Slice imaging of the photodissociation of acetaldehyde at 248 nm. Evidence of a roaming mechanism

Rubio-Lago, Luis; Amaral, G. A.; Arregui, Andrés; Izquierdo, J. G.; Wang, F.; Zaouris, D. K.; Kitsopoulos, Theofanis N.; Bañares, Luis

doi:10.1039/b708310f

Cited by 65 publications

(107 citation statements)

References 16 publications

(28 reference statements)

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“…A preliminary quasiclassical trajectory (QCT) study by Shepler et al (14) indicated that trajectories initiated from the conventional TS (representing exclusively the conventional TS pathway) produced a CO rotational distribution much hotter than experiment, whereas the distribution obtained from trajectories initiated from the equilibrium CH 3 CHO geometry are in good agreement with the HK experiment. More recent experiments at 248 nm suggested a greater contribution to the CH 4 ϩ CO roaming channel (12), although at this energy more than one pathway can lead to formation of CO, making interpretation of the experiment potentially ambiguous.…”

mentioning

confidence: 99%

Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation

Heazlewood

Jordan²,

Kable³

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

194

267

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Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed ''roaming,'' has been described in the photodissociation of H 2CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH 3CHO to CH4 ؉ CO. The CH 4 product is found to have extreme vibrational excitation, with the vibrational distribution peaked at Ϸ95% of the total available energy. Quasiclassical trajectory calculations on fulldimensional potential energy surfaces reproduce these results and are used to infer that the major route to CH 4 ؉ CO products is via a roaming pathway where a CH 3 fragment abstracts an H from HCO. The conventional saddle-point TS pathway to CH 4 ؉ CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the CH 3 roaming mechanism is attributed to the fact that the CH3 ؉ HCO radical asymptote and the TS saddle-point barrier to CH 4 ؉ CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as H 2CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory.reaction dynamics ͉ roaming mechanisms ͉ photochemistry ͉ quasiclassical trajectories ͉ transition state S ince its introduction by Eyring in 1935 (1), the concept of the ''transition state'' (TS) has been central to chemistry because the products, rates, and dynamics of a reaction are often determined by this special molecular configuration (2). For reactions with potential barriers, the TS is a transient molecular structure at the highest point along the minimum energy path connecting reactants to products and thus is a central construct in reaction rate theory and the classification of reaction types. In transition state theory (TST), the reaction rate coefficient is obtained from the ''one-way'' flux through a dividing surface containing the TS. In the more general variational version of TST, denoted VTST, the TS dividing surface is chosen to minimize the reactive flux. This theory is widely used in mathematical modeling of reaction rates in combustion, atmospheric, and biological chemistry, impacting fields as diverse as energy production (3), climate change (4), and enzyme function (5).Although the conventional transition state paradigm will remain essential to chemists, several reaction pathways have been reported in the last 8 years (6-13) in which it is not obvious how to use present implementations of TST or VTST. If such mechanisms are common, they may represent a significant challenge for reaction rate theories. In this report, we show that the ''roaming atom mechanism'' in formaldehyde (H 2 CO) dissociation (9) is not unique to H 2 CO but also occurs in acetaldehyde (CH 3 CHO) dissociation. Moreover, in CH 3 CHO, we find that the roa...

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mentioning

confidence: 99%

Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation

Heazlewood

Jordan²,

Kable³

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

194

267

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“…The branching to each pathway obtained herein is averaged over all the J levels, similar to that by Lee et al based on integration of all rotational states, 4 but different from most J-dependence results by using ion imaging method. 2,7,8,14 Note that the low-J component for CH 3 CHO photolyzed at 308 nm is free from interference by triple fragmentation (CH 3 + CO + H). 15 The HCO obtained even at 266 nm may not undergo secondary decomposition.…”

mentioning

confidence: 99%

“…In addition, an alternative roaming pathway is initiated from the barrierless radical products (HCO + CH 3 ) on the ground state surface. [4][5][6][7][8] The incipient radicals meander with slow motion around varied configuration spaces in the weak attractive field of the moieties and ultimately undergo intramolecular abstraction to form the same molecular products.…”

mentioning

confidence: 99%

Communication: Photodissociation of CH3CHO at 308 nm: Observation of H-roaming, CH3-roaming, and transition state pathways together along the ground state surface

Tsai

Hung

et al. 2015

The Journal of Chemical Physics

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“…12 Trajectory calculations initiated at the S0 TS leads to rotationally excited CO products and partitioning of a large fraction of the available energy into translational recoil, in agreement qualitative with the experimental 5 observations. 13,14 Acetaldehyde was the second molecule to show evidence of photochemical roaming, with the observation of low-J CO formed in conjunction with highly internally excited CH4, [15][16][17][18] consistent with the dynamical signatures established for roaming in formaldehyde. [19][20][21] Quasiclassical trajectory calculations on ab initio potential surfaces supported this interpretation and indicated that roaming involved frustrated dissociation and subsequent intramolecular reaction between the CH3 + HCO radical products.…”

Section: Introductionmentioning

confidence: 97%

“…Ion imaging measurements have also detecting H atoms directly. 16 In this paper, we report a study of acetaldehyde photochemistry following excitation at wavelengths between 265-328 nm. Ion imaging experiments using single-photon VUV ionization of CH3 radicals show three distinct competitive photochemical mechanisms.…”

Section: Introductionmentioning

confidence: 99%

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

Toulson

Fishman

Murray

2018

Phys. Chem. Chem. Phys.

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Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265-328 nm. Ion images recorded probing CH3 radicals with single-photon VUV ionization show different dissociation dynamics in three distinct wavelength regions. At the longest photolysis wavelengths, λ > 318 nm, CH3 radicals are formed over tens of nanoseconds with a speed distribution that is consistent with statistical unimolecular dissociation on the S0 surface following internal conversion. In the range 292 nm ≤ λ ≤ 318 nm, dissociation occurs almost exclusively on the T1 surface following intersystem crossing and passage over a barrier, leading to the available energy being partitioned primarily into photofragment recoil. The CH3 speed distributions become bimodal at λ < 292 nm, in addition to the translationally fast T1 products a new translationally slow, but non-statistical, component appears and grows in importance as the photolysis wavelength is decreased. Photofragment excitation (PHOFEX) spectra of CH3CHO obtained probing CH3 and HCO products are identical across the absorption band, indicating that three-body fragmentation is not responsible for the non-statistical slow component. Rather, translationally slow products are attributed to dissociation on S0, accessed via a conical intersection between the S1 and S0 surfaces at extended C-C distances. Time-resolved ion images of CH3 radicals measured using a picosecond laser operating at a photolysis wavelength of 266 nm show that product formation on T1 and S0 via the conical intersection occurs with time constants of 240 and 560 ps, respectively.3

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Slice imaging of the photodissociation of acetaldehyde at 248 nm. Evidence of a roaming mechanism

Cited by 65 publications

References 16 publications

Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation

Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation

Communication: Photodissociation of CH3CHO at 308 nm: Observation of H-roaming, CH3-roaming, and transition state pathways together along the ground state surface

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

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