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

Evidencing an elusive conical intersection in the dissociative photoionization of methyl iodide

The valence-shell dissociative photoionization of methyl iodide (CH3I) is studied using double imaging photoelectron photoion coincidence (i2 PEPICO) spectroscopy in combination with highly-tunable synchrotron radiation from synchrotron SOLEIL. The experimental results are complemented by new high-level ab initio calculations of the potential energy curves of the relevant electronic states of the methyl iodide cation (CH3I+). An elusive conical intersection is found to mediate internal conversion from the initially populated first excited state, CH3I+(Ã2A1), into the ground cationic state, leading to the formation of methyl ions (CH3+). The reported threshold photoelectron spectrum for CH3+ reveals that the ν5 scissors vibrational mode promotes the access to this conical intersection and hence, the transfer of population. An intramolecular charge transfer takes place simultaneously, prior to dissociation. Upon photoionization into the second excited cationic state, CH3I+(B̃2E), a predissociative mechanism is shown to lead to the formation of atomic I+.

Photodissociation dynamics of the ethyl radical via the Ã2A'(3s) state: H-atom product channels and ethylene product vibrational state distribution

The photodissociation dynamics of jet-cooled ethyl radical (C2H5) via the Ã2A'(3s) states are studied in the wavelength region of 230-260 nm using the high-n Rydberg H-atom time-of-flight (TOF) technique. The H + C2H4 product channels are reexamined using the H-atom TOF spectra and photofragment translational spectroscopy. A prompt H + C2H4(X̃1Ag) product channel is characterized by a repulsive translational energy release, anisotropic product angular distribution, and partially resolved vibrational state distribution of the C2H4(X̃1Ag) product. This fast dissociation is initiated from the 3s Rydberg state and proceeds via a H-bridged configuration directly to the H + C2H4(X̃1Ag) products. A statistical-like H + C2H4(X̃1Ag) product channel via unimolecular dissociation of the hot electronic ground-state ethyl (X̃2A') after internal conversion from the 3s Rydberg state is also examined, showing a modest translational energy release and isotropic angular distribution. An adiabatic H + excited triplet C2H4(ã3B1u) product channel (a minor channel) is identified by energy-dependent product angular distribution, showing a small translational energy release, anisotropic angular distribution, and significant internal excitation in the C2H4(ã3B1u) product. The dissociation times of the different product channels are evaluated using energy-dependent product angular distribution and pump-probe delay measurements. The prompt H + C2H4(X̃1Ag) product channel has a dissociation time scale of <10 ps, and the upper bound of the dissociation time scale of the statistical-like H + C2H4(X̃1Ag) product channel is <5 ns.

An ab initio study of the photodissociation of CH2I and CH2I. Phys Chem Chem Phys 2023. [PMID: 37465906 DOI: 10.1039/d3cp01460f]

Photodissociation of the CH₂I radical and the CH₂I⁺ cation is studied by means of high-level ab initio calculations, including spin-orbit effects. Potential-energy curves (PEC) along the dissociating bond distances involved in some fragmentation pathways of these species are computed for the ground and several excited electronic states. Based on the PECs obtained, the possible photodissociation mechanisms are analyzed and suggested. Significant differences are found between the fragmentation dynamics of the neutral radical and that of the cation. While a relatively simple dissociation dynamics is predicted for CH₂I, more complex fragmentation mechanisms involving internal conversion and couplings between different excited electronic states are expected for CH₂I⁺. The species studied here are relevant to atmospheric chemistry, and the present work can help to understand better how their photodissociation may affect chemical processes in the atmosphere.

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.

An ab initio study of the photodissociation of the vinyl radical

Photodissociation of the vinyl radical through pathways CH₂CH → CH₂C + H, CH₂CH → CHCH + H, and CH₂CH → CH₂ + CH is investigated by means of high-level ab initio calculations. Potential-energy curves (PECs) along the corresponding dissociating bond distance associated with the ground and several excited electronic states involved in the above fragmentation pathways, as well as the nonadiabatic couplings connecting the different states, are obtained. The findings of several experiments on vinyl photodissociation performed at different excitation wavelengths are analyzed and explained qualitatively in the light of the present PECs. A two-dimensional representation (consisting of radial and angular coordinates to represent one of the H atoms of the CH₂ group) is also used to calculate the electronic states. The surfaces obtained reflect a rich variety of conical intersections, exit barriers, and nonadiabatic couplings leading to predissociation in different regions of energy and of the two coordinates, suggesting a complex photodissociation dynamics of the CH₂CH → CHCH + H pathway, with rather different fragmentation mechanisms involved. The two-dimensional results also provide interesting information on the mechanism of in-plane hydrogen migration from the CH₂ group to the CH one through a high-lying transition state.

Ultraviolet Photodissociation Dynamics of the Cyclohexyl Radical: The H-Atom Product Channel

The ultraviolet (UV) photodissociation dynamics of the jet-cooled cyclohexyl (c-C6H11) radical is studied using the high-n Rydberg atom time-of-flight (HRTOF) technique. The cyclohexyl radical is produced by the 193 nm photodissociation of chlorocyclohexane and bromocyclohexane and is examined in the photolysis wavelength region of 232-262 nm. The H-atom photofragment yield (PFY) spectrum contains a broad peak centered at 250 nm, which is in good agreement with the UV absorption spectrum of the cyclohexyl radical and assigned to the 3p Rydberg states. The translational energy distributions of the H-atom loss product channel, P(ET)'s, are bimodal, with a slow (low ET) component peaking at ∼6 to 7 kcal/mol and a fast (high ET) component peaking at ∼44-48 kcal/mol. The fraction of the average translational energy in the total excess energy, ⟨fT⟩, is in the range of 0.16-0.25 in the photolysis wavelength region of 232-262 nm. The H-atom product angular distribution of the slow component is isotropic, while that of the fast component is anisotropic with an anisotropy parameter of β ≈ 0.5-0.7. The bimodal product translational energy and angular distributions indicate two dissociation pathways to the H + C6H10 products in cyclohexyl. The high-ET anisotropic component is from a repulsive, prompt dissociation on a repulsive potential energy surface coupling with the Rydberg excited states to produce H + cyclohexene. The low-ET isotropic component is consistent with the unimolecular dissociation of hot radical on the ground electronic state after internal conversion from the Rydberg states. The similarity of the photodissociation dynamics of the cyclohexyl radical to the previously studied small linear and branched alkyls expands on the understanding of the dissociation dynamics of alkyl radicals to include larger cyclic alkyl radicals.

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

The photodissociation of jet-cooled cyclohexyl was studied by exciting the radicals to their 3p Rydberg state by 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 an ∼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.

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.

Ultraviolet photochemistry of ethane: implications for the atmospheric chemistry of the gas giants

Chemical processing in the stratospheres of the gas giants is driven by incident vacuum ultraviolet (VUV) light. Ethane is an important constituent in the atmospheres of the gas giants in our solar system. The present work describes translational spectroscopy studies of the VUV photochemistry of ethane using tuneable radiation in the wavelength range 112 ≤ λ ≤ 126 nm from a free electron laser and event-triggered, fast-framing, multi-mass imaging detection methods. Contributions from at least five primary photofragmentation pathways yielding CH2, CH3 and/or H atom products are demonstrated and interpreted in terms of unimolecular decay following rapid non-adiabatic coupling to the ground state potential energy surface. These data serve to highlight parallels with methane photochemistry and limitations in contemporary models of the photoinduced stratospheric chemistry of the gas giants. The work identifies additional photochemical reactions that require incorporation into next generation extraterrestrial atmospheric chemistry models which should help rationalise hitherto unexplained aspects of the atmospheric ethane/acetylene ratios revealed by the Cassini-Huygens fly-by of Jupiter.