Comparison of the Benzynes, Pyridynes, and Pyridynium Cations and Characterization of the Bergman Cyclization of Z-But-1-en-3-yn-1-yl Isonitrile to the meta Diradical 2,4-Pyridyne

Debbert, S. L.; Cramer, C. J.
Int. J. Mass Spectrom. 2000, 201, 1.

The geometries and relative energies of the singlet and triplet states of the 3 didehydrobenzenes (benzynes), 6 didehydropyridines (pyridynes), and 6 didehydropyridinium (pyridynium) cations are compared at the density functional, multiconfigurational self-consistent-field, and coupled cluster levels of theory. The title Bergman cyclization, which produces a meta diradical, is characterized at the DFT level and compared to the analogous Bergman cyclizations generating all of the para diradicals in the subject aryne series. Singlet-triplet splittings in the pyridynium cations and ¹H hyperfine couplings in the N-protonated pyridyl radicals are found to be well correlated by proportionality constants determined previously for other arynes. Heats of formation (298 K) are predicted for all 6 pyridynes.

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