Dynamics of chemical bond: general discussion

Roaming Under the Microscope: Trajectory Study of Formaldehyde Dissociation

“…Transition-state theory has been a bedrock method for calculating the rate constants of chemical processes, − yet, at least in its most common form, it is based on the notion that molecular motions predominantly follow a “minimum energy path.” There is increasing evidence that highly excited molecules often bypass this minimum energy path and react in unexpected ways from unexpected geometries. An early, confirmed example of this behavior, often called “roaming,” was stimulated by and speculated about in experimental work on formaldehyde dissociation in the research groups of Moore − and then confirmed by the research groups of Suits − and Bowman. ,− Reviews of this research − and further studies of formaldehyde dissociation and the H + HCO reaction ensued. − Similar roaming behavior has subsequently been found in other unimolecular dissociations, including those of acetaldehyde, − larger aldehydes, methyl formate, − acetone, alkanes, NO 3 , ,, nitromethane, − methyl nitrite, carbon dioxide, and Criegee intermediates . Roaming in bimolecular reactions has also been observed and discussed. − Several reviews of this active area of research have recently appeared. − Consequently, it is now well-established that many systems react via trajectories that deviate strongly from the minimum energy path.…”

Rovibrationally Excited Molecules on the Verge of a Triple Breakdown: Molecular and Roaming Mechanisms in the Photodecomposition of Methyl Formate

“…Roaming has been originally found to occur in the photodissociation of formaldehyde H 2 CO and later discovered in acetaldehyde CH 3 CHO, − methyl formate, , and propionaldehyde observing CO fragments, but this mechanism has been also found in the photodissociation of NO 3 and of even more complex molecules such as nitrobenzene C 6 H 5 NO 2 . In the case of the aldehydes, recent papers, − including extensions to higher members of the series, have shown that, as the molecular size increased, the direct pathway decreased in importance with respect to the roaming one, to the point of becoming negligible (see ref for a résumé of some milestones and ref for contributions to current debates).…”

“…In the present work, we study the photolysis of methyl formate by focusing on the product CO, which, because of its well-characterized spectroscopic features, is a favorite probe in the laboratory and in various contexts, including astrophysics. , In our recent studies − of the photodissociation performed at various different wavelengths, we determined the energy threshold for the triple fragmentation into H, CH 3 O, and CO, measuring by ion imaging the CO translational energies for sets of selected rotational states of CO in its ground and first excited vibrational levels (υ = 0,1); additionally we performed measurements on the H and HCO fragments. The substantial accompanying support of quantum chemical descriptions of the selective rupture of chemical bonds and of molecular dynamics simulations of the evolution of their breakdown along dissociation paths, pointed out the involvement of nonadiabatic effects near a conical intersection in connection with the presence of signatures of roaming.…”

Roaming: A Phase Space Perspective

“…This work was made possible by the timely development of experimental and computational methods that could be employed in tandem, leading to significantly deeper understanding of the details of the reac-tion mechanism than had previously been possible. Since this pioneering work, there have been numerous review papers dealing with different aspects of roaming (4)(5)(6)(7)(8)(9).…”