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Roaming-mediated isomerization in the photodissociation of nitrobenzene


Roaming reactions comprise a new class of reaction in which a molecule undergoes frustrated dissociation to radicals, followed by an intramolecular abstraction reaction. Nitro compounds have long been known to dissociate to give NO as a major product. However, rates based upon isomerization via calculated tight transition states are implausibly slow, so the key dissociation pathway for this important class of molecules remains obscure. Here, we present an imaging study of the photodissociation of nitrobenzene with state-specific detection of the resulting NO products. We observe a bimodal translational energy distribution in which the slow products are formed with low NO rotational excitation, and the fast component is associated with high rotational excitation. High-level ab initio calculations identified a ‘roaming-type’ saddle point on the ground state. Branching ratio calculations then show that thermal dissociation of nitrobenzene is dominated by ‘roaming-mediated isomerization’ to phenyl nitrite, which subsequently decomposes to give C6H5O + NO.

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Figure 1: Direct current sliced images of NO from the photodissocation of nitrobenzene excitation at ~226 nm.
Figure 2: Total translational energy distributions derived from the images in Fig. 1.
Figure 3: State-correlated plot showing NO(J) versus total translational energy or relative velocity derived from analysis of the images.
Figure 4: Schematic potential surfaces for the ground singlet and lowest triplet states of nitrobenzene.
Figure 5: Plot of the branching ratio for dissociation of nitrobenzene versus energy relative to the simple bond fission threshold (loss of NO2).

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This work was supported by the US Army Research Laboratory and the US Army Research Office (contract/grant no. W911NF-10-1-0531). R.S.Z. acknowledges funding from the Office of Naval Research (contract no. N000140810106) and M.C.L. thanks Taiwan's NSC and MoE/ATU programmes for their support. M.L.H. also acknowledges the Office of the Vice President for Research at Wayne State University for a post-doctoral fellowship. A.G.S. acknowledges many fruitful discussions with J.M. Bowman.

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M.H. analysed the data, performed experiments and wrote portions of the paper. N.H. performed experiments and analysed data. R.S.Z. performed the calculations. A.G.S. conceived and designed the experiments. M.C.L. conceived and designed the calculations. All authors contributed to writing sections of the paper.

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Correspondence to Arthur G. Suits.

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Hause, M., Herath, N., Zhu, R. et al. Roaming-mediated isomerization in the photodissociation of nitrobenzene. Nature Chem 3, 932–937 (2011).

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