Hydrogen atom adducts to nitrobenzene: Formation of the phenylnitronic radical in the gas phase and energetics of Wheland intermediates

The phenylnitronic radical (1) was prepared in the gas phase by collisional electron transfer to stable C6H5NO2H+ cation (1(+)) and found to be stable on the microsecond time scale. The major unimolecular dissociation of 1 wa s loss of OH radical to form nitrosobenzene as determined by variable-time neutralization-reionization mass spectrometry. Ab initio calculations at th e effective QCISD(T)/6-311+G(3df,2p) level and combined Moller-Plesset and density functional theory calculations identified loss of OH as the lowest- energy dissociation of 1 that proceeded at the thermochemical threshold wit h no reverse activation barrier. Dissociations of 1 by loss of syn- and ant i-MONO. and a hydrogen atom were more endothermic than loss of OH and had a ctivation barriers above the thermochemical thresholds. The internal energy of 1 formed by electron transfer in the ground electronic state (X) was in sufficient to cause the observed dissociations. The dissociations are postu lated to take place from the metastable excited electronic B state formed b y vertical electron transfer. Wheland intermediates for hydrogen atom addit ions to the ortho (2), meter (3), para (4), and ipso (5) positions in nitro benzene were calculated to be 75, 98, 78, and 101 kJ mol(-1) less stable th an 1. Radicals 2-4 existed in substantially deep potential energy wells to allow their generation as transient intermediates. Radical 5 resided in a s hallow potential energy minimum and was predicted to dissociate exothermica lly to benzene and NO2. Relative thermal rate constants for hydrogen atom a dditions to nitrobenzene were calculated and found to correlate with the re gioselectivities for additions of other radicals.