Question number 228 posted by Professor Arunan
Professors K.-C Lin and V Aquilanti answered The question posted by Professor Arunan on
whether there is documentation on roaming other than in photofragmentations and isomerizations
( i.e. whether the phenomenon is essentially limited to decomposition of highly excited molecules
and to supra-thermal unimolecular processes), has been circulated widely since the nomenclature
was introduced a decade ago. Even previous extensive chemical reactions experimentally
investigated in decades of chemical kinetics and theoretically tackled by quantum mechanics or
simulated by classical trajectories, have been carefully scrutinized to possibly discover previously
overlooked manifestations of roaming, specifically in bimolecular elementary chemical processes.
The search gave so far no convincing evidence, although the updated review by Bowman1 cites as
candidates three recent papers on bimolecular reactions by classical trajectories and a couple of
quantum investigations of the MgH2 system. Here we add similar behavior found in SN2
bimolecular reactions,2 although in references to this important subject no use is normally made of
the term “roaming” to characterize a possibly similar dynamical phenomenon. Inspired by crossed
molecular beams studies with oriented reactants,3 in a recent quantum mechanical first-principles
study of the OH + HBr  H2O + Br reaction,4 which shows a striking anti Arrhenius temperature
dependence, we found arguable to label as roaming the effect occurring in the search of the severely
restricted mutual orientation of reactants, necessary for the favorable H-atom exchange.. This
represents an example of a stereodynamical origin of roaming in a bimolecular chemical reaction,
for which the phenomenology is still meager. Work on extending transition-state based theories to
cope with these situations is cited in Ref1.
References
1 J. M, Bowman, Roaming, Mol. Phys.,2014,112,2516-2528
2 J. Mikosch, S. Trippel, C. Eichhorn, R. Otto, U. Lourderaj, J. X. Zhang, W. L. Hase, M.
Weidemuller, R. Wester, Imaging Nucleophilic Substitution Dynamics, Science, 2008, 319,183-186
3 T. Kasai, D.-C. Che, M. Okada, P.-Y. Tsai, K.C. Lin, F. Palazzetti, V. Aquilanti. Directions of
chemical change: experimental characterization of the stereodynamics of photodissociation and
reactive processes: Phys. Chem. Chem. Phys. 16, 9776-9790 (2014)
4 N.D. Coutinho, V.H.C. Silva, H.C.B. de Oliveira, A.J. Camargo, K.C. Mundim, V. Aquilanti.
Sterodynamical origin of anti-Arrhenius kinetics: negative activation energy and roaming for a fouratom reaction.to be submitted, 2015.