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 was loss of OH radical to form nitrosobenzene as determined by variable-time neutralization-reionization mass spectrometry. Ab initio calculations at the 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 with no reverse activation barrier. Dissociations of 1 by loss of syn- and anti-MONO. and a hydrogen atom were more endothermic than loss of OH and had activation barriers above the thermochemical thresholds. The internal energy of 1 formed by electron transfer in the ground electronic state (X) was insufficient to cause the observed dissociations. The dissociations are postulated to take place from the metastable excited electronic B state formed by vertical electron transfer. Wheland intermediates for hydrogen atom additions to the ortho (2), meter (3), para (4), and ipso (5) positions in nitrobenzene were calculated to be 75, 98, 78, and 101 kJ mol(-1) less stable than 1. Radicals 2-4 existed in substantially deep potential energy wells to allow their generation as transient intermediates. Radical 5 resided in a shallow potential energy minimum and was predicted to dissociate exothermically to benzene and NO2. Relative thermal rate constants for hydrogen atom additions to nitrobenzene were calculated and found to correlate with the regioselectivities for additions of other radicals.