Flowing afterglow study: The effect of nitrogen oxidation on the energetics and dynamics of diradical forming dissociation reactions

Abstract

Mixed σ and π heteroaromatic diradicals were studied in the gas phase. Homolytic bond dissociation energies and heats of formation for a series of diradicals were determined using negative ion thermochemical cycles. Dissociation energies from collision-induced dissociation reactions were used to assess the electronic structures of the diradicals. The gas-phase regioselectivity of pyridine deprotonation was determined using ion-molecule reactions and branching ratio techniques, with the 4-position being the most acidic. Hammett-Taft formalism was applied to quantitate the electronic effects of the fused nitrogen, with σF and σ R values for the nitrogen in the ring are determined to be -0.18 and 0.76, respectively. The properties of the fused nitrogen in pyridine were compared to those of the oxidized nitrogen. Pyridinium nitrogen was found to be a strong electron acceptor, regardless of the complexing agent. When oxidized, however, it was found that the pyridinium nitrogen accepted electrons either from the complexed oxygen or the aromatic system, dependant upon the reaction center electron demand. The apparent heats of formation for the uncomplexed and the oxidized α,n-dehydro-3-picoline diradicals were determined by measuring the threshold energy (Eϒ ), for dissociation of the halo-3-picolinyl precursor anions and incorporating this value into a thermochemical cycle. The effect of the oxidation of the nitrogen on the electronic structure of the diradical was to preferentially stabilize the triplet state in the 4-position, and the singlet state in the 2-position