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.
Bibliographical noteFunding Information:
We are grateful for high-performance vector and parallel computing resources made available by the Minnesota Supercomputer Institute and the University of Minnesota-IBM Shared Research Project, respectively. This work was supported in part by the National Science Foundation. Eric Patterson is thanked for research assistance.
Copyright 2018 Elsevier B.V., All rights reserved.