Roaming Reaction Pathways Along Excited States

• Published in: Science (American Association for the Advancement of Science) Vol. 335; no. 6072; pp. 1054 - 1055
• Main Authors: Jordan, Meredith J. T., Kable, Scott H.
• Format: Journal Article
• Language: English
• Published: Washington American Association for the Advancement of Science 02-03-2012; The American Association for the Advancement of Science
• Subjects: Atoms; Chemical reactions; Chemicals; Coordinate systems; Ground state; Molecular chemistry; PERSPECTIVES; Photolysis
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.1218767

The photodissociation of NO
3
into NO and O
2
, an important atmospheric reaction, has no transition state but proceeds via an O atom roaming around the NO
2
core.
Transition state theory (TST) describes chemical reactions in terms of “reaction coordinates,” usually the coordinates of atoms involved in breaking and forming bonds. Typically, there is an energetic barrier, the transition state (TS), between reactants and products. In 2004, a reaction mechanism was reported that seemingly defied the tenets of TST (
1
). In the photodissociation of H
2
CO, one of the hydrogen (H) atoms “roamed” around the periphery of the HCO core, with no apparent reaction coordinate, and abstracted the other H atom to form H
2
and CO. The roaming products showed characteristic product-state distributions, distinct from those arising from a standard TS mechanism. On page 1075 of this issue, Grubb
et al.
(
2
) use detailed state-selective correlated experiments, together with theoretical calculations, to show that the photodissociation of NO
3
into NO and O
2
, an important reaction in the atmosphere, occurs via roaming reactions on both the electronic excited state and the ground state of NO
3
.