The Onset of H + Ketene Products from Vinoxy Radicals Prepared by Photodissociation of Chloroacetaldehyde at 157 nm

Gas-phase detection of oxirene

Oxirenes-highly strained 4π Hückel antiaromatic organics-have been recognized as key reactive intermediates in the Wolff rearrangement and in interstellar environments. Predicting short lifetimes and tendency toward ring opening, oxirenes are one of the most mysterious classes of organic transients, with the isolation of oxirene (c-C₂H₂O) having remained elusive. Here, we report on the preparation of oxirene in low-temperature methanol-acetaldehyde matrices upon energetic processing through isomerization of ketene (H₂CCO) followed by resonant energy transfer of the internal energy of oxirene to the vibrational modes (hydroxyl stretching and bending, methyl deformation) of methanol. Oxirene was detected upon sublimation in the gas phase exploiting soft photoionization coupled with a reflectron time-of-flight mass spectrometry. These findings advance our fundamental understanding of the chemical bonding and stability of cyclic, strained molecules and afford a versatile strategy for the synthesis of highly ring-strained transients in extreme environments.

Photodissociation Dynamics of Vinoxy Radical via the B̃2A″ State: The H + CH2CO Product Channel

Photodissociation dynamics of the jet-cooled vinoxy radical (CH₂CHO) via the B̃²A″ state was studied in the near-ultraviolet (near-UV) region of 308-328 nm using high-n Rydberg H atom time-of-flight (HRTOF) and resonance-enhanced multiphoton ionization (REMPI) techniques. The vinoxy radical beam was produced by 193 nm photolysis of ethyl vinyl ether followed by supersonic expansion. The H + CH₂CO product channel was observed directly in the H atom TOF spectra. The H atom photofragment yield (PFY) spectra were obtained by integrating the H atom TOF spectra and measuring the H atom REMPI signals, and showed several vibronic bands of the B̃²A″ state. The translational energy distributions of the H + CH₂CO products, P(E_T)'s, were obtained at several vibronic transitions. The P(E_T) distributions were broad, peaking at a low energy of ∼3500 cm^-1. The product translational energy release was moderate; the average translational energy release in the maximum available energy, ⟨f_T⟩, was in the range of 0.24-0.27. The product angular distributions in this wavelength region were slightly anisotropic, with the β parameter in the range of 0.10-0.24. The near-UV photodissociation mechanism of the H + CH₂CO product channel of the vinoxy radical is consistent with unimolecular dissociation on the electronic ground state (X̃²A″) following internal conversion from the B̃²A″ state to the òA' state and then to the X̃²A″ state (although unimolecular dissociation from the first excited òA' may also contribute).

Photoinduced C–H bond fission in prototypical organic molecules and radicals

We survey and assess current knowledge regarding the primary photochemistry of hydrocarbon molecules and radicals.

Primary Product Branching in the Photodissociation of Chloroacetaldehyde at 157 nm

We used crossed laser-molecular beam scattering to study the primary photodissociation channels of chloroacetaldehyde (CH₂ClCHO) at 157 nm. In addition to the C-Cl bond fission primary photodissociation channel, the data evidence two other photodissociation channels: HCl photoelimination and C-C bond fission. This is the first direct evidence of the C-C bond fission channel in chloroacetaldehyde, and we found that it significantly competes with the C-Cl bond fission channel. We determined the total primary photodissociation branching fractions for C-Cl fission:HCl elimination:C-C fission to be 0.65:0.07:0.28. The branching between the primary channels suggests the presence of interesting excited state dynamics in chloroacetaldehyde. Some of the vinoxy radicals from C-Cl photofission and most of the ketene cofragments formed in HCl photoelimination have enough internal energy to undergo secondary dissociation. While our previous velocity map imaging study on the photodissociation of chloroacetaldehyde at 157 nm focused on the barrier for the unimolecular dissociation of vinoxy to H + ketene, this work shows that the HCl elimination channel contributed to the high kinetic energy portion of the m/z = 42 signal in that study.

Dissociative Photoionization of the Elusive Vinoxy Radical

These experiments report the dissociative photoionization of vinoxy radicals to m/z = 15 and 29. In a crossed laser-molecular beam scattering apparatus, we induce C-Cl bond fission in 2-chloroacetaldehyde by photoexcitation at 157 nm. Our velocity measurements, combined with conservation of angular momentum, show that 21% of the C-Cl photofission events form vinoxy radicals that are stable to subsequent dissociation to CH₃ + CO or H + ketene. Photoionization of these stable vinoxy radicals, identified by their velocities, which are momentum-matched with the higher-kinetic-energy Cl atom photofragments, shows that the vinoxy radicals dissociatively photoionize to give signal at m/z = 15 and 29. We calibrated the partial photoionization cross section of vinoxy to CH₃⁺ relative to the bandwidth-averaged photoionization cross section of the Cl atom at 13.68 eV to put the partial photoionization cross sections on an absolute scale. The resulting bandwidth-averaged partial cross sections are 0.63 and 1.3 Mb at 10.5 and 11.44 eV, respectively. These values are consistent with the upper limit to the cross section estimated from a study by Savee et al. on the O(³P) + propene bimolecular reaction. We note that the uncertainty in these values is primarily dependent on the signal attributed to C-Cl primary photofission in the m/z = 35 (Cl⁺) time-of-flight data. While the value is a rough estimate, the bandwidth-averaged partial photoionization cross section of vinoxy to HCO⁺ calculated from the signal at m/z = 29 at 11.53 eV is approximately half that of vinoxy to CH₃⁺. We also present critical points on the potential energy surface of the vinoxy cation calculated at the G4//B3LYP/6-311++G(3df,2p) level of theory to support the observation of dissociative ionization of vinoxy to both CH₃⁺ and HCO⁺.

Photodissociation Dynamics of the i-Methylvinoxy Radical at 308, 248, and 225 nm Using Fast Beam Photofragment Translational Spectroscopy

The photodissociation dynamics of the i-methylvinoxy (CH₃COCH₂) radical have been studied by means of fast beam coincidence translational spectroscopy. The radical was produced by photodetachment of the i-methylvinoxide anion at 700 nm, followed by dissociation at 225 nm (5.51 eV), 248 nm (5.00 eV), and 308 nm (4.03 eV). At all three dissociation energies, the major products were found to be CH₃ + CH₂CO, with a small amount of CO + C₂H₅ produced at the higher dissociation energies. Photofragment mass distributions and translational energy distributions were recorded for each wavelength. Comparison of the mass distributions with dissociation of fully deuterated i-methylvinoxy aided the assignment of the observed channels. Electronic structure calculations were performed to determine the relative energies of minima and transition states involved in the dissociation and to aid interpretation of the experimental results. The proposed dissociation mechanism involves internal conversion from the initially excited electronic state, followed by dissociation over a barrier on the ground state.