The geometries and relative energies of the singlet and triplet states of the three didehydrobenzenes (benzynes), six didehydropyridines (pyridynes), and six 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 density functional theory 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 six pyridynes. (C) 2000 Elsevier Science B.V.
- Enediyne cyclization
- Singlet-triplet splitting