Ab initio studies of the structure, vibrational frequencies; and intensities have been carried out on the open-chain species HNnH, HNnF, and FNnF (n = 3, 4). Particular attention has been focused on the species HN3H, which is isoelectronic with NO2 and exhibits many of the same features in its electronic structure. HN3H is shown to have a planar trans structure with R(N-N) = 1.252 Angstrom, indicating considerable double-bond character. HN3H also exhibits a low-lying (2)A(2) State separated from the ground state by about 36 kcal mol(-1). Replacement of one or more of the hydrogens in HN3H by fluorines alters the electron distribution, spin density, and geometry, most importantly the NNN angle. The biradical species N-4 in its trans planar structure is shown to be of significantly higher energy than two Nz molecules.