogen is transferred to the main alkyl chain, forming CO and an alkane: R–(C=O)–H → CO + RH. Both roaming and TS decarbonylation mechanisms have been observed in gas-phase reactions of small aldehydes . Figure  shows TS decarbonylation thresholds for the aldehydes in the test set.

One trend apparent in Fig.  is that extension of the alkyl chain has little effect on the decarbonylation threshold, with formaldehyde through pentanal all predicted to have ∼350 kJ/mol thresholds. The predicted decrease in threshold upon branching at the α position is <5 kJ/mol.

The decarbonylation thresholds in the α,β-unsaturated enals, acrolein and methacrolein, are ∼10 kJ/mol higher than those for saturated aldehydes as decarbonylation involves breaking a resonance-stabilised C–C bond. Further delocalisation of the π system increases the threshold to 375 kJ/mol for crotonaldehyde.

The α-dicarbonyls are predicted to have the lowest decarbonylation thresholds, which is consistent with the two electron-withdrawing oxygen substituents reducing the electron density between the carbonyl moieties and weakening the α-C–C bond. Four decarbonylation thresholds were found for glyoxal (Table S1 and Fig. S3 in Sect. S2 of the Supplement), with the lowest energy threshold of 225 kJ/mol corresponding to a non-planar TS and formation of formaldehyde and CO. This threshold is lower than the 251 kJ/mol threshold to form hydroxymethylene, HOCH and CO. A similar non-planar TS is found for decarbonylation in methylglyoxal to form acetaldehyde, which has a slightly higher threshold of 235 kJ/mol. Here, substitution strengthens the central C–C bond due to electron donation from the CH3 substituent, and there is also a steric penalty at the TS (Fig. S3a vs. S3e).

All calculated decarbonylation thresholds are accessible at the maximum tropospheric photon energy of 400 kJ/mol, although, with the exception of formaldehyde , CO-loss QYs are low. For example, they are measured to be ∼0.02 in acetaldehyde and ∼0.01 for butanal .

The thresholds for roaming decarbonylation reactions are linked to the S0 NTI asymptotic energies . For example, the roaming threshold in formaldehyde is ∼ 2 kJ/mol below the radical dissociation threshold , and both H and CH3 roaming has been observed in acetaldehyde . As such, the majority of aldehydes in the test set have roaming thresholds that are similar to or lower in energy than the TS decarbonylation threshold, with formaldehyde a notable exception. Roaming pathways associated with NTI may also be present in ketones . This suggests decarbonylation will be energetically accessible under tropospheric conditions for all carbonyls.

We define the TF reaction as a concerted S0 reaction forming three products via a single TS, in which one of these products is H2. For example, the TF reaction in the saturated aldehyde, propanal, is CH3–CH2–(C=O)H → H2C=CH2 + H2 + CO. For enals, the hydrocarbon product will be an alkyne. In dicarbonyls, TF forms H2, CO, and either a second CO or ketene. Predicted S0 TF thresholds are shown in Fig. . All of the TF reactions shown, except that for glyoxal, involve a five-centre TS and H loss from a β-hydrogen. In glyoxal, the four-centre TS involves the two formyl hydrogens. The TSs are shown explicitly in Fig. S4 of the Supplement and are “late”, resembling the photolysis products. As a result, TF threshold energies are strongly influenced by the stability of the products.

The TF thresholds for the saturated aldehydes are not significantly affected by chain extension, with thresholds of 295, 299 and 294 kJ/mol for propanal, butanal and pentanal, respectively. The effect of alkyl branching at the α position is inconsistent and small. It raises the thresholds to 304 kJ/mol for 2-methylpropanal and 306 kJ/mol for 2,2-dimethylpropanal but lowers it for methacrolein (338 kJ/mol) compared to acrolein (343 kJ/mol). These differences are close to the likely accuracy of the B2GP-PLYP-D3 calculations.

The TF thresholds of the enals are significantly higher than those of the saturated aldehydes at ∼340–350 kJ/mol. This increase is larger than the ∼10 kJ/mol increase seen in decarbonylation thresholds for breaking a delocalised C–C bond and arises because of bond angle strain in the five-centre TF TSs of the α,β-unsaturated species. As shown in Fig. S4, the C–C–C backbone angles in the TSs deviate significantly from their equilibrium values, and the energetic penalty is increased, relative to saturated species, because of the rigidity of the delocalised C–C bonds.

The predicted S0 TF thresholds of the two α-dicarbonyls, glyoxal and methylglyoxal, differ markedly by 83 kJ/mol. This arises because their TS structures and reaction products are qualitatively different (Fig. S4j and k). Methylglyoxal has a five-centre TS that involves strain across the O=C–C=O backbone and also forms a relatively high-energy ketene product. Glyoxal, in contrast, forms exothermically favourable products: H2 and two CO molecules, resulting in a significantly lower S0 TF threshold.

Glycolaldehyde is an atypical carbonyl and has the lowest energy TF pathway, calculated at 229 kJ/mol. Here, the H2-loss channel arises from the combination of the formyl hydrogen and the OH hydrogen, which is more labile than the backbone C–H hydrogens. The C–OH angle in the glycolaldehyde TS also involves minimal ring strain (Fig. S4l). This pathway has not been previously proposed for glycolaldehyde and occurs via a five-centre TS, which is lower in energy than the four-centre TF TS for H2 formation from the formyl and a β CH2 hydrogen. Indeed it is the lowest energy dissociation pathway we have found in glycolaldehyde.

Alongside the Norrish type I and II reactions, the lesser known “Norrish type III” reaction (NTIII) was proposed by to explain the observation of acetaldehyde and propene as products of 3-methylbutan-2-one photoexcitation. NTIII involves a four-centre TS, with β-hydrogen transfer from the backbone to the carbonyl moiety leading to the formation of an aldehyde and an alkene; for example, RCH2CH2(C=O)R′ → R–CH=CH2 + H(C=O)R′. Experimentally, NTIII has been found to be a minor channel . However, since the alkene product from NTIII can also be formed in other S0 reactions (TF, H2 loss), NTIII needs to be well understood to disambiguate these mechanisms.

Figure  shows our calculated NTIII thresholds. These vary across a large energy range from 310 to 417 kJ/mol. In glyoxal, methylglyoxal and methyl vinyl ketone the NTIII thresholds are at or above the actinic maximum energy of 400 kJ/mol. The NTIII thresholds of the other α,β-unsaturated carbonyls lie between 380 and 400 kJ/mol. The high thresholds arise from resonance stabilisation of the breaking C–C bond and suggest NTIII is unlikely to be important in these species under tropospheric conditions.

The saturated carbonyls in Fig.  have NTIII thresholds in the range of 310–355 kJ/mol. The NTIII thresholds decrease significantly when the main alkyl chain is lengthened past the β position: there is a 38 kJ/mol lowering of the NTIII threshold from butanone to pentan-2-one, and a 21 kJ/mol lowering from propanal to butanal. The effect of alkyl chain lengthening beyond the β position is also present in α,β-unsaturated carbonyls, although it is smaller, with only a 7 kJ/mol decrease in threshold from acrolein to crotonaldehyde. The NTIII threshold, however, is unchanged with further alkyl chain lengthening. For example, butanal and pentanal have almost the same NTIII threshold (311 and 310 kJ/mol, respectively), and there is only a 4 kJ/mol difference in threshold between butanone and pentan-3-one. This suggests chain lengthening past the γ position will not result in any change to the reaction threshold. The NTIII threshold also appears to be independent of the “spectator” alkyl substituent in the ketones.

Branching at the α position decreases the NTIII threshold, though to a lesser extent than the addition of a γ-carbon. The decreases are as follows: 2 kJ/mol from propanal to 2-methylpropanal, 7 kJ/mol from 2-methylpropanal to 2,2-dimethylpropanal, 15 kJ/mol from acrolein to methacrolein, and 23 kJ/mol ketone to methyl isopropenyl ketone. These decreases in threshold energy exceed the expected uncertainty of our B2GP-PLYP-D3 calculations, except for the smallest 2 kJ/mol energy difference. The trends in NTIII threshold with both alkyl chain length 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 .

H2 can be formed on S0 via a four-centre TS where adjacent hydrogen atoms form an H–H bond and dissociate as H2, leaving an 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 H2-loss mechanisms can be distinguished by the point of unsaturation in the co-product.

The S0 reaction thresholds for the possible H2-loss channels are given in Table  and are shown in Fig. , where solid lines denote H2 loss from the formyl and α positions, dashed lines H2 loss from the α and β positions, and dot-dashed lines H2 loss from the β and γ positions. Note that the y axis of Fig.  is broken into three energy sections to indicate energetic separation between the different mechanisms.

Figure  indicates that thresholds for H2 loss from the α and β positions are higher than from the formyl and α positions, with H2-loss thresholds from the β and γ positions higher still. For a given carbonyl, the highest H2-loss threshold is predicted for removal of a hydrogen from the terminal carbon. Like the NTIII reaction, the TSs for four-centre H2 loss are “late”, and the threshold energies are related to the stability of the forming alkene product. Increasing substitution around the double bond increases alkene stability , and so products with a terminal C=C bond are comparatively less stable than products from H2 loss at other sites. For example, there is an approximately 20 kJ/mol decrease in threshold for formyl + α H2 loss from acetaldehyde to propanal. Like NTIII, there is little effect on the formyl and α H2-loss threshold upon further chain extension. For example, the formyl and α H2-loss thresholds for propanal, butanal and pentanal are all ∼315 kJ/mol. Branching at the α position also has little effect on the formyl and α H2-loss threshold (cf. propanal and 2-methylpropanal). Similar trends are seen for H2 loss from the α and β and β and γ positions.

The results in Table  and Fig.  reinforce, for multiple carbonyl species, that only H2 loss from the formyl and α positions is energetically accessible in the actinic energy range. Indeed, the four-centre H2-loss channel in acetaldehyde has recently been observed experimentally under tropospherically relevant conditions . In the absence of a formyl hydrogen, none of the ketone H2-loss channels are accessible in the troposphere. Similarly, H2 loss is not accessible in species lacking an α-hydrogen, like 2,2-dimethylbutanal and methacrolein.

The thresholds for H2 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 they are close to the maximum actinic energy, suggesting S0 H2 loss is unlikely to be important in α,β-unsaturated aldehydes.

Glycolaldehyde is calculated to have the lowest H2-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 four-centre TS and the hydroxyketene product. Glycolaldehdye is also the only carbonyl with an energetically accessible β-H-loss channel. Loss of the OH and β-hydrogen forms glyoxal and H2 with a 384 kJ/mol threshold, close to the actinic maximum energy.

In ketones, migration of an α-H atom to the “other” α-carbon via a four-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 S0 reaction thresholds for the lowest energy AE channels are given in Table  and are shown in Fig. . In asymmetric ketones two AE channels are possible, and these are described in Sect. S6 of the Supplement.

Figure  shows our calculated AE thresholds vary from 329 to 363 kJ/mol, with all accessible under tropospheric conditions. The TSs for AE are shown in Fig. S6.

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 of formation , although methylketene 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 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.

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.

Carbonyls can exist in two tautomeric forms: a keto form (encompassing, here, both ketones and aldehydes) and an enol form in which 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 S0 carbonyls in aqueous solution at room temperature, although the keto tautomer is thermodynamically favoured . Keto–enol tautomerisation has been observed in gas-phase photolysis experiments on acetaldehyde , and the authors suggest it may occur in many other carbonyls under tropospheric conditions. In solution or at higher gas pressures, keto–enol isomerisation is also readily catalysed by species containing acidic hydrogens – including water, alcohols, and organic and inorganic acids.

The lowest energy calculated keto–enol tautomerisation thresholds for the relevant carbonyls in the test set are given in Table  and shown in Fig. . 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 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 the 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, and all optimised saddle point geometries are shown in Fig. S7 of the Supplement. As described in more detail in 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. , 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 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 the formation of the alternate trans-enol within ∼1 kJ/mol of those shown in Fig. .

In α,β-unsaturated carbonyls, the keto–enol tautomerisation thresholds are low when tautomerisation involves an H atom from an aliphatic group, for example, the CH3 group in methyl vinyl ketone and crotonaldehyde. Tautomerisation thresholds involving olefinic 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 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 and in acetone and methyl vinyl ketone , this tautomerisation may be important under tropospheric conditions in these species.

There has been recent interest in the formation of ketenes as atypical and relatively uncharacterised products of carbonyl photolysis . As seen above, the formyl + α H2-loss mechanism forms ketenes in aldehydes, and the AE mechanism forms ketenes in ketones. In enals there is also an S0 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. , together with the calculated B2GP-PLYP-D3 threshold energies. The enal–ketene tautomerisation threshold in methacrolein, 285 kJ/mol, is ∼ 14 kJ/mol lower in energy than the 299 kJ/mol G3X-K//M06-2X/6-31G(2df,p) threshold previously calculated by . This difference is consistent with the variation between B2GP-PLYP-D3 and G3X-K thresholds for other reactions considered in methacrolein and methyl vinyl ketone (see Supplement) and may be due to the treatment of dispersion in the saddle point geometries.

Figure  shows that the key structural parameters describing the TSs for S0 enal–ketene tautomerisation are almost identical for the three enals in the test set. This indicates that α-branching and main alkyl chain extension have a negligible effect on the TS geometry and little effect on the threshold energy. The predicted enal–ketene tautomerisation thresholds are all well below the maximum actinic energy of 400 kJ/mol and suggest that, in appropriate species, enal–ketene tautomerisation may be competitive in the troposphere.

For saturated aldehydes, the lowest energy dissociation pathway on S0 is concerted TF with a threshold of ∼300 kJ/mol. At wavelengths and internal energies where excited state chemistry is energetically inaccessible, TF may therefore be a significant photo-induced mechanism. Triple fragmentation has been observed as a primary photolysis mechanism in propanal and 2-methylpropanal, with QYs of 4 % and 9 %, respectively, at 1 atm pressure of N2 . On the basis of kinetic modelling and in the absence of excited state primary pathways, both sets of TF products were found to be consistent with S0 reaction. The increased QY for 2-methylpropanal over propanal was attributed to increased reaction path degeneracy rather than a lower reaction threshold. This suggests TF QYs will be higher in branched aldehydes in proportion to the ratio of β-hydrogens. For example, we predict 2,2-dimethylpropanal, with nine β-hydrogens, to have a higher TF QY than 2-methylpropanal, with six β-hydrogens, and propanal, with three β-hydrogens.

The next lowest energy S0 dissociation pathway is either H2 loss from the formyl and α-H atoms in the smaller species (acetaldehyde, propanal, methylpropanal) or NTIII in the larger species (butanal, pentanal, 2,2-dimethylpropanal). For formyl + α H2 loss, acetaldehyde has three α-hydrogens and the highest threshold at 337 kJ/mol. This channel has been observed experimentally, with a QY of ∼1 % at 305 nm (392 kJ/mol) and 1 atm of N2, and is demonstrated to be a primary process by single molecule velocity map ion imaging (VMI) experiments . Although classical trajectory simulations initiated at the H2-loss TS could not reproduce the VMI product state distributions, they are consistent with an S0 roaming mechanism . The equivalent H2-loss thresholds for the other saturated aldehydes, which have two rather than three α-hydrogen atoms, are significantly lower at ∼315 kJ/mol. Thus we expect H2-loss reactions to be present in other saturated aldehydes with similar or slightly higher QYs than in acetaldehyde. Indeed, under tropospheric conditions, methylketene and dimethylketene have been observed as minor products following the photolysis of propanal and 2-methylpropanal, respectively, with kinetic modelling consistent with S0 reaction . In saturated aldehydes, the NTIII thresholds are similar to those for H2 loss. Experimental QYs for this pathway, however, have not been reported. The NTIII mechanism shares common products with TF (alkene) and decarbonylation (CO), which may complicate its experimental identification. Our calculations suggest NTIII is energetically competitive, and it may be important in interpreting photolysis QYs of saturated aldehydes.

With the exception of formaldehyde, the next lowest energy S0 dissociation threshold is for ground state NTIa, that is, α-bond cleavage to the alkyl substituent. This reaction is typically assumed to occur only on T1, but the S0 reaction has been identified in formaldehyde , acetaldehyde and acetone . Because it is barrierless we expect S0 NTIa to dominate as internal energy increases above its threshold. Thus, we expect to observe NTIa dissociation products at energies well below the T1 threshold.

Decarbonylation (CO loss) has the highest calculated S0 dissociation thresholds for all the saturated carbonyls in the test set, bar formaldehyde, for which the decarbonylation threshold is slightly below that of NTI. All decarbonylation thresholds are predicted to be 340–350 kJ/mol, and experimental CO-loss QYs are correspondingly low: in the actinic energy range, the atmospheric pressure decarbonylation QY for acetaldehyde is ∼0.5 % and ∼0.2 % for 2-methylbutanal . Although two S0 decarbonylation mechanisms are known, roaming reactions and reaction via a TS, there is no known excited state decarbonylation mechanism. Given the relatively high threshold energies, we predict other S0 pathways will be favoured in larger saturated aldehydes over decarbonylation, which will be at most a minor channel.

Our calculations predict keto–enol tautomerisation to be the lowest energy pathway on S0 for all saturated aldehydes, lying about 20 kJ/mol below TF. Unlike dissociation, however, tautomerisation is reversible. Collisional cooling into the enol well therefore competes with tautomerisation back to the parent aldehyde . Dissociation of the aldehyde reduces its concentration, and hence, while the rate coefficient is constant, the rate of enol formation is reduced. Although experimentally observed in ketones, the only aldehyde for which photo-initiated keto–enol tautomerisation has been observed is acetaldehyde , in which TF is not available, and master equation modelling confirms that it is an S0 process . Whilst it is likely keto–enol tautomerisation is occurring in other saturated aldehydes, QYs are likely small, and the reactivity of the enol will make characterisation difficult.

Following excitation by an actinic photon, the relevant S0 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 H2-loss pathways. The only S0 dissociation pathways in ketones are NTIII, AE and NTI. The thresholds for AE and NTI are similar 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 – assumed S0 – NTIII mechanism in the literature .

The keto–enol tautomerisation thresholds for saturated ketones are slightly lower than in saturated aldehydes. This, combined with the absence of low-energy S0 dissociation pathways and the experimental observation of keto–enol tautomerisation in acetone under thermal conditions, where reaction must occur on S0 , suggests photo-initiated tautomerisation may be important in ketones.

In general, because of resonance stabilisation of the bond between α-carbons and formyl carbons, we calculate higher S0 thresholds in 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 S0. Further, unlike many other carbonyls, the electronic structure of the α,β-unsaturated carbonyls promotes rapid electronic relaxation from S1 to S0 , suggesting high S0 internal energies and therefore a relatively high probability of S0 reaction.

Triple fragmentation is the lowest energy S0 dissociation pathway for the unsaturated aldehydes in the test set. The TF thresholds are ∼40 kJ/mol higher than in saturated aldehydes and are slightly lower than the S0 NTIb asymptotic energies and decarbonylation thresholds. The other possible S0 dissociation reactions (H2 loss, NTIII and S0 NTIa) have thresholds >370 kJ/mol and are unlikely to be significant under tropospheric conditions.

Unsaturated ketones, like their saturated counterparts, have fewer S0 dissociation pathways, with only NTIII, AE, NTIa and 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 S0 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 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 . Photo-induced keto–enol and enal–ketene tautomerisations, however, have thresholds under 300 kJ/mol, with crotonaldehyde having one of the lowest energy keto–enol tautomerisation thresholds of 133 kJ/mol. Tautomerisation has been observed in methyl vinyl ketone under thermal conditions , and S0 tautomerisation may therefore be competitive for appropriate α,β-unsaturated species . 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.

The α-dicarbonyls have low-energy π+* excited states , red-shifted absorption spectra compared to other carbonyls (Fig. ) 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 S1 and ∼300 kJ/mol on T1 , with both accessible in the troposphere. The S0 asymptotic energies for NTIa are slightly lower than the triplet thresholds, giving an energetic window for S0 radical dissociation. Indeed, in glyoxal, two distinct, wavelength-dependent mechanisms of HCO⚫ formation have been observed and attributed to dissociation on two electronic states . Decarbonylation to form H2CO + CO and TF to form H2 + 2 CO 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 . Decarbonylation in methylglyoxal also has a low threshold energy, 235 kJ/mol, and we expect it to be the dominant S0 dissociation mechanism. In diacetyl, without a formyl hydrogen, NTIa has the lowest dissociation threshold. We therefore predict, for larger α-dicarbonyls, there will be an actinic window from ∼300 to 266 kJ/mol with no accessible dissociation pathways. In this case, collisional cooling and thermalisation to the parent carbonyl are the likely fate.

The remaining carbonyl in the test set is glycolaldehyde, with an actinic range between 352 and 400 kJ/mol . The NTIa thresholds on S1 and T1 for glycolaldehyde are 379 and 334 kJ/mol, respectively , and this channel dominates at all actinic energies , with cleavage of the C–OH bond to form ⚫OH also reported . Thus, despite having the lowest calculated TF and H2-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 the formation of HO2⚫ and formic acid , although it has not been experimentally observed.