  Since roaming was found as a new but common reaction path of isomerization,
many of its properties, especially those of roaming transition state (TSR),
have been studied recently on many systems. However, the mechanism of roaming
is still not clear at the atomic level. In this work, we used first-principles
calculations to illustrate the detailed structure of TSR in an internal
isomerization process of nitrobenzene. The calculations distinctively show its
nature of antiferromagnetic coupling between two roaming fragments. Moreover,
the effect of dispersion is also revealed as an important issue for the
stability of the TSR. Our work provides a new insight from the view of
electronic structure towards the TSR and contributes to the basic understanding
of the roaming systems.
