Heteroatomic substitution in aromatic σ biradicals:: the six pyridynes

Interactions between the nitrogen lone pair and the biradical centers are weak in 3,4- and 3,5-pyridyne,but can be large enough in the remaining pyridynes (where at least one radical site is adjacent to nitrogen) to change singlet-triplet splittings by up to 19.6 kcal/mol compared to the corresponding benzynes. Singlet pyridynes having one dehydro site at the 2-position are stabilized relative to analogous benzynes because of increased contributions from zwitterionic resonance structures. this stabilization is not available to the triplets. Singlet-triplet splittings in the pyridynes and H-1 hyperfine couplings in the pyridyl radicals are well correlated. (C) 1998 Elsevier Science B.V. All rights reserved.