Meredith J. T. Jordan’s research while affiliated with The University of Sydney and other places

Formaldehyde, HCHO, is the highest-volume carbonyl in the atmosphere. It absorbs sunlight at wavelengths shorter than 330 nm and photolyses to form H and HCO radicals, which then react with O2 to form HO2. Here we show HCHO has an additional HO2 formation pathway. At photolysis energies below the energetic threshold for radical formation we directly detect HO2 at low pressures by cavity ring-down spectroscopy and indirectly detect HO2 at 1 bar by Fourier-transform infrared spectroscopy end-product analysis. Supported by electronic structure theory and master equation simulations, we attribute this HO2 to photophysical oxidation (PPO): photoexcited HCHO relaxes non-radiatively to the ground electronic state where the far-from-equilibrium, vibrationally activated HCHO molecules react with thermal O2. PPO is likely to be a general mechanism in tropospheric chemistry and, unlike photolysis, PPO will increase with increasing O2 pressure.

Carbonyls are among the most abundant volatile organic compounds in the atmosphere. They are central to atmospheric photochemistry as absorption of near-UV radiation by the C=O chromophore can lead to photolysis. If photolysis does not occur on electronic excited states, non-radiative relaxation to the ground state will form carbonyls with extremely high internal energy. These “hot” molecules can access a range of ground state reactions. Up to nine potential ground state reactions are investigated at the B2GP-PLYP-D3/def2-TZVP level of theory for a test set of 20 representative carbonyls. Almost all are energetically accessible under tropospheric conditions. Comparison with experiment suggests the most significant ground state dissociation pathways will be concerted triple fragmentation in saturated aldehydes, Norrish type III dissociation to form another carbonyl, and H2 loss involving the formyl H atom in aldehydes. Tautomerisation, leading to more reactive unsaturated species, is also predicted to be energetically accessible and is likely to be important when there is no low-energy ground state dissociation pathway, for example in α,β-unsaturated carbonyls and some ketones. The concerted triple fragmentation and H2-loss pathways have immediate atmospheric implications for global H2 production, and tautomerisation has implications for the atmospheric production of organic acids.

Carbonyls are among the most abundant volatile organic compounds in the atmosphere. They are central to atmospheric photochemistry as absorption of near-UV radiation by the C=O chromophore can lead to photolysis. If photolysis does not occur on electronic excited states, non-radiative relaxation to the ground state will form carbonyls with extremely high internal energy. These “hot” molecules can access a range of ground state reactions. Up to nine potential ground state reactions are investigated at the B2GP-PLYP-D3/def2-TZVP level of theory for a dataset of 20 representative carbonyls. Almost all are energetically accessible under tropospheric conditions. Comparison with experiment suggests the most significant ground state dissociation pathways will be concerted triple fragmentation in saturated aldehydes, Norrish type III dissociation to form another carbonyl, and H2-loss involving the formyl H atom in aldehydes. Tautomerisation, leading to more reactive unsaturated species, is also predicted to be energetically accessible and is likely to be important when there is no low-energy ground state dissociation pathway, for example in α,β-unsaturated carbonyls and some ketones. The concerted triple fragmentation and H2-loss pathways have immediate atmospheric implication to global H2 production and tautomerisaton has implication to the atmospheric production of organic acids.

Carbonyls are among the most abundant volatile organic compounds in the atmosphere, and their C=O chromophores allow them to photolyse. However, carbonyl photolysis reactions are not restricted to the excited state: the C=O chromophore allows relaxation to, and reaction on, the ground state, following photon absorption. In this paper, the energetic thresholds for eight ground state reactions across twenty representative carbonyl species are calculated using double-hybrid density functional theory. Most reactions are found to be energetically accessible within the maximum photon energy available in the troposphere, but are absent in contemporary atmospheric chemistry models. Structure–activity relationships are then elucidated so that the significance of each reaction pathway for particular carbonyl species can be predicted based upon their class. The calculations here demonstrate that ground state photolysis pathways are ubiquitous in carbonyls and should not be ignored in the analysis of carbonyl photochemistry.</div

Photolysis thresholds are calculated for the Norrish Type II (NTII) intramolecular γ-hydrogen abstraction reaction in 22 structurally informative carbonyl species. The B2GP-PLYP excited state S ₁ and T ₁ thresholds agree well with triplet quenching experiments. However, many linear-response methods deliver poor S ₁ energetics, which is explained by a S ₁/ S ₀ conical intersection in close proximity to the S ₁ transition state. Multiconfigurational CASSCF calculations confirm a conical intersection features across all carbonyl classes. Structure–activity relationships are determined that could be used in atmospheric carbonyl photochemsitry modelling. This is exemplified for butanal, whose NTII quantum yields are too low when used as a ‘surrogate’ for larger carbonyls, since butanal lacks the γ-substitution that stabilises the 1,4- biradical. Reaction on T ₁ dominates only in species where the S ₁ thresholds are high — typically ketones. The α, β-unsaturated carbonyls cannot cleave the α–β bond, causing them to photoisomerise. A concerted S ₀ NTII mechanism is calculated to be viable and may explain the recent detection of NTII photoproducts in the photolysis of pentan-2-one below the T ₁ threshold.</div

We approach the problem of predicting excitation energies of diverse, larger (5–6 carbons) carbonyl species central to earth’s tropospheric chemistry. Triples contributions are needed for the vertical excitation energy (E^vert), while EOM-CCSD//TD-DFT calculations provide acceptable estimates for the S₁ relaxation energy (E^relax), and (TD-)DFT suffices for the S₀ → S₁ zero-point vibration energy correction (∆E^ZPVE). Perturbative triples corrections deliver E^vert values close in accuracy to full iterative triples EOM-CC calculations. The error between EOM-CCSD and triples-corrected E vert values appears to be systematic and can be accounted for with scaling factors. However, saturated and α,β-unsaturated carbonyls must be treated separately. Double-hybrid S₀ minima can be used to calculate E^vert with negligible loss in accuracy, relegating the O(N⁵) expense of CCSD to only single-point energy and excitation calculations. This affordable protocol can be applied to all volatile carbonyl species. E⁰⁻⁰ predictions do overestimate measured values by ∼8 kJ/mol due to a lack of triples contribution in E relax, but this overestimation is systematic and the mean unsigned error is within 4 kJ/mol once this is accounted for.</div

Duality of roaming mechanism in H 2 CO The phenomenon of roaming in chemical reactions (that is, bypassing the minimum energy pathway from unlikely geometries) has attracted a great deal of attention in the chemical reaction dynamics community over the past decade and still demonstrates unexpected results. Using velocity-map imaging of state-selected H 2 products of H 2 CO photodissociation, Quinn et al. discovered the bimodal structure of rotational distribution of the other product fragment, CO. Quasiclassical trajectories showed that this bimodality originates from two distinctive reaction pathways that proceed by the trans or cis configuration of O–C–H⋯H, leading to high or low rotational excitations of CO, respectively. Whether such a mechanism is present in the many other chemical reactions for which roaming reaction pathways have been reported is yet to be determined. Science , this issue p. 1592

Norrish Type I (NTI) -bond cleavage is the dominant photolysis mechanism in small carbonyls and is an important source of radicals in the troposphere. In non-symmetric species two cleavages are possible, NTIa and NTIb, forming larger and smaller alkyl radicals, respectively. For a dataset of 20 small, atmospherically relevant carbonyls we predict NTIa and NTIb thresholds on the S0, S1 and T1 electronic states. The calculated NTIaT1 thresholds give a mean absolute deviation (MAD) of 5.8 kJ/mol with respect to the available experimental thresholds of 5 carbonyls. In addition, the intrinsic barrier heights to dissociation on the S0, S1 and T1 electronic states are predicted. We find RI-B2GP-PLYP/def2-TZVP calculations on S0 and unrestricted RI-B2GP-PLYP/def2-TZVP calculations on T1 give MADs of 6.1 kJ/mol for S0 asysmptotic energies and 6.3 kJ/mol for S0 → T1 0–0 excitation energies, with respect to available experimental data. A composite method is used to determine S1 thresholds, with bt-STEOM-CCSD/cc-pVQZ calculation of vertical excitation energies and TD-RI-B3LYP/def2-TZVP calculations on S1, which achieves a MAD of 7.2 kJ/mol, with respect to experimental 0–0 excitation energies. Our calculations suggest, with the exception of bifunctional carbonyls and enones, NTI reactions on S1 are unlikely to be important at tropospherically relevant photolysis energies (< 400 kJ/mol). In contrast, at these energies almost all possible NTI channels on T1 are open and all barrierless S0 NTI dissociations are accessible. Our calculations allow a number of structural effects on both 0–0 excitation energies and intrinsic reaction barriers, on a given electronic state, to be elucidated and rationalised.

The first experimental observation of the primary photochemical channel of acetaldehyde leading to the formation of ketene (CH2CO) and hydrogen (H2) molecular products is reported. Acetaldehyde (CH3CHO) was photolysed in a molecular beam at 305.6 nm and the resulting H2 product characterized using velocity-map ion (VMI) imaging. Resonance-enhanced multiphoton ionization (REMPI), via two-photon excitation to the double-well EF 1∑g+ state, was used to state-selectively ionize the H2 and determine angular momentum distributions for H2 (ν = 0) and H2 (ν = 1). Velocity-map ion images were obtained for H2 (ν = 0 and 1, J = 5), allowing the total translational energy release of the photodissociation process to be determined. Following photolysis of CH3CHO in a gas cell, the CH2CO co-fragment was identified, using Fourier transform infrared spectroscopy, by its characteristic infrared absorption at 2150 cm–1. The measured quantum yield of the CH2CO + H2 product channel at 305.0 nm is φ = 0.008 ± 0.002 for both 15 Torr of neat CH3CHO and a mixture with 745 Torr of N2. Although small, this result has implications to the atmospheric photochemistry of carbonyls and this reaction represents a new tropospheric source of H2. Quasi-classical trajectory (QCT) simulations on a zero-point energy corrected reaction-path potential are also performed. The experimental REMPI and VMI image distributions are not consistent with the QCT simulations, indicating a non reaction-path mechanism should be considered.