Revisiting the photodissociation dynamics of the phenyl radical

Cole-Filipiak, Neil C.; Shapero, Mark; Negru, Bogdan; Neumark, Daniel M.

doi:10.1063/1.4894398

“…268 Further support for such a fragmentation mechanism is provided by traditional PTS studies at 193 nm, which identify a (minor) channel yielding C2H2 (along with n-C4H3) products in addition to one or more channel yielding H atom products. 269 Theory suggests that the molecular fragments, and part of the H atom yield formed at these short excitation wavelengths, arise from the unimolecular decay after ring-opening on the ground state PES. 270 Such behaviour, i.e.…”

Section: Phenyl Benzyl and Larger Aromatic Radicalsmentioning

confidence: 99%

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

Ashfold

¹

,

Ingle

²

,

Karsili

³

et al. 2019

Phys. Chem. Chem. Phys.

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“…We assume 𝐸 𝑖 ≈ 0 kcal/mol for the radical beam following supersonic expansion, in line with previous findings. [63][64][65] During dissociation, the available energy 𝐸 𝑎𝑣𝑎𝑖𝑙 is partitioned into either product internal energy 𝐸 𝑖𝑛𝑡 or relative product translational energy 𝐸 𝑇 . Conservation of momentum in each dissociation event requires that photofragment pairs acquire equal and opposite momenta.…”

Section: Discussionmentioning

confidence: 99%

Photodissociation Dynamics of the Cyclohexyl Radical from the 3p Rydberg State at 248 nm

Ramphal

¹

,

Shapero

²

,

Neumark

³

2021

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The photodissociation of jet-cooled cyclohexyl was studied by exciting the radicals to their 3p Rydberg state using 248 nm laser light and detecting photoproducts by photofragment translational spectroscopy. Both H-atom loss and dissociation to heavy fragment pairs are observed. The H-atom loss channel exhibits a two-component translational energy distribution. The fast photoproduct component is attributed to impulsive cleavage directly from an excited state, likely the Rydberg 3s state, forming cyclohexene. The slow component is due to statistical decomposition of hot cyclohexyl radicals that internally convert to the ground electronic state prior to H-atom loss. The fast and slow components are present in a ~0.7:1 ratio, similar to findings in other alkyl radicals. Internal conversion to the ground state also leads to ring-opening followed by dissociation to 1-buten-4-yl + ethene in comparable yield to H-loss, with the C4H7 fragment containing enough internal energy to dissociate further to butadiene via H-atom loss. A very minor ground-state C5H8 + CH3 channel is observed, attributed predominantly to 1,3pentadiene formation. The ground-state branching ratios agree well with RRKM calculations, which also predict C4H6 + C2H5 and C3H6 + C3H5 channels with similar yield to C5H8 + CH3. If these channels were active it was at levels too low to be observed. I.Previous photodissociation studies from the 3s and 3p Rydberg states of ethyl by Zhang, Fischer, Bañares and co-workers detected H atoms ejected with a bimodal kinetic energy distribution. [14][15][16][17] The 'fast' H atoms result from direct dissociation from the excited electronic state, while the 'slow' population is attributed to statistical dissociation following internal conversion to the ground state. The observation of angular distributions that are anisotropic for the fast H atoms and isotropic for the slow H atoms supports these mechanisms. These fast and slow dissociation channels occur in a roughly 0.4:1 ratio depending on the initial and final states of ethyl used in the experiment. For ethyl excited to the 3s state, the fast dissociation channel has a dissociation time constant of ~300 fs, while for the slow channel it is ~6-7 ps. 18

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“…We assume ≈ 0 kcal/mol for the radical beam following supersonic expansion, in line with previous findings. [62][63][64] During dissociation, the available energy is partitioned into either product internal energy or relative product translational energy . Conservation of momentum in each dissociation event requires that photofragment pairs acquire equal and opposite momenta.…”

Section: Discussionmentioning

confidence: 99%

Photodissociation Dynamics of the Cyclohexyl Radical from the 3p Rydberg State at 248 nm

Ramphal

¹

,

Shapero²,

Neumark³

2021

Preprint

Self Cite

View full text Add to dashboard Cite

The photodissociation of jet-cooled cyclohexyl was studied by exciting the radicals to their 3p Rydberg state using 248 nm laser light and detecting photoproducts by photofragment translational spectroscopy. Both H-atom loss and dissociation to heavy fragment pairs are observed. The H-atom loss channel exhibits a two-component translational energy distribution. The fast photoproduct component is attributed to impulsive cleavage directly from an excited state, likely the Rydberg 3s state, forming cyclohexene. The slow component is due to statistical decomposition of hot cyclohexyl radicals that internally convert to the ground electronic state prior to H-atom loss. The fast and slow components are present in a ~0.7:1 ratio, similar to findings in other alkyl radicals. Internal conversion to the ground state also leads to ring-opening followed by dissociation to 1-buten-4-yl + ethene in comparable yield to H-loss, with the C4H7 fragment containing enough internal energy to dissociate further to butadiene via H-atom loss. A very minor ground-state C5H8 + CH3 channel is observed, attributed predominantly to 1,3-pentadiene formation. The ground-state branching ratios agree well with RRKM calculations, which also predict C4H6 + C2H5 and C3H6 + C3H5 channels with similar yield to C5H8 + CH3. If these channels were active it was at levels too low to be observed.

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Revisiting the photodissociation dynamics of the phenyl radical

Cited by 16 publications

References 60 publications

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

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

Photodissociation Dynamics of the Cyclohexyl Radical from the 3p Rydberg State at 248 nm

Photodissociation Dynamics of the Cyclohexyl Radical from the 3p Rydberg State at 248 nm

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