The Roaming Atom: Straying from the Reaction Path in Formaldehyde Decomposition

Quantum chemistry of C(3)H(6)O molecules: structure and stability, isomerization pathways, and chirality changing mechanisms

Electronic structure calculations were carried out to study the various isomers of formula C(3)H(6)O, as a part of our current quantum chemical and dynamical approaches to intra- and intermolecular kinetics for the C(n)H(2n)O (n = 1, 2, 3) molecules. The usefulness of the GRRM (global reaction route mapping) program developed by Ohno and Maeda in predicting the structure of all isomers and of the transition states connecting them is fully exploited. All the isomers are identified as local minima on the MP2/CC-PVDZ potential energy surface. Acetone is the most stable isomer. In increasing order of stability the others are propanal, 2-propenol, 1-propenol, allyl alcohol, methyl vinyl ether, cyclopropanol, propylene oxide, and oxetane. Various isomerization paths connecting them are identified. All the transition states are fully characterized using intrinsic reaction coordinate calculations. The isomerization reactions may proceed through a single step or involve an intermediate species which is either a carbene or a diradical. Special attention is devoted to propylene oxide, a favorite molecule in current photochemical and stereodynamical studies because of its chiral nature. It is a rigid molecule, and chirality switching is found to be supported by its isomers. Two different chirality switching mechanisms which are assisted by propanal and allyl alcohol are presented.

A new crossed molecular beam apparatus using time-sliced ion velocity imaging technique

A new crossed molecular beam apparatus has been constructed for investigating polyatomic chemical reactions using the time-sliced ion velocity map imaging technique. A unique design is adopted for one of the two beam sources and allows us to set up the molecular beam source either horizontally or vertically. This can be conveniently used to produce versatile atomic or radical beams from photodissociation and as well as electric discharge. Intensive H-atom beam source with high speed ratio was produced by photodissociation of the HI molecule and was reacted with the CD(4) molecule. Vibrational-state resolved HD product distribution was measured by detecting the CD(3) product. Preliminary results were also reported on the F+SiH(4) reaction using the discharged F atom beam. These results demonstrate that this new instrument is a powerful tool for investigating chemical dynamics of polyatomic reactions.

Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation

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(2)CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH(3)CHO to CH(4) + CO. The CH(4) product is found to have extreme vibrational excitation, with the vibrational distribution peaked at approximately 95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional 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 CH(3) + 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(2)CO, 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.

Roaming atoms and radicals: a new mechanism in molecular dissociation

The detailed description of chemical reaction rates is embodied in transition state theory (TST), now recognized as one of the great achievements of theoretical chemistry. TST employs a series of simplifying assumptions about the dynamical behavior of molecules to predict reaction rates based on a solid foundation of quantum theory and statistical mechanics. The study of unimolecular decomposition has long served as a test bed for the various assumptions of TST, foremost among which is the very notion that reactions proceed via a single well-defined transition state. Recent high-resolution ion imaging studies of formaldehyde unimolecular decomposition, in combination with quasiclassical trajectory calculations from Bowman and coworkers, have shown compelling evidence, however, for a novel pathway in unimolecular decomposition that does not proceed via the conventional transition state geometry. This "roaming" mechanism involves near dissociation to radical products followed by intramolecular abstraction to give, instead, closed shell products. This phenomenon is significant for a number of reasons: it resists easy accommodation with TST, it gives rise to a distinct, highly internally excited product state distribution, and it is likely to be a common phenomenon. These imaging studies have provided detailed insight into both the roaming dynamics and their energy-dependent branching. The dynamics are dominated by the highly exoergic long-range abstraction of H from HCO by the "roaming" hydrogen atom. The energy-dependent branching may be understood by considering the roaming behavior as being descended from the radical dissociation; that is, it grows with excess energy relative to the conventional molecular dissociation because of the larger A-factor for the radical dissociation. Recent work from several groups has identified analogous behavior in other systems. This Account explores the roaming behavior identified in imaging studies of formaldehyde and considers its implications in light of recent results for other systems.