Arenium ions deactivating groups

The nitro group is deactivating and directs the incoming electrophile to the meta position. Again, examination of the arenium ions provides an explanation for these results. First, consider the arenium ion produced by attack of the electrophile at an ortho position ... 

The situation is entirely different in the gas phase, especially at low pressures, where coUisional deactivation cannot prevent extensive fragmentation and isomerization of the primary, excited intermediates. This is clearly demonstrated by an apparent t/ b ratio below imity, by the predominant meta orientation, attributable to isomerization of the primary arenium ions into (V) and by the large yields of tritiated products (such as CeHsT from benzene and CeHsCTa from toluene) whose formation requires extensive isotopic scrambling, group migration, and fragmentation. 

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