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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

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.

Norrish Type I (NTI) α-bond cleavage is the dominant photolysis mechanism in small carbonyls and is an important source of radicals in the troposphere. In nonsymmetric species two cleavages are possible, NTIa and NTIb, forming larger and smaller alkyl radicals, respectively. For a data set of 20 small, atmospherically relevant carbonyls we predict NTIa and NTIb thresholds on the S0, S1, and T1 electronic states. The calculated NTIa T1 thresholds give a mean absolute deviation (MAD) of 5.8 kJ/mol with respect to the available experimental thresholds of five 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 asymptotic 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 (

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 1Σ+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 = 0.0075 ± 0.0025 for both 15 Torr of neat CH3CHO and a mixture with 745 Torr of N2. Although small, this result has implications for 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.

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.

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 S1 and T1 thresholds agree well with triplet quenching experiments. However, many linear-response methods deliver poor S1 energetics, which is explained by a S1/S0 conical intersection in close proximity to the S1 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 T1 dominates only in species where the S1 thresholds are high — typically ketones. The α, β-unsaturated carbonyls cannot cleave the α–β bond, causing them to photoisomerise. A concerted S0 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 T1 threshold.

The correlated speed and rotational energy distributions of the CO fragment from photodissociation of CH3CHO have been measured at a range of wavelengths from 308 to 328 nm. The distributions are bimodal, showing low J, slow speed, and high J, fast speed components. The cold component disappears for λ > 325 nm. This threshold corresponds to C–H bond cleavage and we assign these CO products as arising from roaming of a H-atom about a CH3CO core. We attribute the hot component to CO formed through CH3-roaming. No evidence was observed for the presence of a transition state mechanism. This is the first time two distinct roaming channels have been observed from the same electronic state. The results support the growing understanding that roaming can be significant in chemical reactions and outweigh traditional pathways.

The concentrations of organic acids, key species in the formation of secondary organic aerosols, are underestimated by atmospheric chemistry models by a factor of ∼2. Vinyl alcohol (VA, CH2═CHOH, ethenol) has been suggested as a precursor to formic acid, but sufficient tropospheric sources of VA have not been identified. Here, we show that VA is formed upon irradiation of neat acetaldehyde (CH3CHO) in the actinic ultraviolet region, between 295 and 330 nm. Besides the well-known photochemical products CO and CH4, we infer up to a 15% quantum yield of VA at 20 Torr acetaldehyde pressure and a photolysis wavelength of 330 nm. The experiments confirm a recent model predicting phototautomerization of acetaldehyde to VA and imply that photolysis of small aldehydes and ketones could provide tropospheric sources of enols sufficient to impact organic acid budgets. We also report absolute infrared absorption cross sections of VA.

Unexpected and significant isotope exchange is observed in the near-threshold photodissociation of isopically labelled acetaldehyde. Theoretical modelling indicates that, at the lowest energies considered, an average of 20 H- or D-shifts occur before dissociation — evidence for extensive isomerization.

The dynamics of CO production from photolysis of H2CO have been explored over a 8000 cm−1 energy range (345 nm–266 nm). Two-dimensional ion imaging, which simultaneously measures the speed and angular momentum distribution of a photofragment, was used to characterise the distribution of rotational and translational energy and to quantify the branching fraction of roaming, transition state (TS), and triple fragmentation (3F) pathways. The rotational distribution for the TS channel broadens significantly with increasing energy, while the distribution is relatively constant for the roaming channel. The branching fraction from roaming is also relatively constant at 20% of the observed CO. Above the 3F threshold, roaming decreases in favour of triple fragmentation. Combining the present data with our previous study on the H-atom branching fractions and published quantum yields for radical and molecular channels, absolute quantum yields were determined for all five dissociation channels for the entire S1←S0 abs...

The photodissociation dynamics of H2CO molecules at energies bracketing the triple fragmentation threshold were investigated using velocity map ion imaging of the H-atom fragments. An algorithm was developed to model the experimental results as a two-step process: initially barrierless C-H bond fission on the S0 potential energy surface to form H + HCO, followed by secondary fragmentation of those HCO radicals with sufficient internal energy to overcome the small exit channel barrier on the HCO surface to form H + CO. Our model treats the first step using phase space theory (PST) and the second using a combined PST-impulsive model, with a tunneling correction. Experimentally, triple fragmentation reaches 25% of the radical (H + HCO) channel photochemical yield at energies about 1500 cm(-1) above the barrier for breaking the second bond. In addition, the triplet (T1) channel appears to reduce in importance after the barrier on the T1 surface is exceeded, slowly decreasing to10% of the total radical yield at higher energy. The double PST-impulsive model provides a good fit to the experimental H-atom speed and energy distributions for H2CO dissociation on S0 spanning7000 cm(-1) of available energy.

The clearest dynamical signature of a roaming reaction is a very cold distribution of energy into the rotational and translational degrees of freedom of the roaming donor fragment (e.g. CO) and an exceptionally hot vibrational distribution in the roaming acceptor fragment (e.g. H2, CH4). These signatures were initially identified in joint experimental/theoretical investigations of roaming in H2CO and CH3CHO and are now used to infer the presence of roaming mechanisms in other photodissociation reactions. In this paper we construct a phase space theory (PST) model of triple fragmentation (3F) and show that the dynamical signature of 3F is similar to that of the roaming donor fragment. The PST model starts with a calculation of two-body fragmentation (2F) of a generic molecule, ABC into AB + C. Every AB fragment with sufficient energy to undergo subsequence spontaneous dissociation is allowed to dissociate and the PST distribution of energy into A + B products is calculated for every initial AB state. Using CH3CHO --> HCO + CH3 --> H + CO + CH3 as an example, we calculate that the energy disposal into the rotational and translational degrees of freedom of the 3F products is very low, and is similar to the dynamical signature expected for production of CO via a roaming mechanism. We compare the 3F PST model with published experimental data for photodissociation of CH3CHO and CH3OCHO at energies above the 3F threshold.

Enols in the Atmosphere? Keto/enol tautomerization (HC−C=O→C=C−OH) plays a central role in the chemistry of carbonyl compounds in a solution in which solvent and catalytic acids or bases can facilitate the proton transfer from C to O and back again. In contrast, analyses of atmospheric chemistry tend to exclude enol structure, on the assumption that tautomerization does not proceed regularly in gas phase. Andrews et al. (p. 1203 , published online 16 August) used isotopic labeling to probe the photoisomerization pathway of gaseous acetaldehyde in the lab and discovered evidence for an enol. Subsequent modeling indicates that photogenerated enols could build up sufficiently in the troposphere to account for previously puzzling observations of organic acids in the atmosphere.

The dynamics of the photodissociation of CH(3)CHO into CH(3) + HCO products have been investigated at energies between 30,953 and 31,771 cm(-1), spanning the threshold for radical production on the triplet (T(1)) surface. A barrierless pathway to CH(3) + HCO radical products formed on the ground state (S(0)) surface was discovered and established to be an important reaction channel in acetaldehyde photodissociation throughout this wavelength range. HCO laser induced fluorescence (LIF) spectra recorded from CH(3)CHO dissociated above and below the T(1) barrier energy are quite different; HCO produced on S(0) yields a more congested LIF spectrum with sharp rotational transitions, while HCO formed on the T(1) surface displays fewer, more intense, Doppler-broadened lines. These differences have been further explored in the populations of the HCO K(a) = 1 doublets. Despite the upper and lower levels being almost isoenergetic, HCO formed on T(1) preferentially populates the upper K(c) state due to the geometry of the T(1) transition state structure. In contrast, HCO formed on S(0) produces equal population in each of the upper and lower K(a) = 1 components. Product state distributions (PSDs) showed that HCO formed on S(0) is born with an approximately statistical distribution of population in the available product states, modeled well by phase space theory. HCO formed on the T(1) surface, in contrast, has a PSD that can be characterized as arising from "impulsive" dynamics. Previous discrepancies in the height of the T(1) barrier are discussed following the observation that, once the T(1) channel is energetically accessible, there is competition between the S(0) and T(1) pathways, with the dominance of the triplet channel increasing with increasing photolysis energy.

Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed “roaming,” has been described in the photodissociation of H 2 CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH 3 CHO to CH 4 + CO. The CH 4 product is found to have extreme vibrational excitation, with the vibrational distribution peaked at ≈95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to CH 4 + CO products is via a roaming pathway where a CH 3 fragment abstracts an H from HCO. The conventional saddle-point TS pathway to CH 4 + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the CH 3 roaming mechanism is attributed to the fact that the CH 3 + HCO radical asymptote and the TS saddle-point barrier to CH 4 + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as H 2 CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory.

Transition state theory (TST) describes chemical reactions in terms of “reaction coordinates,” usually the coordinates of atoms involved in breaking and forming bonds. Typically, there is an energetic barrier, the transition state (TS), between reactants and products. In 2004, a reaction mechanism was reported that seemingly defied the tenets of TST ( 1 ). In the photodissociation of H2CO, one of the hydrogen (H) atoms “roamed” around the periphery of the HCO core, with no apparent reaction coordinate, and abstracted the other H atom to form H2 and CO. The roaming products showed characteristic product-state distributions, distinct from those arising from a standard TS mechanism. On page 1075 of this issue, Grubb et al. ( 2 ) use detailed state-selective correlated experiments, together with theoretical calculations, to show that the photodissociation of NO3 into NO and O2, an important reaction in the atmosphere, occurs via roaming reactions on both the electronic excited state and the ground state of NO3.