ChemInform Abstract: Roaming Atoms and Radicals: A New Mechanism in Molecular Dissociation
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, USA. Accounts of Chemical Research
(Impact Factor: 22.32).
08/2008; 41(7):873-81. DOI: 10.1021/ar8000734
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.
Available from: Stephen Wiggins
- "The general relation between configuration space topology (distribution of saddles) and phase transitions is also of great current interest . Several examples of non-MEP or non- IRC reactions [45, 57–62] and 'roaming' mechanisms       have been identified in recent years; the dynamics of these reactions is not mediated by a single conventional transition state associated with an index 1 saddle. Higher index saddles can also become mechanistically important for structural transformations of atomic clusters  when the range of the pairwise potential is reduced . "
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ABSTRACT: We study the phase space geometry associated with index 2 saddles of a potential energy surface and its influence on reaction dynamics for $n$ degree-of-freedom (DoF) Hamiltonian systems. For index 1 saddles of potential energy surfaces (the case of classical transition state theory), the existence of a normally hyperbolic invariant manifold (NHIM) of saddle stability type has been shown, where the NHIM serves as the "anchor" for the construction of dividing surfaces having the no-recrossing property and minimal flux. For the index 1 saddle case the stable and unstable manifolds of the NHIM are co-dimension one in the energy surface, and act as conduits for reacting trajectories in phase space. The situation for index 2 saddles is quite different. We show that NHIMs with their stable and unstable manifolds still exist, but that these manifolds by themselves lack sufficient dimension to act as barriers in the energy surface. Rather, there are different types of invariant manifolds, containing the NHIM and its stable and unstable manifolds, that act as co-dimension one barriers in the energy surface. These barriers divide the energy surface in the vicinity of the index 2 saddle into regions of qualitatively different trajectories exhibiting a wider variety of dynamical behavior than for the case of index 1 saddles. In particular, we can identify a class of trajectories, which we refer to as "roaming trajectories", which are not associated with reaction along the classical minimum energy path (MEP). We illustrate the significance of our analysis of the index 2 saddle for reaction dynamics with two examples.
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ABSTRACT: Electrical stimulation of neural tissue requires charge injection into the biological environment. This is achieved through both Faradaic and non-Faradaic reactions at the electrode/tissue interface. Some Faradaic reactions have the potential to dramatically alter pH levels, leading to tissue damage. The present study looked to investigate the effects of stimulus induced pH changes for a variety of stimulation parameters in a retinal implant. Electrodes were stimulated using monophasic and biphasic pulses at different intensities and in different mediums. Stimulus frequency and pulse width were maintained consistent for all tests. pH levels were recorded using a pH microelectrode and verified using a pH color indicator (phenol red). As expected, no significant pH change could be detected in buffered saline or balanced salt solution. However, stimulation parameters causing pH changes could be detected in unbuffered saline solution. While electrode stimulation using biphasic charge-balanced current pulses showed minimal pH change, stimulation using monophasic pulses showed significant pH shifts. The extent of pH change was related to duration of stimulation. The results from this study provide an insight to the electrochemical mechanisms at the interface of the electrolyte medium and retinal stimulation electrodes.
Available from: berkeley.edu
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ABSTRACT: Anion photoelectron spectroscopy (PES) has become one of the most versatile techniques in chemical physics. This article briefly reviews the history of anion PES and some of its applications. It describes efforts to improve the resolution of this technique, including anion zero electron kinetic energy (ZEKE) and the recently developed method of slow electron velocity-map imaging (SEVI). Applications of SEVI to studies of vibronic coupling in open-shell systems and the spectroscopy of prereactive van der Waals complexes are then discussed.
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