Heteroatom (N, P, As, Sb, Bi) Effects on the Resonance-Stabilized 2-, 3-, and 4-Picolyl Radicals

Important recent experimental studies have allowed the isomer-selective identification of the 2-, 3-, and 4-picolyl radicals. The picolyl radicals and their valence isoelectronic P, As, Sb, and Bi congeners are investigated here. For the three observed parent radicals, the theoretical ionization potentials agree with experiment to within 0.02 eV. Two rules are proposed for predicting vertical ionization potentials (E_VIE) and relative energies. The E_VIE values for these radicals will be higher when large percentages of the SOMO orbitals are distributed on the atoms with greater electronegativities. The cations of these systems were also studied along with the closed-shell methylpyridines and their P, As, Sb, and Bi analogs. The energies for the cationic species will lie lower when high percentages of π natural localized molecular orbitals occur on the more electronegative atoms. The structures of the 2- and 4-isomers strongly depend upon the heteroatoms, with the C-C linkages adopting a single-double alternating bond manner when the heteroatoms become heavier. The 3-isomers adopt roughly equal C-C bond distances with small changes from N to Bi.