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 t he three didehydrobenzenes (benzynes), six didehydropyridines (pyridynes), and six didehydropyridinium (pyridynium) cations are compared at the densit y functional, multiconfigurational self-consistent field, and coupled clust er levels of theory. The title Bergman cyclization, which produces a meta d iradical, is characterized at the density functional theory level and compa red to the analogous Bergman cyclizations generating all of the para diradi cals in the subject aryne series. Singlet-triplet splittings in the pyridyn ium cations and H-1 hyperfine couplings in the N-protonated pyridyl radical s are found to be well correlated by proportionality constants determined p reviously 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 Sci ence B.V.