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

Abstract

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

Virtual Issue on Photodissociation: From Fundamental Dynamics and Spectroscopy to Photochemistry in Planetary Atmospheres and in Space

“…Although most photodissociation studies focus on molecules that are stable in their ground states and are sufficiently volatile to be prepared in a molecular beam, photolytic or mass-spectrometric methods can also be used to prepare gaseous samples of free radicals, biradicals such as Criegee intermediates, or molecular cations. ,, The photodissociation dynamics of these reactive intermediates can then be explored using aforementioned techniques such as VMI, HRA PTS, and other time-of-flight mass spectrometry schemes. ,, Metal complexes can also be studied using these same methods, provided they have sufficient vapor pressure or can be transferred to the gas phase by laser ablation or desorption methods. Observation of fragmentation pathways, such as ligand detachment from metal complexes and measurement of kinetic energy release to the fragments, provides useful information about the nature of the bonding, the thermodynamics of ligand binding, and ligand dissociation pathways. ,, …”

“…9,22,28 The photodissociation dynamics of these reactive intermediates can then be explored using aforementioned techniques such as VMI, HRA PTS, and other time-of-flight mass spectrometry schemes. 3,4,29 Metal complexes can also be studied using these same methods, provided they have sufficient vapor pressure or can be transferred to the gas phase by laser ablation or desorption methods. Observation of fragmentation pathways, such as ligand detachment from metal complexes and measurement of kinetic energy release to the fragments, provides useful information about the nature of the bonding, the thermodynamics of ligand binding, and ligand dissociation pathways.…”

Imaging the elusive C–C bond dissociation channel of photoexcited ethyl radical

“…The natural next step in the characterization of the ethyl radical photodynamics is the study of the C-C bond scission. To the best of our knowledge, the tert-butyl radical [5][6][7] and the cyclohexyl radical [8] are the only alkyl radicals for which the CH 3 -loss channel has been described. Negru et al [7] found for tert-butyl that the translational energy release of produced CH 3 matched that of the CH 3 -coproduct, the dimethylcarbene.…”

“…More recently, the photodissociation of cyclohexyl radicals was found to yield CH 3 in correlation with 1,3-pentadiene with a branching ratio of around 1%. [8] A slow translation energy distribution was reported and assigned to a statistical dissociation over exit barriers, this channel requiring a ring-opening and several internal H-atom abstractions prior to dissociation.…”

Site-Specific Carbon–Carbon Bond Fission in Photoexcited Propyl Radicals Leads to Isomer-Selective Carbene and Radical Products

“…To date, C–C bond fission has been observed only in the photodissociation of tert -butyl and cyclohexyl radicals . For tert -butyl radicals, C–C bond fission led to formation of dimethylcarbene plus methyl radicals; C–H bond fission produced isobutene + H. From the translational energy distributions in both channels, it was concluded that the reactions were direct rather than via internal conversion to the ground state …”