Photodissociation of Acetaldehyde, CH<sub>3</sub>CHO → CH<sub>4</sub> + CO:  Direct ab Initio Dynamics Study

Kurosaki, Yasuo; Yokoyama, Keiichi

doi:10.1021/jp020998f

Cited by 41 publications

(37 citation statements)

References 48 publications

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“…However, the internal energy of the CH 4 is most highly populated with Ϸ52% of the available energy. Finally, the v-J vector correlation was also reported from the quasiclassical trajectory calculations (9), which showed almost perfect vЌJ alignment for dissociation via the 3-center TS.…”

Section: Discussionmentioning

confidence: 83%

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Houston

Kable²

2006

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

200

254

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Product state distributions of the CO produced in the 308-nm photolysis of acetaldehyde show clear evidence of two dissociation mechanisms. One is attributed to the conventional transition state mechanism predicted by theory, with high rotational and translational energy of the CO and a pronounced vЌJ vector correlation. However, as much as 15% of the reaction flux proceeds via another pathway that produces low CO rotational and translational energy, very high CH 4 internal energy, and no correlation between the CO velocity and angular momentum vectors. The attributes of this channel are dynamically similar to the recently reported ''roaming atom'' mechanism in formaldehyde. We therefore speculate that the second pathway in acetaldehyde also occurs via a roaming mechanism in the CH 3 ؉ HCO exit channel that decays into the CH4 ؉ CO channel.photochemistry ͉ photodissociation dynamics ͉ product state distributions ͉ roaming atom ͉ transition state T he concept of the transition state (TS) is central to chemistry. It is the transient structure at the highest point of the minimum energy pathway (reaction coordinate) between reactants and products. Reaction mechanisms are often described with reference to this structure. For example, an S N 2 reaction is defined by the transient 5-center carbon atom at the TS, or a 3-center elimination reaction refers to the transient 3-membered ring at the TS. Interpretation of the kinetics and thermodynamics of reactions is also based on the energy and entropy of the TS-from the simplest Arrhenius model to more sophisticated TS theories, including their variational forms.What would happen if reactions were found to bypass the TS? The result would be a new class of reaction mechanisms, with reaction products and kinetics that cannot be predicted by current TS theories. Recently, a mechanism of this type was reported, in which the photodissociation of H 2 CO to H 2 ϩ CO was observed to occur via a second, non-TS mechanism (1). The conventional 3-center elimination of H 2 from H 2 CO has long been known to produce very high relative translational energy of the departing fragments, highly rotationally excited CO, and modest vibrational energy of the H 2 fragment (2). The new mechanism revealed a very different signature: rotationally cold CO, coupled with very high vibrational excitation of H 2 and low relative translational energy. The mechanism was interpreted by reference to quasiclassical trajectory calculations on a high-level ab initio potential energy surface (1). These calculations revealed that all of the reactions that produced the unusual signature circumvented the TS. The reaction starts out looking like a conventional C-H bond cleavage, which would produce the radical products of H 2 CO photolysis: H ϩ HCO. In a significant number of trajectories, just enough energy is tied up in internal motion of the HCO moiety so that the H-atom cannot quite escape; it turns back at very long distance (several angstroms) from the HCO but is still loosely bound. At this range, vibrationa...

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Section: Discussionmentioning

confidence: 83%

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Houston

Kable²

2006

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

200

254

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“…Acetaldehyde ͑CH 3 CHO͒ photodissociation has been the subject of considerable interest for many years both from the experimental 1-20 and theoretical [21][22][23][24][25][26][27][28][29][30][31][32][33][34] points of view. In fact, this molecule is a key case in the field of reaction dynamics, representing a beautiful example where theoretical work has played a fundamental role in the explanation of the vast experimental data.…”

Section: Introductionmentioning

confidence: 99%

“…1͒, the geometry of which has been determined by ab initio calculations. 11,25,26,28 The theoretical data available at that time were enough to describe the main experimental findings: a relatively large kinetic energy release due to the steep down slope on the PES once the TS ‫ء‬ is surmounted, a large rotational excitation and no vibrational excitation in CO, and vibrationally hot CH 4 . However, in 2006, Houston and Kable 14 described the existence of another type of CO fragments after the photodissociation of acetaldehyde at 308 nm.…”

Section: Introductionmentioning

confidence: 99%

Imaging the radical channel in acetaldehyde photodissociation: Competing mechanisms at energies close to the triplet exit barrier

Amaral

Arregui

Rubio-Lago

et al. 2010

The Journal of Chemical Physics

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The photodissociation of acetaldehyde in the radical channel has been studied at wavelengths between 315 and 325 nm using the velocity-map imaging technique. Upon one-photon absorption at 315 nm, the molecule is excited to the first singlet excited state S(1), which, in turn, undergoes intersystem crossing to the first excited triplet state T(1). On the triplet surface, the molecule dissociates into CH(3) and HCO radicals with large kinetic energy release (KER), in accordance with the well characterized exit barrier on T(1). However, at longer wavelengths (>320 nm), which correspond to excitation energies just below the triplet barrier, a sudden change in KER is observed. At these photolysis wavelengths, there is not enough energy to surpass the exit barrier on the triplet state, which leaves the possibility of unimolecular dissociation on S(0) after internal conversion from S(1). We have characterized the fragments' KER at these wavelengths, as well as determined the energy partitioning for the radical fragments. A new accurate estimate of the barrier height on T(1) is presented.

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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: 52%

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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Photodissociation of Acetaldehyde, CH₃CHO → CH₄ + CO: Direct ab Initio Dynamics Study

Cited by 41 publications

References 48 publications

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Imaging the radical channel in acetaldehyde photodissociation: Competing mechanisms at energies close to the triplet exit barrier

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

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Photodissociation of Acetaldehyde, CH3CHO → CH4 + CO: Direct ab Initio Dynamics Study

Cited by 41 publications

References 48 publications

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Imaging the radical channel in acetaldehyde photodissociation: Competing mechanisms at energies close to the triplet exit barrier

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

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Photodissociation of Acetaldehyde, CH₃CHO → CH₄ + CO: Direct ab Initio Dynamics Study