Roaming Dynamics in Acetone Dissociation

The Effect of β-Hydrogens on the Tropospheric Photochemistry of Aldehydes: Norrish Type 1, Triple Fragmentation, and Methylketene Formation from Propanal

Wavelength and pressure dependent quantum yields (ϕ, QYs) of propanal photolysis have been measured for photolysis wavelengths, λ = 300-330 nm, and buffer gases of 3-10 Torr propanal and 0-757 Torr N2. Following laser photolysis, three photochemical pathways were established, using Fourier transform infrared spectroscopy of the stable end-products. Photolysis is dominated by the Norrish Type 1 reaction, which has been reported previously, but with inconsistent quantum yields. The propanal α-hydrogen leads to a 4-center elimination of H2, as observed in CH3CHO, here leading to methylketene. The presence of hydrogen attached to the β-carbon allows a new photochemical pathway: concerted triple fragmentation into CO + H2 + C2H4 via a 5-center transition state. Neither of these channels has been reported previously. No evidence for the previously reported C2H6 + CO, C2H4 + H2CO or CH3 + CH2CHO channels, nor for phototautomerization to 1-propenol (CH3CH═CHOH) was found. Modeling of the wavelength, pressure and collision partner dependence of the QYs allows us to reconcile the previous NT1a results and make recommendations for the quantum yields of all three channels under tropospheric conditions. The general impact of β-hydrogen atoms in the photochemistry of aldehydes is to open up new pathways from cyclic transition states and to reduce the importance of other photolysis or isomerization channels.

Photodissociation dynamics of methylamine in the blue edge of the A-band. I. The H-atom elimination channel

The photodissociation dynamics of methylamine (CH3NH2) upon excitation in the blue edge of the first absorption A-band, in the 198-203 nm range, are investigated by means of nanosecond pump-probe laser pulses and velocity map imaging combined with H(2S)-atom detection through resonance enhanced multiphoton ionization. The images and corresponding translational energy distributions for the H-atoms produced show three different contributions associated with three reaction pathways. The experimental results are complemented by high-level ab initio calculations. The potential energy curves computed as a function of the N-H and C-H bond distances allow us to draw a picture of the different mechanisms. Major dissociation occurs through N-H bond cleavage and it is triggered by an initial geometrical change, i.e., from a pyramidal configuration of the C-NH2 with respect to the N atom to a planar geometry. The molecule is then driven into a conical intersection (CI) seam where three outcomes can take place: first, threshold dissociation into the second dissociation limit, associated with the formation of CH3NH(Ã), is observed; second, direct dissociation after passage through the CI leading to the formation of ground state products; and third, internal conversion into the ground state well in advance to dissociation. While the two last pathways were previously reported at a variety of wavelengths in the 203-240 nm range, the former had not been observed before to the best of our knowledge. The role of the CI and the presence of an exit barrier in the excited state, which modify the dynamics leading the two last mechanisms, are discussed considering the different excitation energies used.

Excited-State Dynamics during Primary C-I Homolysis in Acetyl Iodide Revealed by Ultrafast Core-Level Spectroscopy

In typical carbonyl-containing molecules, bond dissociation events follow initial excitation to nπC═O* states. However, in acetyl iodide, the iodine atom gives rise to electronic states with mixed nπC═O* and nσC-I* character, leading to complex excited-state dynamics, ultimately resulting in dissociation. Using ultrafast extreme ultraviolet (XUV) transient absorption spectroscopy and quantum chemical calculations, we present an investigation of the primary photodissociation dynamics of acetyl iodide via time-resolved spectroscopy of core-to-valence transitions of the I atom after 266 nm excitation. The probed I 4d-to-valence transitions show features that evolve on sub-100-fs time scales, reporting on excited-state wavepacket evolution during dissociation. These features subsequently evolve to yield spectral signatures corresponding to free iodine atoms in their spin-orbit ground and excited states with a branching ratio of 1.1:1 following dissociation of the C-I bond. Calculations of the valence excitation spectrum via equation-of-motion coupled cluster with single and double substitutions (EOM-CCSD) show that initial excited states are of spin-mixed character. From the initially pumped spin-mixed state, we use a combination of time-dependent density functional theory (TDDFT)-driven nonadiabatic ab initio molecular dynamics and EOM-CCSD calculations of the N4,5 edge to reveal a sharp inflection point in the transient XUV signal that corresponds to rapid C-I homolysis. By examining the molecular orbitals involved in the core-level excitations at and around this inflection point, we are able to piece together a detailed picture of C-I bond photolysis in which d → σ* transitions give way to d → p excitations as the bond dissociates. We also report theoretical predictions of short-lived, weak 4d → 5d transitions in acetyl iodide, validated by weak bleaching in the experimental transient XUV spectra. This joint experimental-theoretical effort has thus unraveled the detailed electronic structure and dynamics of a strongly spin-orbit coupled system.

The dynamics of CO production from the photolysis of acetone across the whole S1 ← S0 absorption spectrum: Roaming and triple fragmentation pathways

The unimolecular photodissociation dynamics of acetone spanning the entire S1 ← S0 absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of λ > 305.8 nm, all photoproducts are formed on the S0 state after internal conversion. A roaming mechanism forming C2H6 + CO is active in the window λ = 311.2–305.8 nm. From λ = 305.8 to 262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C–C bond cleavage on the lowest excited triplet state, T1. At higher energy ( λ < 262 nm), an increasing fraction of CH3CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH3 + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH3 radical, allowing us to determine the height of the energetic barrier to CH3CO decomposition as 68 ± 4 kJ mol−1, which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248 nm. We see no evidence of the return of roaming, or any other S0 reaction, in this higher energy region of the first electronic absorption band.

Substitution Reactions in the Pyrolysis of Acetone Revealed through a Modeling, Experiment, Theory Paradigm

The development of high-fidelity mechanisms for chemically reactive systems is a challenging process that requires the compilation of rate descriptions for a large and somewhat ill-defined set of reactions. The present unified combination of modeling, experiment, and theory provides a paradigm for improving such mechanism development efforts. Here we combine broadband rotational spectroscopy with detailed chemical modeling based on rate constants obtained from automated ab initio transition state theory-based master equation calculations and high-level thermochemical parametrizations. Broadband rotational spectroscopy offers quantitative and isomer-specific detection by which branching ratios of polar reaction products may be obtained. Using this technique, we observe and characterize products arising from H atom substitution reactions in the flash pyrolysis of acetone (CH₃C(O)CH₃) at a nominal temperature of 1800 K. The major product observed is ketene (CH₂CO). Minor products identified include acetaldehyde (CH₃CHO), propyne (CH₃CCH), propene (CH₂CHCH₃), and water (HDO). Literature mechanisms for the pyrolysis of acetone do not adequately describe the minor products. The inclusion of a variety of substitution reactions, with rate constants and thermochemistry obtained from automated ab initio kinetics predictions and Active Thermochemical Tables analyses, demonstrates an important role for such processes. The pathway to acetaldehyde is shown to be a direct result of substitution of acetone's methyl group by a free H atom, while propene formation arises from OH substitution in the enol form of acetone by a free H atom.

Photodissociation Dynamics of CO-Forming Channels on the Ground-State Surface of Methyl Formate at 248 nm: Direct Dynamics Study and Assessment of Generalized Multicenter Impulsive Models

The photodissociation dynamics of methyl formate in the electronic ground state S₀, initiated by a 248 nm-wavelength laser, is studied by direct dynamics simulations. We analyze five channels, where four of them have as products CH₃OH + CO, one leading to the formation of three fragments, H₂CO + H₂ + CO, and a channel characterized by a roaming transition state. The analysis of energy distribution among the degrees of freedom of the product and the comparison with experimental results previously published by other groups provide the ingredients to distinguish the examined dissociation pathways. The interpretation of the results proves that the characterization of dissociation mechanisms must rely on a dynamics approach involving multiple electronic states, including considerations on the features of the S₁/S₀ conical intersection. Here, we also assess the generalized multicenter impulsive model, GMCIM, that has been designed for dissociation processes with exit barriers, and the energy distribution in the products is predicted on the basis of information from the saddle points and the intrinsic reaction coordinates. Main features, advantages, limits, and future perspectives of the method are reported and discussed.

Site-specific hydrogen-atom elimination in photoexcited alkyl radicals

A prompt site-specific hydrogen-atom elimination from the α-carbon atom (Cα) has been recently reported to occur in the photodissociation of ethyl radicals following excitation at 201 nm [Chicharro et al., Chem. Sci., 2019, 10, 6494]. Such pathway was accessed by means of an initial ro-vibrational energy characterizing the radicals produced by in situ photolysis of a precursor. Here, we present experimental evidence of a similar dynamics in a series of alkyl radicals (C2H5, n-C3H7, n-C4H9, and i-C3H7) containing the same reaction coordinate, but different extended structures. The main requirements for the site-specific mechanism in the studied radicals, namely a rather high content of internal energy prior to dissociation and the participation of vibrational promoting modes, is discussed in terms of the chemical structure of the radicals. The methyl deformation mode in all alkyl radicals along with the CH bending motion in i-C3H7 appear to promote this fast H-atom elimination channel. The photodissociation dynamics of the simplest unsaturated alkyl radical, the vinyl radical (C2H3), is also discussed, showing no signal of site-specific fast H-atom elimination. The results are complemented with high-level ab initio electronic structure calculations of potential energy curves of the vinyl radical, which are compared with those previously reported for the ethyl radical.

Roaming Dynamics and Conformational Memory in Photolysis of Formic Acid at 193 nm Using Time-resolved Fourier-transform Infrared Emission Spectroscopy

In photodissociation of trans-formic acid (HCOOH) at 193 nm, we have observed two molecular channels of CO + H₂O and CO₂ + H₂ by using 1 μs-resolved Fourier-transform infrared emission spectroscopy. With the aid of spectral simulation, the CO spectra are rotationally resolved for each vibrational state (v = 1-8). Each of the resulting vibrational and rotational population distributions is characteristic of two Boltzmann profiles with different temperatures, originating from either transition state pathway or OH-roaming to form the same CO + H₂O products. The H₂O roaming co-product is also spectrally simulated to understand the interplay with the CO product in the internal energy partitioning. Accordingly, this work has evaluated the internal energy disposal for the CO and H₂O roaming products; especially the vibrational-state dependence of the roaming signature is reported for the first time. Further, given a 1 μs resolution, the temporal dependence of the CO/CO₂ product ratio at v ≥ 1 rises from 3 to 10 of study, thereby characterizing the effect of conformational memory and well reconciling with the disputed results reported previously between absorption and emission methods.

The 3s versus 3p Rydberg state photodissociation dynamics of the ethyl radical

The photodissociation dynamics of the ethyl radical following excitation into the 3s and 3p Rydberg states are revisited in a joint experimental and theoretical study. Two different methods to produce the ethyl radical, pyrolysis and in situ photolysis, are employed in order to modify the initial ro-vibrational energy distribution characterizing the ethyl radical beam. H-atom velocity map images following excitation of the radical at 243 nm and at 201 nm are presented and discussed along with ab initio potential energy curves focussing on the bridged C2v geometry. The reported results show that the dynamics following excitation to the 3s Rydberg state is insensitive to the initial internal energy of the parent radical, in contrast to the dynamics on the 3p Rydberg state, which is strongly modified. The role of the bridged C2v geometry on both photodynamics is highlighted and discussed.

Dynamics and quantum yields of H2 + CH2CO as a primary photolysis channel in CH3CHO

A new ketene + H₂ channel in CH₃CHO photolysis is not modelled by quasi-classical trajectories over the transition state.

Ultrafast α -CC bond cleavage of acetone upon excitation to 3p and 3d Rydberg states by femtosecond time-resolved photoelectron imaging

The radiationless electronic relaxation and α -CC bond fission dynamics of jet-cooled acetone in the S₁ (nπ^*) state and in high-lying 3p and 3d Rydberg states have been investigated by femtosecond time-resolved mass spectrometry and photoelectron imaging. The S₁ state was accessed by absorption of a UV pump photon at selected wavelengths between λ = 320 and 250 nm. The observed acetone mass signals and the S₁ photoelectron band decayed on sub-picosecond time scales, consistent with a recently proposed ultrafast structural relaxation of the molecules in the S₁ state away from the Franck-Condon probe window. No direct signatures could be observed by the experiments for CC dissociation on the S₁ potential energy hypersurface in up to 1 ns. The observed acetyl mass signals at all pump wavelengths turned out to be associated with absorption by the molecules of one or more additional pump and/or probe photons. In particular, absorption of a second UV pump photon by the S₁ (nπ^*) state was found to populate a series of high-lying states belonging to the n = 3 Rydberg manifold. The respective transitions are favored by much larger cross sections compared to the S₁  ←  S₀ transition. The characteristic energies revealed by the photoelectron images allowed for assignments to the 3p and 3d_yz states. At two-photon excitation energies higher than 8.1 eV, an ultrafast reaction pathway for breaking the α -CC bond in 50-90 fs via the 3d_yz Rydberg state and the elusive ππ^* state was observed, explaining the formation of acetyl radicals after femtosecond laser excitation of acetone at these wavelengths.

Photodissociation dynamics of acetone studied by time-resolved ion imaging and photofragment excitation spectroscopy

The photodissociation dynamics of acetone has been investigated using velocity-map ion imaging and photofragment excitation (PHOFEX) spectroscopy across a range of wavelengths spanning the first absorption band (236-308 nm). The radical products of the Norrish Type I dissociation, methyl and acetyl, as well as the molecular product ketene have been detected by single-photon VUV ionization at 118 nm. Ketene appears to be formed with non-negligible yield at all wavelengths, with a maximum value of Φ ≈ 0.3 at 280 nm. The modest translational energy release is inconsistent with dissociation over high barriers on the S₀ surface, and ketene formation is tentatively assigned to a roaming pathway involving frustrated dissociation to the radical products. Fast-moving radical products are detected at λ ≤ 305 nm with total translational energy distributions that extend to the energetic limit, consistent with dissociation occurring near-exclusively on the T₁ surface following intersystem crossing. At energies below the T₁ barrier a statistical component indicative of S₀ dissociation is observed, although dissociation via the S₁/S₀ conical intersection is absent at shorter wavelengths, in contrast to acetaldehyde. The methyl radical yield is enhanced over that of acetyl in PHOFEX spectra at λ ≤ 260 nm due to the onset of secondary dissociation of internally excited acetyl radicals. Time-resolved ion imaging experiments using picosecond duration pulses at 266 nm find an appearance time constant of τ = 1490 ± 140 ps for CH₃ radicals formed on T₁. The associated rate is representative of S₁ → T₁ intersystem crossing. At 284 nm, CH₃ is formed on T₁ with two distinct timescales: a fast <10 ns component is accompanied by a slower component with τ = 42 ± 7 ns. A two-step mechanism involving fast internal conversion, followed by slower intersystem crossing (S₁ → S₀ → T₁) is proposed to explain the slow component.

Photodissociation dynamics of bromoiodomethane from the first and second absorption bands. A combined velocity map and slice imaging study

Ion imaging is applied to disentangle the selective bond cleavage in the photodissociation of bromoiodomethane from the two first absorption bands.

Halogen-atom effect on the ultrafast photodissociation dynamics of the dihalomethanes CH2ICl and CH2BrI

Real time photodissociation of dihalomethanes has been measured by femtosecond velocity map imaging to disentangle the effect of the halogen-atom on the carbon–iodine cleavage dynamics.

Slice imaging of the UV photodissociation of CH2BrCl from the maximum of the first absorption band

The photodissociation dynamics of bromochloromethane (CH₂BrCl) have been investigated at the maximum of the first absorption band, at the excitation wavelengths 203 and 210 nm, using the slice imaging technique in combination with a probe detection of bromine-atom fragments, Br(²P_3/2) and Br*(²P_1/2), via (2 + 1) resonance enhanced multiphoton ionization. Translational energy distributions and angular distributions reported for both Br(²P_3/2) and Br*(²P_1/2) fragments show two contributions for the Br(²P_3/2) channel and a single contribution for the Br*(²P_1/2) channel. High level ab initio calculations have been performed in order to elucidate the dissociation mechanisms taking place. The computed absorption spectrum and potential energy curves indicate the main contribution of the populated 4A″, 5A', and 6A' excited states leading to a C-Br cleavage. Consistently with the results, the single contribution for the Br*(²P_1/2) channel has been attributed to direct dissociation through the 6A' state as well as an indirect dissociation of the 5A' state requiring a 5A' → 4A' reverse non-adiabatic crossing. Similarly, a faster contribution for the Br(²P_3/2) channel characterized by a similar energy partitioning and anisotropy than those for the Br*(²P_1/2) channel is assigned to a direct dissociation through the 5A' state, while the slower component appears to be due to the direct dissociation on the 4A″ state.

New experimental evidence to support roaming in the reaction Cl + isobutene (i-C4H8)

The reaction Cl + isobutene (i-C₄H₈) was reported by Suits et al. to proceed via, in addition to abstraction, an addition-elimination path following a roaming excursion of Cl; a near-zero translational energy release and an isotropic angular distribution observed at a small collision energy characterized this mechanism. We employed a new experimental method to further characterize this roaming mechanism through observation of the internal distribution of HCl (v, J) and their temporal behavior upon irradiation of a mixture of Cl₂C₂O₂ and i-C₄H₈ in He or Ar buffer gas. With 1–3 Torr buffer gas added to approach the condition of small collision energy, the intensities of emission of HCl (v = 1, 2) and the HCl production rates increased significantly; Ar shows a more significant effect than He because Ar quenches Cl more efficiently to reduce the collisional energy and facilitate the roaming path. According to kinetic modeling, the rate of addition-elimination (roaming) increased from k_E ≈ 2 × 10⁵ s⁻¹ when little buffer gas was present to ~1.9 × 10⁶ s⁻¹ when 2–3 Torr of Ar was added, and the branching ratio for formation of [HCl (v = 2)]/[HCl (v = 1)] increased from 0.02 ± 0.01 for abstraction to 0.06 ± 0.01 for roaming.

Two roaming pathways in the photolysis of CH3CHO between 328 and 308 nm

We attribute the two product-state distributions previously seen in CH₃CHO photodissociation to CH₃-roaming and H-roaming, unifying all previous experimental results.

Exploring Multiple Potential Energy Surfaces: Photochemistry of Small Carbonyl Compounds

In theoretical studies of chemical reactions involving multiple potential energy surfaces (PESs) such as photochemical reactions, seams of intersection among the PESs often complicate the analysis. In this paper, we review our recipe for exploring multiple PESs by using an automated reaction path search method which has previously been applied to single PESs. Although any such methods for single PESs can be employed in the recipe, the global reaction route mapping (GRRM) method was employed in this study. By combining GRRM with the proposed recipe, all critical regions, that is, transition states, conical intersections, intersection seams, and local minima, associated with multiple PESs, can be explored automatically. As illustrative examples, applications to photochemistry of formaldehyde and acetone are described. In these examples as well as in recent applications to other systems, the present approach led to discovery of many unexpected nonadiabatic pathways, by which some complicated experimental data have been explained very clearly.

Imaging the molecular channel in acetaldehyde photodissociation: roaming and transition state mechanisms

The roaming dynamics in the photodissociation of acetaldehyde is studied through the first absorption band, in the wavelength interval ranging from 230 nm to 325 nm. Using a combination of the velocity-map imaging technique and rotational resonance enhanced multiphoton ionization (REMPI) spectroscopy of the CO fragment, the branching ratio between the canonical transition state and roaming dissociation mechanisms is obtained at each of the photolysis wavelengths studied. Upon one photon absorption, the molecule is excited to the first singlet excited S(1) state, which, depending on the excitation wavelength, either converts back to highly vibrationally excited ground S(0) state or undergoes intersystem crossing to the first excited triplet T(1) state, from where the molecule can dissociate over two main channels: the radical (CH(3) + HCO) and the molecular (CO + CH(4)) channels. Three dynamical regions are characterized: in the red edge of the absorption band, at excitation energies below the T(1) barrier, the ratio of the roaming dissociation channel increases, largely surpassing the transition state contribution. As the excitation wavelength is increased, the roaming propensity decreases reaching a minimum at wavelengths ∼308 nm. Towards the blue edge, at 230 nm, an upper limit of ∼50% has been estimated for the contribution of the roaming channel. The experimental results are interpreted in terms of the interaction between the different potential energy surfaces involved by means of ab initio stationary points and intrinsic reaction coordinate paths calculations.