Evidence for a second molecular channel in the fragmentation of formaldehyde

Zee, Roger D. van; Foltz, M. Frances; Moore, C. Bradley

doi:10.1063/1.465335

Cited by 138 publications

(158 citation statements)

References 34 publications

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“…The CO rotational distributions reported here for Mechanism B are quite similar to the CO rotational distributions observed in the photodissociation of formaldehyde (2). In that study, when the photolysis energy was below the threshold for the radical H ϩ HCO channel, the CO distribution was also Gaussian, and peaked at high J.…”

Section: Discussionsupporting

confidence: 66%

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Houston

Kable²

2006

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

199

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: Discussionsupporting

confidence: 66%

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Houston

Kable²

2006

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

199

254

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“…striking signature of the second pathway, the lack of significant alignment of the CO molecular rotation relative to the velocity vector of the separating CH 4 and CO products. This implies that the abstraction of the H atom in HCO by the CH 3 fragment occurs at a variety of relative orientations.…”

Section: Attempts To Develop Fully Quantum Versionsmentioning

confidence: 99%

“…' Houston and Kable (1) report another likely example of the ''roaming'' pathway in the photodissociation of CH 3 CHO. In this case, it is the methyl radical that may be roaming about the HCO fragment and abstracting the H atom to form vibrationally excited CH 4 (and rotationally cold CO). By contrast, the CH 4 molecular product formed by the TS is not vibrationally very excited, but the CO product is rotationally excited.…”

mentioning

confidence: 99%

Skirting the transition state, a new paradigm in reaction rate theory

Bowman

2006

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

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“…When excited by photons of sufficient energy, the formaldehyde molecule can dissociate via two channels: H2CO → H + HCO (radical channel) or H2CO → H2 + CO (molecular channel). In earlier work, van Zee et al (3) found that, above the energy threshold for the H + HCO dissociation channel, the CO rotational state distribu-tion exhibited an intriguing shoulder at lower rotational levels correlated with a hot vibrational distribution of H2 coproduct. This observed product state distribution did not fit well with the traditional picture of the dissociation of formaldehyde via the well-characterized saddle point transition state for the molecular channel.…”

Section: Formaldehydementioning

confidence: 99%

Roaming: A Phase Space Perspective

Mauguière

Collins

Kramer

et al. 2017

Annu. Rev. Phys. Chem.

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In this review we discuss the recently described roaming mechanism for chemical reactions from the point of view of nonlinear dynamical systems in phase space. The recognition of the roaming phenomenon shows the need for further developments in our fundamental understanding of basic reaction dynamics, as is made clear by considering some questions that cut across most studies of roaming: Is the dynamics statistical? Can transition state theory be applied to estimate roaming reaction rates? What role do saddle points on the potential energy surface play in explaining the behavior of roaming trajectories? How do we construct a dividing surface that is appropriate for describing the transformation from reactants to products for roaming trajectories? How should we define the roaming region? We show that the phase space perspective on reaction dynamics provides the setting in which these questions can be properly framed and answered. We illustrate these ideas by considering photodissociation of formaldehyde. The phase-space formulation allows an unambiguous description of all possible reactive events, which also allows us to uncover the phase space mechanism that explains which trajectories roam, as opposed to evolving toward a different reactive event.

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Evidence for a second molecular channel in the fragmentation of formaldehyde

Cited by 138 publications

References 34 publications

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Photodissociation of acetaldehyde as a second example of the roaming mechanism

Skirting the transition state, a new paradigm in reaction rate theory

Roaming: A Phase Space Perspective

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