Tuesday, February 21, 2012

Roaming-mediated isomerization in the photodissociation of nitrobenzene

M. L. Hause, N. Herath, R. Zhu, M. C. Lin, A. G. Suits Nature Chemistry 2011, 3, 932 (Paywall)
Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry with permission

The roaming mechanism has gained some traction as a recognizable model.1,2 This mechanism involves typically the near complete dissociation of a molecule into two radical fragments. But before they can completely separate they form a loose complex on a flat potential energy surface. The two fragments can then wander about each other (the “roaming” part of the mechanism), eventually finding an alternative exit channel. The first example was the dissociation of formaldehyde which forms the complex H + CHO.3 The hydrogen atom roams over to the other side of the HCO fragment and then abstracts the second hydrogen atom to form H2 and CO – with the unusual signature of a hot H2 molecule and CO in low rotational/vibrational states.

The photodissociation of nitrobenzene is now suggested to also follow a roaming pathway.4 Bimodal distribution is found for the NO product channel. There is a slow component with low J and a fast component with high J. This suggests two different operating mechanisms for dissociation.

G2M(CC1)/UB3LYP/6-311+G(3df,2p) computations provide the two mechanisms. Near dissociation to phenyl radical and NO2 can lead to a roaming process that eventually leads to recombination to form phenyl nitrite, which can then dissociate to the slow NO product. The fast NO product is suggested to come from rearrangement of nitrobenzene to phenylnitrite on the triplet surface, again eventually leading to loss of NO, but with high rotational excitation.

References
(1) Herath, N.; Suits, A. G., "Roaming Radical Reactions," J. Phys. Chem. Lett. 2011, 2, 642-647, DOI: 10.1021/jz101731q

(2) Bowman, J. M.; Suits, A. G., "Roaming reactions: The third way," Phys. Today 2011, 64, 33-37, DOI: 10.1063/PT.3.1330

(3) Townsend, D.; Lahankar, S. A.; Lee, S. K.; Chambreau, S. D.; Suits, A. G.; Zhang, X.; Rheinecker, J.; Harding, L. B.; Bowman, J. M., "The Roaming Atom: Straying from the Reaction Path in Formaldehyde Decomposition," Science 2004, 306, 1158-1161, DOI: 10.1126/science.1104386.

(4) Hause, M. L.; Herath, N.; Zhu, R.; Lin, M. C.; Suits, A. G., "Roaming-mediated isomerization in the photodissociation of nitrobenzene," Nat. Chem 2011, 3, 932-937, DOI: 10.1038/nchem.1194