Heteroatomic Substitution in Aromatic Sigma Biradicals: The Six Pyridynes

Cramer, C. J.; Debbert, S.
Chem. Phys. Lett. 1998, 287, 320.

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 ¹H hyperfine couplings in the pyridyl radicals are well correlated.

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