Systematic 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

The geometries and relative energies of the singlet and tripler 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 H-1 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) an predicted for all six pyridynes. (Int J Mass Spectrom 201 (2000) 1-15) (C) 2000 Elsevier Science B.V.